Die komplizierte Struktur der Energielandschaft wurde am Beispiel des +/- J Spinglas-Modells untersucht. Sie ist in glasartigen Systemen der Schlüssel zum Verständnis einer verlangsamten Dynamik. Es wurde ein enger Zusammenhang zwischen der Dynamik und der Energielandschaft nachgewiesen. Die Energielandschaft wird in +/- J Spinglas Modellsystemen durch Cluster charakterisiert, die infolge ihrer Konnektivität größere Objekte (z.B. Täler) bilden. Einzelne Cluster, aber auch ganze Täler, sind miteinander durch sogenannten Sattelcluster verknüpft. Die physikalischen Eigenschaften werden durch die Strukturen der Verknüpfungen und durch die innere Struktur der Cluster geprägt. Zur Beschreibung der Energielandschaften wurde die genaue Kenntnis der Zustände benutzt. Auf der Grundlage des "branch-and-bound" Verfahrens war es möglich, für kleine Systeme alle Zustände bis zu der dritten Anregung zu bestimmen. Danach wurden die Konfigurationen so sortiert, dass die Beziehungen zwischen ihnen, wie z.B. Nachbarschaften und Clusterzugehörigkeiten, einfach zu finden waren. Es gelang, die exakte Landschaft für Systeme bis L=6 aufzubauen. Für größere Systeme ist es zur Zeit unmöglich, alle niederenergetischen Zustände zu finden. Eine alternative Möglichkeit, die Struktur zu beschreiben, erhält man durch Untersuchung der Verteilung der Overlap. An der Gestalt der Verteilung erkennt man, ob die niederenergetische Struktur kompliziert oder einfach ist. Bei genaueren Untersuchungen ist es sogar möglich, die Anzahl der existierenden Täler abzuschätzen. Die Untersuchungen der Overlap bei 8555 3D Systemen (L=4) weisen darauf hin, dass bei kleineren Grundzustandsenergien die Struktur durch zwei spiegelsymmetrische Täler geprägt ist. Mit wachsender Grundzustandsenergie wird die Struktur der Systeme immer komplizierter. Eine weitere wichtige Komponente der Energielandschaft ist die innere Struktur der Sattelcluster. Ein Sattelcluster besteht aus wenigstens drei Gruppen von Konfigurationen. Zwei Gruppen enthalten Konfigurationen, die mit den Grundzustandsclustern verbunden sind, und die dritte Gruppe verbindet die beiden. Es passiert oft, dass die Konfigurationsgruppen, die verschiedene Grundzustandscluster verbinden, weit voneinander entfernt liegen. Dies wurde als ein wichtiger Aspekt erkannt, der zu einer Verlangsamung dynamischer Prozesse führt. Der andere Aspekt der Energielandschaft ist ihr Zusammenhang mit dem Realraumbild. Das Realraumbild ist als die Lage der Spins auf dem Gitter zu verstehen. Spins kann man zu verschiedenen Spindomänen zusammenfassen, die dann auf natürliche Weise die Struktur der Energielandschaft generieren. Für die Größe der einzelnen Cluster sind die freien Spins verantwortlich. Es wurde bestätigt, dass die Existenz einzelner Täler durch Spindomänen erklärt werden kann. Dabei wird durch das kollektive Umdrehen aller Spins in einer solcher Domäne ein anderer Cluster in einem anderen Tal erzeugt. Neben dem Zusammenhang von Spindomänen und Energielandschaft konnte der Einfluss von bestimmten zusammenhängenden Strukturen freier Spins genauer aufgeklärt werden. Hier ergeben sich Ansatzpunkte für weitergehende Untersuchungen.

Le sujet principal de cette thèse est la physique des verres de spin. Les verres de spin ont été introduits au début des années 70 pour décrire alliages magnétiques diluées. Ils ont désormais été considerés pour comprendre le comportement de liquides sousrefroidis. Parmis les systèmes qui peuvent être décrits par le langage des systèmes desordonnés, on trouve les problèmes d’optimisation combinatoire. Dans la première partie de cette thèse, nous considérons les modèles de verre de spin avec intéraction de Kac pour investiguer la phase de basse température des liquides sous-refroidis. Dans les chapitres qui suivent, nous montrons comment certaines caractéristiques des modèles de verre de spin peuvent être obtenues à partir de résultats de la théorie des matrices aléatoires en connection avec la statistique des valeurs extrêmes. Dans la dernière partie de la thèse, nous considérons la connexion entre la théorie desverres de spin et la science computationnelle, et présentons un nouvel algorithme qui peut être appliqué à certains problèmes dans le domaine des finances. / The main subject of this thesis is the physics of spin glasses. After their introduction in the 70s in order to describe dilute magnetic alloys, spin-glass models have been considered prototype models to understand the behavior of supercooled liquids. Among the systems that can be described and analyzed using the language of disordered systems, there are problems of combinatorial optimization. In the first part of the thesis, we consider spin-glass models with Kac interactions in order to investigate the supercooled phase of glass-forming liquids. Afterwards, we show how some features of spin-glass models can be described by ubiquitous results of Random Matrix Theory in connection with Extreme Value Statistics. Finally, from the interaction of spin-glass theory and computer science, we put forward a new algorithm of immediate application in Financial problems.

