Neutron scattering study of the spin ordering in amorphous Tb45Fe55 and Tb25Fe75

Abstract

Small-angle neutron scattering measurements (SANS) have been made on Tb45Fe55 and Tb25Fe75 as a function of temperature. The SANS results show that long-range ferromagnetic order is quenched in these alloys and is replaced by a spin-glass-like state. For T>Tc, where Tc is the transition temperature, both samples exhibit a conventional Lorentzian line shape (in q), but they depart from this form below Tc. We have fitted the low-temperature line shapes with the Lorentzian plus Lorentzian-squared form appropriate for random field systems. As the temperature of the Tb45Fe55 alloy is lowered, the correlation length ξ rises to a rounded maximum of about 80 Å at 250 K (Tc =298 K) and decreases to about 60 Å at 5 K. In contrast, ξ in the Tb25Fe75 alloy exhibits a cusplike form reaching a resolution limited value between 400 and 410 K before declining slowly to 55 Å at low T. In both alloys the coefficient of the Lorentzian term rises sharply as T→0, whereas the Lorentzian-squared coefficient follows approximately the square of the order parameter divided by the correlation length. Both systems thus lend support to the suppression of long-range order by the random anisotropy field.