MAGNETIC ANISOTROPY IN THE Fe-BORACITES

Abstract

Abstract.-Using the Mossbauer technique we have investigated the electric and magnetic hyperfine interaction in Fe-boracites Fe3B7013X (X = C1, Br, I). The analysis of the Mossbauer spectra below the NBel temperature leads to 3 lattice sites in the case of Fe-CI-and Fe-Br-boracite and to at least 4 sites in the Fe-I-boracite, which are different with regard to the sizes of the magnetic hyperfine fields and their directions with respect to the electric field gradient tensor. The Nkel temperature turned out to be 33.2 + 0.2 K and 11.8 & 0.2 K for the Fe-I-and Fe-CI-boracite, respectively.The experimental values for the hyperfine fields are compared with calculations based on Ligand Field Theory including spin orbit coupling and molecular field approach. Within this model the strong orbital contribution to the hyperfine field and the magnetic anisotropy can be explained. Introduction.-Recently much attention has been paid to the compound boracite M3B7OI3X, where M stands for a divalent metal ion and X for one of the halogens C1, Br or I. These boracites have nearly the same crystal structure as the natural boracite Mg3B70,,CI [I, 2, 31. The crystal structure is orthorhombic below and cubic above the ferroelectric Curie Temperature. The existence of ferroelectricity in the orthorhombic structure is a characteristic property of all boracite. Mossbauer measurements on iron boracite by J. M. Trooster [4] showed a further phase transition from orthorhombic to trigonal structure.The magnetic properties of the 3d-metal boracites have been investigated by measuring the susceptibility [5, 61. It turned out that the boracites show antiferromagnetic ordering at low temperatures in the ferroelectric phase. The antiferromagnetism is accompanied by a weak ferromagnetism.Recently Mijssbauer measurements on Fe-Br-boracite in the magnetically ordered phase have been published by R. Jagannathan et al. 1111. It was the aim of these Mossbauer measurements to investigate the hyperfine fields at the "Fe nucleus in all three ironboracites below the NBel-temperature.