Magnetic properties of random-anisotropy amorphous magnets (R_xFe_1-x)₈₀Si₁₂B₈ with R = Pr, Nd, Sm, Tb, Dy and Er

Magnetic properties of rapidly quenched amorphous alloys (R_xFe_1-x)₈₀Si₁₂B₈ with R = Pr, Nd, Sm, Tb, Dy and Er (0.1 ≤ x ≤ 1.0) have been investigated systematically by Mössbauer absorption and magnetization measurements in fields of up to 300 kOe. Mössbauer experiments have shown that the averaged Fe magnetic moment decreases rapidly with increasing x. Low-field magnetic measurements have exhibited a spin-freezing behavior at low temperatures in all R-rich samples. The freezing temperature T_f rapidly decreases with increasing magnitude of magnetic field H. High-field magnetization isotherms at low temperatures showed a large hysteresis, in which the coercive field H_c increases rapidly with decreasing temperature. In H-T space, the lines of H_c(T) and T_f(H) are found to be almost identical, which have therefore been regarded as a boundary between reversible and irreversible regions in the magnetic phase diagram. Upward concavity in the high-field magnetization curve has been observed in R = Tb and Dy samples with x = 0.2, which was suggested to be an incipient stage of a transition from sperimagnetic to asperomagnetic state, based on the numerical calculations.