Magnetic Anisotropy and Noncollinear Spin Textures in CoV2O4 Thin Films

The spinel vanadate CoV2O4 is a magnetically frustrated system with an interesting non-collinear spin texture at low temperatures. The magnetic frustration in the system arises from the arrangement of atoms in the structure of the material, and the competition of various anisotropy energies leads to cooperative phase transitions. In this dissertation, we report on the growth of CoV2O4 in thin film form to induce structural perturbations, to attempt to control the magnetic properties in the material. Tuning of the crystal structure is achieved through strain induced in the material by growth on a substrate with different lattice parameter. By growing thin films of CoV2O4 on SrTiO3 substrates, we successfully perturb the structure from its bulk cubic form to a stable orthorhombic unit cell, confirmed with synchrotron x-ray diffraction. The perturbations to the structure lead to interesting magnetic properties in the CoV2O4 films, as the structural change distorts the nearest neighbor distances throughout the unit cell, and in turn, the exchange interactions between the ions. We present both macroscopic and microscopic magnetic behavior in the material as it is cooled through different magnetic transitions. In the different chapters of this dissertation, we use macroscopic magnetometry measurements to find the transition temperatures in the films, neutron diffraction measurements to investigate the arrangement of spins (magnetic structure), and torque magnetometry to probe the anisotropy energies associated with these magnetic structures. Clear differences in magnetic properties are seen in our thin film materials compared to previous reports of bulk CoV2O4. We find three magnetic transitions, rather than the two transitions typically seen in bulk, including one transition that appears to be field-dependent. While the low temperature magnetic structure of our films is similar to the non-collinear spin texture seen in bulk, our structural perturbations have significantly increased the canting angle of this non-collinearity, and induced very strong anisotropy in the magnetic structure. Differences in the magnetic behavior between thin films and bulk, in both magnetic structure and anisotropy, provide evidence that structural tuning of spinel vanadates is a promising avenue to stabilize different exotic spin textures via strain-engineering.