Nuclear Resonance and Magnetic Ordering an a Random Spin System: (LaGd) Al2

Abstract

The results of Al27 zero-field nuclear quadrupole resonance (NQR) and spin-lattice relaxation measurements in the (LaGd)Al2 magnetic alloy system are examined for features dependent on the spatial disorder of the Gd impurity spin system. Persistence of the NQR signal. well below the magnetic ordering temperature TO indicates that the Al27 static hyperfine field is zero at an appreciable fraction of Al sites. This result is attributed to inhomogeneity in the Gd spontaneous magnetization. No clear evidence was obtained for critical-point behavior of the relaxation time near TO. The rapid relaxation observed below TO indicates the presence of a large fluctuating A127 hyperfine field. Thus the observed nuclei must be near regions of large Gd spin correlation, which in turn suggests that the Gd magnetization in-homogeneities are microscopic in size. For T >> TO the relaxation times, appropriately scaled, are comparable to those observed in the intermetallic compound PrA12 in both magnitude and temperature dependence. Hence any major effect of spatial disorder on the spin system dynamics disappears in this temperature range, and the temperature-dependent contribution to relaxation in both random and periodic spin systems is dominated by short-range interspin correlation.