Dissertations / Theses on the topic 'Magnetocaloric materials, hard magnetic materials'

The magnetic properties of NiMnGe1−xAlx, Ni50Mn35(In1−xBx)15, Ni50Mn35In14.5B0.5 (Bulk, As-Solidified and Annealed melt-spun ribbon) and RE-Infuse Carbon Nanotubes, have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. Partial substitution of Al for Ge in NiMnGe1−xAlx results in a first-order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ∆SM = -17.6 J/kg K for ∆H = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. This value is comparable to those of well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (∆ST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ∆SM of -5.8 and -4.8 J/kg K for ∆H = 5 T and up to 9T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5-400 K ). Direct adiabatic temperature change (ΔTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. The structural phase transition temperatures, phase structure, and parameters of the magnetocaloric effect (MCE) of Ni50Mn35In14.5B0.5 as Bulk, As-Solidified and Annealed melt-spun ribbon has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (oH) up to 5 T, and in the temperature interval 5–400 K). Magnetic and structural transitions in Ni50Mn35In14.5B0.5 as ribbons were found to coincide in Ni50Mn35In14.5B0.5 bulk sample, leading to a large magnetocaloric effects associated with the first-order magnetostructural phase transition. In comparison to the bulk Ni50Mn35In14.5B0.5 alloys, both the martensitic transition temperature (TM) and Curie temperature (TC) shifted to lower temperatures. Magnetic measurements revealed that the ribbons undergo a structure transformation similar to the bulk material at the martensitic transformation. The temperature of the transformation depends strongly on lattice parameters of the ribbons. MST shows a weak broad magnetic transition at TCM∼ 160 K, while the Curie temperature of AST TCA is ∼ 297 K. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X = In, Al, and Ge) Heusler alloys. These results suggest the possibility to control the martensitic transition in Ni50Mn35In14.5B0.5 through rapid solidification process. A comparison of magnetic properties and magnetocaloric effects in Ni50Mn35In14.5B0.5 alloys as Bulk, As-Solidified and Annealed ribbons is discussed. Carbon nanotube (CNT)/metal-cluster-based composites are envisioned as new materials that possess unique electronic properties which may be utilized in a variety of future applications. Super paramagnetic behavior was reported for CNTs with Gd ions introduced into the CNT openings by internal loading with an aqueous GdCl3 chemical process. In the current work, the magnetic properties of the CNT/Gd composites were obtained by the joining and annealing of Gd metal and CNTs at 850 °C for 48 h. Energy dispersive X-ray analysis shows the presence of Gd intermingled with the CNT walls with maximum and average Gd concentrations of about 20% and 4% (by weight), respectively. The Gd clusters have a non-uniform distribution and are mostly concentrated at the ends of the CNTs. A ferromagnetic-type transition at TC ∼ 320 K, accompanied by jump like change in magnetization and temperature hysteresis typical for the temperature induced first order phase transitions has been observed by magnetization measurements. It was found that Gd infused into the CNTs by the annealing results in a first order paramagnetic-ferromagnetic transition at TC = 320 K.

The effect of doping on the crystal structure, magnetic, magnetocaloric and transport properties of MnM′Ge (M′ = Ni, Co) intermetallic compounds and NiMnX (X = Sn, In) Heusler alloys have been studied by room temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. The studied magnetic systems include Ni1-xCrxMnGe1.05 (0 ≤ x ≤ 0.120), Mn1-xAlxCoGe (0 ≤ x ≤ 0.05), MnCo1-xZrxGe (0.01 ≤ x ≤ 0.04), Mn1-xAgxCoGe (0.01 ≤ x ≤ 0.10), Ni50-xRxMn35Sn15 (x = 0, 1 and R = La, Pr, Sm), Ni43-xRxMn46Sn11 (x = 0, 1 and R = Pr, Gd, Ho, Er), and Ni50Mn35In15-xBix (0 ≤ x ≤ 1.5).A temperature induced first-order structural transition characterized by a change in crystal structure from high temperature austenite phase (AP) with Ni2In-type Hexagonal structure to low temperature martensite phase (MP) with TiNiSi-type orthorhombic structure was observed at T = TM (martensitic transition temperature) in some of the MnM′Ge-based compounds. The partial substitution of doping elements such as Cr, Al, Zr, and Ag resulted in a decrease in TM and at certain concentration, TM was found to decrease below / coincide with the ferromagnetic transition temperature (TC) of AP. Therefore, such system show a first-order magnetostructural transition (MST).In Ni1-xCrxMnGe1.05, a MST from antiferromagnetic (AFM) orthorhombic to ferromagnetic (FM) hexagonal phase was observed for 0.105 ≤ x ≤ 0.120. Both direct and inverse MCE were observed in this compound. The peak values of the magnetic entropy change (ΔSMpeak ) in the vicinity of TC for ΔH = 5T were found to be 4.5 J/kg K, 5.6 J/Kg K, and 5.1 J/Kg K for x = 0.105, 0.115, and 0.120 respectively. A magnetic field-induced transition from an AFM to a FM state in the martensite structure was observed in annealed Ni0.895Cr0.105MnGe1.05 melt-spun ribbons, which led to a coupled MST from a FM martensite to a PM austenite phase with a large change in magnetization. As a result of the field-induced MST, a large ΔSMpeak value of 16.1 J kg-1 K-1 (which is about a four times larger than the bulk) and Refrigeration capacity (RC-1) =144 J kg-1 at μ0∆H = 5 T was found. It was also found that the ribbon samples showed excellent magnetic reversibility that is important for application. MCE parameters, adiabatic temperature change (∆Tad) and |〖∆S〗_M |, with maximum value of ~ 2.6 K (µoH = 10 T) and 4.4 J kg-1 K-1(µo∆H = 5 T), respectively, were observed in the vicinity of TC. The ∆Tad (T) curves obtained for µoΔH = 10 T and magnetization isotherms were found to be completely reversible, which indicates the reversibility of the MCE in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ∆Tad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC in accordance with Landau’s theory of phase transitions.In MnCoGe compounds doped with Al, Zr, and Ag, a tunable MST from the paramagnetic hexagonal to ferromagnetic orthorhombic phase was observed. The maximum ΔSM values of about 18, 7.2, and 22 J kg-1 K-1for ∆H = 5T at TM was observed for Al, Zr, and Ag doped compounds, respectively. The corresponding maximum value of RC was found to be (303, 266, and 308) JKg-1.The new compounds containing low concentration of rare earth (R) metals: Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, with R = La, Pr, Sm, Gd, Ho, Er and Ni50Mn35In15-xBix were synthesized. The compounds crystallized in the cubic L21 austenite phase (AP) or a mixture of AP and low temperature martensitic phase (MP) at room temperature. For Ni50-xRxMn35Sn15 and Ni43-xRxMn46Sn11 alloys, TM shifted towards higher temperature with rare-earth doping, thus stabilizing the MP at higher temperature. A maximum shift in TM by ~ 60-62 K relative to the parent compound (TM = 190-195 K) was observed for the Ni49LaMn35Sn15 and Ni42PrMn46Sn11. TM shifted towards lower temperature if Bi is placed in In position in Ni50Mn35In15-xBix. A maximum shift of ~ 36 K was detected for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for Bi concentration ≥ 0.5.The ground state magnetization decreased with the rare-earth doping and increasing Bi content. The compounds exhibit both inverse and normal magnetocaloric effects. Large values of ∆SM = 12 (Ni49PrMn35Sn15), 32 Jkg-1K-1(Ni42PrMn46Sn11), 28 Jkg-1K-1 (Ni42GdMn46Sn11), 25 Jkg-1K-1 (Ni42HoMn46Sn11), 40 J/kg K (Ni50Mn35In15) and 34 J/kg K (Ni50Mn35In15-xBix, x = 0.25) were found at TM for ∆H = 5T that can be tuned in a wide temperature range. RC values ranging from 267-336 Jkg-1 at TC, 182 -250 Jkg-1 at TM and 144-165 Jkg-1 at TC were found with ∆H = 5T for Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, and Ni50Mn35In15-xBix, respectively. Significant magnetoresistance (MR) values of -30%, -20 % and -30% were observed in Ni49LaMn35Sn15, Ni42GdMn46Sn11, and Ni50Mn35In14.5Bi0.5 compounds, respectively, at TM and ∆H = 5T. A large exchange bias effect with HEB ~ 1.1 kOe at 10 K was observed for the Ni42PrMn46Sn11 compound in its MP. Thus, the pronounced multifunctional properties such as shape memory effects, MCE, EB, and MR make these new systems promising for the ongoing development of magnetocaloric and multifunctional technologies.

