A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials. A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with different types of anisotropies while allowing us to draw analogies with more common characterization techniques. The angular FMR setup has been used to study coupled systems, such as those coupled through the Ruderman–Kittel–Kasuya–Yosida interaction as well as exchange-biased structures. These types of coupled systems have technological impacts and are highly applied in the components of magnetoresistive random access memory. Using this new characterization technique, properties of synthetic antiferromagnets have been revealed which had not been observed before. In addition to these experiments, magnetic susceptibility and FMR in exchange biased systems have been investigated at temperatures as low as 2 K. This investigation used a new FMR spectrometer and was one of the first studies to use this instrument. For the first time a new method of identifying several types of coupling which can be present in layered nanostructures is presented and supported through comparison with known techniques, thus connecting a new characterization technique for layered structures with decades-old procedures. Many results within this work are also supported theoretically with computer simulations.

Železo-rhodium (FeRh) je látka procházející magnetickou fázovou přeměnou prvního druhu z antiferomagnetické (AF) do feromagnetické (FM) fáze, ke které dochází při zahřátí materiálu nad teplotu fázové přeměny nebo působením dostatečně velkého magnetického pole. Tato fázová přeměna je mimo jiné provázena výraznou změnou entropie, magnetizace a elektrického odporu, přičemž její tvar a poloha teploty přeměny je silně závislá na stechiometrii krystalu, na příměsích, tlaku a v případě tenkých vrstev na napjatosti vrstvy způsobené substrátem. Tato práce se zaměřuje na studium magnetotransportních vlastností drátů připravených z tenkých FeRh vrstev rostlých na substrátech indukujících různou napjatost vrstvy. Jedním z hlavních jevů studovaných v této práci je anizotropní magnetorezistance (AMR) projevující se změnou odporu pro různé natočení magnetických momentů v látce vůči směru elektrického proudu. AMR byla studována jak ve FM fázi, tak i v AF fázi FeRh. Byla změřena hodnota AMR ve vysokoteplotní FM fázi a objeveno neočekávané chování AMR ve zbytkové FM fázi v nízkoteplotním stavu. Dále byla pozorována výrazná závislost AMR na orientaci měřených segmentů vůči krystalografickým směrům FeRh.

No description available.

The magnetocaloric effect (MCE) is the temperature change in a magnetic material due to a change in an applied magnetic field. How the MCE behaves in different magnetic materials and at different phase transitions is fundamental to understand. The driver of the MCE is a change in entropy which has multiple contributions: magnetic, lattice, and electron. In this thesis the MCE is studied in a simple antiferromagnetic (AFM) model andin a realistic noncollinear spin glass Neodymium model using Monte Carlo and Atomistic Spin Dynamics simulations. For the simple AFM system, no clear results were achieved, indicating that MCE in AFM materials is not due to a change solely in the magnetic entropy. For the complex magnetic material Nd, a more clear result is seen, indicating that frustration in the system might be important to the MCE in noncollinear materials. Nd results also signify more phase transitions than previously reported.

