Academic literature on the topic 'Magneto-electric Coupling - Single Crystals'

Electromechanical coupling in piezoelectric materials allows direct conversion of electrical energy into mechanical energy and vice versa. Here, we demonstrate lead-free (KxNa1−x)NbO3 single crystals with an ultrahigh large-signal piezoelectric coefficient d33* of 9000 pm V−1, which is superior to the highest value reported in state-of-the-art lead-based single crystals (~2500 pm V−1). The enhanced electromechanical properties in our crystals are realized by an engineered compositional gradient in the as-grown crystal, allowing notable reversible non-180° domain wall motion. Moreover, our crystals exhibit temperature-insensitive strain performance within the temperature range of 25°C to 125°C. The enhanced temperature stability of the response also allows the materials to be used in a wider range of applications that exceed the temperature limits of current lead-based piezoelectric crystals.

We report an effective substitution of aluminum for gallium in langasite-type La3Ta0.5Ga5.5O14 (LTG) crystals for use in a pressure sensor at high temperature. Al-substituted LTG (La3Ta0.5Ga5.5-xAlxO14; LTGAx) single crystals up to the solubility limit of x=0.5 have been grown by the conventional Czochralski technique. The electric properties of the LTGAx crystals were investigated and compared with those of LTG. By Al substitution, the piezoelectric constant d11 and the electromechanical coupling factors (k12) became slightly larger. The LTGAx crystals showed a lower temperature dependence of d11 and a higher electric resistivity ρ than those of the LTG crystals.

Pyroelectric materials have a broad spectrum for practical application. Apart from the established infrared sensor technology, recently the pyroelectric effect has been employed unconventionally in waste heat recovery, X-ray generation or water disinfection. This coupling phenomenon is the temperature dependence of a ferroelectric's spontaneous polarisation. A crystal structure that allows pyroelectricity cannot have an inversion centre, if it had a polar axis would not exist. Hence the well-known perovskite strontium titanate, crystallizing in the space group Pm-3m, is known to be dielectric. Nonetheless, under an external electric field of 1MV/m charged defects like oxygen vacancies redistribute in a strontium titanate single crystal, leading to a distortion of the unit cell and subsequently to the formation of a defect structure called the migration-induced field-stabilized polar (MFP) phase [1]. Raman scattering shows that the MFP phase of strontium titanate may exhibit broken centrosymmetry, suggesting the existence of a polar axis. Here, we investigate the pyroelectric properties of strontium titanate single crystals at room temperature during these electroformation cycles with a modified Sharp-Garn method [2]. Our frequency and field dependent measurements indicate the pyroelectricity of the MFP phase. Additionally the measurement method elucidates the kinetics of the oxygen vacancy migration as well as electric properties during electroformation. Inducing pyroelectricity in a centro-symmetric crystal structure opens the scope for a new class of pyroelectric materials.

The Bi2Fe2WO9 ceramic was prepared using a standard solid-state reaction technique. Preliminary analysis of X-ray diffraction pattern revealed the formation of single-phase compound with orthorhombic crystal symmetry. The surface morphology of the material captured using scanning electron microscope (SEM) exhibits formation of a densely packed microstructure. Comprehensive study of dielectric properties showed two anomalies at 200[Formula: see text]C and 450[Formula: see text]C: first one may be related to magnetic whereas second one may be related to ferroelectric phase transition. The field dependent magnetic study of the material shows the existence of small remnant magnetization ([Formula: see text]) of 0.052[Formula: see text]em[Formula: see text]/g at room temperature. The existence of magneto-electric (ME) coupling coefficient along with above properties confirms multi-ferroic characteristics of the compound. Selected range temperature and frequency dependent electrical parameters (impedance, modulus, conductivity) of the compound shows that electric properties are correlated to its microstructure. Detailed studies of frequency dependence of ac conductivity suggest that the material obeys Jonscher’s universal power law.

