Neutron scattering studies of the anomalous magnetic alloyFe0.7Al0.3

Abstract

Small-angle total and inelastic neutron scattering measurements were performed on a single crystal of the ordered alloy ${\mathrm{Fe}}_{0.7}$${\mathrm{Al}}_{0.3}$. The behavior of the $Q$ dependence of the small-angle intensity can be classified into four temperature regions which correspond to the bulk behavior: (i) $Tg{T}_{c}=510$ K (paramagnetic region); the line shape is Lorentzian with $\ensuremath{\kappa}\ensuremath{\rightarrow}0$ as $Tg{T}_{c}$. (ii) $300 \mathrm{K}lTl{T}_{c}$ (ferromagnetic region); $I(Q)\ensuremath{\sim}{Q}^{\ensuremath{-}2}$ as expected from spin waves. (iii) $100 \mathrm{K}lTl300 \mathrm{K}$ (near ${T}_{c}^{\mathrm{inv}}$); $I(Q)\ensuremath{\sim}{Q}^{\ensuremath{-}\ensuremath{\alpha}}$, with $\ensuremath{\alpha}$ being $T$ dependent and reaching a maximum value $\ensuremath{\alpha}=2.6$. In this region the $T$ dependence of the intensity exhibits a peak which is $Q$ dependent. Near ${T}_{c}^{\mathrm{inv}}\ensuremath{\approx}160$ K, a thermal hysteresis and novel time dependence of the scattering is observed. (iv) $Tl100$ K (spin-glass regime); the line shape is again Lorentzian with $\ensuremath{\kappa}$ increasing with decreasing $T$. No anomaly is observed at ${T}_{f}\ensuremath{\approx}90$ K. The inelastic measurements reveal that spin waves exist in the ferromagnetic regime but disappear as the temperature is lowered. For temperatures less than ${T}_{c}^{\mathrm{inv}}$, an elastic central peak appears which increases as $T$ is decreased. The results are interpreted in terms of random-field effects.