Diamagnetic to paramagnetic transition in LaCoO3

The diamagnetic to paramagnetic spin state transition in LaCoO3 (LCO) that occurs in the temperature range 30-120 K is generally attributed to the small energy gap between the Co3+ t(2g) and e(g) states. Evidence for this thermally activated transition has been interpreted as leading to either the intermediate spin state, t(2g)(5)e(g)(1)(S=1), or, alternatively, to the high-spin state, t(2g)(4)e(g)(2)(S=2) of the Co3+ ion, with the issue proving highly controversial. In an effort to obtain a consistent description of the temperature dependence of the magnetic and thermal properties of this system, we have made measurements of both the magnetization in applied fields of up to 33 T and the specific heat at 0 and 9 T on a single crystal of LCO. In addition, EPR measurements were made on the same sample using high-field EPR spectrometers. The spin-Hamiltonian parameters are consistent with the previous pulsed-field EPR work and support the atomic-like energy level description of the Co ion. The low-lying first-excited state is part of the T-5(2g) (D-5) set and is a triplet state with effective spin S-eff=1. The magnetization results are analyzed using a mean-field model allowing for antiferromagnetic correlations between the spins. The model is used to estimate the spin contribution to the specific heat.