Dissertations / Theses on the topic 'Ferromagnetic Heusler alloys'

博士
國立中正大學
物理系研究所
104
The effects of lattice constant and valence electron concentration e/a (electrons per atom) on the magnetocaloric effects of Ni–Mn-Sn Heusler alloys are invetigated. At first, the effect of Ge substitution for Sn on phase evolution and magnetocaloric effects (MCEs) of Ni48Mn39Sn13-xGex (x=0-3) and Mn49Ni41Sn10-xGex (x=0-2) ribbons has been studied. When Mn content is over 45 at%, the Ge atoms prefer to form a Mn-rich phase with Mn atoms. This Mn-rich phase might suppress the martensitic transition of the alloy ribbons, resulting in the decrease of maximal magnetic entropy change, ΔSMmax. Secondly, for Co-substituted Mn49-xCoxNi41Sn10 (x=0-4) ribbons, the ferromagnetic behavior of austenite is improved with incresing Co content originated from the enhancement of ferromagnetic exchange interaction. For these series samples, an average ΔSMmax value of 12.5 J/kg·K is easily obtained in different operation temperature region. On the other hand, the Fe-Mn based ferromagnetic shape memory alloys (FSMAs) is also an interesting functional material to study. In order to observe the relationship of exchange bias (EB) effect with phase composition, tunning the composition of Fe-Mn-Ga ternary alloy is nessessary. For Fe50Mn25Ga25 ribbon, the results showed that if its FCC phase undergoes field cooling, giant EB properties could be observed. The HEB=2.5 kOe and iHc=11 kOe were obtained at 5 K. Furthermore, if the content of Fe atoms is over 50 at% and Mn atoms lower than 25 at%, ribbons are perfer to form a B2 phase. In contrast, if Mn content is higher than 25at%, the ribbons are perfer to form a pure L12 phase (FCC). Next, the effect of Ni substitution for Fe on the crystal structure, magnetic state and magnetocaloric effect of the Fe-Ni-Mn-Ga Heusler alloy ribbons is also presented. Results show that with the increase of Ni content, the phase changed from FCC structure into B2 structure. An average ΔSMmax value is 1.5 Jkg/K in these samples. A relatively broader temperature range at the half maximum of ΔS¬M peak (~90 K), low-cost and nontoxic elements make Fe-Ni-Mn-Ga-based ribbons the promising candidates for magnetic refrigeration applications close to room temperature. Keywords: Heusler alloys, Magnetic refrigerant materials, Magnetocaloric effect and Exchange bias.

Ni-Mn-X (X – Ga, Sn, In, Sb) based ferromagnetic Heusler alloys are deemed to be multiferroic materials with the multiferroicity realized by engineering reversibility through composition tuning, of an inherent first-order diffusionless martensitic phase transformation and a second-order magnetic transition accompanying the first with the application of an electromagnetic field. Martensitic phase transformation transforms high-temperature austenite phase to low-temperature martensite phase while magnetic transition changes the electromagnetic ordering in the phases. The engineered reversibility of the magnetostructural transformations is brought with remarkable functional changes such as change in entropy, saturation magnetization, strain recovery, etc. that offer great scope for applications. There are examples in literature of such alloys with great potential for magnetic refrigeration by virtue of a giant entropy change, or with potential for magnetic shape memory effect because of giant magnetic field-induced strains. However, the most interesting characteristics of these alloys is their ability in the conversion of waste heat into electricity on account of the sudden thermally induced surge in saturation magnetization around the phase transformation, suitably utilized to generate electricity. A majority of these ferromagnetic Heusler alloys have been synthesised by liquid processing method of arc/induction-melting of high purity elements in a controlled environment usually followed by annealing. Synthesis procedures which are a sequel to arc/induction melting such as rapid solidification by melt-spinning, directional solidification etc. are also prevalent. Solid processing through powder metallurgy is sparsely employed on pre-alloyed powders of these alloys and not elemental powders even though use of elemental powders is advantageous for reasons such