NMR, magnetism and Mössbauer effect in icosahedral Al 63Cu 24.5Fe 12.5 — a thermodynamically stable quasi-periodic alloy

Abstract

We report results of 27Al nuclear magnetic resonance (NMR), magnetism and iron-site Mössbauer experiments on the thermodynamically stable and “perfectly” quasicrystalline icosahedral alloy Al 63Cu 24.5Fe 12.5. NMR experiments were performed at 11.10, 17.8 and 45.7 MHz and at temperatures as low as 50 K. Magnetization was measured at 295, 100 and 5 K, while iron site Mössbauer was measured at 295 and 4.2 K. We find very small NMR Knight shifts and long relaxation times that we interpret as consistent with a pseudo-gap in the density of states near the Fermi level. NMR line shapes in AlCuFe do not show quadrupolar structure consistent with a single aluminum site. We report results of numerical simulations that effectively reproduce the features and trends in observed line shapes by means of a broad distribution of electric field gradients (EFGs) at aluminum sites. Our magnetization and Mössbauer effect experiments show that there is a very small fraction of the material that is magnetically ordered at low temperatures. This magnetic behavior was only observed well below the lowest temperature of NMR experiments and cannot be responsible for the broad NMR lines. Iron-site Mössbauer lines show significant broadening characteristic of a distribution of EFGs that is qualitatively similar to that indicated for the aluminum site.