Investigation of spin and orbital ordered phases in spinel oxides

Abstract

In this study, I want to demonstrate how spin configuration and orbital state can be investigated by Nuclear Magnetic Resonance (NMR). The strong correlation between four degrees of freedoms, i.e. spin, charge, orbital and lattice, brings rich emergent properties to solids. In order to investigate how they interact with each other, a long range ordered phase in which spin, charge, and orbital are well defined would be suitable. As a playground for the four degrees of freedom, spinel oxides are recently focused due to its versatility of charge and orbital states, producing large numbers of spinel materials with different properties. Spin configurations and orbital states of several spinel oxides were investigated in this thesis. First, the spin configuration of nanocrystalline Zn-ferrite was figured out. From the NMR results in the presence of external magnetic field, the coexistence of antiferromagnetic ordering in B sites and ferrimagnetic ordering between A and B sites was observed. Second, both electronic valence and spin configuration of Mn-ferrite were investigated. Mn and Fe NMR spectrum uncovered the valence states of Mn and Fe ions, and in the presence of external magnetic field, the spin configuration of $Fe^{3+}$ in B sites found to be abnormal. Finally, spin and orbital ordered phase of both $MnV_2O_4$ and $Mn_3O_4$ were studied. Inprior to the NMR experiment, the orbital dependence of anisotropic hyperfine field wascalculated to determine the exact orbital state from field-angle resolved NMR experiment. With the rotation of electron spin around three axes, NMR frequency shift due to the anisotropic hyperfine field clearly re°ects spatial electron distribution, i.e. the orbital state. According to the field-angle resolved NMR result, the orbital state of $Mn^{3+}$ in $Mn_3O_4$ was found to be $d_{z^2}$, as expected from the lattice symmetry, which means that the orbital state of $Mn_3O_4$ is determined by the crystal via the strong ...