The non-centrosymmetric bismuth-based oxyborate crystals have been extensively studied for non-linear optical, opto-electric and piezoelectric applications. In this work, single crystal growth and electro-elastic properties of α-BiB3O6 (α-BIBO) and Bi2ZnB2O7 (BZBO) crystals are reported. Centimeter-sized α-BIBO and BZBO crystals were grown by using the Kyropoulos method. High resolution X-ray diffraction tests were performed to assess the crystal quality. The full-width at half-maximum values (FWHM) of the rocking curves were evaluated to be 35.35 arcsec and 47.85 arcsec for α-BIBO and BZBO samples, respectively. Moreover, the electro-elastic properties of α-BIBO and BZBO crystals are discussed and summarized, based on which the radial extensional and the face shear vibration modes were studied for potential acoustic device applications. The radial extensional mode electro-mechanical coupling factors kp were evaluated and found to be 32.0% and 5.5% for α-BIBO and BZBO crystals, respectively. The optimal crystal cuts with face shear mode were designed and found to be (YZt)/−53° (or (YZt)/37° cut) for α-BIBO crystal, and (ZXt)/±45° cut for BZBO crystal, with the largest effective piezoelectric coefficients being in the order of 14.8 pC/N and 8.9 pC/N, respectively.

Perovskite rare-earth ferrites (REFeO3) have attracted great attention for their high ferroelectric and magnetic transition temperatures, strong magnetoelectric coupling, and electric polarization. We report on the flux method growth of rare-earth iron oxide Lu1−xScxFeO3 single crystals through a K2CO3-B2O3-Bi2O3 mixture as a flux solution, and give a detailed characterization of the microstructure, magnetism, and ferroelectric properties. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements revealed that the obtained single crystals can be designated to three different crystal structures of different chemical compositions, that is, Lu0.96Sc0.04FeO3 (perovskite phase), Lu0.67Sc0.33FeO3 (hexagonal phase), and Lu0.2Sc0.8FeO3 (bixbyite phase), respectively. Magnetic measurements indicate that the perovskite Lu0.96Sc0.04FeO3 is an anisotropic hard ferromagnetic material with a high Curie transition temperature, the bixbyite Lu0.2Sc0.8FeO3 is a low temperature soft ferromagnetic material, and the hexagonal Lu0.67Sc0.33FeO3 exhibits multiferroic properties. Lu0.67Sc0.33FeO3 possesses a weak ferromagnetic transition at about 162 K. We further investigate the ferroelectric domain structures in hexagonal sample by scanning electron microscope and the characteristic atomic structures in ferroelectric domain walls by atomically resolved scanning transmission electron microscope. Our successful growth of perovskite Lu1−xScxFeO3 single crystals with distinct crystal structures and stochiometric Lu-Sc substitutions is anticipated to provide a useful ferrites system for furthering exploitation of their multiferroic properties and functionalities.

We report a cluster spin glass behavior, multiferroicity and magnetoelectric (ME) effects in the single crystals of 0.46[Formula: see text]-0.54[Formula: see text]. The crystals are found to possess a pseudo-cubic structure with a weak tetragonal distortion and show ferroelectricity at room temperature with a ferroelectric curie temperature of [Formula: see text] K. The cluster spin glass state in the crystal is evidenced by detailed dc and ac magnetic experiments, including thermo-remnant magnetization, aging effect, memory effect, etc. Magnetodielectric effects, poling enhanced magnetism, and electric field induced acceleration of magnetization relaxation in the spin glass state are observed and ascribed mainly to spin-lattice coupling. These results might suggest an effective route to improve ferromagnetism and ME effects by constructing a spin glass state in [Formula: see text]-based antiferromagnetic multiferroics.

