Free-electron-like Hall effect and deviations from free-electron behavior in Ca-Al amorphous alloys

Abstract

The Hall coefficients of Ca-Al amorphous alloys have been measured at 4.2 K over a wide range of compositions. It is shown that the magnitude of the Hall coefficients are close to the nearly-free-electron (NFE) prediction for low Ca concentrations but deviate significantly from the NFE values for Ca concentration greater than 45 at. %. The deviations from the free-electron values have previously been attributed to the effects of s-d hybridization, while a reduction in magnitude by Au doping has been argued to result from the side-jump effect.The Hall coefficient has been investigated in quite a wide range of amorphous alloys' and can deviate significantly from the nearly-free-electron (NFE)-like behavior. Indeed, in some transition-metal-based alloys it can even be positive. The origin of these deviations in the Hall coefficient still remains a controversy. There are two effects that may inhuence the magnitude and sign of the Hall coefficient: s-d hybridization and the side-jump mechanism.Both the magnitude and sign of the Lorentz force contribution are affected by the s dhybridi-zation depending on the position of the Fermi energy relative to the d band. The side-jump effect leads to an extra contribution to the Hall effect arising from the spinorbit interaction which again can be positive or negative depending on the position of the Fermi level in the d band. Ca-Al is a simple-metal-based amorphous alloy. Despite this, all previous measurements have shown significant deviations from the NFE-like behavior one might expect in a simple-metal alloy. ' For all these measurements the concentration of Ca has been greater than -SO at. %. In our previous paper we argued that the deviations from nearly-free-electron-like behavior were the result of the Fermi level being pushed into the tail of Ca d band by Al increasing the effects of s-d hybridization. The side-jump contribution in this system is enhanced by increasing the strength of spin-orbit scattering and we show this using Au doping. Both effects of s-d hybridization and the side-jump mechanism are expected to lead to a Hall coefficient that is not freeelectron like. In this paper, we present an extension of the Hall-coefficient measurements of Ca-Al over a wider range of compositions and show that for concentration of Ca less than 45 at. %, the NFE model adequately describes the magnitude of the Hall coefficient. The measurements were made at 4.2 K on sputtered samples in magnetic fields up to 7 T. Experimental details have been outlined by Mayeya and Howson.Figure 1 shows the Hall coefficients of Ca-Al as a function of Ca concentration and the NFE calculation made by assuming a contribution of three free electrons per atom from Al and two from Ca. The Hall coefficients are negative over the entire composition range. Their magni-2.0x10 ' I Ĩ 1.0% Au 1.5x10 ' O E 1.0x10" z CC 0.5x10 '0 0 I 20 40 %Ca I 60 I 80 100 FIG. 1. Hall coefficients of Ca-Al amorphous alloys as a function of Ca concentration.tudes however differ significantly for low and high Ca content samp...