Electric quadrupole effects and nuclear spin relaxation in aAl72.4Pd20.5Mn7.1icosahedral quasicrystal studied by NMR

Abstract

The ${}^{27}\mathrm{Al}\mathrm{}\mathrm{NMR}$ spectrum and the spin-lattice relaxation rate of an icosahedral ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ quasicrystal were studied in the temperature range from 300 to 4 K at two magnetic fields. The results are compatible with the hypothesis that a screening electron cloud is formed around the Mn ions in an aluminum-rich environment, which together with the core electrons produces a large electric-field gradient (EFG) at the sites of the Al nuclei. The EFG is temperature dependent due to the polarization of the screening cloud and the core electrons by the magnetic moments of the Mn unpaired d electrons. The shift of the ${}^{27}\mathrm{Al}$ central line shows an inverse magnetic-field dependence and appears to be of an electric quadrupolar origin. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation rate shows an ${aT+bT}^{3}$ temperature dependence that can be explained by approximating the quasicrystalline electronic structure by a two-band model, consisting of a broad s and a narrow d conduction bands. The ${T}^{3}$ term originates from the narrowness of the d band. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation in ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ is governed by two relaxation mechanisms of similar strength, the electric quadrupole and the magnetic hyperfine. The quadrupole mechanism is stronger at high temperatures such as room temperature whereas the magnetic relaxation dominates at low temperatures.