NMR features of a decagonal Al72.6Ni10.5Co16.9 quasicrystal

The nuclear magnetic resonance (NMR) spectra of a decagonal Al72.6Ni10.5Co16.9 single-grain quasicrystal (QC) were investigated by field- and frequency-sweep techniques in two magnetic fields and the Al-27 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 Al-27 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 Co-59 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 Al-27 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 pi/5. The orientation dependence is stronger for rotation about the twofold axis, where the rotation pattern exhibits a periodic symmetry 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.