As a unique electric-optics material, liquid crystals (LCs) have been used in various light-control applications. In LC-based light-control devices, the structural alignment of LC molecules is of great significance. Generally, additional alignment layers are required for LC lens and microlens, such as rubbed polyimide (PI) layers or photoalignment layers. In this paper, an electrically controlled liquid crystal microlens array (EC-LCMLA) based on single-crystal graphene (SCG) coupling alignment is proposed. A monolayer SCG with high conductivity and initial anchoring of LC molecules was used as a functional electrode, thus no additional alignment layer is needed, which effectively simplifies the basic structure and process flow of conventional LCMLA. Experiments indicated that a uniform LC alignment can be acquired in the EC-LCMLA cell by the SCG coupling alignment effect. The common optical properties including focal lengths and point spread function (PSF) were measured experimentally. Experiments demonstrated that the proposed EC-LCMLA has good focusing performance in the visible to near-infrared range. Moreover, the plenoptic imaging in Galilean mode was achieved by integrating the proposed EC-LCMLA with photodetectors. Digital refocusing was performed to obtain a rendering image of the target.

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.

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.

博士
國立交通大學
電子工程學系 電子研究所
102
This dissertation mainly researches the photoluminescence characteristics of single quantum dot. We report on the magnetic responses of neutral exciton (X), biexcitons (XX) and positive/negative trions (X+/X-) in single self-assembled InAs/GaAs quantum dots. Unlike the conventional quadratic diamagnetic shift for neutral excitons, the observed X- diamagnetic shifts are small and nonquadratic. In particular, we also observed a reversal in sign of the conventional diamagnetic shift. A theoretical analysis indicates that such anomalous behaviors for X- arise from an apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction induced by the hole in its initial state. This effect can be very pronounced in small quantum dots, where the electron wave function becomes weakly confined and extended much into the barrier region. When the electrons gradually lose confinement, the magnetic response of X- will transit gradually from the usual quadratic diamagnetic shift to a quartic dependence, and finally into a special paramagnetic regime with an overall negative energy shift. On the other hand, we purpose to study the coupling effect between single quantum dot and photonic crystal cavity, a method for designing H1 photonic crystal cavity is introduced to enhance its quality factor (Q factor). The highest theoretical Q factor of 120,000 is obtained. The Fourier transformation of field distribution shows that the enhancement arises from the component reduction of leaky mode. The Q-factor improvement has also been demonstrated experimentally with the highest value of 11700. Our design could be useful for studying light-matter interaction in H1 cavity as the mode volume only increases slightly. Finally, we successfully demonstrated the strong coupling effect in the H1 photonic crystal cavity embedded single InAs/GaAs quantum dot. Two polariton states arise from the hybridization of the cavity mode and quantum dot, which reflect in the alterations of observed emission characteristics, such as emission wavelength, full width half maximum and intensity. Via analysis, the strongest coupling effect occurs at about 37.75 K, while Rabi splitting is equal to 156.7 μeV.

The prefix “piezo” (pronounced pie-ease-o) comes from the Greek word for pressure or mechanical force. Piezoelectricity refers to the linear coupling between mechanical stress and electric polarization (the direct piezoelectric effect) or between mechanical strain and applied electric field (the converse piezoelectric effect). The equivalence between the direct and converse effects was established earlier using thermodynamic arguments (Section 6.2). The principal piezoelectric coefficient, d, relates polarization, P, to stress, X, in the direct effect (P = dX) and strain, x, to electric field E (x = dE). Thus the units of d are [C/N] or [m/V] which are equivalent to one another. Typical sizes for useful piezoelectric materials range from about 1 pC/N for quartz crystals to about 1000 pC/N for PZT (lead zirconate titanate) ceramics. To understand how the piezoelectric effect varies with direction and how it is affected by symmetry, it is necessary to determine how piezoelectric coefficients transform between coordinate systems. Since polarization is a vector and stress a second rank tensor, the physical property relating these two variables must involve three directions: . . . Pj = djklXkl . . . . In the new coordinate system . . . P'i = aijPj = aijdjklXkl . . . . Transforming the stress to the new coordinate system gives . . . P'i= aijdjklamkanlX'mn = d'imnX 'mn. . . . Thus piezoelectricity transforms as a polar third rank tensor. . . . d'imn = aijamkanldjkl . . . . In general there are 33 = 27 tensor components, but because the stress tensor is symmetric (Xij = Xji), only 18 of the components are independent. Therefore the piezoelectric effect can be described by a 6 × 3 matrix.

