Ground-state properties of the mixed-valence cobaltites Nd0.7Sr0.3CoO3, Nd0.7Ca0.3CoO3 and Pr0.7Ca0.3CoO3

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: T_C = 130 K, 55 K and 25 K and m = 0.68 μ_B, 0.34 μ_B and 0.23 μ_B for Nd_0.7Sr_0.3CoO₃, Pr_0.7Ca_0.3CoO₃ and Nd_0.7Ca_0.3CoO₃, respectively. The moment reduction with respect to the moment of the conventional ferromagnet La_0.7Sr_0.3CoO₃ (T_C = 230 K, m = 1.71 μ_B) in the so-called low spin/intermediate spin (IS/LS) state for Co³⁺/Co⁴⁺ 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 Nd³⁺, we find, however, that the ferromagnetic phase in Nd_0.7Ca_0.3CoO₃ and likely also in Pr_0.7Ca_0.3CoO₃ 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 Nd_0.7Sr_0.3CoO₃ 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 4f³ (Nd³⁺), 4f² (Pr³⁺) and 4f¹ (Ce³⁺ or Pr⁴⁺) are calculated and their magnetic characteristics are presented.