Magnetic properties of Ge_1−xMn_xTe ferromagnetic semiconductors doped with gadolinium

We report on magnetic measurements and X-band electron paramagnetic resonance for Ge_1-x-yMn_xGd_yTe (x=0.15-0.40, y=0, 0.02) single crystals. Ferromagnetic behavior was observed with the Curie temperature T_C=70-90 K depending on the alloy composition. Effective magnetic moment p_eff per Mn ion estimated from the Curie constant increases with increasing x from 2.8 up to 4.3 μ_B, while saturation magnetization per magnetic ion at 4.2 K gives 1.7-3.4 μ_B. EPR spectra at low temperatures are described by Dysonian type line, which splits into two lines at T ≈ 150 K. At temperature lower this value we observe distinct anomalies on temperature dependencies of the linewidths and g-factors. In paramagnetic region anomalous broadening of the linewidth with decreasing the temperature is revealed, while the g-factor is practically independent both on temperature and alloy composition.