Magnetoelastic coupling at Fe2+ions in antiferromagnetic CoCl2

Abstract

The57Fe Mossbauer spectrum of a single crystal of (Fe0.002Co0.998)Cl2at 4.2K has been recorded and analysed in terms of the effective-field approximation to the hyperfine interactions. At this temperature, CoCl2(space group R3m) is antiferromagnetically ordered, with moments aligned in the a-b plane of the hexagonal unit cell. It was found that the Mossbauer spectrum, which indicated both magnetic dipole and electric quadrupole splitting, could not be adequately explained using a static crystal-field model (a 'rigid-crystal' model) for the wavefunctions of the Fe2+ion in a site of 3m (D3d) symmetry. In order to obtain a satisfactory interpretation of the data it was necessary to allow the orbit-lattice interaction to generate symmetry-breaking strains at the Fe2+site as a result of the transverse exchange interaction. The electric field gradient asymmetry parameter eta proved to be a sensitive indicator of the presence of such strains. The theory of calculating these strains is outlined. The magnitudes of the strains required to satisfy the Mossbauer data are consistent with the calculations, although estimates of the values of the orbit-lattice coupling parameters are very approximate. These results are analogous to the magnetoelastic effects produced in the uniaxial antiferromagnet FeCl2by a transverse applied field, and they indicate the possible observability of magnetoelastic effects in susceptibility and Mossbauer spectra of non-magnetic crystals of appropriate symmetry.