Nuclear Spin-Lattice Relaxation in theLa1−cGdcAl2Intermetallic Compounds

Abstract

The contribution of Gd impurities to the spin-lattice relaxation of ${\mathrm{La}}^{139}$ and ${\mathrm{Al}}^{27}$ nuclei in the compound ${\mathrm{La}}_{1-c}{\mathrm{Gd}}_c{\mathrm{Al}}_2$ is studied for values of $c$ from 0.0013 to 0.10, applied magnetic fields between 3.6 and 24.4 kOe, and temperatures from 1.2 to 300 K. The impurity component of the magnetization recovery for ${\mathrm{Al}}^{27}$ is characterized by an $\exp [-{\left(\frac{t}{{\tau }_1}\right)}^{\frac{1}{2}}]$ time dependence, indicating diffusionless relaxation of the nuclei to the impurity. The recovery rate $\frac{1}{{\tau }_1}$ has a temperature and magnetic field dependence described by the derivative of the Brillouin function, and is found to be proportional to the impurity concentration. Longitudinal dipolar fluctuations of the Gd moments are the principal source of the impurity relaxation, and these fluctuations result from a spin-spin interaction between impurities. An analysis in the free-electron approximation indicates that a Ruderman-Kittel-Kasuya-Yosida coupling is the source of this interaction and yields a value of $J_{\mathrm{sf}}\simeq 0.09$ eV for the strength of the conduction-electron-rare-earth coupling constant. A comparison is made with previous magnetization, susceptibility, and paramagnetic-resonance studies in the same system.