Ce travail de thèse présente un dispositif expérimental original permettant la mesure, dans des conditions comparables, des fluctuations de l'aimantation d'un échantillon et de sa réponse à un champ magnétique. Une comparaison quantitative permet, via le théorème de fluctuation-dissipation, une mesure absolue de la température lorsque l'échantillon est à l'équilibre thermodynamique. Pour des systèmes vitreux, la "température effective" ---une extension conservant le formalisme du théorème de fluctuation-dissipation de la température pour les système à faible production d'entropie--- est rendue accessible. Un échantillon "verre de spin" ($CdCr_{1,7}In_{0,3}S_4$) aux propriétés vitreuses étudiées depuis de nombreuses années a permis cette mesure. Le régime fortement vieillissant, non-stationnaire, est étudié; la mesure nécessaire des fluctuations thermiques est très délicates (l'amplitude de ces fluctuations correspond à la réponse de l'échantillon à des variations de l'ordre du millionième du champ magnétique terrestre). <br> Les résultats obtenus montrent en premier lieu une dynamique de quasi-équilibre, confirmant des résultats précédents. Le régime fortement vieillissant est maintenant également atteint. Toutefois, les mesures ne peuvent pas être traduites directement en terme de température effective, car expérimentalement, on observe systématiquement la coexistence d'une dynamique stationnaire et de la dynamique de vieillissement. Une analyse par scaling est proposée pour séparer ces deux contributions. Sous réserve de validité de cette analyse, les mesures confirment alors les principales caractéristiques attendues pour la température effective, et notamment son indépendance en fonction de l'âge du système. <br> Les différents modèles connus ne permettent cependant pas d'expliquer complètement toutes les caractéristiques de la température effective mesurée, certaines d'entre elles paraissant encore antinomiques...

Les applications d’électronique de spin telles les mémoires à accès aléatoire (MRAM), les capteurs (e.g.les têtes de lecture des disques durs d’ordinateurs) et les éléments de logique magnétique utilisent les interactionsd’échange ferromagnétique/antiferromagnétique (F/AF) dans le but de définir une direction de référence pour lespin des électrons de conduction. Les MRAM à écriture assistée thermiquement (TA-MRAM) utilisent mêmedeux bicouches F/AF : une pour le stockage de l’information et l’autre comme référence. Ces dernièresapplications technologiques impliquent des étapes de nanofabrication des couches minces continues pour formerune matrice de cellules mémoires individuelles. La qualification industrielle du produit final impose de sérieusescontraintes sur la largeur des distributions des propriétés magnétiques - y compris d’échange F/AF - de cellulemémoire à cellule mémoire. Des phases verres de spin réparties de manière aléatoire sur la couche continue, àl’interface F/AF ou dans le coeur de l’AF pourraient contribuer de manière significative à ces distributionsd’échange F/AF dans les dispositifs, après nanofabrication ; comme supposé il y a de cela quelques années. Lebut de cette thèse est d’étudier factuellement le possible lien entre verre de spin répartis dans des couches mincesF/AF et dispersions de propriétés d’échange de cellule mémoire à cellule mémoire dans les dispositifs TAMRAMcorrespondants. Avant cela, l’origine de ces régions verre de spin a été étudiée et une attention plusparticulière a été portée au rôle joué par les diffusions de Mn. Ces dernières ont été directement observées,comprises et l’utilisation de barrières complexes pour les réduire et par là même pour diminuer la quantité dephases verre de spin a été mise en oeuvre avec succès. En guise d’alternative pour varier la quantité de verres despin, l’utilisation d’AFs composites a été également étudiée dans le cadre de cette thèse. Ce type d’AF permet dumême coup de varier la stabilité thermique des grains AF et de répondre à un autre problème identifié pour lesTA-MRAM qui consiste à trouver des matériaux AF avec des propriétés de rétention et d’écriture intermédiairespar rapport aux matériaux actuellement utilisés. Finalement, ces AFs composites ont été utilisés comme moyende varier la quantité de verres de spin dans des dispositifs TA-MRAM réels et de prouver le lien direct avec ladispersion de propriétés de cellule mémoire à cellule mémoire. / Spintronics applications such as magnetic random access memories (MRAM), sensors (e.g.. hard diskdrive read head) and logic devices use ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) interactionsto set the reference direction required for the spin of conduction electrons. Thermally-assisted (TA-) MRAMapplications even use two F/AF exchange biased bilayers: one for reference and one for storage. Suchtechnological applications involve patterning full sheet wafers into matrix of individual bit-cells. Industrialproducts qualification imposes stringent requirements on the distributions of the magnetic properties from cell tocell, including those related to EB. It was supposed few years ago that randomly spread spin-glass like phases atthe F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB propertiesin devices after processing. This thesis aimed at factually studying