Concentrated Spin-Glass Behavior and Thermal Expansion Anomaly in Amorphous Y-Fe Alloys

摘要

Magnetic field and annealing effects on the spin-glass state in amorphous Y-Fe alloys have been investigated. In low magnetic fields, the differential magnetic susceptibility dM/dH versus T curves exhibit the peaks, indicating that the spin-glass state occurs through two steps. These two freezing temperatures are defined from these curves, i. e. the higher temperature is T_g and the lower one is T_g. In the M versus T curves, it is indicated that the magnetization starts to decrease at T and a thermal irreversibility appears below T_f with decreasing temperature. With the increase in the applied magnetic field, the third peak appears around at 150 K in the dM/dH versus T curve for amorphous Y_(16)Fe_(84) Hoy. The position of this peak corresponds to the upper inflection point in the M versus T curve is defined as the Curie temperature T_c. Therefore, re-entrant behavior appears in this system by applying the magnetic field. The concentration dependences of T_c and T_f for in 500 Oe are very similar to those obtained in zero field for other amorphous RE-Fe alloys (RE : rare earth metal) such as Lu-Fe and Ce-Fe systems. Amorphous Y-Fe alloys annealed at 523 K for 30 min exhibit re-entrant spin-glass behavior in a similar manner she as-prepared samples in an applied magnetic field. The Curie temperature is enhanced but the spin freezing temperatures T_f is depressed by annealing. These facts suggest that the Fe-Fe interatomic distance which significantly influences the magnetic properties is increased by annealing. Actually, the increases in the atomic distance of Fe-Fe pairs and in the coordination number of the nearest neighbor Fe atoms by annealing have been confirmed by X-ray [fraction analysis. A large thermal expansion anomaly has been observed in amorphous Y_(7.5)Fe_(92.5) and Y_(16)Fe_(84) alloys over a wide temperature range from the spin-glass to the paramagnetic state. This anomaly fades away around 450 K, implying that c magnetic excitation to small amplitudes of local moments is saturated above this temperature. The inverse magnetic susceptibility influenced by the excitation becomes convex upwards up to around 450 K and varies linearly above this temperature obeying. a new Curie-Weiss law.