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

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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.