Spin State Transition in ${\mathrm{LaCoO}}_3$ Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge, we reveal that the spin state transition in ${\mathrm{LaCoO}}_3$ can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that ${\mathrm{LaCoO}}_3$ at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.

Figure 1

Experimental Co-$L_{2,3}$ XAS spectra taken from ${\mathrm{LaCoO}}_3$ at various temperatures between 20 and 650 K, together with the corresponding theoretical isotropic spectra calculated using a ${\mathrm{CoO}}_6$ cluster in the LS-HS scenario. For clarity, only the 20, 300, and 650 K spectra are shown.

Figure 2

Energy level diagram of a ${\mathrm{CoO}}_6$ cluster [43] as a function of the ionic part of the crystal field splitting $10Dq$.

Figure 3

(a) HS population from XAS data. (b) Corresponding effective activation energy between the LS and the lowest HS state. The inset sketches the role of lattice relaxations. (c) Magnetic susceptibility measured by VSM (solid line), calculated from the cluster (triangles) using the HS population of Fig. 3a, and extracted from MCD data (squares) of Fig. 4.

Figure 4

Top curves: experimental Co-$L_{2,3}$ XAS spectra taken from ${\mathrm{LaCoO}}_3$ at 60, 110, and 300 K using circularly polarized x rays with the photon spin aligned parallel (dotted line, ${\sigma }^{+}$) and antiparallel (solid line, ${\sigma }^{-}$) to the 6 Tesla magnetic field. Middle curves: experimental MCD spectra. Bottom curves: theoretical MCD spectra calculated in the LS-HS scenario.