Magnetism in Complex Oxides Probed by Magnetocaloric Effect and Transverse Susceptibility

Magnetic oxides exhibit rich complexity in their fundamental physical properties determined by the intricate interplay between structural, electronic and magnetic degrees of freedom. The common themes that are often present in these systems are the phase coexistence, strong magnetostructural coupling, and possible spin frustration induced by lattice geometry. While a complete understanding of the ground state magnetic properties and cooperative phenomena in this class of compounds is key to manipulating their functionality for applications, it remains among the most challenging problems facing condensed-matter physics today. To address these outstanding issues, it is essential to employ experimental methods that allow for detailed investigations of the temperature and magnetic field response of the different phases. In this PhD dissertation, I will demonstrate the relatively unconventional experimental methods of magnetocaloric effect (MCE) and radio-frequency transverse susceptibility (TS) as powerful probes of multiple magnetic transitions, glassy phenomena, and ground state magnetic properties in a large class of complex magnetic oxides, including La0.7Ca0.3-xSrxMnO3 (x = 0, 0.05, 0.1, 0.2 and 0.25), Pr0.5Sr0.5MnO3, Pr1-xSrxCoO3 (x = 0.3, 0.35, 0.4 and 0.5), La5/8−xPrxCa3/8MnO3 (x = 0.275 and 0.375), and Ca3Co2O6. First, the influences of strain and grain boundaries, via chemical substitution and reduced dimensionality, were studied via MCE in La0.7Ca0.3-xSrxMnO3. Polycrystalline, single crystalline, and thin-film La0.7Ca0.3-xSrxMnO3 samples show a paramagnetic to ferromagnetic transition at a wide variety of temperatures as well as an observed change in the fundamental nature of the transition (i.e. first-order magnetic transition to second order magnetic transition) that is dependent on the chemical concentration and dimensionality. Systematic TS and MCE experiments on Pr0.5Sr0.5MnO3 and Pr0.5Sr0.5CoO3 have uncovered the different nature of low-temperature magnetic phases and demonstrate the importance of coupled structural/magnetocrystalline anisotropy in these half-doped perovskite systems. These findings point to the existence of a distinct class of phenomena in transition-metal oxide materials due to the unique interplay between structure and magnetic anisotropy, and provide evidence for the interplay of spin and orbital order as the origin of intrinsic phase separation in manganites. While Pr0.5Sr0.5MnO3 provides important insights into the influence of first- and second-order transitions on the MCE and refrigerant capacity (RC) in a single material, giving a good guidance on the development of magnetocaloric materials for active magnetic refrigeration, Pr1-xSrxCoO3 provides an excellent system for determining the structural entropy change and its contribution to the MCE in magnetocaloric materials. We have demonstrated that the structural entropy contributes significantly to the total entropy change and the structurally coupled magnetocrystalline anisotropy plays a crucial role in tailoring the magnetocaloric properties for active magnetic refrigeration technology. In the case of La5/8−xPrxCa3/8MnO3, whose bulk form is comprised of micron-sized regions of ferromagnetic (FM), paramagnetic (PM), and charge-ordered (CO) phases, TS and MCE experiments have evidenced the dominance of low-temperature FM and high-temperature CO phases. The dynamic strain liquid state is strongly dependent on magnetic field, while the frozen strain-glass state is almost magnetic field independent. The sharp changes in the magnetization, electrical resistivity, and magnetic entropy just below the Curie temperature occur via the growth of FM domains already present in the material, even in zero magnetic field. The subtle balance of coexisting phases and kinetic arrest are also probed by MCE and TS experiments, leading to a new and more comprehensive magnetic phase diagram. A geometrically frustrated spin chain compound Ca3Co2O6 provides an interesting case study for understanding the cooperative phenomena of low-dimensional magnetism and topological magnetic frustration in a single material. Our MCE studies have yielded new insights into the nature of switching between multi-states and competing interactions within spin chains and between them, leading to a more comprehensive magnetic phase diagram