The ground state properties of the mixed-valence cobaltites Nd$_{0.7}$Sr$_{0.3}$CoO$_3$, Nd$_{0.7}$Ca$_{0.3}$CoO$_3$ and Pr$_{0.7}$Ca$_{0.3}$CoO$_3$

TL;DR

This study investigates the magnetic, electric, and thermal properties of three mixed-valence cobaltites with 30% hole doping, revealing how cation size influences ferromagnetic behavior and ground state phases, supported by experimental and theoretical analysis.

Contribution

It provides new insights into the uniform versus inhomogeneous ferromagnetic phases in cobaltites and clarifies the ground state configurations using experimental data and theoretical modeling.

Findings

Ferromagnetic Curie temperature decreases with smaller cation size.

Ground state in Nd$_{0.7}$Ca$_{0.3}$CoO$_3$ and Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ is uniformly distributed.

Nd$_{0.7}$Sr$_{0.3}$CoO$_3$ exhibits an inhomogeneous ground state due to phase competition.

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 - $T_C=130$ K, 55 K and 25 K and $m = 0.68 \mu_B$, 0.34 $\mu_B$ and 0.23 $\mu_B$ for Nd$_{0.7}$Sr$_{0.3}$CoO$_3$, Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ and Nd$_{0.7}$Ca$_{0.3}$CoO$_3$, respectively. The moment reduction with respect to moment of the conventional ferromagnet La$_{0.7}$Sr$_{0.3}$CoO$_3$ ($T_C=230$ K, $m = 1.71 \mu_B$) in so-called IS/LS state for Co$^{3+}$/Co$^{4+}$, was originally interpreted using phase-separation scenario. Based on the present results, mainly the analysis of Schottky peak originating in Zeeman splitting of the ground state Kramers doublet of Nd$^{3+}$, we find, however, that ferromagnetic phase in Nd$_{0.7}$Ca$_{0.3}$CoO$_3$ and likely also Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ is uniformly distributed over all 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 \textit{et al.} [Phys. Rev. B \textbf{80}, 024423 (2009)]. The ground state of Nd$_{0.7}$Sr$_{0.3}$CoO$_3$ with an intermediate cobalt moment is inhomogeneous due to competing of 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.