Investigation of crystal field, magnetic frustration and magnetization reversal effects in 4f and 3d oxides

Abstract

This thesis deals with the investigation of single-ion excitations, magnetic frustration, and magnetization reversal phenomena. We have employed neutron scattering, μ+SR, and computational methods for these studies. Inverse spinel oxide Co2VO4 belongs to the family of spinel vanadates where cobalt and vanadium ions occupy the spinel B-site equally. This system exhibits magnetization reversal for temperatures below 65 K. Magnetization studies reveal three anomalies involving collinear and non-collinear ferrimagnetism phase and magnetization reversal crossover. Neutron diffraction analysis of Co2VO4 unveils that the evolution of relative balance between the two sublattice moments leads to ferrimagnetic phase and magnetization reversal. DFT calculation and μ+SR results suggest delocalization -localization crossover as the underlying microscopic mechanism for magnetization reversal. Rare earth oxide SrTm2O4 belongs to the family of SrLn2O4 where two inequivalent Tm3+s form two zig-zag chains along the orthorhombic c-axis. An earlier study on SrTm2O4 reports the absence of long- or short-range order down to 65 mK. The crystal fields and exchange interactions in SrTm2O4 were studied to examine the absence of order. The crystal fields in SrTm2O4 were modeled using DFT and the effective charge (EC) model. The EC model describes the system well and suggests ꞁJ,mj > = ꞁ6,0> dominates the ground state of both Tm3+s. The exchange interactions extracted from low-energy dispersing excitations using random phase approximation suggest that Tm2 chains are frustrated and Tm1 chains could form dimers. The critical ratio calculated from exchange interaction indicates SrTm2O4 cannot undergo a thermal second-order phase transition confirming the absence of order. μ+SR results show oscillations in polarization evolution spectra; these are typically associated with muon precession due to the system’s long-range order. Modeling precession frequency vs. temperature reveals that these oscillations originate due to nuclear hyperfine enhancement. Additionally, the magnetic field application induces long-range order, with Tm2 function as polarized paramagnet whereas Tm1 above 4 T transitions into XY-AFM phase.