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

Abstract

We have investigated the temperature dependence of the magnetic susceptibility χ(T ) of rareearth cobaltites RCoO3 (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 χ(T ) observed in LaCoO3 and the anomalously large pressure effect (d ln χ/dP ∼ −100 Mbar −1 for T = 78 K) are analysed in the framework of a two-level model with energy levels difference ∆. 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 LaCoO3, indicate a significant increase in ∆ with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO3 (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 ∆ 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 RCoO3 compounds, which we have observed at T = 300 K.