Parmi les aimants sans terres raresactuellement étudiés, τ-MnAl ferromagnétique est uncandidat à haut potentiel, car il possède despropriétés magnétiques intrinsèques prometteuses.Dans cette thèse, Mn(Fe)AlC a été synthétisé parbroyage mécanique. Les effets du carbone sur lamicrostructure et les propriétés magnétiques ont étéétudiés. Les résultats montrent qu’une pureté élevéede τ-MnAl(C) pouvait être obtenue avec un dopage à2% en atomes de carbone, montrant clairement l’effetstabilisant du carbone. L’alliage Mn54.2Al43.8C2possède les meilleures propriétés magnétiques :aimantation à 2T M2T = 414 kAm-1, aimantationrémanente Mr = 237 kAm-1, coercivité HC = 229 kAm-1et |BH|max = 11,2 kJm-3. HC augmente inversementproportionnellement avec la taille des cristallites de laphase τ et proportionnellement à la teneur en C. Descalculs ab initio confirment l’effet stabilisant etindiquent les positions interstitielles préférentielles ducarbone dans la maille quadratique de la phase τ-MnAl.Les alliages Mn51-xFexAl47C2 (x = 0,25, 0,5, 1, 2, 4, 6)ont également été synthétisés par broyagemécanique, montrant une pureté élevée de la phaseτ jusqu'à un taux de substitution de 2% du Mn par duFe. L'ajout de Fe dans MnAl(C) réduit l'aimantationet TC, mais augmente légèrement la valeur de HC. Laspectrométrie 57Fe Mössbauer à 300K a été utiliséepour sonder l'environnement local dans ε-, τ-, β- etγ2-MnFeAl(C). γ2-, ε- et β-MnFeAl(C) présentent unestructure hyperfine quadripolaire alors que τ-Mn50.5Fe0.5Al47C2 montre une structutr hyperfinemagnétique assez complexe. Une expérience despectrométrie Mössbauer effectuée à bassetempérature (10K) et sous champ magnétique (8T)montre un ordre ferromagnétique local non colinéairedes moments magnétiques de Fer par rapport à ladirection du champ appliqué. Le champ hyperfin del’alliage MnFeAl calculé par Wien2k confirme lespropriétés magnétiques et les résultats despectrométrie Mössbauer
Among currently investigated rare-earth-free magnets, ferromagnetic τ-MnAl is a highly potential candidate as having promising intrinsic magnetic properties. In my thesis, Mn(Fe)AlC was synthesized by mechanical alloying method. Effects of carbon on microstructure and magnetic properties were systematically investigated. It was found that high purity of τ-MnAl(C) could be obtained at 2 at.% C doping, showing clearly stabilizing effect of carbon. Mn54.2Al43.8C2 has the best magnetic properties: magnetization at 2T M2T = 414 kAm-1, remanent magnetization Mr = 237 kAm-1, coercivity HC = 229 kAm-1, and |BH|max = 11.2 kJm-3. HC increased inversely with the crystallite size of τ phase and proportionally with C content. Moreover, first principle calculation showed both stabilizing effect and preferable interstitial positions of carbon in tetragonal τ-MnAl. Mn51-xFexAl47C2 (x= 0.25, 0.5, 1, 2, 4, 6) alloys were also synthesized by mechanical alloying method, showing high purity of τ phase up to 2 at.% Fe doping. Adding of Fe on MnAl(C) reduced both magnetization and TC but likely increased slightly HC. 57Fe Mössbauer spectrometry at 300K was used to probe local enviroment in ε-, τ-, β-, and γ2-MnFeAl(C). In which, γ2-, ε-, and β-MnFeAl(C) exhibited a quadrupolar structure while τ -Mn50.5Fe0.5Al47C2 spectrum showed a rather complex magnetic hyperfine splitting. The interaction between Fe and Mn examined by in-field Mössbauer measurement at 10 K and 8 T showed a non-collinear magnetic structure between Fe and Mn with different canting angles at different sites. Hyperfine field of MnFeAl alloy calculated by Win2k supported both magetic properties and Mossbauer results

The attention for materials displaying high magnetocaloric effect (MCE) has grown during the past 30 years. One of the most important properties of MCE is the adiabatic temperature change ( ). The main aim of this work was to develop a method to measure the temperature change ( ) for magnetocaloric materials in a changing magnetic field.  A technique was developed where maximum reached  for Gadolinium was 1.19 K in a changing magnetic field of 1.3 T, however, this is lower value in comparison with previous studies (3.3 K in a changing magnetic field of 1 T, Bjørk, et al., 2010) which makes the developed method not sufficient enough to measure . Furthermore, finding novel materials displaying high MCE is of great interest. MnFePSiB alloys display promising MCE properties but processing method is expensive and time consuming. Therefore, a MnFePSiB compound was simply remelted several times and heat treated to enhance its properties. The MnFePSiB alloy was remelted 1, 2 and 3 times after initial casting. Melting the material 3 times resulted improvement in both the magnetic and magnetocaloric properties due to enhanced homogeneity. The material melted 3 times was further heat treated to improve its magnetic magnetocaloric properties. Heat treating the material for 5 hours at 1373K improved the magnetic entropy change more than 10 times compared to the as cast sample,  was moved closer to room temperature and maximum  of 0.71 K was obtained.

This thesis concerns and combines the results of experimental studies of magnetocaloric, multiferroic and spin-glass materials, using SQUID magnetometry as the main characteriza-tion technique.  The magnetocaloric effect offers an interesting new technology for cooling and heating applications. The studies of magnetocaloric materials in this thesis are focused on experimen-tal characterization of fundamental magnetic properties of Fe2P-based materials. These are promising magnetocaloric materials with potential industrial use. It is found that the magneto-caloric properties of Fe2P can be optimally tuned by substitution of manganese for iron and silicon for phosphorus. Furthermore, a simple device to measure the magnetocaloric effect in terms of the adiabatic temperature change was constructed.  Materials that simultaneously exhibit different types of ferroic order, for example magnetic and electrical order, are rare in nature. Among these multiferroic materials, those in which the ferroelectricity is magnetically-induced, or vice versa the magnetism is electrically-induced, are intensively studied due to a need for new functionalities in future data storage and logic devices. This thesis presents results on two materials: Co3TeO6 and Ba3NbFe3Si2O14, which belong to the group of magnetically-induced ferroelectrics and exhibit strong coupling be-tween the magnetic and the electrical order parameter. Their ordering properties were studied using magnetic and electrical measurement techniques. The coupling between the magnetic and electronic degrees of freedom was investigated using high-field and low-temperature Raman spectroscopy.  Spin-glass materials exhibit complex magnetism and disorder. The influence of the spin dimensionality on the low and high magnetic field properties of spin glasses was investigated by studying model Heisenberg, XY and Ising spin-glass systems. Significant differences were found between the non-equilibrium dynamics and the hysteresis behavior of Heisenberg systems compared to those of XY and Ising spin glasses.

The overall goals of the present PhD research are to explore the giant magnetoimpedance (GMI) and giant magnetocaloric (GMC) effects in functional magnetic materials and provide guidance on the optimization of the material properties for use in advanced magnetic sensor and refrigeration applications. GMI has attracted growing interest due to its promising applications in high-performance magnetic sensors. Research in this field is focused on the development of new materials with properties appropriate for practical GMI sensor applications. In this project, we have successfully set up a new magneto-impedance measurement system in the Functional Materials Laboratory at USF. We have established, for the first time, the correlation between sample surface, magnetic softness, critical length, and GMI in Co-based amorphous ribbon materials, which provide a good handle on selecting the suitable operating frequency range of magnetic materials for GMI-based field sensor applications. The impact of field-induced magnetic anisotropy on the GMI effect in Co-based nanocrystalline ribbon materials has also been investigated, providing an important understanding of the correlation between the microstructure, magnetic anisotropy, and GMI in these materials. We have shown that coating a thin layer of magnetic metal on the surface of a magnetic ribbon can reduce stray fields due to surface irregularities and enhance the magnetic flux paths closure of the bilayer structure, both of which, in effect, increase the GMI and its field sensitivity. This finding provides a new way for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors. We have also introduced the new concepts of incorporating GMI technology with superparamagnetic nanopthesiss for biosensing applications and with carbon nanotubes for gas and chemical sensing applications. GMC forms the basis for developing advanced magnetic refrigeration technology and research in this field is of topical interest. In this project, we have systematically studied the ferromagnetism and magnetocaloric effect in Eu8Ga16Ge30 clathrate materials, which are better known for their thermoelectric applications. We have discovered the GMC effect in the type-VIII clathrate and enhanced refrigerant capacity in the type-I clathrate. We have successfully used the clathrates as excellent host matrices to produce novel Eu8Ga16Ge30-EuO composite materials with desirable properties for active magnetic refrigeration technologies. A large refrigerant capacity of 794 J/kg for a field change of 5 T over a temperature interval of 70 K has been achieved in the Eu8Ga16Ge30-EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K - 100 K. The excellent magnetocaloric properties of the Eu8Ga16Ge30-EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

The design and development of nanoparticles is of great interest in the current energy and electronic industry. However, based on the current materials available the production cost can be high with insignificant magnetic and mechanical properties. Specifically, rare-earth magnetic materials composed of neodymium and samarium are known for their high magnetic performance, however, due to the cost of development there is a need to develop a versatile and cost effective material. Alternatively, cobalt carbide nanomaterials have shown to be a promising alternative for rare-earth free magnets as they exhibit comparable properties as hexaferrite magnetic materials. The primary goal of this dissertation focuses on the development of nanoparticles for permeant magnetic, and magnetic refrigeration applications. The first part of this work focuses on the synthesis of cobalt carbide (CoxC, x=2,3) nanoparticles using a novel polyol synthesis method by introducing a small amount of Ru, Cu, or Au as nucleating agent. It was found that the morphology and magnetic properties of the as-synthesized CoxC nanoparticles change as a result of directional growth of nanoparticles using nucleating agents. Needle-like particle morphology ranges from 20-50 nm in width and as long as 1 µm in length were synthesized using Ru as nucleating agent. These particles exhibit magnetization saturation of 33.5 emu/g with a coercivity of 2870 Oe and a maximum energy product 1.92 MGOe (BHmax) observed. Particle morphology is a critical aspect in the development of magnetic nanoparticles as anisotropic particles have shown increased coercivity and magnetic properties. These CoxC nanomaterials have a higher maximum energy product compared to previous work providing further insight into the development of non-rare earth magnetic material. The second part of this dissertation work focuses on the sol-gel synthesis of perovskite LaCaMnO3 (LCMO) nanomaterials. In this process, various chain lengths of polyethylene glycol (PEG) was added into a solution consisting of La, Ca, and Mn salts. The solution was left for the gelation process, and high temperature sintering to obtain the final product. By varying the polymer chain of the PEG, the size of the as synthesized LaCaMnO3 nanomaterials were altered. The as-synthesized LCMO nanomaterials have shown a maximum change in magnetic entropy (-ΔSM) was found to be 19.3 Jkg-1K-1 at 278 K for a field change of 0-3 T and 8.7 Jkg-1K-1 for a field change of 0-1 T. This is a significant improvement in comparison to current literature of the material suggesting that this is a promising alternative to Gd materials that is prone to oxidation. With additional development, LCMO or related maganites could lead to application in commercial technologies.

