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

Published magnetic data for LaCoOare successfully analyzed with coexistingand low-spin (LS) cobalt states. Energy levels of the two states are derived in analytic forms. To this end, fictitious orbital angular momentumof magnitude one defines the Γ() state. Our Hamiltonian includes the spin-orbit interaction, and a cubic crystal field embellished by a trigonal distortion9B20(lz2-2/3)-80B40(lz2-9/10). 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 thestate 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 theHamiltonian. 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 theand LS states. Calculated Bragg diffraction patterns gathered with x-rays tuned to the Co-edge reveal potential to refine the current crystal structure and to shed light on the origin of the coexisting states.