Magnetic behaviour of Cu2FeGeSe4

Measurements of magnetic susceptibility χ as a function of temperature T and of magnetisation M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu2FeGeSe4. The χ versus T data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at ∼8 K, indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below ∼70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well fitted by a Langevin-type of equation, and the variation of the fitting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and effectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55x1018/cm3. The analysis gave a value of 213 μB for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 A