This thesis documents the work and research effort on the design, fabrication and testing of a magnetocaloric MEMS microcooler, focusing on the testing of the microcooler at low magnetic fields. The phenomenon of magnetocaloric effect (MCE), or adiabatic temperature change, which is obtained by heating or cooling magnetic materials due to a varying magnetic field, can be exploited in the area of magnetic refrigeration as a reliable, energy-efficient cooling system. In particular, its applications are being explored primarily in cryogenic technologies as a viable process for the liquefaction of hydrogen. The challenge for magnetic refrigeration is that the necessary MCE is most easily achieved with high magnetic fields (5-6 Tesla) provided by superconducting magnets. However, a significant magnetocaloric effect can be exhibited at lower magnetic fields (1-2 Tesla) by carefully controlling initial temperature conditions as well as by selecting, preparing and synthesizing the optimal fabrication process of Silicon (Si) wafers. A microcooler was integrated based on previous works of others and tested. Finally, testing of the magnetocaloric effect was conducted and results analyzed. Experimental results in these domains demonstrate that magnetic refrigeration can be part of the best current cooling technology, without having to use volatile, environmentally hazardous fluids. The MEMS magnetocaloric refrigerator demonstrated a ~ -12°C change in the temperature of cooling fluid at a magnetic field of 1.2 T.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references.
(cont.) portable consumer electronics, such as PDAs, cell phones, music players, digital cameras, etc. make a relatively modest but fast growing market for ultrahigh areal density HAMR-based HDDs. HAMR-based HDD for portable applications could very well be a disruptive technology in the magnetic recording industry. Companies that intend to profit from this technology need to invest on its development and must try to be first-to-volume production to benefit from economies of scale and to build the necessary expertise that could give them leadership roles in future magnetic recording.
The magnetic recording industry keeps up with the demand of high capacity hard disk drives by improving the areal recording density of these devices. The use of conventional longitudinally magnetized media will be truncated by the challenges it faces nowadays, which are related to the instability of the stored information, produced by the aggressive decrease in the volume of the grains in the media. To overcome this problem, the use of large magnetic anisotropy energy density alloys is necessary, but the write fields that are required by such alloys can be prohibitively large, rendering these media effectively unwritable. Fortunately, the magnetocrystalline anisotropy energy density decreases with increasing temperature and so does the required write field. Heat assisted magnetic recording allows the use of such magnetically hard alloys by using both a magnetic and a thermal field during the writing process. Research in HAMR is centered in three major fields: the heat delivery system, the magnetic recording media and the heat dissipation technology. Based on an analysis of several US patents related to HAMR, one can see the real value of such patents is in negotiating and cross-licensing between companies to guarantee the right to participate in the manufacture of HDDs. Trade secrets and know-how are valuable assets for corporations. However, information exchange exists due to the great mobility of highly trained personnel between competing companies. Because the basic application of HAMR is in supplying the computer industry with affordable storage devices, there is a well established market that makes the research efforts in HAMR advisable for individuals, universities and companies. Besides that traditional market,
by Diego A. Méndez del la Luz.
M.Eng.

The structural, electronic, magnetic, magnetocaloric, and transport properties of doped Ni-Mn-(In, Sn) based Heusler alloys were studied using neutron diffraction, x-ray diffraction (XRD), differential scanning calorimetry (DSC), high field magnetization, specific heat, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and hydrostatic pressure measurements. The adiabatic temperature change (∆Tad) by a direct method and through thermomagnetic measurements in magnetic fields up to 14 T has been performed for these alloys. Also the mixed effect of pressure and magnetic field on the transition temperature of these alloys are discussed. In order to develop new magnetocaloric and multifunctional materials, the synthesis and characterization of Heusler alloys in reduced dimensions, i.e., ribbons and thin films has been performed. In addition, the structural, magnetic, and magnetocaloric properties of Ni-based binary alloys were investigated, including saturation magnetization and Curie temperature (TC) for the possible applications in self controlled magnetic hyperthermia applications.

Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.

Nanoparticle system research and characterization is the focal point of this research and dissertation. In the research presented here, magnetite, cobalt, and ferrite nanoparticle systems have been explored in regard to their magnetocaloric effect (MCE) properties, as well as for use in polymer composites. Both areas of study have potential applications across a wide variety of interdisciplinary fields. Magnetite nanoparticles have been successfully dispersed in a polymer. The surface chemistry of the magnetic nanoparticle proves critical to obtaining a homogenous and well separated high density dispersion in PMMA. Theoretical studies found in the literature have indicated that surface interface energy is a critical component in dispersion. Oleic acid is used to alter the surface of magnetite nanoparticles and successfully achieve good dispersion in a PMMA thin film. Polypyrrole is then coated onto the PMMA composite layer. The bilayer is characterized using cross-sectional TEM, cross-sectional SEM, magnetic characterization, and low frequency conductivity. The results show that the superparmagnetic properties of the as synthesized particles are maintained in the composite. With further study of the properties of these nanoparticles for real and functional uses, MCE is studied on a variety of magnetic nanoparticle systems. Magnetite, manganese zinc ferrite, and cobalt ferrite systems show significant broadening of the MCE and the ability to tune the peak temperature of MCE by varying the size of the nanoparticles. Four distinct systems are studied including cobalt, cobalt core silver shell nanoparticles, nickel ferrite, and ball milled zinc ferrite. The results demonstrate the importance of surface characteristics on MCE. Surface spin disorder appears to have a large influence on the low temperature magnetic and magnetocalorie characteristics of these nanoparticle systems.

Orientadores: Adelino de Aguiar Coelho, Lisandro Pavie Cardoso
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho propomos o estudo de algumas propriedades magnéticas e do efeito magnetocalórico (EMC) de compostos da família TbxPr1-xAl2, onde x = 0,1; 0,2; 0,25; 0,3; 0,4; 0,5 e 0,75. É fato conhecido que dois elementos terras-raras se acoplam antiferromagneticamente quando inseridos em uma rede cristalina do tipo (R,R¿)Al2 se um deles é um elemento terra-rara leve o outro pesado. Desta forma, era esperado que as amostras desta série apresentassem um ordenamento ferrimagnético, o que foi confirmado pelos nossos dados. Comparamos nossos resultados com sistemas semelhantes e com eles fomos capazes de entender a origem de algumas características e aspectos físicos dos materiais, incluindo o EMC. Alguns compostos estudados aqui apresentaram o efeito magnetocalórico inverso, que acontece devido à competição entre as duas subredes magnéticas acopladas antiferromagneticamente. Observamos também os comportamentos da temperatura de transição magnética e dos parâmetros de rede em função da concentração de Tb, e verificamos o deslocamento dos picos do EMC associados à temperatura de transição (TC). Algumas propriedades importantes na aplicação destes materiais em refrigeração magnética foram levantadas e comparadas com a literatura. Verificamos que dois membros da série apresentaram o efeito de Exchange Bias (EB) e, em analogia com sistemas bicamadas FM-AFM em filmes finos, fomos capazes de entender e explicar este efeito. Acreditamos que o efeito de Exchange Bias está associado à forte anisotropia apresentada pelos compostos Tb0.3Pr0.7Al2 e Tb0.25Pr0.75Al2 que ocorre em temperaturas extremamente baixas e à magnetização dos elétrons polarizados de condução. Identificamos também as temperaturas de compensação (propriedades características deste tipo de sistema) nestes compostos
Abstract: In this work we studied some magnetic properties and the magnetocaloric effect (MCE) of the series of compounds TbxPr1-xAl2, where x = 0.1, 0.2, 0.25, 0.3, 0.4, 0.5 and 0.75. It is known from the literature that the moments of a light and a heavy rare earth are antiferromagnetically coupled when they are inserted in a crystal lattice and this was confirmed by our analysis of the experimental data. We compared our results with similar systems and we were able to understand the origin of some features and physical aspects of those materials, including the magnetocaloric effect (MCE). Some compounds show the inverse MCE, associated to the competition between the two magnetic sublattices antiferromagnetically coupled. We also observed the behavior of the magnetic transition temperature (TC) and lattice parameters as a function of the concentration ('x¿), with a corresponding MCE peak shift. Some important properties regarding the application of these materials in magnetic refrigeration are discussed. Two members of the family of compounds show a shifted hysteresis curve which was assigned to the Exchange Bias (EB) effect in analogy with FM-AFM bilayers systems in thin films. We believe that the EB effect is associated with strong unidirectional anisotropy of the magnetic sublattices appeared in Tb0.3Pr0.7Al2 and Tb0.25Pr0.75Al2, which occurs at low temperatures, and with the magnetization of the polarized conduction electrons. We also identified the compensation temperature (characteristic properties of this type of system) in these compounds
Doutorado
Física
Doutor em Ciências

