Magnetic Field Dependent Thermal Conductivity in FeCl₂ and FeBr2

Abstract

FeCl₂ and FeBr₂ are two antiferromagnetic insulators whose magnetic properties have been widely investigated these last years (1)–(6). Besides, other properties, such as anomalous thermal conductivity (7), pressure (4)–(5) and strain (6) dependence of magnetic properties and change of the lattice constants at the Neel temperature (8), have suggested the presence of a large magnetoelastic interaction in these two compounds. A first measurement of the thermal conductivity in FeCl₂ (7) has revealed a pronounced dip in the thermal conductivity, in a wide temperature range, the minimum being at 1 7 K, a value lower than the Neel temperature. In a previous paper (9), we showed that this anomalous behaviour could be accounted for by making the assumption of a resonant interaction between magnons and phonons. We now report measurements of the thermal conductivity in magnetic field of FeCl₂ and FeBr₂. The measurements were made using a steady state heat-flow method in a pumped helium cryostat between 1 K and 50 K. Magnetic fields up to 75 kOe were applied with a superconducting solenoid; the sample being rotated in the cryostat so as to apply the field either parallel or perpendicular to the c axis.