the link between spin-glasses spread overF/AF thin films and bit-cell dispersions of EB in corresponding TA-MRAM. Prior to that the origin of the spinglasslike regions and more specifically the role of Mn-diffusion was consolidated. Mn-diffusion was directlyobserved, understood and the use of complex barriers to reduce such diffusion and consequently to minimize theamount of spin-glass was successfully studied. Mixing AFs as another way to tune the amount of spin-glass likephases was also evidenced in the framework of this thesis. All at once, this last solution also tuned the AF grainsthermal stability and this solved another issue related to TA-MRAM, i.e. finding AF-materials with intermediateretention and write properties compared to the AFs presently used. Finally, those mixed antiferromagnets werethe mean chosen to tune the amount of spin-glass like phases in real TA-MRAM devices and to factually provetheir link with bit-cell distributions of EB properties.

Les applications d’électronique de spin telles les mémoires à accès aléatoire (MRAM), les capteurs (e.g.les têtes de lecture des disques durs d’ordinateurs) et les éléments de logique magnétique utilisent les interactionsd’échange ferromagnétique/antiferromagnétique (F/AF) dans le but de définir une direction de référence pour lespin des électrons de conduction. Les MRAM à écriture assistée thermiquement (TA-MRAM) utilisent mêmedeux bicouches F/AF : une pour le stockage de l’information et l’autre comme référence. Ces dernièresapplications technologiques impliquent des étapes de nanofabrication des couches minces continues pour formerune matrice de cellules mémoires individuelles. La qualification industrielle du produit final impose de sérieusescontraintes sur la largeur des distributions des propriétés magnétiques - y compris d’échange F/AF - de cellulemémoire à cellule mémoire. Des phases verres de spin réparties de manière aléatoire sur la couche continue, àl’interface F/AF ou dans le coeur de l’AF pourraient contribuer de manière significative à ces distributionsd’échange F/AF dans les dispositifs, après nanofabrication ; comme supposé il y a de cela quelques années. Lebut de cette thèse est d’étudier factuellement le possible lien entre verre de spin répartis dans des couches mincesF/AF et dispersions de propriétés d’échange de cellule mémoire à cellule mémoire dans les dispositifs TAMRAMcorrespondants. Avant cela, l’origine de ces régions verre de spin a été étudiée et une attention plusparticulière a été portée au rôle joué par les diffusions de Mn. Ces dernières ont été directement observées,comprises et l’utilisation de barrières complexes pour les réduire et par là même pour diminuer la quantité dephases verre de spin a été mise en oeuvre avec succès. En guise d’alternative pour varier la quantité de verres despin, l’utilisation d’AFs composites a été également étudiée dans le cadre de cette thèse. Ce type d’AF permet dumême coup de varier la stabilité thermique des grains AF et de répondre à un autre problème identifié pour lesTA-MRAM qui consiste à trouver des matériaux AF avec des propriétés de rétention et d’écriture intermédiairespar rapport aux matériaux actuellement utilisés. Finalement, ces AFs composites ont été utilisés comme moyende varier la quantité de verres de spin dans des dispositifs TA-MRAM réels et de prouver le lien direct avec ladispersion de propriétés de cellule mémoire à cellule mémoire. / Spintronics applications such as magnetic random access memories (MRAM), sensors (e.g.. hard diskdrive read head) and logic devices use ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) interactionsto set the reference direction required for the spin of conduction electrons. Thermally-assisted (TA-) MRAMapplications even use two F/AF exchange biased bilayers: one for reference and one for storage. Suchtechnological applications involve patterning full sheet wafers into matrix of individual bit-cells. Industrialproducts qualification imposes stringent requirements on the distributions of the magnetic properties from cell tocell, including those related to EB. It was supposed few years ago that randomly spread spin-glass like phases atthe F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB propertiesin devices after processing. This thesis aimed at factually studying the link between spin-glasses spread overF/AF thin films and bit-cell dispersions of EB in corresponding TA-MRAM. Prior to that the origin of the spinglasslike regions and more specifically the role of Mn-diffusion was consolidated. Mn-diffusion was directlyobserved, understood and the use of complex barriers to reduce such diffusion and consequently to minimize theamount of spin-glass was successfully studied. Mixing AFs as another way to tune the amount of spin-glass likephases was also evidenced in the framework of this thesis. All at once, this last solution also tuned the AF grainsthermal stability and this solved another issue related to TA-MRAM, i.e. finding AF-materials with intermediateretention and write properties compared to the AFs presently used. Finally, those mixed antiferromagnets werethe mean chosen to tune the amount of spin-glass like phases in real TA-MRAM devices and to factually provetheir link with bit-cell distributions of EB properties.