Magnetic recording media refers to the disc shaped thin film magnetic medium present inside the hard disk drive of a computer. Magnetic recording is an important function of the hard disk drive by which information such as text, pictures, audio and videos are stored. Information is broken down to a simple binary format and is stored as magnetised bits along the tracks of the disk forming the hard drive. Over the years advancements in research on the type of magnetic materials used has allowed increased data storage capacities by reducing magnetic bit sizes. It is with this advancement in magnetic data storage, that we have today's hard disk drive technology, which uses a perpendicular magnetic medium to store data. A perpendicular magnetic medium is a multi-layered magnetic thin film structure with the topmost layer comprising nanoscale magnetic grains of high perpendicular anisotropy. The topmost recording layer (RL) is mapped into individual bits of 80-100 nm² area that consist of 5-10 nm diameter CoCrPt grains, embedded in an oxide matrix. A bit area is defined to ensure a significant number of stable grains allowing data to be stored in each bit as a ‘0' or a ‘1' depending on its switched magnetic state. The magnetic grains if sputtered below a threshold grain size tend to suffer from thermal fluctuation and instability due to super-paramagnetic effects, hence bringing limitations to grain size. As a result of this, research in recent years has been directed at introducing a softer magnetic exchange coupled composite (ECC) layer above the recording layer. This layer facilitates the delicate balance of switching smaller grains with strong magneto-crystalline anisotropy at lower magnetic fields, by exchange coupling with the CoCrPt grains in the recording layer. However this technique of increasing the efficiency in the perpendicular magnetic medium by introducing ‘facilitating' layers is an area that is still being widely researched and understood. Although numerous surface and bulk analysis techniques exist to study magnetic and surface properties of these materials, there is limited information on the structural and magnetic properties of these materials at the nanoscale level. The reported work investigates the structural and magnetic properties of the magnetic grains and multi-layers in the perpendicular magnetic medium using polarised neutron scattering and reflectivity techniques. The work investigates the structural and magnetic properties of the CoCrPt grains, apart from understanding the CoCrPt magnetic grain switching. The work also investigates the magnetisation in the layers of the thin film perpendicular media structure using polarised neutron reflectivity (PNR). Using polarised small angle neutron scattering (PolSANS), it has been shown that ferromagnetic ordered core region of the CoCrPt grain in the recording layer is smaller than the physical CoCrPt granular structure. The magnetic switching behaviour of the CoCrPt grain at different magnetic fields is also analysed and the experimental PolSANS data is fitted with non-interacting size-dependent analytical grain switching models. This result provides significant evidence that the magnetic anisotropy increases with grain size, with larger magnetic grains having larger magnetic anisotropy. Polarised neutron scattering experiments are carried out with the magnetically softer exchange coupled composite (ECC) layer included in the thin film magnetic structure. The first experiments investigate if the ECC layer contributes to the nuclear and magnetic interference scattering term in the experimenting scattering data. The experiments clearly show that there is no contribution from the ECC layer in the nuclear and magnetic scattering interference term. The role of the ECC layer in the magnetic switching process is then investigated at different magnetic fields. The ECC layer was found to influence the size-dependent magnetic grain switching of the CoCrPt grains in the recording layer and a detailed investigation is presented in the reported work. Polarised neutron reflectivity (PNR) experiments have also been carried out with the ECC layer on the perpendicular magnetic media samples. These experiments investigate the composition and thickness of the thin film structure, while also providing information on the magnetic state of the thin films under the influence of an in-plane magnetic field. The in-plane magnetisation in the recording and ECC layer is determined at different in-plane magnetic fields. The magnetisation values determined for the ECC layer and the recording layer (RL) at different in-plane magnetic fields help better understand the differences in their magnetic properties.

In this thesis both the static and dynamic magnetic behaviour of complex three-dimensional nanoscale commercial hard disk drive write heads and thin film structures of interest to emerging spintronic devices have been investigated using a plurality of experimental techniques. The magneto-optical Kerr effect (MOKE) provides the basis for an optical microscopy technique sensitive to the magnetisation of a sample, detectable as a change in polarisation of light reflected from the sample surface. With a modelocked laser light source, synchronised electrical pulse generator and lock-in amplifier (LIA), a stroboscopic technique has been used to observe the magnetisation dynamics of hard disk drive write heads at 600 nm spatial resolution and 10 ps time resolution in response to a driving electrical pulse. The equilibrium magnetic state of these devices has been directly imaged by x-ray photo-emission electron microscopy (XPEEM), as well the stability of the equilibrium state in response to the application of an external bias field. Direct images of the equilibrium state obtained by XPEEM were found to agree with inferences made from MOKE images. Time-resolved scanning Kerr microscopy (TRSKM) images of magnetisation dynamics showed that flux does not form in ‘beams’ as commonly believed, but instead nucleates in separate sites across the writer. Static and time-resolved x-ray techniques have also been used to investigate a number of thin films of interest to spintronics. Spin pumping and spin transfer torque in Co2MnGe / Ag / Ni81Fe19 spin valves were explored using time-resolved x-ray ferromagnetic resonance (XFMR) carried out at Diamond Light Source (DLS), a as well as static x-ray magnetic circular dichroism (XMCD) for sample characterisation. This has provided element-specific measurements of the spin state in the source and sink layers of the spin valve, revealing a clear sign of spin transfer torque, while also investigating the role of sink layer thickness in spin pumping and damping. Ferrimagnetic yttrium iron garnet (Y3Fe2(FeO4)3) (YIG), a material of great interest in spintronics, has been studied by static and dynamic XMCD in comparison with ferromagnetic Co. While static and dynamic spectra for Co were identical, those for YIG differed markedly. While this may hint at a phase difference between the precession of Fe moments on different lattice sites, the true source of this difference has not been identified. Comparisons between vector network analyser ferromagnetic resonance (VNA-FMR) and XFMR measurements further suggest the presence of long-range inhomogeneities in the YIG. The spin dynamics of an antiferromagnet being driven by a ferromagnet have also been investigated using XMCD and x-ray magnetic linear dichroism (XMLD). A CoO / Fe / Ni81Fe19 trilayer wherein the thickness of the CoO layer varies across the sample has been thoroughly characterised by static XMCD and XMLD, providing information necessary to fully interpret time-resolved MOKE measurements on these samples. Measurements have shown that even small amounts of ordered CoO significantly modify the resonant field and linewidth of the adjacent ferromagnetic layers. Phase-resolved measurements of CoO spins have shown these spins to precess in phase with those of the adjacent Fe. The viability of dynamic XMLD measurements has also been confirmed. Finally, potential directions for future work in each project are discussed.

Mes travaux de thèse visent à récupérer, grapiller, la chaleur fatale de bas à très bas niveau pour produire de l'énergie électrique et ainsi alimenter des petits systèmes autonomes (µW à mW). Le générateur développé convertit l'énergie en trois étapes. Tout d'abord l'énergie thermique est convertie en énergie magnétique au travers d'un cycle thermodynamique opéré à l'aide d'un matériau magnétocalorique. Cette première conversion est intimement liée à la seconde, conversion de l'énergie magnétique en énergie mécanique, car le déplacement du matériau magnétocalorique contrôle aussi le champ appliqué et les échanges thermiques avec les réservoirs. C'est l'imbrication de ces deux cycles, thermodynamique et dynamique, qui permet au système d'auto-osciller. L'énergie mécanique du système pseudo-oscillant est finalement convertie en énergie électrique via des éléments piézoélectriques. Mes travaux expérimentaux, théoriques et numériques ont cherché à maximiser l'énergie électrique récupérée tout en assurant l'auto-oscillation de la structure. Les dispositifs développés sont en mesure d'auto-osciller pour des écarts de température de 35 °C tout en produisant de l'énergie électrique. Notre prototype le plus performant présente une énergie de 10,6 μJ par cycle pour une fréquence de 0,41 Hz, soit une puissance de 4,2 μW (240 μW/cm3). Ces travaux mettent l'accent sur les cycles associés à la conversion d'énergie
The main objective of my thesis is the design and development of a device suitable to recover, and scavenge, low grade heat to produce electrical energy and thus supply small autonomous systems (μW to mW). The developed generator converts energy in three steps. First of all, thermal energy is converted into magnetic energy through a thermodynamic cycle operated by a magnetocaloric material. This first conversion is closely linked to the second, conversion of magnetic energy into mechanical energy, because the displacement of the magnetocaloric material also controls the applied field and the heat exchanges with the reservoirs. It is the interweaving of these two cycles, thermodynamic and dynamic, which allows the system to self-oscillate. The mechanical energy of the pseudo-oscillating system is converted into electrical energy via piezoelectric elements. My experimental, theoretical and numerical works aimed to maximize the electrical energy recovered while ensuring the self-oscillation of the structure. All devices developed are able to self-oscillate for temperature difference of 35 °C while producing electrical energy. Our most efficient prototype has an energy of 10.6 μJ per cycle for a frequency of 0.41 Hz, i.e. a power of 4.2 μW (240 μW/cm3). This work, especially, focuses on the cycles associated with energy conversion

La réfrigération magnétique attire beaucoup d’attention ces dernières années parce qu’elle est considérée comme une technologie respectueuse de l’environnement et énergétiquement économique. Aujourd’hui, cette technologie avancée est encore en phase de recherche que des dispositifs de réfrigérations magnétiques soient déjà opérationnels. Ce travail de thèse consiste à étudier la potentialité de résistance à la corrosion de différents types de matériaux magnétocaloriques (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga et Pr0.66Sr0.34MnO3) en contact avec un fluide caloporteur. Afin de comprendre les propriétés magnétocaloriques des matériaux, nos recherches se sont aussi focalisées sur les relations entre la transition magnéto-structurales d’alliages Heusler Ni2Mn0.75Cu0.25Ga et (i) la distribution cationique au sein de la structure cristalline et/ou (ii) la microstructure. Finalement, le diagramme de phase magnétique et nucléaire en lien avec les effets magnétocalorique obtenu grâce à la diffraction de neutrons et de pérovskite Pr1-xSrxMnO3 (0.25≤x≤0.45) est également présenté
Magnetic refrigeration has gained lot of importance and attention as they are highlighted to be environmental friendly, energy efficient. Presently, though at research stage, the magnetic refrigerators are pushed towards realization in domestic application with extensive work on materials and with few working models. One critical issue, the potential resistance to corrosion in case of different class of magnetocaloric materials (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga and Pr0.66Sr0.34MnO3) against the heat transport fluid is addressed. To better understand and improve the observed magnetocaloric properties in Heulser alloys Ni2Mn0.75Cu0.25Ga and to elaborate the same with the magneto-structural relation, studies on (i) cation distribution with in crystal structure and/or (ii) microstructural dependence are presented. Nuclear and magnetic phase diagram based on detailed neutron diffraction and magnetism studies for magnetocaloric perovskite oxide Pr1-xSrxMnO3 (0.25≤x≤0.45) is also presented

