Nuclear magnetic resonance and unstable rare-earth magnetism inCeAl3

Abstract

${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ nuclear magnetic resonance (NMR) has been studied in the unstable-moment rare-earth (RE) compound ${\mathrm{CeAl}}_3$ to obtain information on local magnetic behavior. The experiments were carried out at temperatures well above a characteristic temperature $T_{\mathrm{ch}\mathrm{\sim }0.5}$ K, below which the system can be described as a degenerate Fermi fluid. A change of slope in the relation between the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ isotropic frequency shift $K_i$ and the bulk susceptibility χ is found below ∼10 K, and is attributed to a temperature-dependent transferred hyperfine field in this temperature range. This hyperfine-field anomaly is probably not the same as that previously noted at T≃$T_{\mathrm{ch}}$ in other RE compounds (${\mathrm{CeSn}}_3$, YbCuAl), where the moment instability is associated with intermediate valence. The temperature range is more nearly characteristic of crystalline electric field (CEF) splittings. In ${\mathrm{CeAl}}_2$ an anisotropic hyperfine interaction in the presence of CEF splitting has been invoked to explain a similar shift anomaly. The temperature dependence of the effective Ce-spin fluctuation rate, obtained from measured spin-lattice relaxation rates 1/$T_1$, indicates the onset of near-neighbor spatial correlations between dynamic Ce-spin fluctuations at low temperatures, but the nature of these correlations cannot be elucidated from NMR data alone. An effective near-neighbor number $n_{\mathrm{eff}=7\mathrm{}\pm 2}$ is obtained at 300 K. The Korringa product ($K_i^2$ $T_1$T${)}_{4f}$ is not strongly enhanced at low temperatures, which is strong evidence against the applicability of a paramagnonlike model to explain the paramagnetism of this compound.