Inhomogeneous magnetism in decagonal Al$_{69.8}$Pd$_{12.1}$Mn$_{18.1}$

Abstract

We report the results of measurements of $^{27}$Al and $^{55}$Mn NMR spectra, the related spin-lattice relaxation rates, and the dc magnetic susceptibility of the stable decagonal quasicrystal Al$_{69.8}$Pd$_{12.1}$Mn$_{18.2}$. The temperature-variation of the magnetic susceptibility $\chi(T)$ reveals that the Mn ions carry only an average magnetic moment of approximately $2\mu_{\mathrm{B}}$, and confirms a spin-glass type freezing of the Mn-moments at $T_f=12$K. The NMR spectra reveal two partially resolved lines for the $^{27}$Al-nuclei, indicating that there are two different sets of environments for the Al-sites. The integrated intensity of the Mn line in the spectra suggests that about half of the Mn ions carry no magnetic moment. Below 50K, and upon decreasing the temperature, the $^{27}$Al NMR-linewidth $w$ and the spin-lattice relaxation rate $T_1^{-1}$ both grow with an increasingly negative slope, as it is often observed and interpreted as a critical "slowing down" of magnetic moments in systems approaching a spin-glass transition. Various features, such as a broad maximum in $T_1^{-1}(T)$ and a slope change in the $w(\chi)$ plot, both around 120K, suggest a gradual reduction of the number of Mn moments with decreasing temperature below 120K.