La réfrigération magnétique est une technologie innovante de production de froid, qui peut remplacer la technique classique de compression-détente de fluides frigorigènes. Son principe est basé sur l'effet magnétocalorique qui se traduit par le refroidissement ou l'échauffement de certains matériaux sous l'action d'un champ magnétique. Ce travail de thèse s'est déroulé dans le cadre d'un projet " CARNOT Energies du futur " et s'oriente vers l'étude magnétothermique et fluidique de systèmes de réfrigération. Pour cela, un outil numérique a été développé à l'aide du logiciel FLUENT afin de décrire le comportement thermique de différents régénérateurs, cœur même des systèmes de RM. En parallèle, deux systèmes de réfrigération magnétique ont été développés et améliorés, chacun d'eux présentant des performances intéressantes. Ces résultats ont permis de comprendre et définir les facteurs les plus influents sur leurs performances, et en déduire ainsi leurs conditions de fonctionnement optimales

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Conselho Nacional de Desenvolvimento Científico e Tecnológico
In this work, magnetic entropy change of nonstoichiometric Ni54[Mn(1-x)Fex]19Ga27 Heusler alloys was experimentally verified by partial substitution (up to 50%) of the Mn atoms by magnetic Fe atoms. Such study was mainly concentrated in the magneto-structural transformation region, at low temperatures. The analysis of X-ray diffraction patterns indicate that the partial substitution of Mn atoms by Fe atoms causes predominance of the L21 -phase, however accompanied by spurious phases. The magnetization measurements as a function of the temperature in the low magnetic field reveals that, in all concentration range (0 x 0.5), the system presents a magnetic transition (Ferromagnetic Paramagnetic) at a temperature, TC, near the room temperature, when the material lay in the austenitic phase. Moreover, as many others Heusler alloys, the material undergoes a martensitic structural transition at low temperatures, TM. As the Fe concentration increases, the Curie temperature, TC, undergoes a little variation, increasing around 5%, while TM decreases slowly and monotonically. The magnetic entropy change, for a field of 5T, presents a maximum SM = - 9,3 J/kg.K, for x = 0.1, at a temperature of 250K, and than decreases for x 0.3, changing linearly with the maximum applied field.
Neste trabalho, verificou-se experimentalmente a variação da entropia magnética nas ligas Heusler não-estequiométricas Ni54[Mn(1-x)Fex]19Ga27, substituindo-se parcialmente (até 50%) os átomos de Mn por átomos de Fe. Tal estudo focalizou-se principalmente na região de transformação magneto-estrutural a baixas temperaturas. A análise dos difratogramas de raios-X indica que a substituição parcial de Mn por Fe nessa faixa de concentração, ocasiona uma predominância da fase- L21, porém acompanhada de outras fases espúrias. As medidas de magnetização em função da temperatura realizadas a campos baixos revelam que, em toda a faixa de concentração estudada (0 x 0,5), o sistema apresenta uma transição magnética (Ferromagnético Paramagnético) ao atingir a temperatura de Curie, TC, um pouco acima da temperatura ambiente, quando o material se encontra na fase austenítica. E, assim como muitas outras ligas Heusler, o material sofre uma transição estrutural ao ser resfriado a temperaturas inferiores à temperatura de transição martensítica, TM. À medida que se aumenta a concentração de Fe, a temperatura de Curie, TC, sofre uma pequena variação, aumentando em torno de 5%, enquanto que TM diminui lenta e monotonicamente. A variação da entropia, para um campo de 5T, apresenta um máximo SM = - 9,3 J/kg.K para x = 0,1, a uma temperatura T = 250K e, então, diminui para x 0,3, variando linearmente com o campo aplicado.

Orientador: Mario Antonio Bica de Moraes
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Técnicas de condensação de vapor são úteis na preparação de ligas magnéticas cujos componentes têm pouca, o mesmo nenhuma, solubilidade em condições de equilíbrio Neste trabalho, uma dessas técnicas ¿ sputtering ¿ foi empregada para fabricar ligas metaestáveis de GdXCr1-X, cujas propriedades magnéticas foram investigadas em função da concentração de Gd, x. Difratometria de raios-X de baixo ângulo (GAXRD) e espectroscopia de retroespalhamento Rutherford foram utilizados para determinar a estrutura do filme e sua composição elementar, respectivamente. As análises de GAXRD mostraram que a estrutura da fase de Gd, é hcp quando x ³ 0,88; e amorfa quando 0,16 £ 0,76. Uma estrutura bcc , para a fase de Cr, foi observada nos difratogramas quando x < 0,16, e amorfa quando x ³ 0,16 Para investigar as propriedades magnéticas utilizamos um magnetometro SQUID e m PPMS. O primeiro foi utilizado para as medidas de momento magnético em função do campo estático e temperatura. O PPMS foi tilizado nas investigações de susceptibilidade-AC em função da freqüência de oscilação do campo, temperatura e campo estático. A complexa natureza magnética dos filmes de Gd-Cr foi observada através das isotermas MxH, que não apresentaram saturação em baixas temperaturas, nem comportamento linear em altas temperaturas. Pela análise dos dados magnéticos, observamos que as amostras admitem um comportamento ferromagnético para x ³ 0,5 e paramagnético para as outras concentrações de Gd. A temperatura de Curie (TC) apresenta um aumento monotônico de 170 para 290 K quando x aumenta de 0,5 para 1,0. A temperatura de Curie-Weiss (q C) mostra um aumento monotônico com x. A partir das isotermas MxH a 2 K, o momento de saturação foi calculado, sendo independente de x e aproximadamente constante com um valor médio de 7,3 µB. Medidas de susceptibilidade em campos estáticos e dinâmicos revelaram a existência de comportamentos de vidros magnéticos em todas as amostras abaixo da temperat ra de freezing (Tf). Observamos, nas ligas com altas concentrações de Gd, a presença de comportamentos ferromagnéticos e cluster-glass em baixas temperaturas. Concluímos que a interação de troca entre os átomos de Gd dentro dos clusters de Gd não é do tipo RKKY, e sim do tipo supertroca. O efeito magnetocalórico (MCE) foi investigado através da variação de entropia magnética ( D SM) em função da temperatura, para a remoção de um campo de 50 kOe Curva de D SMxT para as amostras com x < 0,2 apresentaram um formato típico de superferromagneto, consistente com a existência de clusters Gd nos filmes. Nas outras amostras em que o EMC foi analisado, a presença de clusters é observada pelo comportamento dessas curvas a baixa temperatura; a altas temperaturas o comportamento de D SM com T indica fortemente a presença de mais fases magnéticas no filme. O diagrama de fase baseado em TC e Tf, e sua dependência com x é apresentado
Abstract: Vapor condensation techniques are very useful for preparation of alloys whose components have no mutual solubility under equilibrium conditions In this work, one of these techniques ¿ sputtering ¿ has been used to fabricate metastable GdXCr1-X alloys whose magnetic properties were investigated as a function of the Gd concentration, x. Grazing incidence angle X-ray diffraction (GAXRD) and Rutherford backscattering spectroscopy were employed to characterize the film structure and elemental composition, respectively. The GAXRD measurements revealed, for the Gd fraction, a hcp structure for x ³ 0,88; for 0,16 £ x £ 0,76 the Gd fraction was amorphous. The existence of a bcc structure for x < 0,16 was observed in the diffractograms for the Cr phase, which was amorphous for x ³ 0,16. To investigate the magnetic properties, a SQUID magnetometer and a PPMS were used. The former was employed for magnetic moment measurements as a function of applied static field and temperature. The PPMS was used for ac-susceptibility determinations as a function of the frequency of the ac driving field, temperature, and applied static field. The complex magnetic nature of the Gd-Cr films was revealed from the MxH isotherms which did not show saturation even at the lowest temperatures, and did not exhibit a linear behavior at higher temperatures. Processing of the magnetic data has shown that the films exhibit a ferromagnetic behavior for x ³ 0,5 and paramagnetic one for all other Gd concentrations. The Curie temperature (TC) increased monotonically from 170 to 290 K as x increased from 0,5 to 1,0. A monotonical increase in the Curie-Weiss temperature ( q C ) with x was also observed for all films. From the extrapolated MxH isotherm at 2 K (saturation magnetization), the saturation moments were calculated and found to be nearly constant at about 7.3 µB. Both static and dynamic susceptibility measurements revealed the existence of a magnetic glassy behavior in all alloys, occurring below a freezing temperature Tf . For the higher concentration alloys, the ferromagnetic and the cluster glass state were observed at low temperatures. It was thus concluded that the exchange interactions within Gd atoms in the clusters were not of the RKKY but of the superexchange type. The magnetocaloric effect (MCE) was investigated from the magnetic entropy change ( DSM) as a function of temperature, for the removal of a 50 kOe field. Samples with x < 0,2 exhibited DSMxT curves whose shapes are typical of a superferromagnet, consistently with the existence of Gd clusters in the films. For all the other alloys whose MCE was investigated, the presence of clusters is manifested from the behavior of these curves at low temperatures; at higher temperatures, the evolution of DSM with T strongly indicated the presence of more than one magnetic phase in the alloys A magnetic phase diagram based on the Tf and Tc transition temperatures and their dependence on x is presented in this thesis
Doutorado
Física da Matéria Condensada
Doutor em Ciências

