Multiferroic materials, showing strong coupling between ferroelectric and ferromagnetic interaction, have been the subject of intense research recently. These materials are potential candidates for the next generation in microelectronic device. CdCr2S4, one of the few multiferroics with spinel structure proposed in 2005, is in the limelight of scientific research, because of the complex interaction among lattice, spin and dipole. To investigate further, we have measured the magnetic and electric properties of single crystal CdCr2S4. The spins are coupled ferromagnetically below Curie temperature TC = 84 K. With increasing magnetic field, ferromagnetic transition is found to be shifted towards higher temperature. The magnetic hysteresis is satisfied with the characteristic of soft ferromagnet. Electric field induced permanent metal-insulator transition (MIT) and colossal magnetoresistance (CMR) are observed in the resistivity measurement. From dielectric constant measurement, two interesting dipolar ordering states have been observed. The peak near TC ~ 84 K is due to the glassy type dipolar state stimulated by the onset of ferromagnetic ordering and the other near 54 K, might be the ferroelectric ordered state induced by externally applied electric field. Both electric and magnetic field dependent physical properties have been studied and reveal significant evidences of strong spin-dipole coupling in the present system.

This thesis presents the synthesis and investigations of physical and chemical properties of multiferroic materials experimentally as well as theoretically. Multiferroics are materials in which at least two of the three ferroic orders, ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Multiferroics, have the potential to be used as a four state as well as cross switchable memory devices. The thesis is organized into seven Chapters. Chapter 1 gives a brief overview of the different facets of multiferroics, explaining the origin of Multiferroicity and magnetoelectric coupling, their possible technological applications and the challenges involved. Chapter 2-4 concerns the experimental aspects and chapter 5-7 concerns the theoretical aspects. Chapter 2 deals with experimental investigations on nanoscale charge-ordered rare earth manganites. It shows with decreasing particle size the ferromagnetic interaction increases and the charge-ordering vanishes down to the lowest sizes. Chapter 3 describes magneto-dielectric, magnetic and ferroelectric properties of hexagonal LuMnO3. It also describes the Raman spectroscopy of this compound through the magnetic and ferroelectric transition temperatures. Chapter 4 deals with the anisotropic multiferroic properties in single crystals of hexagonal ErMnO3. In chapter 5 a brief introduction of density functional theory (DFT) is given. Chapter 6 deals with the magneto-structural changes, spin-phonon couplings and crystal field splittings for the different magnetic orderings LuMnO3. Chapter 7 elucidates the role of Lu d0-ness for the ferroelectricity observed of this compound.

Excessive energy dissipation in CMOS devices during switching is the primary threat to continued downscaling of computing devices in accordance with Moore’s law. In the quest for alternatives to traditional transistor based electronics, nanomagnet-based computing [1, 2] is emerging as an attractive alternative since: (i) nanomagnets are intrinsically more energy-efficient than transistors due to the correlated switching of spins [3], and (ii) unlike transistors, magnets have no leakage and hence have no standby power dissipation. However, large energy dissipation in the clocking circuit appears to be a barrier to the realization of ultra low power logic devices with such nanomagnets. To alleviate this issue, we propose the use of a hybrid spintronics-straintronics or straintronic nanomagnetic logic (SML) paradigm. This uses a piezoelectric layer elastically coupled to an elliptically shaped magnetostrictive nanomagnetic layer for both logic [4-6] and memory [7-8] and other information processing [9-10] applications that could potentially be 2-3 orders of magnitude more energy efficient than current CMOS based devices. This dissertation focuses on studying the feasibility, performance and reliability of such nanomagnetic logic circuits by simulating the nanoscale magnetization dynamics of dipole coupled nanomagnets clocked by stress. Specifically, the topics addressed are: 1. Theoretical study of multiferroic nanomagnetic arrays laid out in specific geometric patterns to implement a “logic wire” for unidirectional information propagation and a universal logic gate [4-6]. 2. Monte Carlo simulations of the magnetization trajectories in a simple system of dipole coupled nanomagnets and NAND gate described by the Landau-Lifshitz-Gilbert (LLG) equations simulated in the presence of random thermal noise to understand the dynamics switching error [11, 12] in such devices. 3. Arriving at a lower bound for energy dissipation as a function of switching error [13] for a practical nanomagnetic logic scheme. 4. Clocking of nanomagnetic logic with surface acoustic waves (SAW) to drastically decrease the lithographic burden needed to contact each multiferroic nanomagnet while maintaining pipelined information processing. 5. Nanomagnets with four (or higher states) implemented with shape engineering. Two types of magnet that encode four states: (i) diamond, and (ii) concave nanomagnets are studied for coherence of the switching process.

