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

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_f. In the M versus T curves, it is indicated that the magnetization starts to decrease at T_g 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_Fe_ alloy. 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 to the 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 diffraction analysis. A large thermal expansion anomaly has been observed in amorphous Y_Fe_ and Y_Fe_ alloys over a wide temperature range from the spin-glass to the paramagnetic state. This anomaly fades away around 450 K, implying that the 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