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

Abstract

Crystal and magnetic structures of the $x=0.2$ member of La${}_{0.8-\mathrm{x}}$Tb${}_{\mathrm{x}}$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.

8 pages, 11 figures

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