as good compaction characteristics and ease of obtaining new alloy compositions through precise control of stoichiometry. Ferromagnetic Heusler alloys of compositions discussed herein are meant to transform martensitically and vice versa, irrespective of the way they are synthesized. Do these alloys necessarily have to be synthesized through liquid processing? The question is broader in concept and the lack of literature on solid processed ferromagnetic shape memory Heusler alloys provided scope for investigation of the feasibility of adopting conventional pressure-less sintering process as a cost-effective alternative to casting. This research therefore endeavoured to explore the feasibility of solid processing of ferromagnetic Heusler alloys through conventional sintering using elemental powders. The starting composition was Ni45Co5Mn40Sn10. The choice of composition was the singular magnetostructural behaviour at phase transformation of the cast composition that drove its potential as an energy material similar to or even better than thermoelectric materials. Two sets of alloys were prepared – a quinary Ni45Co5Mn40(Sn,Cu)10 and quaternary Ni45Co5Mn40Sn10 using the conventional press and sinter procedure. The quinary compacts were prepared at a compaction load of 70 KN (184 MPa) and sintered at two different temperatures of 950°C and 1050°C for 24 h in order to first study the feasibility of powder processing in fabricating these alloys. The addition of Cu in the quinary composition was to understand the effect of compositional change on the transformation temperatures. With the results of the quinary alloy turning out to be favourable, quaternary alloys were then synthesized in order to understand the effect of process parameters such as compaction pressure, temperature and time of sintering on the magnetostructural characteristics of these alloys. The quaternary compacts were prepared at two different compaction loads of 70 KN (184 MPa) and 80 KN (210 MPa). Sintering on them was carried out at two different temperatures of 950 °C and 1050 °C at sintering times of 12, 24, 72 and 144 h. With the use of standard characterization techniques of differential scanning calorimetry, optical/electron microscopy including transmission electron microscopy, Xray diffraction and magnetization testing various issues concerning synthesis and characterization of ferromagnetic Heusler alloys using powder metallurgy are reported and discussed.
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2019

Tesis (Lic. en Física)--Universidad Nacional de Córdoba, Facultad de Matemática, Astronomía, Física y Computación, 2016.
Las aleaciones de Ni2MnGa se caracterizan por poseer memoria de forma magnética (FSMA). La clave de este comportamiento es la transformación martensítica que experimentan cuando son enfriadas por debajo de una temperatura crítica, que es muy diferente para variaciones de composición alrededor de la estequiometrica. El mecanismo que da origen a la FSMA tiene lugar en esta fase martensítica e involucra el desplazamiento de maclas, inducido por un campo magnético aplicado. La magnitud del efecto FSMA es importante en monocristales de esta fase, siendo en general despreciable en muestras policristalinas. Sin embargo, se ha observado recientemente que es posible lograr con técnicas de colada por succión cilindros policristalinos de esta aleación, de uno y dos milímetros de diámetro, con granos orientados de manera tal que la textura cristalográfica y el estado de tensiones internas en el material da origen a FSMA.
In the martensite phase, monocrystalline Heusler alloys with composition near Ni2MnGa exhibit Ferromagnetic Shape Memory (FSM). The mechanism leading to the FSM involves twin boundaries displacement, induced for a magnetic field applied; the magnitude of this FSM effect may be important in monocrystals but it is in general negligible in polycrystalline samples. In this work, we report FSM in highly textured polycrystalline cylinders of this alloy, obtained by the Suction Casting method, with diameters of about 1-2 mm. The microstructure and the magnetic properties of these FSM cylinders are discussed.
Fil : Deghi, Sebastián Esteban. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina.