archives@tulane.edu / The electronic, spin, phonon and magnetic behavior govern the electrodynamics of solid materials. The different compositions and symmetries mix all the degrees of freedom leading to varieties of interesting phenomena such as metal-to-insulator transition, nonreciprocal directional dichroism and topological states. The study of the behavior of electrons, spins and phonons is crucial to reveal the physics behind the mysterious phenomena. The nature of terahertz time domain spectroscopy (THz TDS) which has low photon energy and contains phase information makes this technique very powerful to probe the physics of spins, electrons, phonons and magnons where the resonance energy is in the THz range. The multiferroic materials are studied by using THz-TDS with strong dc magnetic field. Multiferroic material is one of complex materials that simultaneously contain ferroelectricity and magnetism. Many fascinating physical phenomena are discovered in multiferroics, including magneto-dielectric effect and nonreciprocal directional dichroism. The magneto-dielectric effect, change in dielectric function in applied magnetic field, is studied in multiferroic CaBaCo4O7. We analyze the dynamics of phonons to clarify the individual phonon contribution to the magneto-dielectric effect. We observe giant nonreciprocal directional dichroism in the multiferroic material FeZnMo3O8, which is defined as the difference in absorption coefficient for linearly polarized light waves travelling in the opposite direction. A spin excitation is determined as the origin of nonreciprocal effect in the multiferroic FeZnMo3O8 by using THz-TDS. The nonreciprocal effect from magneto-chiral dichroism is also observed in BaCoSiO4 crystal where the material simultaneously possesses the chiral structure and magnetization. The polarimetry of transmitted THz light through BaCoSiO4 is carefully analyzed. We attribute the change in polarization in the zero magnetic field to the chirality of the structure. Nonlinearity of semiconductor InSb due to intense THz electric field is investigated quantitatively by using THz-TDS. The effective mass approximation breaks down when the intense THz pulse is applied to the semiconductor. We develop a predictive model that replaces the effective mass with a realistic band structure and retains the Drude parameters, the electron density and scattering rate, to accurately calculate the experimental observations (saturable absorption and amplitude-dependent refractive index) in InSb. / 1 / Shukai Yu

In recent years multiferroic materials have great application potential in the ferroelectric random access memory and emerging spintronics development setting off the boom of the multiferroic materials research. It was assume that the ferroelectric and magnetic properties cannot coexist at the same temperature range or their coupling is weak making the searching for multiferroics in dream. Multiferroic materials have been discovered in recent years. Multiferroics may even be induced due to the strong coupling between artificial layers by growing multilayer technique. [1] [2] Among of which the BiFeO3 compound contains of antiferromagnetic and ferroelectric properties at room temperature. The BiFeO3 thin films manifests a larger spontaneous porlarization than that of the bulk and is highly capable of industrial application. In this study, Pb doped BFO (BPFO) this films were grown on top the conductive SrRuO3 layers. By varying BPFO¡¦s thickness, the strain effect the relative physical properties were studied. It is found that the lattice constants of SRO is totally different to that of STO substrate indicating strain relaxation. Similar phenomena are also discovered for BPFO thin film which is believed due to the tilting effect on the SRO/STO interface.

With combination of the both ferroelectric and ferromagnetic and mutural coupling properties, multiferroics attracts a lot of researcher¡¦s attentions. Among these very popular materials, such as REMnO (113 series, 125 series), BiFeO3 etc¡K , the BiFeO3 interests us the most for it manifests multiferroic effects above the room temperature. However, the pure BiFeO3 phase is very difficult to form and a serious lost of Bi creates complex grain boundaries that produces enormous electric leaking. In this study, Pb is doped into BiFeO3 in Bi sites and we hope this doping effect may increase the stability and the ferroelectricity of Bi1-xPbxFeO3. It is found that the crystal structure is changed dramatically, because of the doping of Pb, from the rhombohedra structure of the parent pure BiFeO3 to the cubic structure of Bi0.85Pb0.15FeO3. By the Pb doping, the compounds exhibit free from impurity phases and the dielectric constants are enhanced qualitatively to the doping levels.

