A theory of the spin-crossover phenomena in the rare earth perovskite LaCoO3 induced by temperature or a magnetic field

Abstract

Published magnetic data for LaCoO3 are successfully analyzed with coexisting 5 D and low-spin (LS) cobalt states. Energy levels of the two states are derived in analytic forms. To this end, fictitious orbital angular momentum l of magnitude one defines the Γ5 (5 D) state. Our Hamiltonian includes the spin–orbit interaction, and a cubic crystal field embellished by a trigonal distortion . A singlet ground state with an energy gap to the first excited doublet is realized for certain values of the parameters. The temperature-independent paramagnetic susceptibility (TIPS) of the 5 D state has a finite value, which accords with the observation. Whereas, TIPS is symmetry forbidden in the LS state. A rigorous calculation is made of the excitation spectrum in the LS state. The elementary excitation is modeled as a creation of an electron–hole pair that results in an energy level scheme in which the first excited quartet lies above the singlet ground state. The electron spin resonance data are successfully equated with transitions within the excited quartet. Available magnetization data delineate parameters in the 5 D Hamiltonian. The temperature dependence of the susceptibility of our coexisting model is qualitatively reasonable. To improve on a quantitative outcome, we are led to introduce a temperature dependent concentration for the 5 D and LS states. Calculated Bragg diffraction patterns gathered with x-rays tuned to the Co K-edge reveal potential to refine the current crystal structure and to shed light on the origin of the coexisting states.