UC Berkeley

Studies of magnetism at reduced scales have revealed new phenomena that are distinctly different from their bulk counterparts providing insight to the fundamental mechanisms that govern magnetism and other correlated properties. To this end, the use of heteroepitaxy and heterostructures is invaluable for investigating magnetism at reduced dimensions and at surfaces and interfaces. This dissertation is a compilation of investigations examining the magnetic properties of spinel-structure oxide thin films and heterostructures. Of particular interest are non-collinear spin systems as closely competing exchange interactions between magnetic moments give rise to a plethora of ground state degeneracies and phenomena inaccessible in the bulk.

The first part of this dissertation highlights the use of heteroepitaxial lattice distortions as a method to tune spin functionality and potentially lift ground state degeneracies more broadly in frustrated magnets. It discusses the first synthesis of heteroepitaxial thin films of the frustrated canted-moment ferrimagnet, CuCr2O4, and demonstrates the modification of exchange interaction strengths which results in greater collinear spin ordering and enhanced magnetization compared to the bulk. The data illustrates the sensitivity of the strong competing exchange interactions suggesting that strain is a promising instrument for perturbing the delicate balance of the exchange interactions in frustrated materials.

The second part of this dissertation probes magnetic proximity effects induced by interfaces and unconventional transport properties when CuCr2O4 is incorporated as a barrier layer in magnetic tunnel junctions comprised of ferromagnetic La0.7Sr0.3MnO3 and Fe3O4 electrodes. It is surprising that a heterostructure composed entirely of magnetic materials can achieve distinct magnetic and resistive switching given the complexities present at the two barrier-electrode interfaces. Studies of the CuCr2O4/Fe3O4 interface again illustrate the delicate balance between exchange interactions as proximity effects by Fe3O4 are believed to modulate alignment in the Cr moments. Additionally, the bias dependence of JMR displays a local minimum at zero bias which is believed to be the result of the Fe3O4 band structure and spin filtering properties of the CuCr2O4 barrier.