Multiferroics (MF) have attracted much research attention due to the coexistence of ferroelectric and magnetic ordering as well as magnetoelectric (ME) coupling. At present there are very few room temperature single phase MF except BiFeO3. Multiferroic properties of Aurivillius compound Bi5FeTi3O15 were reported at 80 K. The at 80 K. The at 80 K. The at 80 K. The at 80 K. The magnetization of Bi5FeTi3O15 was significantly improved by substituting parts of Fe cations by Co cations. Bi5FeTi3O15 showed ferromagnetic order above room temperature. The magnetic cations Fe/Co in B-site contribute to the both ferroelectric and ferromagnetic properties, which could possibly induce strong magnetoelectric effect. Aurivillius materials are layered structured materials with formula (Bi (Bi2O2)2+ (A m-1BmO3m+1 )2-. The polarization of Aurivillius materials is mainly in a-b plane . High dense and textured ceramics were fabricated by a two-step spark plasma sintering (SPS) method to improve the polarization of ceramics. The multiferroic properties of of Aurivillius materials with different octahedral layers (m=2, 3, 4 and 5) were investigated. All these materials showed ferroelectric and ferromagnetic order at room temperature except Bi 3Nb 1.1251.1251.1251.1251.125Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 0.750.750.750.75O9 (m = 2). (m = 2). (m = 2). (m = 2). (m = 2). Bi 3.253.253.253.25La 0.750.750.750.75Nb 0.250.250.250.25Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 2.52.52.5O12 (m = 3) was identified to be single phase. (m = 3) was identified to be single phase. Although a small amount of secondary phase (CoFe (CoFe2O4/Co /Co2FeOFeOFeO4) were found in Bi 4.254.254.254.25La 0.750.750.750.75Fe 0.50.50.5Co 0.50.50.5Ti 3O15 (m = 4) and Bi 5.25.25.25La 0.750.750.750.75FeCoTiFeCoTiFeCoTiFeCoTi FeCoTi3O18 (m = 5), ), the intrinsic multiferroicity of the main Aurivillius phase was confirmed by the magnetic controlled ferroelectric domain switching. Clear ME couplings were observed in these materials.

In this study, we study the annealing effect of Bi0.9Pb0.1FeO3 bulk in various time span and the various growth conditions of Bi0.9Pb0.1FeO3 thin film to physical proporties, such as crystal structure, surface amorphous, delectric properties. With these effort, this study wish to find a better growth condition for Bi0.9Pb0.1FeO3 film that exhibit the best ferroelectric property, and to understant the possible mechanism underlaying the growth conditions to the physical properties. It is found that the doping of Pb in Bi0.9Pb0.1FeO3 compound does stabalize the formation of single phase Bi0.9Pb0.1FeO3 ,however, this stabalization can only postpone the decay of Bi0.9Pb0.1FeO3 properties when is annealed in a long period of time. The crystal strucutre of Bi0.9Pb0.1FeO3 is very close to a pseudocubic structure in which oxgyen sites locate noncenter-symmetrically that generates a stronge electric polariztion. The various growth conditions has a very stong influence to the physical properties of Bi0.9Pb0.1FeO3 thin films. For those films grwon at 700oC exibits the best delectricity. The grain size of films grows as grwoth time as resutl of this the thicker the film thelarger the grain size. The electric hysterises property measured by PFM is observed for grain itself, however, the grain boundaries where accumulates many possible defects exhibits a large electric leakage therefore no saturated polarization is observed if a large area of electrode is used.