Seit der Entdeckung des Spin-Torque durch Berger und Slonczewsky im Jahre 1996 gewann dieser Effekt immer mehr an Einfluss in dem Gebiet der Spintronic. Dies geschah besonders durch den Einfluss des Spin-Torque auf die Informationsspeicher und Kommunikationstechnologien (z.B. die Möglichkeit einen magnetischen Zustand eines Speicherelementes mit Hilfe von Strom und nicht wie bisher durch das Anlegen eines magnetischen Feldes zu ändern, oder die Realisierung eines hochfrequenten Spin-Torque-Oszillator (STO). Aufgrund des direkten Zusammenhangs zwischen der Dämpfung in Ferromagneten und der kritischen Stromdichte, die nötig ist um ein Spin-Ventil zu schalten oder ein Präzidieren der Magnetisierung zu induzieren, wurde die Forschung an Ferromagneten mit geringer Dämpfung zunehmend forciert. In dieser Arbeit werden Studien der ferromagnetischen Resonanz (FMR) von NiMnSb Schichten und Transportmessungen an NiMnSb basierten Spin-Ventilen präsentiert. Das Halbmetall NiMnSb ist mit einer theoretischen 100%igen Spinpolarisation prädestiniert für die Verwendung in GMR Elementen. Neben der theoretisch vorhergesagten hohen Spinpolarisation zeigen die durchgeführten FMR Messungen einen überaus geringen Dämpfungsfaktor für dieses Material. Dieser liegt in der Größenordnung von wenigen 10-3. Somit ist die Dämpfung in NiMnSb um den Faktor zwei geringer als in Permalloy und gut vergleichbar mit epitaktisch gewachsenen Eisen-Schichten. Neben den guten Dämpfungseigenschaften zeigen jedoch theoretische Modelle den Verlust der 100%igen Spinpolarisation durch das Brechen der Translationssymmetrie an Grenzflächen und das Kollabieren der Aufspaltung im Minoritäts-Spin-Band. Da ein Wachstum in (111) Richtung diesen Prozess entgegen wirken kann, werden in dieser Arbeit zudem auf (111)(In,Ga)As gewachsene NiMnSb Schichten mittels FMR untersucht. Die Messungen an diesen Proben zeigen, im Vergleich zu (001) orientierten Schichten, eine erhöhte Dämpfung. Zudem kann bei diesen Schichten eine schichtdickenabhängige uni-direktionale magnetische Anisotropie gemessen werden. Im Hinblick auf den möglichen industriellen Einsatz in Speicherelementen werden überdies Messungen an Sub-Mikrometer großen NiMnSb Elementen auf (001) orientierten Substraten präsentiert. Die Elemente wurden mittels Elektronenstrahllithographie hergestellt und mittels FMR vermessen. Auch die so prozessierten Schichten zeigen einen Dämpfungsfaktor im unteren 10-3 Bereich. Das Auftreten von magnetostatischen Moden in den Messungen ist ein weiterer indirekter Nachweis der hohen Qualität der NiMnSb-Schichten. Im Jahre 2001 wurde von Mizukamie und seinen Kollegen eine dickenabhängige Erhöhung der Gilbertdämpfung bei, mit Metallen bedeckten, Permalloy-Schichten beobachtet. Im Jahr darauf wurde von Tserkovnyak, Brataas und Bauer eine Theorie erarbeitet die dieses Phänomen auf ein Pumpen von Spins aus dem Ferromagneten in die Metalschicht zurückführt. Aus diesem Grund werden Messungen von NiMnSb Schichten, die mit verschiedenen Metallen und Isolatoren in-situ vor Oxidation geschützt wurden, präsentiert. Nach diesen materialspezifischen Voruntersuchungen werden auf NiMnSb und Permalloy basierte Pseudo-Spin-Ventile unter Verwendung eines selbst ausrichtenden lithographischen Prozesses hergestellt. Transportmessungen an den Proben zeigen ein GMRVerhältnis von 3,4% bei Raumtemperatur und fast das doppelte bei tiefen Temperaturen. Diese sind sehr gut vergleichbar mit den besten veröffentlichten GMR-Verhältnissen für Einzelschichtsysteme. Überdies kann in den Experimenten eine viel versprechend geringe kritische Stromdichte, die nötig ist, um die magnetische Orientierung zu ändern, gemessen werden. Diese ist vergleichbar mit kritischen Stromdichten aktuellster metallbasierter GMR-Elemente oder auf dem Tunneleffekt basierenden Spin-Ventilen. Das eigentliche Potential der auf NiMnSb basierenden Spin-Ventile wird erst ersichtlich wenn diese als STO zum Emittieren hochfrequenter, durchstimmbarer und schmalbandiger elektromagnetischer Wellen verwendet werden. Auf Heusler basierende STO zeigen einen überdurchschnittlich hohen q-Faktor von 4180, sogar im Betrieb ohne extern angelegtes Magnetfeld. Dieser ist um mehr als eine Größenordnung höher als der höchste veröffentliche q-Faktor eines ohne externes Feld arbeitenden STO. Während die Heusler basierten STO ebenso wie alle anderen STO unter einer geringen Ausgangsleistung leiden, machen die Maßstäbe im Sub-Mikrometer Bereich eine On-Chip Herstellung möglich. Somit kann durch ein Parallelschalten von gekoppelten Oszillatoren eine Erhöhung der Ausgangsleistung erzielt werden
