NMR in CeAl2. Hyperfine fields, fluctuation rates, and spatial correlation in an unstable-moment system

Abstract

$^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements in the antiferromagnetic and paramagnetic states of the intermetallic compound Ce${\mathrm{Al}}_{2}$ have yielded information on the competition between magnetic ordering and moment instability in this system. In the antiferromagnetic state below ${T}_{N}=3.9$ K the zero-field NQR linewidth increases from 12(2) kHz above ${T}_{N}$ to 90(10) kHz at $1.5 \mathrm{K}=0.4{T}_{N}$. This increase is consistent with the onset of a distribution of hyperfine fields in the spatially modulated antiferromagnetic structure suggested on the basis of neutron scattering studies, but the width of the hyperfine field distribution is more than a factor of 10 smaller than consistent with either the "single-$q$" modulated structure of Barbara and co-workers or low-temperature nuclear hyperfine specific-heat measurements. It is suggested that the large fraction of nuclei in small hyperfine fields is a consequence of moment modulation in more than one direction. Observed $^{27}\mathrm{Al}$ spin-lattice relaxation rates below $\ensuremath{\sim}0.65{T}_{N}$ were spatially distributed in the specimen: both "hard," magnonlike fluctuation modes and "soft" fluctuations, presumably associated with Kondo-like states in nodes of the modulated structure, were observed. The activation energy of 0.87(8) meV for the magnonlike modes is in good agreement with neutron results. In the paramagnetic state the ratio of the NMR shift to the bulk susceptibility varies with temperature in a way which is consistent with a simple model of anisotropic hyperfine coupling to the crystal-field-split Ce ionic states. The spin-lattice relaxation behavior above ${T}_{N}$ indicates the onset of spatial short-range order at temperatures below \ensuremath{\sim} 100 K, but the nature of this order is uncertain due to difficulties in reconciling the NMR and neutron scattering data. If such short-range order is assumed to be absent at 300 K, comparison of NMR and neutron results indicates that effectively \ensuremath{\sim} 10 Ce neighbors are hyperfine coupled to a given $^{27}\mathrm{Al}$ nucleus.