Le pavage de Penrose est une structure quasipériodique bidimensionnelle, utilisée dans la description des composés quasicristallins. Cette structure est parfaitement ordonnée, avec une symétrie de rotation cinq et elle est invariante sous un changement d'échelle par un facteur $\tau$ (le nombre d'or). On s'attend à ce que les propriétés d'un modèle d'antiferromagnétisme dans un tel système diffèrent nettement de celles des antiferromagnétiques périodiques. Nous avons étudié les propriétés d'un modèle d'Heisenberg sur le pavage de Penrose construit à partir de losanges, en utilisant une méthode de développement en ondes de spin. Les énergies et fonctions d'ondes des magnons (quantum d'une onde des spins) ont été étudiées dans le cadre d'une théorie linéarisée. A basse énergie, on trouve une loi de dispersion linéaire, comme dans d'autres antiferromagnetiques bipartites, avec une vitesse effective de l'onde de spin inférieure à celle d'un réseau carré équivalent. Les propriétés spatiales des modes propres ont été étudiées en détail. A basse énergie, nous trouvons que les états propres sont relativement étendus. Une analyse multifractale montre qu'ils sont de type “critique”, ayant une distribution d'exposants multifractaux. Aux énergies plus élevées, les états deviennent plus localisés, et, en fonction de l'énergie, l'amplitude de la fonction d'onde est non-nulle autour d'un sous-ensemble de sites d'une valeur de coordinence donnée. <br /><br />L'énergie de l'état fondamental de cette antiferromagnetique, et la distribution des aimantations locales dans cet état ont été calculés. Des projections dans l'espace perpendiculaire montrent la simplicité sous-jacente de ce état "complexe". Un simple modèle analytique, l'étoile de Heisenberg à deux niveaux, a été présenté pour expliquer de la distribution d'aimantation locales dans ce système antiferromagnétique.<br /><br />Dans une dernière partie, les effets de désordre de type “phason” sont considérés. Nous avons progressivement augmenté le désordre géometrique de la structure originale. Nous avons trouvé que l'etat fondamental conserve son ordre de Néel, mais que la forme de la distribution ainsi que la norme des aimantations sont modifiés. Nous montrons, à l'aide d'un développement en ondes des spin ainsi que par Quantum Monte Carlo, que l'aimantation alternée diminue exponentiellement vers une valeur asymptote en fonction du désordre. La distribution spatiale de magnetizations locales devient plus homogène par rapport à pavage parfait. La vitesse des ondes des spin augmente avec le désordre, et les singularités dans le spectre et les functions d'onde sont en partie lissées. Ces résultats sont comparés avec des résultats connus dans des systèmes désordonnés.

Condensed matter physics provides us with an opportunity to explore a large variety of systems with diverse properties. Central to the understanding of these systems is a characterization of the nature of their ground states and low energy excitation. Often, such systems show various forms of emergent properties that are absent in the microscopic level. Identification of such emergent phases of condensed matter form an important avenue of research in the field. In this thesis example of such phases and their associated phase transitions have been studied. The work presented here may be broadly divided into two themes: construction of the theoretical framework for understanding materials already studied experimentally, and, trying to provide new theoretical avenues which may be relevant for understanding future experiments. In these studies we shall explore some unconventional phases and phase transitions that may occur in condensed matter systems. A comprehensive understanding of the properties of such unconventional phases and phase transitions is important in the context of the large array of experimentally studied materials that regularly defy conventional wisdom in more than one way. The thesis consists of two distinct parts. In the first part we study three problems in frustrated magnets. The second part consists of studies of the tunnelling spectroscopy of metal-insulator-superconductor junctions in graphene. Studies in frustrated magnets have opened up the possibility of existence of a whole range of phases beyond the already known magnetically ordered ones. Some of these new phases, like the spin nematic or the valence bond solid, display some other conventional order themselves. Others, like the much sort after spin liquid phases displays a whole new kind of order that cannot be captured through the celebrated Landau’s classification of phases on the basis of symmetry breaking and associated order parameters. The phase transitions in these systems are also equally interesting and lead to intriguing possibilities that demand new modes of analysis. In this part of the thesis we shall study the different properties of three magnets with spin-1/2, 1 and 3/2 respectively. We start by providing an introduction to frustrated spin systems in Chapter [1]. The origin of antiferromagnetic interactions in Mott insulators is discussed and the concept of frustration of magnetic interaction is explained. We also point out the causes that may destroy magnetic order in spin systems, particularly the role of quantum fluctuations in presence or absence of magnetic frustration. This is followed with a brief outline of various magnetically ordered and disordered ground states with particular emphasis on the description of the later. We also give a brief outline of various properties of such phases and associated quantum phase transitions particularly noting the influences of quantum interferences encoded in the Berry phase terms. A brief description of the finite temperature properties is also provided. We end an outline of various experimentally relevant compounds that requires comprehensive understanding, some of which have been addressed in this thesis. In Chapter [2] we study the properties of a spin-nematic state in context of the recently discovered spin-1 Mott insulator Nickel Gallium Sulphide (NiGa2S4). This isotropic triangular lattice compound shows no spin ordering till low temperatures. We propose that it may have a particular type of spin-nematic ground state and explain the experimentally observed properties of the compound on the basis of our proposal. Starting from a two band Hubbard model description, relevant for the compound, we derive the Bilinear Biquadratic spin Hamiltonian. We then show, within mean field theory, that this Hamiltonian describes a transition from the spiral state to a ferro-nematic state as a function of the ratio of bilinear and biquadratic couplings. We also study the possible effects of small pinning disorder andmagnetic field and suggest experiments that can possibly distinguish the proposed nematic state from others. In Chapter [3] we explore the effects of the magneto-elastic coupling in the spin-3/2 B-site chromite spinel Cadmium Chromite (CdCr2O4). In this compound the spins form a pyrochlore lattice. Nearest neighbour spins interact antiferromagnetically. Due to frustration the system does not order at low temperatures and instead goes into a classical spin liquid state. Such a cooperative paramagnet is very susceptible to external perturbations which may relieve their frustration. In CdCr2O4, at lower temperatures the magnetic frustration is relieved by distorting the lattice through a first order magnetoelastic transition. Thus the compound presents a case where the relevant perturbation to the frustrated spin interactions is provided by spin-phonon coupling. An effect of such perturbations on a cooperative paramagnet is of general interest and all aspects of this are not understood presently. We take the initial step of characterizing the spin-phonon interaction in detail. Based on recent sound velocity experiments, we construct a microscopic theory for the sound velocity renormalization due to the spin-phonon coupling and explain the recent experimental data obtained by S. Zherlitsyn et al. using our theory we can explain the dependence of the sound velocity on temperature as well as magnetic field. We also construct a Landau theory to explain (qualitatively) the behaviour of sound velocity across the magneto-structural transition. Further, we discuss the effects due to the small Dzyaloshinskii-Moriya interaction that may be present in these compounds. In Chapter [4] we study the possibility of a direct second order quantum phase transition from spiral to dimer phase in two dimensional antiferromagnets. Such transitions between phases with incompatible symmetries are forbidden within conventional Landau Ginzburg-Wilson paradigm of critical phenomena. Early works showed that when the spiral is destroyed by long wavelength fluctuations a fractionalized Z2 spin liquid is obtained. In this work we show an alternative way–the quantum destruction of the spiral magnet. We argue that, when the defects of the spiral phase proliferate and condense, their associated Berry phase automatically leads to dimerization. We apply our theory to study concrete lattice models where such transitions may be observed. This transition is an example of a Landau forbidden deconfined quantum phase transition. The proposed critical theory is naturally written in terms of fractional degrees of freedom which emerge right at the critical point. These fractional particles interact with each other through emergent gauge fields and are deconfined right at the critical point (but are confined in either of the two adjoining phases). We argue, based on existing results, that the monopoles of the gauge field are dangerously irrelevant right at the critical point rendering the later noncompact. The critical point is characterized by an emergent global U (1) conservation law that is absent in the microscopic model, a typical feature of a deconfined quantum critical point. The resultant field theory belongs to the class of anisotropic NCCP3 class which may be studied numerically in future to understand its critical properties. In modern condensed matter physics the emergence of new and novel phases of matter have often been associated with the presence of strong correlations. Indeed, strongly correlated systems seem to harbour in them the potential to realize some of the most unconventional and exotic emergent phases of matter. However in graphene, which is a single layer of graphite, the emergence of novel properties, as present experiments suggest, is due to its unique band structure and not a fallout of intricate correlation effects. Band structure studies of graphene suggest that the material is a zero gap semiconductor with the low energy excitations resembling massless Dirac quasi-particles. The consequence of this is immediate and interesting. It has lead to the possibility of exploring the physics of relativistic fermions in two spatial dimensions and much of this has been studied with great vigour in the last five years. In our studies, presented in Chapter [5], we explore one of the many consequence of this emergent Dirac structure of the low energy quasi-particles, namely the properties of metal-insulator-superconductor junctions of graphene. The twin effect of Klein tunneling of Dirac fermions (and associated transmission resonances) and Andreev reflection (both specular and retro) sets them aside from their conventional counterparts. The graphene normal metal-insulator-superconductor (NIS) junctions show strikingly different properties like oscillations in the sub-gap tunneling conductance as a function of both barrier strength and width. We make a detailed study of this for arbitrary barrier strengths and widths with and without Fermi-surface mismatch between the normal and the superconducting sides. The amplitude of these oscillations are maximum for aligned Fermi surface and vanishes for large Fermi surface mismatch. We provide an understanding for this unconventional behaviour of graphene NIS junctions. We also suggest experimental tests for our theory. Such experimental verification will reveal one more remarkable emergent property in a condensed matter system.

Hoch manganhaltige Eisenbasislegierungen sind bei Raumtemperatur austenitisch und antiferromagnetisch (afm). Dabei besteht die Besonderheit, dass sich durch Legierung die afm Übergangstemperatur (Néeltemperatur) so einstellen lässt, dass sie nahe Raumtemperatur liegt. FeMn-Basislegierungen zeigen in Abhängigkeit von der Zusammensetzung Transformation Induced Plasticity (TRIP) und/oder Twinning Induced Plasticity (TWIP), d.h. die niedrige Stapelfehlerenergie dieser Legierungen führt zu verformungsinduzierter, metastabiler Phasenbildung (TRIP) bzw. zur Bildung von Verformungszwillingen (TWIP) und dadurch zu außerordentlich hoher Duktilität bei gleichzeitig hoher Verfestigung. Darüber hinaus haben FeMn-Basislegierungen einen ausgeprägten Magnetovolumeneffekt und magnetoelastischen Effekt durch magnetische Ordnung. Daher sind die untersuchten FeMnNiCr-Basislegierungen auch prototypisch für afm Élinvarlegierungen. Da Élinvar jedoch für invariable Elastizität steht, bedingt eine Anwendung als temperaturkompensierte Konstantmodullegierungen die Glättung der ausgeprägten magnetischen Anomalien, die industriell noch in keiner Anwendung realisiert wurde. Der Vorteil dies für eine Anwendung zu erreichen, läge in der Unempfindlichkeit feinmechanischer Bauelemente, gegenüber magnetischen Feldern, die bei den industriell verfügbaren ferromagnetischen Élinvarlegierungen nicht gewährleistet ist. Mit Bezug zu feinmechanischen Schwingsystemen spielen dabei neben der Einstellung der magnetoelastischen Eigenschaften die Prozessierbarkeit, Kaltumformbarkeit und Festigkeit sowie deren wechselseitige Beeinflussung eine große Rolle. Die vorliegende Arbeit befasst sich daher mit der Anwendbarkeit der untersuchten FeMnNiCr-Legierungen. Dabei wurden grundlegende Untersuchungen zur Plastizität durchgeführt, die die mechanische Zwillingsbildung in diesen Legierungen charakterisiert und ein Modell der mechanischen Zwillingsbildung bei kleinen plastischen Dehnungen vorschlägt, das eine Abschätzung der Stapelfehlerenergie erlaubt. Die Untersuchung des Antiferromagnetismus umgeformter Proben zeigt das Auftreten thermoremanenter Magnetisierung (TRM), deren Größe mit dem Umformgrad der untersuchten Proben skaliert. Sie wird den durch Umformdefekte erzeugten unkompensierten Momenten in der afm Spinstruktur zugeschrieben. Diese werden durch eine magnetische Feldkühlung magnetisiert und koppeln durch Austauschwechselwirkung an die umgebende antiferromagnetische Matrix unterhalb der Néeltemperatur. Das komplexe thermomagnetische Verhalten der unkompensierten Momente wird experimentell beschrieben und phänomenologisch gedeutet. Die Weiterentwicklung und Bewertung technischer, ausscheidbarer FeMnNiCrBe- und FeMnNiCr(Ti, Al)-Legierungen wird mit Bezug zu den grundlegenden Untersuchungen dargestellt. Es wird gezeigt, dass die neu entwickelten ausscheidbaren FeMnNiCr(Ti, Al)-Legierungen eine vielversprechende Ausgangsbasis darstellen, afm Élinvarlegierungen technisch umzusetzen. / High manganese iron-base alloys are austenitic and antiferromagnetic (afm) at room temperature. By further alloying it is possible to tune the afm transition temperature (Néel temperature) near room temperature. FeMn-base alloys show extraordinary strain hardening as well as ductility because of Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticty (TWIP), i.e. in dependence on composition the generally low stacking fault energy in these alloys allows for the mechanically induced formation of metastable phases (TRIP) or deformation twinning (TWIP). Furthermore, magnetic order causes distinct magnetovolume and magnetoelastic effects in these afm FeMn-base alloys. The investigated FeMnNiCr-base alloys are therefore prototypic for afm Élinvar alloys. However, as Élinvar is meant for invariant elasticity, an application as temperature compensated alloy with constant elastic modulus requires the smoothing of the pronounced magnetic anomalies, that is not industrially available yet. The advantage of afm Élinvar alloys in precision mechanics applications, would be their impassiveness with respect to magnetic fields that is not achievable by their ferromagnetic counterparts. For precision components like mechanic oscillators not only the tuning of the magnetoelastic properties but also the processing, cold formability and mechanical properties as well as their interplay have strong influence. Therefore this work addresses the applicability of the studied FeMnNiCr alloys. Elementary investigations on plasticity characterise the occurrence of TWIP in these alloys and propose a modell for deformation twinning at low plastic strains that allows for an estimation of the stacking fault energy. The investigations on the antiferromagnetism of deformed samples show the appearance of thermoremanent magnetisation (TRM). Its magnitude scales with the degree of deformation. The TRM is therefore attributed to uncompensated moments in the afm spin structure due to deformation induced defects. These are magnetised by a magnetic field cooling and couple to the afm matrix by exchange interaction below the Néel temperature. The complex thermomagnetic behaviour of the uncompensated moments is experimentally described and phenomenologically explained. The further development and assessment of engineering-grade pecipitable FeMnNiCrBe and FeMnNiCr(Ti, Al) alloys is presented in relation to the aforementioned elementary investigations. It is shown that the newly developped precipitable FeMnNiCr(Ti, Al) alloys are good candidates for afm Élinvar alloys in application.

Dans cette thèse, nous présentons quelques analyses théoriques récentes ainsi que des observations expérimentales de l’effet tunnel quantique macroscopique et des tran- sitions de phase classique-quantique dans le taux d’échappement des systèmes de spins élevés. Nous considérons les systèmes de spin biaxial et ferromagnétiques. Grâce à l’approche de l’intégral de chemin utilisant les états cohérents de spin exprimés dans le système de coordonnées, nous calculons l’interférence des phases quantiques et leur distribution énergétique. Nous présentons une exposition claire de l’effet tunnel dans les systèmes antiferromagnétiques en présence d’un couplage d’échange dimère et d’une anisotropie le long de l’axe de magnétisation aisé. Nous obtenons l’énergie et la fonc- tion d’onde de l’état fondamentale ainsi que le premier état excité pour les systèmes de spins entiers et demi-entiers impairs. Nos résultats sont confirmés par un calcul utilisant la théorie des perturbations à grand ordre et avec la méthode de l’intégral de chemin qui est indépendant du système de coordonnées. Nous présentons aussi une explica- tion claire de la méthode du potentiel effectif, qui nous laisse faire une application d’un système de spin quantique vers un problème de mécanique quantique d’une particule. Nous utilisons cette méthode pour analyser nos modèles, mais avec la contrainte d’un champ magnétique externe ajouté. La méthode nous permet de considérer les transitions classiques-quantique dans le taux d’échappement dans ces systèmes. Nous obtenons le diagramme de phases ainsi que les températures critiques du passage entre les deux régimes. Nous étendons notre analyse à une chaine de spins d’Heisenberg antiferro- magnétique avec une anisotropie le long d’un axe pour N sites, prenant des conditions frontière périodiques. Pour N paire, nous montrons que l’état fondamental est non- dégénéré et donné par la superposition des deux états de Néel. Pour N impair, l’état de Néel contient un soliton, et, car la position du soliton est indéterminée, l’état fondamen- tal est N fois dégénéré. Dans la limite perturbative pour l’interaction d’Heisenberg, les fluctuations quantiques lèvent la dégénérescence et les N états se réorganisent dans une bande. Nous montrons qu’à l’ordre 2s, où s est la valeur de chaque spin dans la théorie des perturbations dégénérées, la bande est formée. L’état fondamental est dégénéré pour s entier, mais deux fois dégénéré pour s un demi-entier impair, comme prévu par le théorème de Kramer / This thesis presents recent theoretical analyses together with experimental observa- tions on macroscopic quantum tunneling and quantum-classical phase transitions of the escape rate in large spin systems. We consider biaxial ferromagnetic spin systems. Using the coordinate dependent spin coherent state path integral, we obtain the quantum phase interference and the energy splitting of these systems. We also present a lucid exposition of tunneling in antiferromagnetic exchange-coupled dimer, with easy-axis anisotropy. Indeed, we obtain the ground state, the first excited state, and the energy splitting, for both integer and half-odd integer spins. These results are then corroborated using per- turbation theory and the coordinate independent spin coherent state path integral. We further present a lucid explication of the effective potential method, which enables one to map a spin Hamiltonian onto a particle Hamiltonian; we employ this method to our models, however, in the presence of an applied magnetic field. This method enables us to investigate quantum-classical phase transitions of the escape rate of these systems. We obtain the phase boundaries, as well as the crossover temperatures of these phase transi- tions. Furthermore, we extend our analysis to one-dimensional anisotropic Heisenberg antiferromagnet, with N periodic sites. For even N, we show that the ground state is non-degenerate and given by the coherent superposition of the two Neél states. For odd N, however, the Neél state contains a soliton; as the soliton can be placed anywhere along the ring, the ground state is, indeed, N-fold degenerate. In the perturbative limit (weak exchange interaction), quantum fluctuation stemming from the interaction term lifts this degeneracy and reorganizes the states into a band. We show that this occurs at order 2s in (degenerate) perturbation theory. The ground state is non-degenerate for inte- ger spin, but degenerate for half-odd integer spin, in accordance with Kramers’ theorem

量子擾動及挫折性兩者均可破壞絕對零溫的磁序，為近代凝態物 理關注的有趣現象。在外加橫場下的三角晶格易辛反鐵磁兼具量子臨 界現象(quantum criticality)及幾何挫折性，可謂量子磁性物質之一典 範理論模型。本論文利用平衡態及非平衡態量子蒙地卡羅(quantum Monte Carlo)方法探測三角晶格易辛反鐵磁之量子相變，其界定零溫 時無磁性的順磁態及具 Z6 對稱破缺的有序態(所謂時鐘態)。這裡的 量子蒙地卡羅方法為運用算符的零溫投射(zero-temperature projector) 及隨機序列展開(stochastic series expansion)演算法。在非平衡模擬 中，我們分別沿降溫過程及量子絕熱過程逼近量子相變點，藉此我們 得到動力學指數，及其它相關臨界指數。 / The destruction of magnetic long-range order at absolute zero temperature arising from quantum fluctuations and frustration is an interesting theme in modern condensed-matter physics. The triangular lattice Ising antiferromag- net in a transverse field provides a playground for the study of the combined effects of quantum criticality and geometrical frustration. In this thesis we use quantum Monte Carlo methods both in equilibrium and non-equilibrium setups to study the properties of the quantum critical point in the triangular lattice antiferromagnet, which separates a disordered paramagnetic state and an ordered clock state exhibiting Z6 symmetry breaking; The methods are based on a zero-temperature projector algorithm and the stochastic series ex- pansion algorithm. For the non-equilibrium setups, we obtain the dynamical exponent and other critical exponents at the quantum critical point approached by slowly decreasing temperature and through quantum annealing.

Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). The first experimental chapter examines the formation of the ferrite from a mixture of reactants.It describes the chemical reactions and phase transitions that lead to the formation of the oxide. Ball milling of the reactants prior to annealing was found to eliminate transient phases from the reaction route and to increase the kinetics of the reaction at lower temperatures. Examination of the thermodynamics of iron oxide (hematite) used for the reactions led to a new route of synthesis of the ferrite frommagnetite and strontium carbonate.This chapter also explores the possibility of synthesis of the material at room temperature using ball milling. ¶ The ferrite strongly interacts with the gas phase so its behaviour was studied under different pressures of oxygen and in carbon dioxide.The changes in ferrite composition have an equilibrium character and depend on temperature and oxygen concentration in the atmosphere. Variations of the oxygen content x were described as a function of temperature and oxygen partial pressure, the results were used to plot an equilibrium composition diagram. The heat of oxidation was also measured as a function of temperature and oxygen partial pressure. ¶ Interaction of the ferrite with carbon dioxide below a critical temperature causes decomposition of the material to strontium carbonate and SrFe12O19 . The critical temperature depends on the partial pressure of CO2 and above the critical temperature the carbonate and SrFe12O19 are converted back into the ferrite.The resulting SrFe12O19 is very resistant towards carbonation and the thermal carbonation reaction does not lead to a complete decomposition of SrFeOx to hematite and strontium carbonate. ¶ The thermally induced oxidation and carbonation reactions cease at room temperature due to sluggish kinetics however,they can be carried out at ambient temperature using ball milling.The reaction routes for these processes are different from the thermal routes.The mechanical oxidation induces two or more concurrent reactions which lead to samples containing two or more phases. The mechanical carbonation on the other hand produces an unknown metastable iron carbonate and leads a complete decomposition of the ferrite to strontiumcarbonate and hematite. ¶ Thermally and mechanically oxidized samples were studied using Mossbauer spectroscopy. The author proposes a new interpretation of the Sr4Fe4O11 (x=2.75) and Sr8Fe8O23 (x=2.875)spectra.The interpretation is based on the chemistry of the compounds and provides a simpler explanation of the observed absorption lines.The Mossbauer results froma range of compositions revealed the roomtemperature phase behaviour of the ferrite also examined using x-ray diffraction. ¶ The high-temperature crystal structure of the ferrite was examined using neutron powder diffraction.The measurements were done at temperatures up to 1273K in argon and air atmospheres.The former atmosphere protects Sr2Fe2O5 (x=2.5) against oxidation and the measurements in air allowed variation of the composition of the oxide in the range 2.56 ≤ x ≤ 2.81. Sr2Fe2O5 is an antiferromagnet and undergoes phase transitions to the paramagnetic state at 692K and from the orthorhombic to the cubic structure around 1140K.The oxidized formof the ferrite also undergoes a transition to the high-temperature cubic form.The author proposes a new structural model for the cubic phase based on a unit cell with the Fm3c symmetry. The new model allows a description of the high-temperature cubic form of the ferrite as a solid solution of the composition end members.The results were used to draw a phase diagramfor the SrFeOx system. ¶ The last chapter summarizes the findings and suggests directions for further research.