Multiferroics and magnetic metal-organic complexes are candidates for sophisticated applications in the future. In thisthesis, the magnetism in BiFeO3 (a multiferroic material with an incommensurate spin cycloidal structure), copper guanidiniam formate (a multiferroic metal-organic complex with a one-dimensional magnetic structure) and CP -RE-COT (a series of \zero-dimensional" single molecule magnets) are discussed. A radio-frequency plasma sputtering thin lm deposition system and a ferroelectric characterisation system were developed for the study of BiFeO3 epitaxial thin lms. A large leakage current was observed in BiFeO3 thin lms, which hindered the investigations on the ferroelectric properties and magnetoelectric coupling in them. An evidence of the spin cycloid in a BiFeO3 thin lm was observed by grazing-incidence small angle neutron scattering. The magnetism of a multiferroic metal-organic complex with a one-dimensional magnetic chain, [C(NH2)3][Cu(HCOO)3], was studied by magnetometry and muon spin spectroscopy. A spin-canted antiferromagnetic order and critical phenomenon in this material were investigated. It was shown that this material possessed an 3D Heisenberg long-range order below 4.6K. The one-dimensional magnetic chain was also studied by muon spin spectroscopy. The correlation length was measured with a eld dependence of H 1. Magnetisation relaxations of a series of single molecule magnets CP -RE-COT (COT = C8H8- CP = C5Me, which show "zero-dimensional" magnetism, were studied using an AC magnetometer and muon spin spectroscopy. Three possible relaxation pathways, including a quantum tunnelling process and two Orbach relaxation processes, were suggested by the relaxation behaviour. The suppression of the quantum tunnelling effect resulting from the entanglement of the ground states, which probably arises from the exchange interactions in CP -RE-COT, was also observed with a 1000 Oe applied magnetic eld. Data that were consistent with long-range magnetic ordering was observed in CP -Dy-COT, which would be the fi rst ever report of long-range magnetic order in a single ion magnet.

Le besoin d'énergies propres et renouvelables, et en calculs numériques de plus en plus performant ont été deux des moteurs de la recherche mondiale. Les multiferroiques (matériaux présentant plusieurs ordres ferroiques couplés) ont pendant longtemps été étudiés pour des applications électroniques. Récemment, leur interaction avec la lumière a été considéré pour des applications photovoltaique. Leur grande bande interdite et la faible mobilité de leur porteurs sont néanmoins des freins à la conversion efficace de l'énergie solaire en électricité.Cependant, les matériaux multiferroiques présentent un nombre important de degrés de libertés, et leur interaction avec la lumière ne peut être réduite au seul effet photovoltaique. Ici, l'interaction lumière-multiferroique est d'abord considéré au travers de l'effet de photostriction (changement de longueur sous illumination). Les calculs ab-initio montrent que, dans le bismuth de ferrite, la photostriction peut être comprise comme un effect d'écrantage de la polarisation à l'échelle de la maille primitive, et de l'effet piézoélectrique inverse. Une solution solide de plomb nickel niobium et de titanate de plomb, présentant un fort effet piézoélectrique à sa frontière morphotropique est ensuite synthétisée et caractérisée pour ces propriétés optiques et électriques. Le rôle des défauts dans la grande conductivité des parois de domaines est aussi étudié, et des calculs de la théorie de la fonctionnelle densité montrent que les défauts se forment préférentiellement à la paroi, et y procure une plus grande densité de charges libres. Enfin, nous détaillons les dernières avancées d'un couplage de type spin-orbite, le couplage angulaire magnéto-électrique, et son application à la génération de champs magnétiques par une lumière polarisée circulairement. / The need for clean and renewable energy, as well as constantly improved numerical performances have been two of the most important driving forces in research worldwide. In this light, multiferroic materials, which are materials presenting several ferroic order, have been widely investigated towards their application in electronics and computation, or as sensors. Recently, they have been also considered for their potential use to generate energy through the photovoltaic effect. However, power conversion have remained poor compared to existing technologies such as p-n junction silicon based solar cells, mainly because of their wide bandgap and low mobility of the carriers. Nevertheless, multiferroic materials often present a vast number of degrees of freedom, and their interaction with light cannot be reduced to the sole photovoltaic effect.In this work, we study from first-principles the interaction of light and strain in the multiferroic bismuth ferrite, and find that the so-called photostriction effect originates from a screening of the polarization at the unit cell scale, which results in a photo-induced strain via the action of the converse piezoelectric effect. A solid solution of lead nickel niobium and lead titanate, exhibiting large electromechanical properties at its morphotropic phase boundary, is then synthesized, and its optical and photoinduced properties are studied. Also, the influence of defects at domain walls in the model ferroelectric lead titanate is studied from ab-initio calculations, in order to understand why domain walls exhibit a large conductivity compared to the domains. It is found that defects are more likely to form at the domain wall, and provide it with extra-carriers. Eventually, the advances in a recently considered spin-orbit energy term, the Angular MagnetoElectric coupling (AME), are considered and applied to the Inverse Faraday Effect (IFE), that is the existence of a magnetic field induced by circularly polarized light.

