Magnetic properties of RCoO\(_3\) cobaltites (R = La, Pr, Nd, Sm, Eu). Effects of hydrostatic and chemical pressure

We have investigated the temperature dependence of the magnetic susceptibility \(\chi(T)\) of rare-earth cobaltites RCoO\(_3\) (R= La, Pr, Nd, Sm, Eu) in the temperature range \(4.2-300\) K and also the influence of hydrostatic pressure up to 2 kbar on their susceptibility at fixed temperatures \(T=78 \) and 300 K. The specific dependence \(\chi(T)\) observed in LaCoO\(_3\) and the anomalously large pressure effect (d ln \(\chi\)/d\(P\sim -100\) Mbar\(^{-1}\) for \(T = 78\) K) are analyzed in the framework of a two-level model with energy levels difference \(\Delta\). The ground state of the system is assumed to be nonmagnetic with the zero spin of Co\(^{3+}\) ions, and magnetism at a finite temperature is determined by the excited magnetic spin state. The results of the analysis, supplemented by theoretical calculations of the electronic structure of LaCoO\(_3\), indicate a significant increase in \(\Delta\) with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO\(_3\) (R= Pr, Nd, Sm, Eu) compounds, the volume of crystal cell decreases monotonically due to a decrease in the radius of R\(^{3+}\) ions. This leads to an increase in the relative energy \(\Delta\) of the excited state (the chemical pressure effect), which manifests itself in a decrease in the contribution of cobalt ions to the magnetic susceptibility at a fixed temperature, and also in a decrease in the hydrostatic pressure effect on the susceptibility of RCoO\(_3\) compounds, which we have observed at \(T=300\) K.