Nous présentons une contribution expérimentale à l’étude des propriétés structurales et magnétiques de ferrites de gallium substituées par des terres rares. Ce travail s’inscrit dans la thématique plus générale des multiferroïques magnétoélectriques dont fait partie le composé Ga2-xFexO3 qui est ferrimagnétique et présente une polarisation spontanée à température ambiante. On s’intéresse à l’influence de la substitution d’atomes de gallium ou de fer par des éléments de terres rares (Sc, Yb, Er, Sm) sur les paramètres structuraux de Ga2-xFexO3 avec 0,9  x  1,2 et à l’effet de la substitution par Sc et Yb sur la température de transition magnétique et l’aimantation à saturation. On présente une étude expérimentale du magnétisme du composé Ga0.99Yb0.01FeO3, réalisée à partir de mesures de la susceptibilité alternative et de l’aimantation en fonction du champ magnétique, H, et de la température T. Un large domaine d’irréversibilité magnétique est observé dans le diagramme de phase H–T. Les résultats suggèrent que Ga0.99Yb0.01FeO3 constitue un verre de spin Heisenberg tridimensionnel. / We present an experimental contribution to the study of structural and magnetic properties for gallium ferrites substituted by rare earth elements. This work is part of the more general theme of magnetoelectric multiferroïc which includes the Ga2-xFexO3 compound ferrimagnetic and with a spontaneous polarization at room temperature. We study the influence of the substitution of gallium or iron atoms with rare earth elements (Sc, Yb, Er, Sm) on the structural parameters of Ga2-xFexO3 with 0.9  x  1.2 and the effect of the substitution by Sc and Yb on the magnetic transition temperature and the saturation magnetization. We present an experimental study of magnetism Ga0.99Yb0.01FeO3 compound, realized from alternative susceptibility and magnetization measurements as a function of magnetic field, H, and the temperature T. A wide irreversible magnetic area is observed in the phase diagram H–T. The results suggest that Ga0.99Yb0.01FeO3 is a three-dimensional Heisenberg spin glass.

Cette thèse porte sur le développement et l'application en physique statistique d'un nouveau paradigme pour les méthodes sans rejet de Monte-Carlo par chaînes de Markov irréversibles, grâce à la mise en œuvre du filtre factorisé de Metropolis et du concept de lifting. Les deux premiers chapitres présentent la méthode de Monte-Carlo et ses différentes applications à des problèmes de physique statistique. Une des principales limites de ces méthodes se rencontre dans le voisinage des transitions de phase, où des phénomènes de ralentissement dynamique entravent fortement la thermalisation des systèmes. Le troisième chapitre présente la nouvelle classe des algorithmes de Metropolis factorisés et irréversibles. Se fondant sur le concept de lifting des chaînes de Markov, le filtre factorisé de Metropolis permet de décomposer un potentiel multidimensionnel en plusieurs autres unidimensionnels. De là, il est possible de définir un algorithme sans rejet de Monte-Carlo par chaînes de Markov irréversibles. Le quatrième chapitre examine les performances de ce nouvel algorithme dans une grande variété de systèmes. Des accélérations du temps de thermalisation sont observées dans des systèmes bidimensionnels de particules molles, des systèmes bidimensionnels de spins XY ferromagnétiques et des systèmes tridimensionnels de verres de spins XY. Finalement, une réduction importante du ralentissement critique est exposée pour un système tridimensionnel de spins Heisenberg ferromagnétiques. / This thesis deals with the development and application in statistical physics of a general framework for irreversible and rejection-free Markov-chain Monte Carlo methods, through the implementation of the factorized Metropolis filter and the lifting concept. The first two chapters present the Markov-chain Monte Carlo method and its different implementations in statistical physics. One of the main limitations of Markov-chain Monte Carlo methods arises around phase transitions, where phenomena of dynamical slowing down greatly impede the thermalization of the system. The third chapter introduces the new class of irreversible factorized Metropolis algorithms. Building on the concept of lifting of Markov chains, the factorized Metropolis filter allows to decompose a multidimensional potential into several unidimensional ones. From there, it is possible to define a rejection-free and completely irreversible Markov-chain Monte Carlo algorithm. The fourth chapter reviews the performance of the irreversible factorized algorithm in a wide variety of systems. Clear accelerations of the thermalization time are observed in bidimensional soft-particle systems, bidimensional ferromagnetic XY spin systems and three-dimensional XY spin glasses. Finally, an important reduction of the critical slowing down is exhibited in three-dimensional ferromagnetic Heisenberg spin systems.