Nous avons developpe des techniques mathematiques efficaces et un ensemble d'outils numeriques specifiques au micromagnetisme, pour calculer des configurations magnetiques a l'equilibre et hors equilibre. Le modele utilise une methode de minimisation qui repose sur l'integration des equations de precession-dissipation de landau-lifshitz-gilbert (llg). Nous avons mis en evidence l'importance de l'etude de stabilite des equations llg. L'approche numerique mise au point a ete appliquee pour etudier le comportement magnetique de systemes reels fabriques au laboratoire. Dans les tri-couches ferromagnetiques smco/zrco/smco', dont la couche centrale douce est prise en sandwich entre deux couches dures, le mecanisme de renversement de la phase dure est du a la propagation d'une paroi depuis la phase douce. L'essentiel des processus de renversement d'aimantation experimentaux a ete decrit correctement par la simulation, seul un desaccord porte sur le renversement d'aimantation de la couche de smco la plus dure. L'etude menee sur les plots de fer epitaxies fe(110)/w(110) a permis de decrire les distributions d'aimantation et d'estimer les champ de retournement. Le champ coercitif calcule pour un plot carre de l = 5000 a de cote et de e = 58 a d'epaisseur est proche de la valeur obtenue experimentalement. Un bon accord entre l'experience et la simulation a ete observe pour la dependance angulaire du champ coercitif d'une particule ronde de diametre 2000 a et d'epaisseur 10 a. Le comportement calcule, comme observe, tendent vers celui decrit par le modele de stoner-wohlfarth. Dans le cas des materiaux nanostructures doux-durs, l'approche micromagnetique utilisee nous a permis d'identifier les parametres pertinents et les mecanismes impliques dans le renversement d'aimantation. Les contributions de chaque phase a la susceptibilite totale ont ete deduites. La contribution reversible a la susceptibilite totale a ete determinee par une approche qui mime le procede experimental. La comparaison de nos resultats avec ceux de l'experience tend a prouver qu'une amelioration des performances des systemes nanostructures reels est possible en diminuant la taille des grains.

In der vorliegenden Arbeit wurden vor allem das Auftreten und der Mechanismus von feldinduzierten Übergängen und der damit verbundenen kritischen Felder untersucht. Die verwendete Magnetisierungsmessmethode ist auf die bestehende Impulsfeldanlage des IFW Dresden abgestimmt. Die Magnetisierung in Feldern bis zu 48 T wurde gemessen. Erstmals wurde für Sm2Fe17N3 der Anisotropiekoeffizient aus der Kombination der Messung des Austauschfeldes mittels inelastischer Neutronenstreuung und der Messung der Anisotropiekonstanten K1 am gleichen Material bestimmt. Für den führenden Anisotropiekoeffizienten konnte mit K1 von rund 13 MJ/m³ der Wert A20&lt;r²&gt; = -28 meV bestimmt werden. Der in SmCo2,5Cu2,5 und SmCo2Cu3 beobachtete Hochfeldübergang konnte mit der Mikrostruktur verknüpft werden. Die laminare Mikrostruktur bestehend aus Phasen mit unterschiedlichem Sm-Anteil ist eine notwendige Bedingung für das Auftreten des Übergangs. Das Koerzitivfeld steigt mit dem Kupfergehalt und erreicht bei tiefen Temperaturen sehr hohe Werte. Das Koerzitivfeld und das Übergangsfeld zeigen eine große magnetische Viskosität. In DyFe6Al6 wird das Verschwinden der spontanen Magnetisierung bei tiefen Temperaturen durch starke antiferromagnetische Kopplungen verursacht. Durch ein feldinduziertes magnetisches Moment an einem ungeordneten Kristallgitterplatz könnte der magnetische Übergang bei tiefen Temperaturen erklärt werden. An hexagonalem DyMn6Ge6 wurde erstmals der Temperaturverlauf des Übergangsfeldes zur gekanteten antiferromagnetischen Struktur gemessen. Oberhalb von 100 K ruft das angelegte Feld den Übergang von der helimagnetischen zu einer Fächerstruktur hervor. Bei tiefen Temperaturen tritt ein Spinflop-Übergang auf, der durch die magnetische Anisotropie des Dysprosiumions unterstützt wird. Bei magnetokalorischen Materialien zeigt sich eine Abhängigkeit der gemessenen Magnetisierung von der Feldänderungsrate. Dies lässt sich qualitativ auf die Messbedingungen zurückführen: So herrschen bei Impulsfeldmessungen adiabatische Bedingungen, während bei statischen Messungen isotherme Verhältnisse vorliegen. Neben herkömmlichen magnetischen Verbindungen wurden auch stark korrelierte Elektronensysteme untersucht. Der gefundene Magnetisierungsübergang bei 43 T in CeNi2Ge2 lässt sich auf das Unterdrücken des Kondoeffekts und das Aufbrechen der antiferromagnetischen Struktur zurückführen. Darüber hinaus wurden Magnetisierungsmessungen an Hochtemperatursupraleitern durchgeführt. Die Messungen im Impulsfeld sind ein Beitrag zur Bestimmung des Phasendiagramms von schmelztexturiertem YBa2Cu3O7-d. Das Irreversibilitätsfeld Hirr konnte an massiven Proben bis zu tiefen Temperaturen bestimmt werden. Hirr(T) zeigt einen unerwarteten linearen Anstieg bis zu tiefen Temperaturen. Aufgrund der hohen Feldänderungsraten und großen Unterschiede von Ummagnetisierungsprozessen in magnetischen Materialien gibt es derzeit keine einheitliche Beschreibung der magnetischen Viskosität für Feldänderungsraten im Bereich von 0,001 bis zu 1000 T/s. Durch die Messung im Impulsfeld konnte die Größenordnung der magnetischen Viskosität in nanokristallinem Bariumferrit bestimmt werden. Magnetisierungsmessungen im Impulsfeld stellen sowohl durch das hohe Magnetfeld als auch aufgrund der hohen bzw. variierenden Feldänderungsrate ein sehr nützliches Instrument zur Untersuchung feld- und zeitabhängiger Eigenschaften von Festkörpern dar
In this work, the occurrence and the mechanism of field induced transitions and the related critical fields were investigated. The way of measuring the magnetisation was designed for the existing pulsed field device of the IFW Dresden. The magnetisation was measured in fields up to 48 T. For the first time, the anisotropy coefficient of Sm2Fe17N3 was obtained in the combined measurement of the exchange field via inelastic neutron scattering and the measurement of the anisotropy constant K1 for the same material. For the leading anisotropy coefficient, a value of A20&lt;r²&gt; = -28 meV was found using K1 of about 13 MJ/m³. It was shown that the observed high field transition in SmCo2.5Cu2.5 and SmCo2Cu3 is connected with the microstructure. The laminar microstructure consisting of phases with different Sm-content is a necessary precondition for the occurrence of the transition. The coercivity increases with the Cu-content and reaches high values at low temperature. The coercivity and the transition field show big magnetic viscosity. In DyFe6Al6, the disappearance of the spontaneous magnetisation at low temperature is caused by a strong antiferromagnetic coupling. The magnetic transition at low temperature could be explained by a field induced magnetic moment on a disordered crystal site. For the hexagonal DyMn6Ge6, the temperature dependence of the transition field towards the canted antiferromagnetic structure was measured for the first time. Above 100 K, the applied field causes the transition from the helimagnetic to the fan structure. At low temperature, a spin flop transition occurs, which is supported by the magnetic anisotropy of the Dy-ion. The magnetisation of magnetocaloric materials exhibits a dependence of the field changing rate. This can be explained qualitatively by the measurement condition: The pulsed field measurement is adiabatic, whereas during static measurements, the condition is isothermal. Besides common magnetic compounds, highly correlated electron systems were also investigated. The magnetic transition at 43 T in CeNi2Ge2 can be explained by the suppression of the Kondo effect and the breaking up of the antiferromagnetic structure. Furthermore, magnetisation of high temperature superconductors was measured. The measurements in the pulsed field are a contribution to the determination of the phase diagram of melt textured YBa2Cu3O7-d. The irreversibility field Hirr was measured for bulk samples down to low temperature. Hirr(T) shows an unexpected linear increase down to low temperature. Because of the high field-changing rates and the big differences of magnetisation processes in magnetic materials, there is no uniform description of the magnetic viscosity for field changing rates in the magnitude from 0,001 up to 1000 T/s. By the measurement in the pulsed field, the magnitude of the magnetic viscosity of nanocrystalline barium ferrite was determined. Magnetisation measurement in pulsed fields is a very useful instrument to investigate field and time dependent properties of solids due to their high magnetic field and their high and varying field changing rate

La réfrigération magnétique est une technologie émergente de production de froid. Elle constitue une alternative à la technique classique utilisant des fluides frigorigènes polluants. L'effet magnétocalorique qui en est à la base se traduit par le refroidissement ou l'échauffement de certains matériaux sous l'action d'un champ magnétique. Ce travail de thèse se situe dans le domaine de l'étude de ces matériaux et des dispositifs de réfrigération magnétique à aimants permanents autour de la température ambiante. Ainsi, des composés à effet magnétocalorique géant à l'ambiante de formule Mn1−x(Ti0.5V0.5)xAs ont été étudiés. D'autre part, un système complet de réfrigération magnétique à aimants permanents initié par une thèse précédente a été réalisé et testé. Enfin, pour relier ces deux thématiques, un dispositif de test a été réalisé. Il permet d'évaluer et de comparer les performances de ces nouveaux matériaux dans les conditions réelles de fonctionnement dans un bloc réfrigérant.