The primary obstacle to continued downscaling of charge-based electronic devices in accordance with Moore's law is the excessive energy dissipation that takes place in the device during switching of bits. Unlike charge-based devices, spin-based devices are switched by flipping spins without moving charge in space. Although some energy is still dissipated in flipping spins, it can be considerably less than the energy associated with current flow in charge-based devices. Unfortunately, this advantage will be squandered if the method adopted to switch the spin is so energy-inefficient that the energy dissipated in the switching circuit far exceeds the energy dissipated inside the system. Regrettably, this is often the case, e.g., switching spins with a magnetic field or with spin-transfer-torque mechanism. In this dissertation, it is shown theoretically that the magnetization of two-phase multiferroic single-domain nanomagnets can be switched very energy-efficiently, more so than any device currently extant, leading possibly to new magnetic logic and memory systems which might be an important contributor to Beyond-Moore's-Law technology. A multiferroic composite structure consists of a layer of piezoelectric material in intimate contact with a magnetostrictive layer. When a tiny voltage of few millivolts is applied across the structure, it generates strain in the piezoelectric layer and the strain is transferred to the magnetostrictive nanomagnet. This strain generates magnetostrictive anisotropy in the nanomagnet and thus rotates its direction of magnetization, resulting in magnetization reversal or 'bit-flip'. It is shown after detailed analysis that full 180 degree switching of magnetization can occur in the "symmetric" potential landscape of the magnetostrictive nanomagnet, even in the presence of room-temperature thermal fluctuations, which differs from the general perception on binary switching. With proper choice of materials, the energy dissipated in the bit-flip can be made as low as one attoJoule at room-temperature. Also, sub-nanosecond switching delay can be achieved so that the device is adequately fast for general-purpose computing. The above idea, explored in this dissertation, has the potential to produce an extremely low-power, yet high-density and high-speed, non-volatile magnetic logic and memory system. Such processors would be well suited for embedded applications, e.g., implantable medical devices that could run on energy harvested from the patient's body motion.

Les matériaux multiferroïques sont des matériaux multifonctionnels qui possèdent simultanément des propriétés magnétiques et ferroélectriques. Les perspectives d’applications sont ainsi très nombreuses dans les domaines de l’électronique (mémoires, dispositifs spintroniques et hyperfréquences). Le nombre restreint de matériaux multiferroïques monophasés a conduit au développement de nanostructures multiferroïques artificielles constituées d'oxydes ferroélectriques et ferrimagnétiques. Ce travail de thèse est axé sur l'effet magnétoélectrique (ME), obtenu pour de telles hétérostructures via la contrainte, qui permet de manipuler la polarisation spontanée ou l’aimantation par l’application d’un champ magnétique (effet ME direct) et d’un champ électrique (effet ME converse) respectivement. Les effets ME peuvent être observés à température ambiante grâce aux effets d’interfaces et de contraintes dans les nanocomposites multiferroïques. La combinaison de matériaux piézoélectriques PbZr0.52Ti0.48O3 (PZT), Ba0.7Sr0.3TiO3 (BSTO), BaTiO3 (BTO) et de matériaux magnétostrictifs CoFe2O4 (CFO) a été largement exploitée pour l’élaboration de nanocomposites multiferroïques. Les travaux issus de la littérature montrent l’existence d’un fort couplage magnétoélectrique à température ambiante dans des films minces épitaxiés (systèmes de connectivité 2-2), mais un verrou est l’effet de « bride » (clamping effect) induit par le substrat. La conception d'architectures innovantes est un défi dans le domaine des nanocomposites multiferroïques. Ce travail est axé sur les composites de type (1-3) au sein desquelles des nanostructures ferrimagnétiques CoFe2O4 unidimensionnelles (1) sont incorporées dans des couches tridimensionnelles PZT, BTO et BSTO (3). De nouvelles approches ont été envisagées pour concevoir trois types de matériaux: i) des réseaux de nanofils CFO unidirectionnels entourés de nanotubes PZT imprégnés dans des membranes d'alumine; ii) des nanopilliers CFO incorporés dans des couches minces de BTO, BSTO et PZT; ii) des réseaux de nanofils CFO interconnectés 3-D intégrés dans une matrice PZT. Nos principaux objectifs visent i) la maîtrise de l’étape d’oxydation des nanofils et des nanopilliers métalliques CoFe2 afin de contrôler la morphologie et la densité des nanostructures CFO, ii) le contrôle des caractéristiques diélectriques des nanocomposites, iii) l’augmentation du couplage magnétoélectrique en optimisant la densité d’interfaces entre les deux phases ferroïques.La première architecture développée est un dépôt par imprégnation sol-gel de nanotubes PZT dans des membranes d'alumine poreuses autosupportées, suivie d'une électrodéposition des nanofils CoFe2 dans les nanotubes PZT et