The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal (B- site) directly result in structural modification through internal pressure. Consequently, atomic distances and bond angles between the transition metals vary. This, intern, influences the nearest neighbour exchange coupling energy and magnetic interaction. A detailed investigation has been carried out on two A-site doped perovskite namely, La0.85Sr0.15CoO3 & La0.5Sr0.5CoO3 and two B-site doped perovskite, LaMn0.5Co0.5O3 & LuMn0.5Ni0.5O3 with a view to study the impact of chemical in homogeneity and lattice distortion on their respective magnetic ground states. The thesis is organized in seven chapters. A brief summary of each is given below: Chapter 1: Provides a brief introduction about the perovskite structure. Origins of lattice distortions and its effect on the magnetic properties are discussed. It includes a discussion on different types of indirect magnetic interactions involved in perovskite oxide structure. The chapter concludes with a description of spin-glass, phase separation/ cluster-glass, memory effect in glassy magnetism, critical behaviour at phase transition and specific heat in magnetic systems. Chapter 2: This chapter outlines basic principles of the experimental techniques employed for the work presented in this thesis. Chapter 3: Details macroscopic as well as microscopic investigations carried out to understand the glassy magnetic anomalies in La0.85Sr0.15CoO3 samples. The origin of phase separation (PS) has been reinvestigated. Since the magnetic behavior of La0.85Sr0.15CoO3 (LSCO15) lies in the border of spin glass (SG) and ferromagnetic (FM) region in the x-T phase diagram, it is subject to controversial debate for the last several years. While some research groups favour PS, others regard SG behaviour as the dominant phenomenon. In-depth investigation carried out to elucidate these views is outlined in this chapter in two sections. The first section deals with the glassy magnetic anomalies in single crystals of LSCO15 grown by optical floating zone method. Since the sample crystallizes from melt, it possesses good compositional homogeneity and the phase purity is confirmed by XRD pattern. Many characteristics of canonical SG systems are discernible in the magnetic study, such as, kink in field-cooling curve below Tf, frequency-dependent peak shift and the time dependent memory effect. The relaxation time in sub-pico second range (~10-13 s) is very similar to that of the typical SG systems. Time dependent transport relaxation study exhibits memory effect and the time evolution of resistance scales with magnetization and strictly adheres to the stretched exponential behaviour as commonly expected for a SG-like disordered system. However, a detailed study on transport mechanism and temperature-dependent inverse susceptibility reveals the existence of nanoscopic PS in the sample. In the second section, the origin of PS has been examined through a comprehensive study on two sets of LSCO15 polycrystalline samples prepared from the same initial mixture but subjected to different heat treatment processes. This study depicts the dependence of PS on the preparation conditions. The contrasting magnetic behaviour of PS and SG was resolved by experiments of dc magnetization, linear & non-linear ac susceptibility, neutron depolarization and field-cooled magnetic relaxation. Both samples conform to the general characteristics of a glassy behaviour: a kink in FC magnetization, frequency-dependent peak shift (Vogel–Fulcher law), dc bias-dependent peak shift in accordance with de Almeida–Thouless relation, and characteristic relaxation time in the range of 10-13/10-14 s. This is despite their internal spin structure and interaction being much different at a microscopic level. It is found that the sample processed through a proper homogenization process mimics the SG behaviour, whereas the sample prepared by the conventional method behaves like the PS phase. It is confirmed from neutron depolarization experiments that no ferromagnetic correlation exists in the SG phase of La0.85Sr0.15CoO3, a result in contrast to that of PS phase. Higher harmonic ac susceptibility measurement complements the above observation by the evidence that of 2nd order harmonics are not present in the SG phase of La0.85Sr0.15CoO3. The field-cooled magnetic relaxation study makes a distinct reference to the relaxation process and the strength of interaction between PS and SG like phases. In essence, a concerted effect is made to identify and resolve the spin-glass phase from phase-separated/ cluster-glass. This work shows that chemical in homogeneity is a key factor responsible for phase separation in La0.85Sr0.15CoO3; also intrinsic differences between PS and SG are identified that can serve as guiding tools for research in other similar magnetic oxide systems. It is concluded that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass in nature. Chapter 4: This chapter contains two sections. In the first part, the origin of the re-entrant spin-glass (RSG) behaviour in La0.5Sr0.5CoO3 has been investigated using the