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

摘要

Spin-glass-like behavior and the random magnetic anisotropy(RMA) for amorphous Er_xCu_(100-x) and Er_xCu_(100-x) alloys prepared by high-rate DC sputtering have been investigated. Their magnetic properties have been compared with those of amorphous Gd_xAl_(100-x) alloys without the random magnetic anisotropy. For these three amorphous alloy systems, the paramagnetic Curie temperature theta_p is positive, showing that the ferromagnetic exchange interactions are predominant, and they exhibit spin-glass-like behavior. The value of theta_p, the spin freezing temperature T_f, the local anisotropy constant D for the amorphous Er_xCu_(100-x) and Er_xCu_(100-x) alloys increase with the Er concentration. However, a steep decrease of theta_p occurs below about x = 50 for the amorphous Er_xCu_(100-x), suggesting a development of some antiferromagnetic exchange interaction. The concentration dependences of T_f and D also show a remarkable change around x = 50 percent 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_xCu_(100-x) 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_(100-x) alloys. The RMA suppresses the ferromagnetic coupling, resulting in a spin-glass-like state. The magnetization per Er atom for the amorphous Er_xCu_(100-x) alloys exhibits maximum around 20 kOe, but that for the amorphous Er_xCu_(100-x) alloys becomes smaller with increasing Er concentration due to the invtrsdr in the RMA. The magnetic anisotropy energy K for the amorphous Er_xCu_(100-x) and Er_xCu_(100-x) alloys is alloys is one or two orders of magnitude larger than that of the amorphous Gd_xAl_(100-x) alloys which have no RMA. It should be noted that the temperature at which the magnetic specific heat of the amorphous Er_(33)Ni_(67) 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=l5/2 into a Kramers doublet is considered to AC 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 percent approx 60 percent of the value expected theoretically, being much larger than that for crystalline dilute spin-glass systems. Moreover, the plot of the magnetic specific neat heat versus T~(3/2) 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.