Crystal-field approach to rare-earth higher borides: Dimerization, thermal, and magnetic properties of R B50 (R=Tb,Dy,Ho,Er,Tm) CRYSTAL-FIELD APPROACH to RARE-EARTH HIGHER . B. Z. MALKIN, S. L. BUD'KO, and V. V. NOVIKOV

The microscopic justification for the dimerization in ladder-type rare-earth higher borides RB50 assumed by Mori [in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner, Jr., J.-C. G. Bünzli, 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 RB50 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 TbB50, DyB50, HoB50 and ErB50 are successfully reproduced within the framework of this approach.