This work presents a novel and elaborate framework to evaluate promising materials for magnetocaloric application. In the first step, the molecular meanfield theory (Bean-Rodbell model) is applied to simulate various magnetic systems covering the first and second order transition. The magnetic systems of second order transition are analyzed to find the appropriate composition for a magnetic field change comparable to practical values (up to 2 Tesla). Moreover, the intrinsic hysteresis occurring in first order materials is estimated, under certain magnetic field change, and its impact on the materials’ cooling capacity is calculated. The most efficient candidate in terms of cooling capacity is detected via the comparison between materials of various transition behavior. A set of samples of La2/3(Ca1-xSrx)1/3MnO3 magnetocaloric family with transitions ranging from second to first order is produced and the data is fitted with the simulation tool to parameters such as spin value and the magnetovolume coupling parameter. Magnetic systems similar to experimental samples are simulated and the cooling capacity of the simulated system and experimental samples are compared to verify the theoretical model. The temperature dependence of the thermal conductivity of the La-Ca-Sr-Mn-O and Mn-Fe-P-Si systems are measured and show up to 50% change within the operating temperature ranges along with fully contrasting behaviour for the two families of magnetocaloric materials. It is also shown that the temperature dependence of thermal conductivity is coordinated with the order of the magnetic transition. By synthesizing La1-x(Ce, Pr, Nd)xFe11.6Si1.4 samples via a combination of induction melting and suction casting techniques, the substitution range is expanded up to x=0.4. The impact of La substitution on the magnetocaloric characteristics including magnetic entropy change, adiabatic temperature change, Tc and hysteresis is investigated. Finally, the phase transition order is studied using methods based on field dependence of magnetocaloric effect (including the Bean-Rodbell model).
Este trabalho apresenta uma abordagem inovadora para a avaliação de materiais promissores para aplicações magnetocalóricas. Inicialmente, a teoria do campo médio molecular (modelo Bean-Rodbell) é aplicada para simular vários sistemas magnéticos que descrevem transições de primeira e de segunda ordem. Os sistemas magnéticos de transição de segunda ordem são analisados para encontrar a composição com performance otimizada para uma mudança de campo magnético comparável aos valores práticos (até 2 Tesla). Além disso, a histerese intrínseca que ocorre em materiais de primeira ordem é estimada, sob certas alterações de campo magnético, e o seu impacto na capacidade de arrefecimento dos materiais é calculado. O candidato mais eficiente em termos de capacidade de refrigeração é detectado através da comparação entre materiais com vários comportamentos diferentes na transição. Um conjunto de amostras da família magnetocalórica La2/3(Ca1-xSrx)1/3MnO3 com transições variando de segunda e primeira ordem foi produzido e os dados foram ajustados com a ferramenta de simulação e parâmetros como valor de spin e o parâmetro de acoplamento magneto-volume. Sistemas magnéticos semelhantes às amostras experimentais foram simulados e a capacidade de arrefecimento do sistema simulado e amostras experimentais foram comparadas para verificar o modelo teórico. A dependência da temperatura da condutividade térmica dos sistemas La-Ca-Sr-Mn-O e Mn-Fe- P-Si foi medida e mostra alterações de até 50% dentro das faixas de temperatura de operação, além de um comportamento totalmente contrastante para as duas famílias de materiais magnetocalóricos. Também é mostrado que a dependência na temperatura da condutividade térmica é coordenada com a ordem da transição magnética. Sintetizando a família de amostras de La1-x(Ce, Pr, Nd)xFe11.6Si1.4 por meio de uma combinação de técnicas de fusão por indução e fundição por sucção, a faixa de substituição é expandida até x = 0.4. O impacto da substituição de La nas características magnetocalóricas, incluindo alteração da entropia magnética, mudança de temperatura adiabática, Tc e histerese, é investigada. Finalmente, a ordem de transição de fase é estudada usando métodos baseados na dependência de campo do efeito magnetocalórico (incluindo o modelo Bean-Rodbell).
Programa Doutoral em Ciência e Engenharia de Materiais

The objective of this work is to assess the potential of Mn-Fe-Si-P for magnetic heat pump applications. Mn-Fe-Si-P is a first order transition magnetocaloric material made from safe and abundantly available constituents. A significant magnetocaloric effect occurs at the transition temperature of the material. The transition temperature can be tuned by changing the atom ratios to a region near room temperature. Mn-Fe-Si-P in magnetic heat pumps is investigated by determining the material's properties, 1D system modeling and experiments in a magnetic heat pump prototype. We characterize six samples of Mn-Fe-Si-P, based on their heat capacity and magnetization. The reversible component of the adiabatic temperature change is found from the entropy diagram and compared to cyclic adiabatic temperature change measurements. Five of the six samples are selected to be formed into epoxy xed crushed particulate beds, which can be installed into a magnetic heat pump prototype. A system model is constructed to understand the losses of the magnetic heat pump prototype. Several experiments are performed with Gd with rejection temperatures around room temperature. Including dead volume and casing losses improves the modeling outcomes to match the experimental results closer. Experiments with Mn-Fe-Si-P are performed. Five materials are formed into modular beds that can be combined into two layer configurations. Six experimental configurations are tested, one single layer regenerator test with a passive lead second layer, and five experiments using two layers with varying transition temperature spacing between the materials. The best performance of the beds was found at close spacing at suitable rejection temperatures. It was found that at far spacing, the performance of stronger materials would produce a lower temperature span than that of weaker materials at close spacing. The experiments provide results that are used to validate the system modeling approach using the material data obtained of the Mn-Fe-Si-P samples. We integrate material properties into a system model. A framework is proposed to take into account the hysteresis. This framework shows an improvement of the predicted trend for a single layer case. The proximity of simulation and experimental multi-layering results are dependent on the rejection temperature. At the higher end of the rejection temperature the modeling results over-predict the temperature span around the active region. At lower rejection temperatures the simulation under-predicts the experimental temperature span. The inclusion of experimental pressure drop improved the trends found at higher rejection temperatures. A further improvement was found varying the interstitial heat transfer term. Modeling future research should focus on characterizing the thermo-hydraulic closure relationships for crushed particulate epoxy xed beds, and improvements to the heat loss model. Mn-Fe-Si-P is able to produce a temperature span, when a suitable set of Mn-Fe- Si-P materials are selected based on minimal hysteresis, making it a viable material for magnetic heat pump applications. The performance of Mn-Fe-Si-P is further improved by layering materials with a closely spaced transition temperature. Future research should focus on increasing the production of Mn-Fe-Si-P materials with low hysteresis, and improving the regenerator matrix geometry and stability.
Graduate

碩士
國立交通大學
機械工程系所
106
This thesis will study on melting and forming of the isotropic NdFeB powder (MQP-S-11-9) by using selective laser melting system. Control the cooling rate by changing process parameters such as multi-beams, single-beam and laser parameters, and study the effects of different parameters on the hard magnetic phase and magnetic properties. By additive manufacturing technology, the limitations such as shape, internal hollow structure, and the same magnetism in one component, are overcome. The experiments are divided into single-beam and multi-beam systems. In the multi-beam experiment, the X-ray diffraction analysis measurement data of the single-layer structure can be used firstly to confirm that the CW mode laser source will cause too much α Fe content in the finished product and low magnetic energy product. During the experiment, we will control repetition rate, pulse duration, scanning speed and hatching distance. The results showed that scanning speed is the main factor on the magnetic properties. As the scanning speed increased, the maximum energy product also improved. In single-beam experiment, scanning speed can overcomes the limitation in multi-beam system, and furthermore increase cooling rate and improve maximum energy product. During the experiment, it was also found that if the thickness of the powder layer was reduced from 40 μm to 25 μm and the overlapping rate of melted track was increased, the magnetic properties could be efficiently improved.