de leur oxydation par traitement thermique. La deuxième architecture repose sur un dépôt par pulvérisation cathodique magnétron en radiofréquence de couches BSTO et BTO et sur un dépôt par sol-gel de couches PZT, sur des réseaux de nanopilliers CoFe2 et CoFe2O4 alignés verticalement sur des substrats Si. L'oxydation de CoFe2 est réalisée in situ lors du dépôt par pulvérisation cathodique de BSTO et BTO. Les réseaux de nanopilliers CoFe2 sont obtenus par électrodéposition dans des structures nanoporeuses en alumine anodisée qui sont ensuite dissoutes. La dernière architecture proposée est obtenue en combinant l'électrodéposition des nanofils CoFe2 dans des membranes polymères poreuses, et le procédé sol-gel. Les nanostructures PZT-CFO sont préparées par imprégnation sol-gel de couches épaisses PZT dans des réseaux de nanofils CoFe2 et leur oxydation simultanée au cours de la cristallisation des couches PZT.Une attention particulière a été accordée aux effets d’interfaces par le biais des études microstructurales et morphologiques des nanocomposites (XRD, HRSEM, TEM et EDX). Les caractérisations magnétiques, diélectriques, ferroélectriques et magnétoélectriques ont permis d’évaluer les performances des différents nanocomposites élaborés. / Multiferroic materials including magnetoelectric materials that combine magnetic and ferroelectric orders have attracted great attention due to a possible strain-mediated coupling leading to potential applications in memories, sensors, detectors, spintronic and microwave devices. The number of single-phase multiferroic materials operating at room temperature being limited, we are exploring artificially designed multiferroic nanostructures consisting of ferroelectric and ferrimagnetic oxides. Current work is focused on strain-mediated magnetoelectric effect, which allows to generate a spontaneous polarization or magnetization by an applied magnetic field (direct ME effect) and electric field (converse ME effect) respectively. ME effects can be observed at room temperature through interface and strain interaction in two-phase multiferroic nanocomposites. The combination of piezoelectric materials PbZr0.52Ti0.48O3 (PZT), Ba0.7Sr0.3TiO3 (BSTO), BaTiO3 (BTO) and magnetostrictive CoFe2O4 (CFO) materials have been intensively studied in multiferroic nanocomposites. The community has been able to demonstrate large magnetoelectric coupling at room temperature in epitaxial thin films, so called 2-2 connectivity system, but a key limitation in epitaxially grown thin films is a substrate imposed clamping effect limiting thin film’s strain. Designing innovative architectures is a challenge in the field of multiferroic nanocomposites. Our work is focused on vertically aligned multiferroic nanostructures, so called (1-3) connectivity nanocomposites, where one-dimensional ferrimagnetic CoFe2O4 nanostructures (1) are embedded into three-dimensional PZT, BTO and BSTO layers (3). New routes were considered to design three kinds of materials: i) vertically aligned CFO nanowire arrays surrounded by PZT nanotubes embedded into alumina membranes; ii) vertically aligned CFO nanopillar arrays embedded in thin BTO, BSTO and PZT layers supported on Si substrates; ii) 3-D interconnected CFO nanowire networks embedded in a thick PZT matrix. The objectives of the present work are to control the oxidation of metallic CoFe2 nanowires and nanopillars to control the morphology and density of CFO nanostructures, to control the resistivity and dielectric losses of the nanocomposites at the interface region, and to increase the magnetoelectric coupling of the multiferroic nanocomposites by increasing the interfacial surface area between the two ferroic phases.The first geometry we are developing is a deposition by sol-gel dip impregnation of PZT nanotube arrays into self-supported porous alumina membranes, followed by an electrodeposition and thermal oxidation of CoFe2 nanowire arrays within PZT nanotubes. The second architecture we are focusing on is a deposition by RF magnetron sputtering of BSTO and BTO layers and by sol-gel dip coating of PZT layers onto vertically aligned CoFe2 and CoFe2O4 nanopillar arrays supported on Si substrates. The CoFe2 oxidation is conducted in-situ during the BSTO and BTO sputter deposition. Free-standing CoFe2 nanopillar arrays are obtained by electrodeposition into anodized alumina nanoporous structures and chemical dissolution of alumina templates. The last geometry is prepared using a combination of electrodeposition into self-supported porous polymer membranes and sol-gel processes. The PZT-CFO nanostructures are prepared using impregnation of thick PZT layers into self-supported CoFe2 3D nanowire networks on Si substrates by sol-gel method and their simultaneous oxidation during PZT layers crystallization. Specific attention was focused on interfaces through microstructural and morphological evaluations of nanocomposites using XRD, HRSEM, TEM and EDS characterizations. The performances of the nanocomposites were evaluated using magnetic, dielectric, ferroelectric and ME measurements, an alternating gradient magnetometer, impedance analyser, PFM and the ME susceptometer operated inside PPMS were utilized, respectively.