La doped HoMn2O5 in studying the influence of the local structure distortion on the magnetic transition has been investigated systematically. It is found that by doping of La ions in a proper percentage, 0.1≤x≤0.2, the formation of the possible impurity RMnO3 (113) phase could be suppressed; single phases of LaxHo1-xMn2O5 (0.1≤x≤0.2) can be formed in one atmosphere of flowing oxygen. For x=0.2, an weak ferromagnetic transition on top of the paramagnetic background appears at 150K and saturated at 75K, which implies that the compound undergoes a ferromagnetic to antiferromagnetic transition. In addition, the lattice didn¡¦t appear a huge phase transition at low temperature, it¡¦s observed local distortion behavior within 100~150K, and back to normal structure as high temperature ones. It¡¦s convinced that will be associated with the magnetic properties, which need further examination and experiment.

Metal oxide materials are a broad class of materials found in many current technologies due to their interesting properties such as magnetism and ferroelectricity. Material properties can be tuned and heavily influenced by disorder at the atomic level, as has been shown in the ferrimagnetic and ferroelectric Al2-x-yGaxFeyO3 materials, which adopt the non-centrosymmetric, orthorhombic GaFeO3 crystal structure-type (Pna21). The significant cation disorder and non centrosymmetric nature of the crystal structure underlie the multiferroic properties in these materials and make them one of the few chemical systems to possess multiferroic ordering near room temperature. Unfortunately, while cation site disorder is responsible for the multiferroic properties observed in these compounds, their complex crystal structure has led to inconsistent reports in the ternary Al2-xFexO3 and Ga2-xFexO3 compounds. X-ray absorption near-edge spectroscopy (XANES) is an element specific technique, which can be used to examine cation site disorder as a function of changes in the average coordination environment around the metal, providing a means of studying these complex materials. In this thesis, XANES was used to investigate factors affecting cation site disorder in a series of Al1-xGaxFeO3 materials (0 ≤ x ≤ 1) adopting the GaFeO3 crystal structure-type. The GaFeO3 crystal structure has four cation sites, of which, the distorted octahedral Fe1 and Fe2 sites are primarily occupied by Fe3+, and the less distorted tetrahedral A1 and octahedral A2 sites are primarily occupied by Al3+ or Ga3+. These materials were initially synthesized using a high temperature ceramic method, and it was found that with increasing Ga3+ content (x) these materials show a decrease in the amount of cation site disorder between the tetrahedral site and the three octahedral sites. This decrease is attributed to the tetrahedral site preference of Ga3+, which inhibits cation site disorder at the A1 site, as opposed to the octahedral site preference observed for Al3+. Additionally, Fe3+ was found to predominantly occupy the three octahedral sites over the tetrahedral site in these materials, likely because of its large ionic size and the strong magnetic coupling between those three sites. The quaternary Al1-xGaxFeO3 materials (0 ≤ x ≤ 1) were synthesized again via two other techniques: a citrate sol-gel method and a co-precipitation method. The oxide network binding the binary metal oxide precursors limits ion mobility in the high temperature ceramic method. The citrate sol-gel and co-precipitation methods were used to generate mixed-metal precursors with a more homogeneous distribution of the metal cations than the binary metal oxide precursors commonly used by the high temperature ceramic method. Mixed-metal precursors reduce the distance the ions have to diffuse, while the nature of the amorphous matrix was found to affect disorder in the resulting material. From analysis of the XANES spectra, the ceramic method showed the least amount of cation site disorder, followed by the citrate sol-gel method and co-precipitation method, respectively. Greater annealing temperatures resulted in an increase in cation site disorder, with the average coordination number of Al3+ and Ga3+ increasing while the average coordination number of Fe3+ decreased. Al1-xGaxFeO3 materials synthesized via the co-precipitation method showed the greatest amount of cation disorder, followed by the citrate sol-gel and high temperature ceramic techniques, respectively. The research presented in this thesis is among the first to examine a large number of materials from the relatively unexplored Al1-xGaxFeO3 system, and has contributed to the growing body of knowledge on the factors affecting cation site disorder in these materials and potentially other systems. Further, despite a simple rationale for understanding the features present in Al L2,3- and Ga K-edge spectra, these studies have shown how effectively XANES can be used to understand subtle changes in the atomic structure of solid-state materials.