Since the discovery of spin torque in 1996, independently by Berger and Slonczewski, and given its potential impact on information storage and communication technologies, (e.g. through the possibility of switching the magnetic configuration of a bit by current instead of a magnetic field, or the realization of high frequency spin torque oscillators (STO), this effect has been an important field of spintronics research. One aspect of this research focuses on ferromagnets with low damping. The lower the damping in a ferromagnet, the lower the critical current that is needed to induce switching of a spin valve or induce precession of its magnetization. In this thesis ferromagnetic resonance (FMR) studies of NiMnSb layers are presented along with experimental studies on various spin-torque (ST) devices using NiMnSb. NiMnSb, when crystallized in the half-Heusler structure, is a half-metal which is predicted to have 100% spin polarization, a consideration which further increases its potential as a candidate for memory devices based on the giant magnetoresistance (GMR) effect. The FMR measurements show an outstandingly low damping factor for NiMnSb, in low 10-3 range. This is about a factor of two lower than permalloy and well comparable to lowest damping for iron grown by molecular beam epitaxy (MBE). According to theory the 100% spin polarization properties of the bulk disappear at interfaces where the break in translational symmetry causes the gap in the minority spin band to collapse but can remain in other crystal symmetries such as (111). Consequently NiMnSb layers on (111)(In,Ga)As buffer are characterized in respect of anisotropies and damping. The FMR measurements on these samples indicates a higher damping that for the 001 samples, and a thickness dependent uniaxial in-plane anisotropy. Investigations of the material for device use is pursued by considering sub-micrometer sized elements of NiMnSb on 001 substrates, which were fabricated by electron-beam lithography and measured by ferromagnetic resonance. The damping remains in the low 10-3 range as determined directly by extracting the Gilbert damping from the line width. Additionally magnetostatic modes are observed in arrays of elements, which is further evidence of high material quality of the samples. By sputtering various metals on top of the NiMnSb, spin pumping from the ferromagnet into the non-magnetic layer is investigated. After these material investigations, pseudo-spin-valves using NiMnSb as one of the ferromagnet, in combination with Permalloy were fabricating using a self-aligned lithography process. These samples show a GMR ratio of 3.4% at room temperature and almost double at low temperature, comparing favourably to the best single stack GMR structures reported to date. Moreover, current induced switching measurements show promisingly low current densities are necessary to change the magnetic orientation of the free layer. These current densities compete with state-of-the-art GMR devices for metal based structures and almost with tunnel junction devices. The true potential of these devices however comes to light when they are operated as spin torque oscillators to emit high frequency, tunable, narrow spectrum electromagnetic waves. These Heusler based STOs show an outstanding q-factor of 4180, even when operating in the absence of an external field, a value which bests the highest value in the literature by more than an order of magnitude. While these devices currently still suffer from the same limited output power as all STO reported to date, their sub-micron lateral dimensions make the fabrication of an on-chip array of coupled oscillators, which is a promising path forward towards industrially relevant output power