Spin-glass-like Behavior, Random Magnetic Anisotropy and Low-temperature Specific Heat of Amorphous Er-Cu, Er-Ni and Gd-Al Alloys

Spin-glass-like behavior and the random magnetic anisotropy(RMA) for amorphous Er_xCu_ and Er_xNi_ alloys prepared by high-rate DC sputtering have been investigated. Their magnetic properties have been compared with those of amorphous Gd_xAl_ alloys without the random magnetic anisotropy. For these three amorphous alloy systems, the paramagnetic Curie temperature θ_p is positive, showing that the ferromagnetic exchange interactions are predominant, and they exhibit spin-glass-like behavior. The value of θ_p, the spin freezing temperature T_f, the local anisotropy constant D for the amorphous Er_xCu_ and Er_xNi_ alloys increase with the Er concentration. However, a steep decrease of θ_p occurs below about x=50 for the amorphous Er_xCu_, suggesting a development of some antiferromagnetic exchange interaction. The concentration dependences of T_f and D also show a remarkable change around x=50 % Er. This means that the RMA becomes weaker, whereas the competition of the exchange interaction becomes stronger with decreasing Er concentration. For the amorphous Er_xNi_ alloys, the competition of ferromagnetic and antiferromagnetic interactions arising from the exchange fluctuations is negligibly small in contrast to that in the amorphous Er_xCu_ alloys. The RMA suppresses the ferromagnetic coupling, resulting in a spin-glass-like state. The magnetization per Er atom for the amorphous Er_xCu_ alloys exhibits maximum around 20 kOe, but that for the amorphous Er_xNi_ alloys becomes smaller with increasing Er concentration due to the increase in the RMA. The magnetic anisotropy energy K for the amorphous Er_xCu_ and Er_xNi_ alloys is one or two orders of magnitude larger than that of the amorphous Gd_xAl_ alloys which have no RMA. It should be noted that the temperature at which the magnetic specific heat of the amorphous Er_Ni_ alloy reaches a maximum value coincides with the spin freezing temperature T_f determined by the AC magnetic susceptibility measurement. The crystal field splitting from J=15/2 into a Kramers doublet is considered to be caused by the RMA, accompanied by a Schottky-type specific heat with a linear temperature dependence. The magnetic entropy at T_f is estimated to be about 45 %?60 % of the value expected theoretically, being much larger than that for crystalline dilute spin-glass systems. Moreover, the plot of the magnetic specific heat versus T^ is linear at low temperatures. Therefore, it is considered that a ferromagnetic-like spin-waves is excited, although its magnetic structure is not ferromagnetic but speromagnetic