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

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) $T>\mathrm{}{T}_c=510\mathrm{}$ K (paramagnetic region); the line shape is Lorentzian with $\kappa \to 0$ as $T>\mathrm{}{T}_c$. (ii) $300 \mathrm{K}<T<\mathrm{}{T}_c$ (ferromagnetic region); $I\left(Q\right)\mathrm{}\sim Q^{-2\mathrm{}}$ as expected from spin waves. (iii) $100 \mathrm{K}<T<\mathrm{}300\mathrm{} \mathrm{K}$ (near $T_c^{\mathrm{inv}}$); $I\left(Q\right)\mathrm{}\sim Q^{-\alpha }$, with $\alpha$ being $T$ dependent and reaching a maximum value $\alpha =2.6\mathrm{}$. In this region the $T$ dependence of the intensity exhibits a peak which is $Q$ dependent. Near $T_c^{\mathrm{inv}}\approx 160$ K, a thermal hysteresis and novel time dependence of the scattering is observed. (iv) $T<\mathrm{}100\mathrm{}$ K (spin-glass regime); the line shape is again Lorentzian with $\kappa$ increasing with decreasing $T$. No anomaly is observed at $T_f\approx 90$ 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.