Effect of Ising-type Tb$^{3+}$ ions on the low-temperature magnetism of La, Ca cobaltite

TL;DR

This study investigates how Ising-type Tb$^{3+}$ ions influence the low-temperature magnetic properties of La, Ca cobaltite, revealing complex magnetic ordering and significant effects on coercivity and heat capacity.

Contribution

It provides detailed neutron diffraction and magnetic measurements showing the role of Tb$^{3+}$ ions in magnetic ordering and anisotropy in La$_{0.8-x}$Tb$_x$Ca$_{0.2}$CoO$_3$.

Findings

Tb$^{3+}$ ions induce collinear and canted magnetic structures.

Large coercivity and remanence increase at low temperatures.

Anomalously large linear heat capacity term linked to Tb$^{3+}$ Ising behavior.

Abstract

Crystal and magnetic structures of the $x=0.2$ member of La$_{\rm 0.8-x}$Tb$_{\rm x}$Ca$_{0.2}$CoO$_3$ perovskite series have been determined from the powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below $T_C=55$ K. The moments evolve at first on Co sites, and ordering of Ising-type Tb$^{3+}$ moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear F$_x$ for cobalt subsystem, while it is canted F$_x$C$_y$ for terbium ions. The rare-earth moments align along local Ising axes within \textit{ab}-plane of the orthorhombic $Pbnm$ structure. The behavior in external fields up to $70-90$ kOe has been probed by the magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb$^{3+}$ Ising character is the observation of a low-temperature region of anomalously large linear term of heat capacity and its field dependence. Similar behaviours are detected also for other terbium dopings $x=0.1$ and 0.3.