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

BARBOSA, R. de L. Propriedades dielétricas intrínsecas de perovskitas duplas RE2CoMnO6 (RE = íon terra-rara). 2017. 69 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-08-23T16:49:17Z No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) / Approved for entry into archive by Giordana Silva (giordana.nascimento@gmail.com) on 2017-08-23T18:16:47Z (GMT) No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) / Made available in DSpace on 2017-08-23T18:16:47Z (GMT). No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) Previous issue date: 2017 / Dielectric ceramics with ordered double perovskite structure and RE2CoMnO6 stoichiometry, in which RE is a rare-earth ion, are usually multiferroic with critical magnetic and electrical transition temperatures depending on the RE ion. This family of materials crystallizes in the P21/n symmetry when there is ordering of the Mn and Co ions. This monoclinic structure has a distortion directly dependent on the RE ionic size, influencing directly the properties of these materials. In this work, we investigate the intrinsic dielectric properties of these samples for RE = Tb, Dy, Ho, Yb and Tm. These properties were investigated by infrared reflection spectroscopy, allowing to estimate both the intrinsic static dielectric constant and the dielectric losses due to the polar phonons, which allows estimating the maximum microwave quality factor obtained when a resonator is formed with the respective materials. Our results show that, as obtained for La2CoMnO6 (LaCMO), the main compound of this family, the intrinsic static dielectric constant is reduced. In the case of LaCMO, dielectric measurements performed by other authors showed a colossal effect, whose origin was extrinsic effects. Therefore, our result allows to predict that any effect that leads to very high dielectric constants is extrinsic in this family of materials. In addition, the analysis of the quality factor showed that such ceramics have a quality factor compatible with those employed in microwave dielectric resonators since the low dielectric constant is not a problem. / As cerâmicas dielétricas com estrutura perovskita dupla ordenada com estequiometria RE2CoMnO6, na qual RE é um íon terra-rara, são usualmente multiferróicas com temperaturas críticas de transição magnética e elétrica dependendo do íon RE. Esta família de materiais cristaliza-se na simetria P21/n quando há o ordenamento dos íons Mn e Co. Esta estrutura monoclínica, tem um distorção diretamente ligada ao tamanho do íon RE, o que influencia diretamente suas propriedades. Neste trabalho investigamos as propriedades dielétricas intrínsecas destas amostras para RE = Tb, Dy, Ho, Yb e Tm. Tais propriedades foram investigadas por espectroscopia de reflexão especular no infravermelho, permitindo estimar tanto a constante dielétrica estática intrínseca devido aos fônons polares, como as perdas dielétricas devido a estes fônons, o que permite estimar o fator de qualidade em micro-ondas máximo obtido quando se conforma um ressonador dielétrico com o respectivo material. Nossos resultados mostram que, assim como obtido para o La2CoMnO6 (LaCMO), principal composto desta família, a constante dielétrica estática intrínseca é reduzida. No caso do LaCMO , medidas dielétricas realizadas por outros pesquisadores mostraram efeito colossal, cuja origem eram efeitos extrínsecos. Portanto, nosso resultado permite prever que qualquer efeito que leve a obter constantes dielétricas muito altas tem natureza extrínseca nesta família de materiais. Além disso, a análise do fator de qualidade mostrou que tais cerâmicas tem fator de qualidade compatível com aqueles empregados em ressonadores dielétricos para micro-ondas desde que a constante dielétrica baixa não seja um problema.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22991号 / 理博第4668号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20699号 / 工博第4396号 / 新制||工||1683(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 西脇 眞二, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM