Results of spin density functional theory (SDFT) calculations were used to construct and check features of a generally applicable semi-quantitative approach to understanding magnetic coupling in gadolinium-containing molecules, clusters, and solids. Using fragments based on structures of metal-rich lanthanide compounds, we have investigated molecular and low-dimensional extended structures, and have shown that open-d-shell clusters facilitate strong ferromagnetic coupling whereas closed-d-shell systems prefer antiferromagnetic coupling. The qualitative features can be interpreted using a perturbative molecular orbital (PMO) model that focuses the influence of the 4f 7- d exchange interaction on the d-based molecular orbitals. The f-d exchange interaction, mediated by spin polarization of both filled and partially-filled metal-metal bonding orbitals, is described for the model system Gd3I6(OPH3)12 n+ using basic perturbation methods. This approach is successful for predicting the magnetic ground state for Gd2Cl3, a semiconducting system for which calculations predict antiferromagnetic ordering of the 4f 7 moments in a pattern consistent with published neutron diffraction data. An attempt to account for the calculated magnetic energies of spin patterns using an Ising model was unsuccessful, indicating that the Ising model is inappropriate. Instead, the d-electron mediated f-f exchange interaction was interpreted using our basic perturbation theory approach. Computed density of states and spin polarization information was used to support the perturbation-theoretic analysis. This method has also been successful evaluating the ground state for Gd[Gd6FeI12]. Using the model [Gd6CoI12](OPH3)6, which has three unpaired electrons in the HOMO, the 4f moments prefer spin alignment with the unpaired electrons in the system and the ferromagnetic 4f 7 spin arrangement is the ground state. We have extended our analysis of R6X12 clusters to include nonmetal interstitial atoms, the bioctahedral cluster compounds Gd10Cl17C4 and Gd10I16C4, and Gd5(O)(OPri)5. Finally, we have shown that we can successfully predict the ground state magnetic structures of several metallic and semiconducting Gd-containing compounds, Gd2Cl3, GdB2C2,alpha-Gd2S3, Gd5Si4, and Gd5Ge4, using semi-empirical calculations which closely simulates the exchange effects exerted by the 4f electrons. In a more speculative vein, ideas concerning the incorporation of anisotropic rare-earth metal atoms to the cluster framework are touched upon.

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.

We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO₂ terminated SrTiO₃ (STO) substrates and have grown epitaxial thin films of LaAlO₃ (LAO), LaGaO₃ (LGO), and RAlO₃ on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R³+ magnetism on the interface between RAlO₃ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10¹⁴ cm[superscript -2] and 4.5 x 10¹⁴ cm[superscript −2], respectively. Rare-earth aluminate films, RAlO₃, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R³+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO₃ films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R³+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering. / text