Influence of the external pressure on the structure and magnetic properties of Nd_0.5Sr_0.5CoO₃ cobaltite

The magnetic properties of the compound-type cobaltites LnACoO₃ (Ln = Lanthanide and A = alkaline-earth) are extremely dependent on the structural deformation, which can be induced by either chemical substitution or applying pressure. These two approaches are not equivalent because the atomic substitution changes the ratio of Co³⁺ and Co⁴⁺. It is well known that the additional electron of Co³⁺ plays a fundamental role in the magnetic and electric properties of these materials. Thus, the pressure is a way to influence the magnetic and electric properties considering only the structural deformation. In this work, pressure was applied to deform the structure of Nd_0.5Sr_0.5CoO₃ and, thus, to elucidate its effect on the structural and magnetic properties. The findings show that even under pressure of 50 kbar, no structural phase transition was detected, but the unit cell volume significantly decreases by 2.8% and the Co-O₁-Co bond angle increases by 2.6%. However, the elasticity parameter (defined as the Co-O apical distance divided by the square root of the octahedral basal area) remained constant over the pressure range. The magnetic properties were evaluated up to 10.9 kbar, and the findings show that T_C varies with a rate of 0.5 K/kbar, and that the magnetic entropy change of the compound remains almost constant at a value of −2 J/kg K for 5 T of magnetic field variation. The Co³⁺ and Co⁴⁺ ions were found to be in an intermediate spin state and remain the same over all studied pressure interval. To conclude, the structural deformation induced by the high pressure seem to not affect the magnetic properties, and the elasticity parameter is a useful tool sensitive to this change. These results indicate that, for strongly correlated electron system, the ratio of Co³⁺ and Co⁴⁺ is much more critical for changing the magnetic properties than the structural deformation.