Orientador: Pascoal José Giglio Pagliuso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-15T19:42:04Z (GMT). No. of bitstreams: 1 Bufaical_LeandroFelixdeSousa_D.pdf: 2467040 bytes, checksum: 4460b75c04d570fb27584b7dfff2c5f3 (MD5) Previous issue date: 2010 / Resumo: Há muitas décadas os óxidos de metais de transição são tema de grande interesse científico devido à grande variedade de propriedades físicas interessantes que apresentam, com suas possíveis aplicações tecnológicas. Mais recentemente, por exemplo, os óxidos de metais de transição com propriedades multiferróicas ganharam destaque na comunidade científica como potenciais dispositivos magneto-eletrônicos. Muitos óxidos de metais de transição se formam na estrutura cristalina chamada perovskita simples, com simetria cúbica ou distorcida. Muitos outros óxidos podem se cristalizar numa variante da perovskita simples, a chamada perovskita dupla ordenada (PDO), que possui fórmula geral A2B¿B¿¿O6, onde o íon A ocupa os vértices do cubo enquanto os cátions B¿ e B¿¿ se alternam nos centros dos octaedros de oxigênio. Dois compostos com estrutura PDO bastante estudados são o Sr2FeReO6 e Sr2FeMoO6 devido ao fato de apresentarem, entre outras propriedades interessantes, comportamento meio-metálico (halfmetal), magnetrorresistência por tunelamento à temperatura ambiente, ferrimagnetismo com TC acima de 400K em ambos os compostos. As propriedades estruturais, eletrônicas e magnéticas dessas PDOs estão altamente conectadas e são fortemente dependentes do grau de hibridização dos orbitais d dos cátions B¿¿. Assim, se fazem importantes os estudos de novos compostos PDO a fim de se investigar as idéias correntes propostas em literatura e, nesse contexto, reportamos aqui os resultados da síntese e caracterização das séries inéditas Ca2-xLaxFeIrO6 e Sr2-xLaxFeIrO6, onde o Ir, assim como o Re e Mo, é metal de transição, no caso com caráter 5d, e pode assumir diferentes estados de valência. As medidas de magnetização indicaram que estes sistemas tendem a evoluir de antiferromagnéticos nas extremidades das séries, x = 0 e x = 2, para ferrimagnéticos em regiões intermediárias da série. Medidas realizadas no composto de maior magnetização da série de Sr, o Sr1.2La0.8FeIrO6, indicaram que este composto se ordena ferrimagneticamente em torno de 700 K, sendo esta a mais elevada TC já reportada para perovskitas duplas. Medidas de resistividade em função da temperatura indicaram que os compostos apresentam comportamento isolante e praticamente nenhum efeito magneto-resistivo. No composto antiferromagnético Sr2FeIrO6 foi estudada a resistividade sob efeito de pressão e, embora não tenha ocorrido nenhuma transição metal-isolante, ocorre uma diminuição sistemática da resistência do material e da inclinação da curva à medida que a pressão aumenta, indicando um comportamento do tipo isolante de Mott nesse composto. Neste trabalho são apresentados também resultados dos estudos realizados na perovskita TbMnO3. Realizamos neste óxido medidas de susceptibilidade magnética, calor específico, Ressonância Paramagnética Eletrônica (EPR) e absorção de microondas para várias temperaturas. A susceptibilidade magnética e o calor específico confirmaram para a amostra estudada as temperaturas de transição de fase magnética (TN = 41 K) e ferroelétrica (Tlock) já reportadas em literatura. Os espectros de EPR mostraram para todo o intervalo de temperatura uma única linha consistente com uma forma de linha Lorentziana e um valor de g independente da temperatura g = 1.96(3) consistente com Mn3+ em um meio isolante. A largura de linha sofreu um alargamento com a temperatura seguindo uma lei do tipo C/T. Esse alargamento impediu a observação dos espectros de ressonância em torno das regiões de temperaturas das transições de fase magnética e ferroelétrica. Devido à forte dependência da constante dielétrica com a freqüência, as medidas realizadas com a cavidade de campo elétrico não permitiram a observação de qualquer anomalia em torno das temperaturas de transições / Abstract: For many decades the transition metal oxides are subject of great scientific interest because of the wide variety of interesting physical properties and their potential technological applications. More recently, for example, oxides of transition metals with multiferroic properties have been considered as potential magneto-electronic devices. Many transition metal oxides form in the perovskite crystalline