Doctoral Dissertations

Title

Nuclear magnetic resonance and magnetization studies of rapidly quenched aluminum-based alloys

January 1988

Keywords

Physics, Condensed Matter

Ph.D.

Abstract

This work presents and discusses nuclear magnetic resonance (NMR) spectra and magnetic measurements of rapidly quenched alloys of La$\sb{100-x}$Al$\sb{x}$ (18 $\leq$ x $\leq$ 45), Al$\sb{100-x}$Mn$\sb{x}$ (17 $\leq$ x $\leq$ 22) and Al$\sb{86}$Ru$\sb{14}$, as well as annealed alloys of Al$\sb{83}$Mn$\sb{17}$ and Al$\sb{86}$Ru$\sb{14}$. The La-Al alloys are in the form of metallic glasses while the rapidly quenched Al-Mn and Al-Ru alloys exhibit phases having non-crystallographic rotational symmetry (icosahedral phase and decagonal phase).^ In order to investigate the electronic and atomic structures of the La-Al metallic glass system, measurements of the magnetic susceptibility have been carried out at room temperature for fields up to 20 kOe as well as the $\sp{27}$Al NMR Knight shift and linewidth for resonance frequencies ranging from 8 to 20 MHz. Concerning the electronic structure, a calculation of the conduction electron Pauli spin contribution to the susceptibility, assuming the free electron model, yields results which are too small to account for the observed susceptibility values. This indicates that there is a significant d-electron contribution to the density of states. The dependence of both the $\sp{27}$Al Knight shift and linewidth on the Al concentration shows a change in behavior at x = 25, which corresponds to the equilibrium crystalline phase, La$\sb3$Al. This correlates with similar behavior in other properties such as the density and magnetic susceptibility, and is discussed in terms of the atomic structure.^ In order to investigate the atomic packing, chemical bonding and electronic band structure in the Al-Mn and Al-Ru alloys, we carried out both pulse and continuous wave measurements for resonance frequencies ranging from 6 MHz to 20 MHz and for temperatures from 4.2 $\sp\circ$K to 300$\sp\circ$K, as well as magnetization measurements over a temperature range from 2.5$\sp\circ$K to 300$\sp\circ$K and for magnetic fields up to 20 kOe. For the icosahedral and decagonal phases, the $\sp{27}$Al spectra are characterized by a distribution in the quadrupole interaction, with none of the spectral features associated with the related crystalline phases. The magnetic behavior of both phases in Al-Mn alloys is characterized by a small concentration of large moment "clusters" and spin-glass-like ordering, while the icosahedral phase in the Al-Ru alloy shows very small susceptibility and no localized moments over the entire temperature range. ^

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