Schottky-type anomaly of linear optical birefringence of the antiferromagnetic LiCoPO4 crystal

Anomalous behavior of the birefringence of linearly polarized light has been observed in the antiferromagnetic magnetoelectric crystal LiCoPO4 at temperatures approaching the Neel temperature. Arguments are present in support of the assumption that the observed nonmonotonic change of the birefringence of light is due to the interplay of two anomalies - a magnetic anomaly, associated with the intensification of the contribution of correlated spin fluctuations, and the Schottky anomaly, due to the changing occupations of the ground and lowest excited electronic states of Co2+ ions. The estimate of the energy splitting from the excited state (45-55 K) agrees satisfactorily with the position of the weakly dispersing branch in the energy spectrum of this crystal, observed in an investigation of the inelastic scattering of neutrons, as well as with the energies of the low-energy bands in the Raman scattering spectrum