A nuclear magnetic resonance and magnetic susceptibility study of the Ca-Al metallic glass system and related crystalline compounds

Abstract

In order to investigate both the atomic and electronic structures, we have carried out steady state and pulsed NMR experiments on the melt spun metallic glass system Ca\sb{\rm 100-x}Al\sb{\rm x} (15 $\le$ x $\le$ 45) and related crystalline compounds. Measurements of the \sp{27}Al NMR Knight shift, spin lattice relaxation time and lineshape have been obtained at 4.2 \sp\circK and room temperature, and for frequencies ranging from 8 to 20 MHz. The experimental results indicate that the Knight shift (k = +0.038% $\pm$ 0.010% at room temperature), spin-lattice relaxation time (T\sb1 = 2.0 $\pm$ 0.2 sec. at 4.2 \sp\circK), and linewidth of the central transition remain constant throughout the entire glassy regime. Furthermore, the small value for the Knight shift and consequently, the long spin-lattice relaxation time, indicate that the local density of s-electron states at the Al sites is small. In addition, spin-echo NMR measurements indicate that the entire \sp{27}Al spectrum is quite broad ($\sim$2 MHz) due to the quadrupole interaction, and also, essentially the same for all compositions. All of the NMR results indicate that certain features of the local environment remain unchanged throughout the entire glassy regime. This has been taken as strong evidence of compositional short-range ordering in these amorphous alloys, contrary to the random packing assumed in the DRPHS model. The symmetry properties of the electric field gradient are compared with those for the related crystalline compounds which include CaAl\sb2 and Ca\sb3Al. This last compound has been found to have two sites for Al. The analysis of the magnetic susceptibility has been carried out by taking into account correlation effects. The magnetic susceptibility results indicate that the amorphous Ca-Al metallic glass system cannot be described simply in terms of a free electron model. This is consistent with the calculated values of the Korringa constant. The difference between the experimental and calculated susceptibilities indicates a small contribution due to the presence of d-like states in the vicinity of the Fermi level.