Paramagnetism, superparamagnetism, and spin-glass behavior in bulk amorphous Pd–Ni–Fe–P alloys

We have investigated the magnetic properties of bulk amorphous Pd40Ni40−xFexP20 (x=0–17.5) alloys. For Pd40Ni40P20 (x=0), the magnetic susceptibility consists of temperature-independent and Curie–Weiss-type terms. Alloys with x⩾5 are paramagnetic at high temperatures. With decreasing temperature, the amorphous alloys become superparamagnetic. At even lower temperatures, and under a weak applied magnetic field, these alloys are spin glasses, as evidenced by static and dynamic magnetic measurements. The spin-freezing temperature increases with increasing iron content and this is attributed to the role of the Ruderman–Kittel–Kasuya–Yosida interaction in creating the spin-glass state. The occurrence of a reentrant spin-glass behavior on cooling (superparamagnetism-to-ferromagnetism-to-spin-glass transition) is also observed for x=17.5 at a field ⩾50 Oe. An unexpected result is that the ferromagnetic state in the present bulk metallic glasses is field induced. Evidence for the field-induced ferromagnetic-like order is obtained from (a) straight regions in the susceptibility versus temperature curves measured at various fields, (b) an Arrott plot, and (c) time-independent magnetization. With increasing applied field, the spin-freezing temperature decreases and the Curie temperature increases, broadening the temperature range of the field-induced ferromagnetic-like state. The temporal decay of the thermoremanent magnetization in the amorphous alloy with x=17.5 is slower than that in typical crystalline spin glasses. The spin-freezing temperature of the amorphous alloy with x=17.5 decreases approximately logarithmically with applied field, which differs from the prediction of Néel’s model for spin glasses.