We have measured the low frequency (undamped) electrooptic and piezoelectric coefficients in several photorefractive crystals using interferometric techniques. The electrooptic and piezoelectric measurements were independent of the frequency (in the range from 10 Hz to 100 kHz) and linear in the magnitude (in the range from 0.1 to 200 V/cm) of the applied electric field. The electrooptic coefficients measured in melt-grown BaTiO3 single crystals at λ = 5145 Å and at T = 23°C are r13 = 33 ± 2 pm/V and r33=124 ± 6 pm/V. The piezoelectric coefficient d13 = 57 ± 3 pm/V was measured separately. The combined Pockels coefficient r C = 84 ± 5 pm/V was measured independently and is in good agreement with the value r c ≡ r33 − (n1/n3)3r13 = 84 ± 5 pm/V computed from the above values of r13 and r33. (n1, and n3 are the ordinary and extraordinary indices of refraction, respectively, of tetragonal BaTiO3.) Similar measurements at λ = 5145 Å in Sr0.6Ba0.4Nb2O6 single crystals yield the electrooptic coefficients r13 = 55 ± 3 pm/V, r33 = 224 ± 11 pm/V, and the piezoelectric coefficient d13 = 47 ± 3 pm/V. The electrooptic and piezoelectric coefficients of LiNbO3 single crystals were also measured. As an independent check on this data, we also measured the coupling between two optical beams in these photorefractive crystals. The ratio of the measured coupling strengths for different optical polarizations agrees with theory using the electrooptic coefficients measured above.

The last decade has witnessed the discovery of materials combining shape memory behavior with ferromagnetic properties (FSMAs), see, e.g., James & Wuttig1, James et al.2. These materials feature the so-called giant magnetostrain effect, which, in contrast to conventional magnetostriction, is due to the motion of martensite twins. It was first observed in NiMn2Ga single crystals, Ullakko et al.3, but later discovered in polycrystals as well, see Ullakko4. This effect has motivated the development of a new class of active materials transducers, which combine intrinsic sensing capabilities with superior actuation speed and improved efficiency when compared to conventional shape memory alloys. The effect has also been found in thin films, Rumpf et al.5, and this technology is currently being developed intensively in order to pave the way for applications in micro- and nanotechnology. As an example, Kohl et al.6,7, recently proposed a novel actuation mechanism based on NiMnGa thin film technology, which makes use of both the ferromagnetic transition and the martensitic transformation allowing the realization of an almost perfect antagonism in a single component part. The implementation of the mechanism led to the award-winning development of an optical microscanner8. Possible applications in nanotechnology arise, e.g., by combination of smart NiMnGa actuators with scanning probe technologies. The key aspect of Kohl’s device is the fact that it employs electric heating for actuation, which requires a thermo-magneto-mechanical model for analysis. The research presented in this paper aims at the development of a model that simulates this particular material behavior. It is based on ideas originally developed for conventional shape memory alloy behavior, (Mueller & Achenbach9, Achenbach10, Seelecke11, Seelecke & Mueller12) and couples it with a simple expression for the nonlinear temperature-and position-dependent effective magnetic force. This early and strongly simplified version does not account for a full coupling between SMA behavior and ferromagnetism yet, and does not incorporate the hysteretic character of the magnetization phenomena either. It can however be used to explain the basic actuation mechanism and highlight the role of coupled magnetic and martensitic transformation with respect to the actuator performance. In particular will we be able to develop guidelines for desirable alloy compositions, such that the resulting transition temperatures guarantee optimized actuator performance.