conventional magnetometer measurements. Polycrystalline samples prepared by the conventional solid-state synthesis exhibit RSG characteristics with a glassy transition at 190 K. The nature of frequency dependence of χ″(T), a pronounced memory effect and the sluggish response in dc magnetization measurement, all of which clearly indicate the re-entrant behaviour. But, once the sample is taken through a rigorous homogenization procedure of repeated grinding and annealing, its phase turns into pure ferromagnetic one. During the course of this homogenization process, the sample loses oxygen with concurrent degeneration of TC to a lower level. In order to regain the oxygen stoichiometry, it is necessary to anneal the sample in oxygen environment at 900 oC, which triggers deleterious ageing effect by which TC falls progressively with time. In the second part, the effect of oxygen stoichiometry on La0.5Sr0.5CoO3 (LSCO50) thin-films has been investigated. The highest TC reported so far for LSCO50 thin film is 250 K, which is significantly less compared to the bulk TC (262 K) of an oxygen stoichiometric compound. This work focuses on achieving the highest ferromagnetic transition temperature (TC) for LSCO50 films under optimized growth conditions. The analysis of experimental data suggests that the Curie temperature can be enhanced to 262 K, irrespective of whether or not, (a) the film on LAO or STO or (b) any induced strain occurs in the LSCO50 film. Apart from different thin-film growth parameters such as oxygen pressure and substrate temperature during the growth, and post-growth annealing temperature and oxygen pressure, the profile of the laser beam used for ablation of bulk material profile also plays an important role. The elevation of Curie temperature observed in thin-films to that close to the bulk value is believed to be a result of improved stoichiometric composition of oxygen facilitated during thin film growth. However, the strong ageing effect seen is quite close to that is observed in oxygen-annealed polycrystalline sample. Chapter 5: Of the three segments constituting this chapter, the first outlines different magnetic anomalies induced by lattice distortion in LaMn0.5Co0.5O3 (LMCO) single crystals. Single crystals of LMCO compound [(100) orientation] have been successfully grown using the optical floating zone method. Powder as well as single crystal x-ray diffraction analyses provides evidence of large strain dependent structural distortion in as-grown crystals. Spatially resolved 2-D Raman scan reveals that the strain generates a distribution of octahedral distortion in the lattice. While some are compressive in nature, others in the nearby territory relate to tensile distortion. The ac susceptibility measurement elucidates distinct changes in the ferromagnetic transition temperature (TC) in the as grown (strained) crystal. It is possible to release strain by rigorous annealing process. Which also results in a uniform TM-O octahedral deformation. Room temperature 2-D Raman spectra bears testimony to this. Upon annealing, the single crystalline order is diminuend by the atomic rearrangement. This causes tilting of the oxygen octahedra, by decreasing intra-octahedral angle θTM-O-TM, and lowering of exchange energy Jex between the magnetic ions. The transition temperature falls and the magnetic phase merges with that in the strain-free polycrystalline material. A detailed critical analysis performed in the vicinity of paramagnetic to ferromagnetic phase transition in both the samples establishes that the ground state magnetic behaviour, assigned to the strain-free LMCO crystal is of 3D Heisenberg type. But the local octahedral distortion present in the as-grown crystal causes mean field like magnetic interaction at few local sites. This serves as a key drive for the critical exponents to distance from the 3D Heisenberg model towards the mean-field type. The second part of this chapter concerns the anomalous re-entrant glassy magnetic behaviour observed in LMCO single crystals. The ac susceptibility study illustrates the low temperature anomalous glassy magnetic ordering in these crystals. The material behaves like a normal magnetic glass, (frequency-dependent peak-shift in ac susceptibility) in conformance with the phenomenological Vogel-Fulcher law, of spin flips time: ~10-4 s. However, the crystal does not respond to the external dc bias and just as well remains free from memory effect. Anomalous behaviour of this kind is rare in magnetic oxides. The magneto-dielectric effect in LMCO is discussed in the third section of this chapter. The real part of dielectric permittivity (ε′) has a colossal value of 1800 at 220 K and 10 kHz. However as the sample is cooled further, ε′ decreases slowly; followed by dielectric relaxation in the region, 120 - 150 K. Detailed analysis of the temperature dependence of the imaginary part of the dielectric permittivity (ε″) show that there is no relaxor-like phenomena in this compound. The frequency dependence of ε″ reveals that the low frequency region is dominated by Maxwell-Wagner relaxation, whereas, at high frequency, a