Ground-state properties of the mixed-valence cobaltites Nd_0.7Sr_0.3CoO₃, Nd_0.7Ca_0.3CoO₃and Pr_0.7Ca_0.3CoO₃

Abstract

The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed: TC = 130 K, 55 K and 25 K and m = 0.68 μB, 0.34 μB and 0.23 μB for Nd0.7Sr0.3CoO3, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3, respectively. The moment reduction with respect to the moment of the conventional ferromagnet La0.7Sr0.3CoO3 (T(C) = 230 K, m = 1.71 μB) in the so-called low spin/intermediate spin (IS/LS) state for Co(3+)/Co(4+) was originally interpreted using a phase-separation scenario. Based on the present results, mainly the analysis of the Schottky peak originating from Zeeman splitting of the ground-state Kramers doublet of Nd(3+), we find, however, that the ferromagnetic phase in Nd0.7Ca0.3CoO3 and likely also in Pr0.7Ca0.3CoO3 is uniformly distributed over the whole sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov et al (2009 Phys. Rev. B 80 024423). The ground state of Nd0.7Sr0.3CoO3 with an intermediate cobalt moment is inhomogeneous due to competition between the LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for 4f(3) (Nd(3+)), 4f(2) (Pr(3+)) and 4f(1) (Ce(3+) or Pr(4+)) are calculated and their magnetic characteristics are presented.