structure, with cubic or distorted symmetry. Many other oxides can crystallize in a variant of the simple perovskite, called the ordered double perovskite (ODP), which has the general formula A2B'B''O6, where the A ion occupies the vertices of the cube while the cations B 'and B'' alternate in the centers of the oxygen octahedra. Sr2FeReO6 and Sr2FeMoO6 are two compounds with the ODP structure which were extensively studied due to their interesting properties such as half-metal behavior, tunneling magnetoresistance at room temperature and ferrimagnetic order (TC above 400 K). The structural, electronic and magnetic properties of these ODPs are highly correlated and are strongly dependent on the strong d orbitals hybridization of the of the B'' cations. Therefore, studies of new ODP compounds are important in order to investigate the current ideas proposed in the literature and improve the understanding of their physical properties. We report here our results of synthesis and characterization of the unpublished series Ca2-xLaxFeIrO6 and Sr2-xLaxFeIrO6, where the Ir such as Re and Mo are transition metal, with d character that can assume different valence states. The magnetic measurements indicated that those systems tend to evolve from antiferromagnetics at the ends of the series, x = 0 and x = 2, to ferrimagnetic for intermediate regions of the series. Measurements performed in the compound of higher magnetization in the Sr serie, Sr1.2La0.8FeIrO6 indicated that this compound orders ferrimagnetic around 700 K, which is the highest TC ever reported for double perovskites. Resistivity measurements as a function of temperature indicated that these compounds also exhibit insulating behavior and virtually no magneto-resistive effect. In the antiferromagnetic compound Sr2FeIrO6, the effect of pressure on the resistivity was investigated, and although no metal-insulator transition was seen, there is a systematic decrease of the resistance and the slope of the curve as the pressure increases, indicating a Mott insulator-like behavior in this compound. This work also presents results on the TbMnO3 perovskite. We have performed magnetic susceptibility, specific heat, Electron Paramagnetic Resonance (EPR) and microwave absorption measurements at various temperatures. Magnetic susceptibility and specific heat data confirmed the ocurrence of a magnetic (TN = 41 K) and ferroelectric (Tlock) phase transition. The EPR spectra showed, for the entire temperature range measured, a single Lorentzian line shape and T independent g-value = 1.96 (3), consistent with the resonance of Mn3+ in an insulating environment. The width line broadens with the decreasing temperature following a C/T law. This broadening prevented the observation of the resonance spectra near the magnetic and ferroelectric phase transitions. Because of the strong frequency dependence of the dielectric constant, the measurements performed with the electric field cavity also did not allow observation of any anomaly around the ferroelectric transition / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Recently, multiferroics-defined as materials with coexistence of at least two of the ferroelectric, ferroelastic and ferromagnetic effects-have attracted enormous research activities. In this thesis, the structure and properties of multiferrioic BiFeO3-x%PbTiO3 and Fe-x%Ga crystalline solutions were investigated. First, the results show that modified BiFeO3-PbTiO3 based ceramics have significantly enhanced multiferroic properties, relative to BiFeO3 single crystals. The data reveal: (i) a dramatic increase in the induced polarization; and (ii) the establishment of a remnant magnetization by a breaking of the translational invariance of a long-period cycloidal spin structure, via substituent effects. In addition, temperature dependent magnetic permeability investigations of BiFeO3-xPbTiO3 crystalline solutions have shown that aliovalent La substitution results in a significant increase in the permeability. Second, room temperature high-resolution neutron and x-ray diffraction studies have been performed on Fe-x%Ga crystals for 12<x<25at%. It has been observed that the structures of both Fe-12%Ga and Fe-25%Ga are tetragonal; however, near the phase boundary between them, an averaged-cubic structure was identified. In addition, an unusual splitting along the transverse direction indicates that the crystals are structurally inhomogeneous. / Master of Science