Debye type relaxation persists. The temperature dependent full-width at half-maximum for this Debye relaxation, peaks at the corresponding TC. The temperature variation of the relaxation time has two domains of different slopes. At zero external field, ε″(ω) has a low activation energy (U = 46.4 meV) in the ferromagnetic region, compared to that in the paramagnetic (60.1 meV) phase. The boundary lies near the corresponding TC. In the presence of external applied field 5 T, U remains unchanged in the ferromagnetic region, but decreases ( U ~ 5 meV) in the paramagnetic phase. These results signify the existence of strong magneto-dielectric coupling in LMCO crystals. The field variation of ε′(ω) at fixed temperature and specific frequency highlights the rise in magnetodielectricity (MD) as well as magneto-loss (ML) with increasing magnetic field. It is perceived that this variation is not due to the magneto resistance of LMCO or caused by LMCO - electrode interfaces. The influence of extrinsic parasitic contributions cannot be ruled out entirely, but the presence of positive MD as well as ML at frequencies above the time constant suggests that the relaxation process and the magneto-dielectric coupling are intrinsic to the LaMn0.5Co0.5O3 system. Chapter 6: This chapter describes the successful synthesis of a new perovskite oxide compound, LuMn0.5Ni0.5O3. The structural characterization employs the Rietveld refinement of powder X-ray diffraction pattern. The compound crystallizes in orthorhombic Pbnm crystal structure. dc magnetization reveals ferromagnetic ordering in the sample. However the low temperature glassy phase spotted in the ac susceptibility measurement might classify it as a re-entrant spin-glass compound. But the display of memory effect until the ferromagnetic transition indicates that intrinsic ant ferromagnetic interaction prevails over the dominant ferromagnetic interaction. A critical behaviour study was carried out in the vicinity of the ferromagnetic to paramagnetic phase transition, which provided the critical exponents: α = 0.37, β = 0.241 ± 0.003, γ = 1.142 ± 0.003 and δ = 5.77 ± 0.03. Interestingly, this set of critical exponents does not match with any of the conventional theories of mean field, 3D Heisenberg, and 3D Ising. Rather it fits quite well with data calculated for the stacked triangular 3D version of the (Z2 × S1) model [α = 0.34 ± 0.06, β = 0.25 ± 0.01, γ = 1.13 ± 0.05 and δ = 5.47 ± 0.27]. This study indicates that the magnetic ground state of LuMn0.5Ni0.5O3 is canted ferromagnetic. Chapter 7: Various important results are summarized in this chapter. It also provides a broad outlook in this area of research.

Recent findings of multiferroicity and magnetoelectric effects in rare earth manganites have fuelled research in this class of materials. These multiferroics can be structurally divided into two classes – orthorhombic and hexagonal. Especially attractive are TbMnO3, HoMnO3 and DyMnO3. Since the ionic radius of Dy is at the boundary that separates the orthorhombic and hexagonal RMnO3, DyMnO3 can be synthesized in both the structures using different synthesis conditions. In this thesis, DyMnO3 single crystals (both hexagonal and orthorhombic) prepared using optical floating zone furnace are studied through structural, magnetic and thermal properties. The influence of rare earth ion on the magnetic phase transitions is revealed in magnetisation, ac susceptibility and specific heat studies. Moreover, doping RMnO3 (small R) with alkaline earth ions creates an arena to test the interesting physics of spin-glass-like phenomena in manganites that arises due to quenched disorder. In this regard, 50% strontium diluted DyMnO3 could be an ideal system to study the effects of quenched disorder and structural/magnetic inhomogeneities that govern the magnetic phases in manganites. Structural phase-coexistence and ensuing anomalous magnetism in Pr–based manganite Pr0.6Sr0.4MnO3 are also presented in this thesis. Details of how the thesis is organized into eight chapters and a brief summary of each chapter follows: Chapter 1 is an introduction to the physics of manganites which progresses into multiferroics and eventually discusses the spin-glass-like effects arising due to size mismatch. A discussion on the phase-coexistence and its effect on physical properties are also presented. Eventually, the scope of the thesis is outlined in the last section. Chapter 2 outlines the basic experimental methods employed in this thesis work. Chapter 3 describes the details of crystal growth by optical floating zone method. DyMnO3 crystals in both hexagonal and orthorhombic structures are grown by employing the ambience of argon and air respectively. The crystals in the two crystallographic variants are characterized by X ray diffraction, Energy dispersive X ray analysis and Inductively coupled plasma atomic emission spectroscopy. The crystal structures are refined using Rietveld method with FULLPROF code and found to be P63cm for hexagonal and Pnma for orthorhombic DyMnO3. Details of crystal growth of Dy1−xSrxMnO3 are also presented. The change in