Magnetic Properties of Transition Metal Oxides from First-principles

Abstract

Due to the strong coupling between the spin, lattice, and orbital degrees of freedom, transition metal oxides exhibit a wealth of exotic phases, such as ferroelectricity, superconductivity, and magnetic ordering. In this thesis, I focus on the magnetic properties of three transition metal oxides. The first study I present in this thesis is on the botallackite cuprate Cu2(OH)3Br. I present the results for the excitation spectrum of the material, obtained through a combination of first-principles methods, linear spin wave theory and exact diagonalization. Our calculations of the dynamical structure factor highlight the coexistence of magnon and spinon excitations in the system, and our results qualitatively agree with experimental results obtained through inelastic neutron scattering.I then turn to the rare-earth titanate (RTiO3) compounds, which are well-known to transition from a predominantly ferromagnetic state to a predominantly G-type antiferromagnetic state with increasing rare earth radius. This extraordinary behavior arises from the high sensitivity of the exchange interactions to the crystal structure of RTiO3. As such, the rare-earth titanates are natural candidates for exploring the possibility of controlling a system’s magnetic behavior through the application of uniaxial or biaxial strain. I discuss the results of our comprehensive study of the rare-earth titanates, in which we used a combination of first-principles and analytical methods to show that the application of uniaxial or epitaxial strain in RTiO3 should lead to a host of magnetic and structural phase transitions. This study is then followed by a description of the collaborative works I have participated in, in which I provided first-principles and analytical calculations to complement experimental and theoretical analyses of RTiO3. I then discuss my contribution to the joint experimental and theoretical investigation of PYCCO. In this work, my coauthors demonstrate that simultaneous first-order spin-state/valence-state/metal-insulator transitions can be experimentally induced in PYCCO with applied epitaxial strain. Studying this system from first-principles, I provide evidence that the strain-tunable phase transitions in PYCCO are directly analogous to the first-order thermal phase transitions observed in PCCO.