27AlNMR study in grain-alignedPrNiAl4

Abstract

${}^{27}\mathrm{Al}$ NMR results are presented in grain aligned ${\mathrm{PrNiAl}}_{4}$ in the temperature range 30--320 K. The Knight shift (K) and the spin-lattice relaxation rate ${(T}_{1}{)}^{\ensuremath{-}1}$ have been measured for the different aluminum sites present in the unit cell of the crystal. Temperature dependence of the $4f$ electron contribution to the Knight shift ${(K}_{f})$ clearly shows the development of ferromagnetic short-range correlations, below 150 K, within the Pr ions in chains along the crystallographic a axis. On the other hand, the variation of ${K}_{f}$ with the bulk susceptibility shows the onset of antiferromagnetic correlation between the two adjacent chains at near 85 K. The NMR results thus confirm the presence of two types of magnetic correlations which was earlier suggested from neutron scattering in this system. An almost temperature-independent behavior of ${(T}_{1f}{)}^{\ensuremath{-}1}$ in the range 85--320 K suggests that the fluctuation frequency of the local electronic moments $({\ensuremath{\omega}}_{e})$ is much greater than the ${}^{27}\mathrm{Al}$ nuclear resonance frequency. However, below this temperature range down to 30 K, a large enhancement of ${(T}_{1f}{)}^{\ensuremath{-}1}$ is observed suggesting the development of magnetic correlations. However, this data cannot distinguish the nature of the correlation as it was reflected in the Knight shift behavior. The temperature dependence of the spin-correlation rate of Pr local moments follows a temperature independent behavior down to 85 K, below this temperature it falls rapidly. This indicates the development of spin correlations at finite $\mathbf{q}$ below 85 K, leading to temperature dependent enhancement of spin-spin relaxation rate suggesting the critical slowing down of the electronic spin fluctuations.