Two Hall Effects of Iron-Cobalt Alloys

The Hall coefficients and the resistivity of Fe-Co alloys have been measured at 77°K, 169°K, and room temperature using fields up to 3.3 webers/${\mathrm{m}}^2$. For up to 0.2% Co in Fe the ordinary Hall coefficient $R_0$ is positive at room temperature but becomes negative at low temperatures; for all other compositions $R_0$ is negative. Analysis with a simple model in which the ($4s$) conduction band consists of a parallel and an antiparallel sub-band shows that, for up to 20% Fe in Co, the parallel $3d$ sub-band is filled. It then empties as the Fe content increases, having about 0.2 holes per atom at 50% Fe, but at 65% Fe the bands apparently shift so that again the parallel $3d$ sub-band is filled. These conclusions remain unchanged if the analysis is based instead upon a model proposed recently in which the $3d$ electrons may be in nonconducting states as well as in the usual conducting-type states. The extraordinary Hall coefficient $R_1$ is positive for less than 25% Co in Fe, but for all other compositions it changes from positive to negative as the temperature decreases from 300°K to 169°K. $R_1$ and the resistivity $\rho$ satisfy the relation $R_1=a+b{\rho }^2$.