Crystal-field approach to rare-earth higher borides: Dimerization, thermal, and magnetic properties ofRB50 (R=Tb,Dy,Ho,Er,Tm)

Abstract

The microscopic justification for the dimerization in ladder-type rare-earth higher borides $R{\mathrm{B}}_{50}$ assumed by Mori [in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner, Jr., J.-C. G. B\unzli, and V. K. Pecharsky (North-Holland, Amsterdam, 2008), Vol. 38, p. 105] is presented. We consider a phenomenological model of crystal-field interactions and introduce Heisenberg antiferromagnetic exchange interactions between the rare-earth ions on the ladder rungs and the indirect exchange fields generated by magnetic interactions along the ladder legs. The crystal-field parameters and the exchange integrals for the rare-earth dimers are determined from modeling the temperature dependences of the heat capacity and static magnetic susceptibility. The field dependence of the low-temperature magnetization of polycrystalline samples of borides $R{\mathrm{B}}_{50}$ is simulated by making use of the proposed empirical expression for the renormalized magnetic moments of rare-earth ions. The characteristic anomalies of the thermal and magnetic properties of $\mathrm{Tb}{\mathrm{B}}_{50}, \mathrm{Dy}{\mathrm{B}}_{50}, \mathrm{Ho}{\mathrm{B}}_{50}$ and $\mathrm{Er}{\mathrm{B}}_{50}$ are successfully reproduced within the framework of this approach.