After the prediction of magnetoelectric effect in Cr2O3, in early 1960's, D. Asrov became the first to experimentally verify this phenomenon. After the pioneering work on magnetoelectric materials in 1960's and 1970's, the discovery of large magnetoelectric effect in orthorhombic rare-earth manganite TbMnO3 has revived great interest in magnetoelectric materials, especially during the last decade. Magnetoelectric multiferroics have great potential in applications such as novel memory storage devices and sensors. As a result of extensive theoretical and experimental investigations conducted on rare-earth magnetoelectric manganites, TbMnO3 has become a prototype magnetoelectric multiferroic material. Orthorhombic rare-earth manganites RMnO3 (R = Gd, Tb and Dy) exhibit improper ferroelectricity where the origin of ferroelectricity is purely magnetic in nature. RMnO3 exhibit diverse and complex magnetic interactions and phases. Doped manganites of the type R1-xAxMnO3 (A = Ca, Sr and Ba) present a rich magnetic and electronic phase diagram. The doping concentration, average ion-size and size mismatch (i.e. disor-der) at A-site, all contribute to determine the ground state. A variety of magnetic phases, competing with each other, are responsible for many functional properties like magnetoelectric effect, colossal magnetoresistance (CMR), magnetostriction and magnetocaloric effect (MCE). In this context, studies of magnetoelectric materials are of great relevance from technical as well as fundamental aspects. Notably, complexity of electronic (and magnetic) phases and experimental difficulties in acquiring reliable measurement-data easily are the most concerning issues in establishing a clear understanding of magnetoelectric materials. In the magnetic phase diagram of RMnO3, GdMnO3 lies on the border between A-type antiferromagnetic and cycloidal antiferromagnetic ground states. Cycloidal spin arrangement is responsible for the induction of ferroelectricity in these materials. There are disparate opinions about the ground state of GdMnO3 (whether the ground state is ferroelectric or not). Understanding of the influence of rare-earth magnetic sublattice on magnetism in GdMnO3 (at low temperature) lacks clarity till date. Neutron scattering studies on GdMnO3 due to high absorption cross-section of Gd ion, yield little success in determining the nature of complex magnetic phases in this material. Interestingly, an earlier report on strontium-substituted gadolinium manganite Gd0.5Sr0.5MnO3 demonstrated the spontaneous electric polarization and related magnetoelectric effect. It was hypothesized that the observed ferroelectricity could be improper and electronic in nature. Strontium doping facilitates quenched disorder that leads to interesting magnetic phases and phase transitions. In order to understand the physical properties of gadolinium manganites and to unravel the relationship between them, it is essential to investigate high quality single crystals of these materials. This thesis deals with growth and investigation of several important physical phenomena of gadolinium manganites such as magnetic, electric, magnetoelectric and magnetocaloric properties. The thesis is organized in seven chapters. A brief summary of each chapter follows: Chapter:1 This chapter provides general introduction to magnetoelectric effect and multiferroicity. The term multiferroicity refers to simultaneous existence of magnetic and electric ordering in a single phase material. Magnetoelectric multiferroics have shown great potential for several applications. They exhibit cross coupling between the electronic and magnetic order parameters, hence basics of various magnetic interactions (and magnetism) are brie y discussed in the rst section of the chapter. It is followed by a brief discussion about the principle of magnetoelectric effect. Magnetoelctric coupling is broadly classified into two types namely, direct coupling and indirect coupling. In the former, the emphasis is given on linear magnetoelectric effect. The concept of multiferroicity is introduced in the next section followed by a brief overview and application potential of multiferroics. Further, classi cation scheme of multiferroic materials is discussed. The concept of improper ferroelectricity and description of subcategories namely, magnetic ferroelectric, geometric ferroelectric and electronic ferroelectric are documented. Magnetic ferroelectric category is considered the most relevant; featuring the type of ferroelectric material as GdMnO3 referred in this thesis. The microscopic theory for mechanism of ferroelectricity in spiral antiferromagnets is presented. While brie ng the thermodynamic background of the magnetocaloric effect, indirect estimation of two important characteristics namely, isothermal magnetic entropy change (∆SM ) and adiabatic change in temperature (∆Tad) under the application of magnetic field are dealt with. In the last part of the chapter, motivation and scope of the thesis is discussed. Chapter:2 This chapter outlines various experimental methodologies adopted in this work. It describes the basic principles of various experimental techniques and related experimental apparatuses used. The chapter starts with the synthesis tech-niques used in the preparation of different compounds studied. The principle of oat-zone method, employed for single-crystal growth, is described. Orientation of single crystals was determined using a home-built back- reflection Laue set up. The basics of Laue reflection and indexing procedure for recorded Laue photographs are described. Various physical properties (electric, magnetic, thermal, magnetoelectric and magnetocaloric properties) were studied using commercial as well as home-built experimental apparatuses. Design and working principle of all the experimental tools are outlined in this chapter. Fabrication details, interfacing of measurement instruments and calibration (standardization) of equipment used in this work are described in appropriate sections. Chapter:3 Chapter-3 describes the investigation of various physical properties of high quality single crystals of magnetoelectric multiferroics, GdMnO3. Synthesis of GdMnO3 is carried out using solid state synthesis route. Single phase nature of the material is confirmed by X-ray powder diffraction technique. Single crystals of GdMnO3 are grown in argon ambience using oat-zone method. As grown crystals are oriented with the help of back-reflection Laue method. GdMnO3 exhibits incommensurate collinear antiferromagnetic phase below 42 K and transforms to canted A-type antiferromagnetic phase below 23 K. Magnetic and specific heat studies have revealed very sharp features near the magnetic transitions which also confirm the high quality of the single crystal. dc magnetization studies illustrate the anisotropic behavior in canted A-type antiferromagnetic phase and clarifies the influence of rare-earth magnetic sub-lattice on overall magnetism (at low temperature). Application of magnetic field (above 10 kOe) along `b' axis helps formation of the cycloidal antiferromagnetic phase. Here, spontaneous electric polarization is induced along `a' axis. The temperature variation plot of dielectric constant, ϵa (under ap- plied magnetic field along `b' axis) shows sharp anomalies in the vicinity of magnetic ordering transitions suggesting magnetodielectric effects. Magnetic field tuning of electric polarization establish the magnetoelectric nature of GdMnO3. Magnetocaloric properties of single crystals of GdMnO3 are investigated using magnetic and magnetothermal measurements. The magnitude of the giant magnetocaloric effect observed is compared with that of other rare-earth manganite multiferroics. Magnetocaloric studies shed light on magnetic ordering of rare-earth ion Gd3+. The phenomenon of inverse magnetocaloric effect observed at low temperature and under low fields is possibly linked to the ordering of Gd3+ spins. Complex interactions between the 3d and 4f magnetic sublattices are believed to influence magnetocaloric properties. Chapter:4 The details of synthesis and single crystal growth of Gd0.5Sr0.5MnO3 using oat-zone method are presented in Chapter 4. Single phase nature of the material is veri ed by carrying out powder x-ray diffraction analysis and confirmation of single crystallinity and orientation through back-reflection Laue method. Electric transport studies reveal semiconductor-like nature of Gd0.5Sr0.5MnO3 until the lowest temperature achieved. This is due to charge localization process which occurs concurrently with decrease in temperature. Gd0.5Sr0.5MnO3 exhibits charge-ordered insulator (COI) phase below 90 K (ac-cording to an earlier report). It is found that under application of magnetic field above a critical value, charge ordering melts and the phase transforms to ferromagnetic metallic (FMM) phase. This transformation is first-order in nature with associated CMR (109%). The first-order phase transition (FOPT) occurs between competing COI and FMM phases and manifests as hysteresis across the FOPT. Strontium doping at A-site induces a large size mismatch at A-site resulting in high quenched disorder in Gd0.5Sr0.5MnO3. The disorder plays a significant role in CMR as well as glass-like dynamics within the low-temperature magnetic phase. ac susceptibility studies and dynamic scaling analysis reveal very slow dynamics inside the low-temperature magnetic phase (below 32 K). According to an earlier report, spontaneous electric polarization and magnetoelectric effect were pronounced near FOPT (at 4.5 K and 100 kOe) between COI and FMM phases. It is prudent to investigate FOPT across COI and FMM phases in Gd0.5Sr0.5MnO3 to understand complex magnetic phases present. Thermodynamic limits of the FOPT (in magnetic field - temperature (H-T) plane), such as supercooling and superheating, are experimentally determined from magnetization and magnetotransport measurements. Interestingly, thermomagnetic anomalies such as open hysteresis loops are observed while traversing the FOPT isothermally or isomagnetically in the H-T plane. These anomalies point towards incomplete phase transformation while crossing the FOPT. Phenomenological model of kinetic arrest is invoked to understand these anomalies. The model put for-ward the idea that while cooling across the FOPT, extraction of specific heat is easier than that of latent heat. In other words, phase transformation across FOPT is thermodynamically allowed but kinetics becomes very slow and phase transformation does not occur at the conventional experimental time scale. Magnetization relaxation measurements (at 89 kOe) with field-cooled magnetization protocol reveal that the relaxation time constant rst decreases with temperature and later, increases non-monotonically below 30 K. This qualita-tive behavior indicates glass-like arrest of the FOPT. Further, thermal cycling studies of zero field-cooled (ZFC) and eld-cooled (FC) magnetization indicate that a low temperature phase prepared with ZFC and FC protocols (at 89 kOe) is not at equilibrium. This confirms the kinetic arrest of FOPT and formation of magnetic phase similar to glass. Chapter:5 Chapter-5 deals with the investigation of the effect of an electric field on charge ordered phase in Gd0.5Sr0.5MnO3 single crystals. As discussed in the previous chapter, application of magnetic field above a critical value collapses the charge ordered phase which transforms to FMM phase. In this view, it is interesting to investigate effect of electric field on the charge ordering. There are various reports on doped manganites such as Pr1-xCaxMnO3 (x = 0:3 to 0:4) that claim melting of charge ordering under application of electric field (or current) above a critical value. In this thesis work, current - voltage (I - V) characteristics of Gd0.5Sr0.5MnO3 are studied at various constant temperatures. Preliminary measurements show that the I-V characteristics are highly non-linear and are accompanied by the onset of negative differential resistance (NDR) above a critical current value. However, we suspect a major contribution of Joule heating in realization of the NDR. Continual I - V loop measurements for five loops revealed thermal drag and that the onset of NDR shifts systematically towards high current values until it disappeared in the current window. Two strategies were employed to investigate the role of Joule heating in realization of NDR: 1) monitoring the sample surface temperature during electric transport measurement and 2) reducing of the Joule heating in a controlled manner by using pulsed current I - V measuremenets. By tuning the duty cycle of the current pulses (or in other words, by controlling the Joule heating in the sample), it was feasible to shift the onset of NDR to any desired value of the current. At low magnitude of the duty cycle in the current range upto 40 mA, the NDR phenomenon did not occur. These experiments concluded that the NDR in Gd0.5Sr0.5MnO3 is a consequence of the Joule heating. Chapter:6 `Chapter-6 deals with the thermal and magnetocaloric properties of Gd0.5Sr0.5MnO3 oriented single crystals. Magnetocaloric properties of Gd0.5Sr0.5MnO3 have been studied using magnetic and magnetothermal measurements. Tempera-ture variation of ∆SM is estimated for magnetic field change of 0 - 70 kOe. The eld 70 kOe is well below the critical magnetic eld required for FOPT between COI and FMM phases. Magnetzation - field (M-H) loop shows minimal hysteresis for measurements up to 70 kOe. The minimal hysteresis behavior al-lows one to make fairly accurate estimation of magnetocaloric properties. ∆Tad was separately estimated from specific heat measurements at different magnetic fields. Specific heat studies show the presence of Schottky-like anomaly at low temperature. Chapter:7 Finally, Chapter-7 summarizes various experimental results, analyses and conclusions. A broad outlook of the work in general with future scope of research in this area are outlined in this chapter.