NMR features of a decagonalAl72.6Ni10.5Co16.9quasicrystal

Abstract

The nuclear magnetic resonance (NMR) spectra of a decagonal ${\mathrm{Al}}_{72.6}{\mathrm{Ni}}_{10.5}{\mathrm{Co}}_{16.9}$ single-grain quasicrystal (QC) were investigated by field- and frequency-sweep techniques in two magnetic fields and the $^{27}\mathrm{Al}$ relaxation rate was determined. The spectra are strongly inhomogeneously broadened by the electric quadrupole interaction, as evident from the magnetic-field independence of the width of the satellite part of the spectrum and the inverse-field dependence of the width of the central line. The temperature-independent $^{27}\mathrm{Al}$ isotropic Knight shift is anomalously low similarly to icosahedral Al-based QCs, whereas the line broadening due to anisotropic Knight shift is minute as compared to the quadrupolar broadening. The $^{59}\mathrm{Co}$ resonance was found centered at its Larmor frequency and its temperature-independent frequency shift demonstrates that cobalt is in a nonmagnetic state. The NMR spectra exhibit orientation dependence in a magnetic field. In the twofold rotation pattern, the satellite intensity shows a pronounced orientation-dependent shape and structure, whereas this was not detected in the tenfold pattern. The tenfold rotation pattern of the $^{27}\mathrm{Al}$ central line, obtained with the Fourier-transform technique, exhibits a tiny, but significant orientation dependence of the line shape and its center of gravity ${M}_{1}$ with the quasiperiodic symmetry $\ensuremath{\pi}∕5$. The orientation dependence is stronger for rotation about the twofold axis, where the rotation pattern exhibits a periodic symmetry $\ensuremath{\pi}$. We introduce a model that we name as ``stacked planar,'' which reproduces the orientation-dependent shape of the NMR spectra of the decagonal AlNiCo QC for rotation about the periodic axis, whereas---due to assumed amorphous order within the atomic planes---it does not yield orientation-dependent spectra for rotation about the quasiperiodic axis.