ambience has no effect in the crystal structure of this doped manganite. A comparison of the growth of undoped and doped systems is given. In a later section, the crystal growth and structure refinement of Pr0.6Sr0.4MnO3 are discussed and the optimized growth parameters are tabulated for various manganite systems grown in the present work. Chapter 4 deals with the magnetic and thermal characterization of hexagonal and orthorhombic DyMnO3 single crystals. Magnetic measurements reveal the importance of rare earth magnetism in these compounds. The antiferromagnetic transition to a stacked triangular antiferromagnet is discernible from the specific heat studies of hexagonal DyMnO3, which is masked in the bulk magnetisation measurements. Various magnetic transitions pertaining to the antiferromagnetic sinusoidal – spiral – incommensurate magnet, are evident in the magnetisation and specific heat of orthorhombic DyMnO3 which belongs to the class of non-collinear magnets. Chapter 5 deals with basic investigations on the spin-glass-like state in Dy0.5Sr0.5MnO3. Preliminary dc magnetisation shows indication of spin-glass state as a split in field-cooled and zero-field-cooled magnetisation cycles. Further, the failure of scaling of M(T) with H/T indicates the absence of superparamagnetism in Dy0.5Sr0.5MnO3. The dynamic susceptibility and its analysis using the theory of critical slowing down yield exponents pertaining to the spin-glasses. However, a four-order magnitude change is observed in the characteristic spin-flip time. This leads to the assumption that in Dy0.5Sr0.5MnO3 the spin entities are not atomic spins as in canonical spin-glasses but clusters of spins. The specific heat is analysed for signatures of spin-glass state and is found that a linear term in temperature is essential in fitting the observed data. The crystalline electric fields of Dy ion is also analysed attempting multiple Schottky-levels instead of two. Chapter 6 concerns with the aging experiments performed in the spin-glass-like state in Dy0.5Sr0.5MnO3. Striking aging and chaos effects are observed through these measurements. However, owing to the clusters of spins present, deviations from the typical time-dependent behavior seen in canonical spin-glass materials are anticipated in Dy0.5Sr0.5MnO3. In fact, the relaxation measurements indicate that the glassy magnetic properties are due to a cooperative and frustrated dynamics in a heterogeneous or clustered magnetic state. In particular, the microscopic spin flip time obtained from dynamical scaling near the spin-glass transition temperature is four orders of magnitude larger than microscopic times found in atomic spin-glasses. Magnetic viscosity, deduced from the waiting time dependence of the zero field cooled magnetisation, exhibits a peak at a temperature T<Tsg. Waiting time experiments prove that the dynamics is collective and that the observed memory effects are not due to superparamagnetism of separate magnetic entities. Chapter 7 discusses the Electron paramagnetic resonance (EPR) studies on single crystals of DyMnO3 in hexagonal as well as orthorhombic structures. The interesting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is also probed using EPR. The lineshapes are fitted to broad Lorentzian in the case of pure DyMnO3 and to modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity and geff derived from the signals are analysed as a function of temperature. The EPR results corroborate well with the magnetisation measurements. The study clearly reveals the signature of frustrated magnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systems persist even above the transition. Moreover, a spinglass-like behaviour in Dy0.5Sr0.5MnO3 is indicated by a step-like feature in the EPR signals at low fields. Chapter 8 deals with the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This crystal undergoes two prominent phase transitions – a paramagnetic to ferromagnetic at Tc~300 K and a structural transition at Tstr ~ 64 K. These phase transitions are evident in the static magnetisation as well as in frequency-dependent susceptibility. In these measurements, the structural transition is associated with a sizeable hysteresis typical of a first-order transition. The M–H curves below Tc show clear indication of anomalous magnetism at low temperatures: the virgin curve lies outside the subsequent magnetisation loops. These observations are explained by assuming structural coexistence of a high–temperature orthorhombic and a low–temperature monoclinic ferromagnetic phases. The nature of static magnetisation data is analysed in the critical region. Modified Arrott’s plots yielded perfect straight lines with the isotherm at ~ 300 K passing through the origin. The exponent values thus should be very close to those expected for the universality class of Heisenberg ferromagnets. The temperature dependence of resistivity also shows critical nature with an exponent belonging to the Heisenberg class. The thesis concludes with a chapter on General conclusions and future scope on these systems.

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