We report the experiment of a laser beam amplified by another beam coming from the same laser after both beams crossed in 0.1-wt. % Ce-doped strontium barium niobate (BaxSr1−xNb2O6 or SBN x = 0.48) crystal. The thirteenfold amplification is obtained (without an external electric field) and we predict that this factor of 13 is not the upper limit. Through this experiment the density of mobile charges Pd in Ce-SBN crystal was calculated to be +4.5 × 1016 cm−3, and the photorefractive index n1 is 1.83 × 10−4, and the maximum intensity of electric field E, which exists between the finger pattern of mobile charges in crystal Ce-SBN, is 7.8 × 104 V/M.

In this paper, single crystal piezoelectric actuators are developed and demonstrated as a means of increasing actuator authority while maintaining strain precision for adaptive optics used large throw deformable mirror applications. Single crystal (PMN-PT crystal) stack actuators with foot print size of 5 mm × 5 mm and height varied from 16 mm to 21 mm have been designed, fabricated and tested. The actuation stroke larger than 50 μm is obtained under driving electric field of 15 KV/cm for single crystal stack actuators. In-plane actuation stroke is about 25 μm under 8KV/cm for single crystal plate actuators. The characterization results show that the single crystal piezoelectric actuators hold enhanced stroke and coupling coefficient (>0.83) under lower driving field comparing with PZT counterpart, while keep the nature of piezoelectric actuations with high precision, high resolution, fast response under high preload stress. The potential benefits of incorporating these actuators in a deformable mirror (DM) design were demonstrated by system modeling using the Integrated Optical Design Analysis Software (IODA) developed by SRS. The modeling shows that low order aberrations with peak to valley errors as large as 40 microns could be corrected to diffraction limited performance with a reasonable number of actuators and acceptable actuator forces and face sheet stresses. In the case of DM correction with 20 microns of 1st-Order Astigmatism-X error (Peak to valley: 38.74 μm, RMS error: 8.13 μm), the corrected wavefront error was reduced to PV of 0.26 μm and RMS error of 0.024 μm, which suggest promise for large throw DM applications.

A three-dimensional nonlinear finite element formulation for ferroelectric materials is developed based on a principle of virtual work. The formulation includes the coupling of three physical fields, namely polarization field, electric field and strain field. The developed finite element formulation is employed to investigate the polarization distribution near a flaw in a ferroelectric single crystal under mechanical loadings. It is found that the polarization switching takes place near the flaw tip if the loadings exceed a critical value. In the simulation, we do not take any prior assumptions, i.e. without any switching criterion, on the polarization switching. The polarization switching is a result of the minimization of the total energy in the simulated system.

We present a new promising photorefractive crystal: lead germanate Pb5Ge3O11. This crystal is ferroelectric and has trigonal symmetry. The crystal itself has been studied before and most of its physical properties are well known. However, to our knowledge, the occurrence of photorefraction in this material has not been reported before. Large (0.9 cm × 0.8 cm × 0.43 cm) samples of Pb5Ge3O11 exhibit absorption of 1.5 cm−1 and optical activity of 8° /mm for green light (514.5 nm). Both parameters are smaller than in commonly used BSO crystal whose optical activity is almost 5 times as large. Since the largest electrooptic coefficient r33 is 15 pm/V compared with 5 pm/V for BSO, this crystal seems to be really interesting as a candidate for holographic recording medium. We studied the photorefractive properties of lead germanate in a conventional two-beam coupling experiment. We found that photorefraction occurs for all argon-ion lines and is strongest for λ = 514.5 nm. Weaker photorefractive sensitivity was found for red light. Therefore we used a He-Ne laser beam to read out the grating written by two green beams. In two-beam coupling measurements we obtained a photorefractive intensity gain coefficient of Γ = 1.46 cm−1. The time constant for grating formation was ~1 s for a total writing beam intensity of 200 mW/cm2. In our experiments we saw the effect of hologram fixing under long illumination. For short exposure the grating was completely erasable. We also studied the influence of external electric dc fields on the grating properties. It was found that an electric field of 5 kV/cm significantly improved the diffraction efficiency (5 times). Switching of this field leads to interesting transient behavior of the diffracted beam intensity (strong oscillations).

Nd:MgO:LiNbO3 is a high gain solid-state laser material. In addition, it allows electronic control of the emitted radiation by virtue of its electrooptic properties. We have operated a miniature Q-switched laser consisting of a single 2- × 2- × 10-mm Nd:MgO:LiNbO3 crystal and a Brewster plate placed between two mirrors spaced by 3 cm. This laser was end pumped by a dye laser beam at 598 nm. Peak powers of 5 W have been achieved with only 0.5% output coupling. Pulses 30-ns wide (FWHM) have been observed. The absorbed pump power threshold was 10 mW. Due to the material photoconductivity, an internal electric field can be generated that screens the external voltage. This screening effect was eliminated by applying the voltage in the form of pulses of alternating polarity. The voltage switching (from ~300 V to zero) excites several acoustic resonances in the crystal (piezoelectric effect) that modulate the crystal birefringence (elastooptic effect). The resulting time-dependent cavity loss affects the time of emission of the Q-switched pulse at low pump levels. However, the acoustic resonances have no influence when pumping more than three times above threshold.

Abstract Recently NRL researchers have embarked on a basic research effort “Tuning Giant Magnetoelectric Properties in Phase Transformation Multiferroics” focused on multifunctional materials for energy transduction and conversion. Multiferroic materials combine at least two coupled ferroic properties and are used in multiple applications including magnetic field sensors, energy harvesting devices, non-volatile memory and antennas. There are very few single phase multiferroic materials, and they normally have relatively low magnetoelectric (ME) coupling coefficient. In contrast, engineered materials such as ME composites fabricated from piezoelectric and magnetostrictive materials can show multiple orders of magnitudes increase in the ME coupling coefficient. The optimal design of ME composites would lead to conditions of maximum response (strain, induced voltage, or field) with minimum applied electric or magnetic fields, providing advanced materials for transduction, sensing, energy harvesting and other applications. That is why NRL researchers are working on piezoelectric materials with enhanced properties due to a phase transformation that would minimize the stimuli needed to achieve large strains. Key to the successful design and fabrication of ME composites is an understanding of interface characteristics as well as individual material components. In this paper we will review the current status of work at NRL on engineered multiferroic composites comprised of piezoelectric and magnetostrictive materials coupled through strain. There are still many open questions about the interfacial properties as well as the individual component materials. Details will be presented from recent work on material characterization under repetitive cycling, interface characteristics, and stress/electric/thermal effects on driving the phase transition in a relaxor ferroelectric single crystal.

Flexoelectric effect occurs in the solid crystalline dielectrics of symmetry or centro-symmetric crystals, which shows the electromechanical coupling of the polarization response and the strain gradient or the stress and the electric field gradient. Thus, a generic stress expression induced by the converse flexoelectric effect is established first in this study. The generic stress expression is simplified to a cantilever beam to evaluate the vibration control effect due to the converse flexoelectric effect. Flexoelectric fiber embedded with a metal core is placed into the cantilever beam to generate inhomogeneous electric field. When the flexoelectric fiber is actuated with the applied voltage, stress induced by the actuator is obtained with the electric field gradient, which results in a control bending moment to the beam. Static displacement control of the cantilever beam is established and the control effect is related to the fiber location and size of the flexoelectric fiber and the metal core. Cases show that the control effect is enhanced when the flexoelectric fiber is far away from the neural surface of the beam. Besides, the control effect can enhance with thinner fiber thickness. Since the piezoelectricity is similar to the flexoelectricity, comparison of the vibration control induced by the piezoelectric fiber is also discussed. The results show that the control effect of the flexoelectric fiber is more effective than the piezoelectric fiber in the cantilever beam.