Local Magnetic Fields in the Vanadium-Manganese Alloy System

A study of the nuclear magnetic resonance (NMR) line shapes, Knight shifts, and linewidths of both ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ and ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ has been made between 1.4 and 300°K in the vanadium-manganese alloy system, and is complemented by measurements of the magnetic susceptibility. The concentration dependence of the magnetic susceptibility shows a gradual decrease and is temperature independent to 30% Mn in V, followed by a temperature-dependent increase. Both V and Mn Knight shifts decrease rapidly above 40% Mn, but without most of the temperature dependence displayed by the susceptibility. Both shifts and susceptibility are analyzed in terms of the details of the $3d$-band structure in this and related alloy systems: A relatively flat $4s$ band is superimposed on two $3d$ subbands whose position is such that the Fermi level falls between their peaks in alloys at 45% Mn in V. In a more detailed consideration of the susceptibility, and both Knight shifts, it is necessary to depart from the rigid-band model by taking into account solute-solvent local electron density differences. The much smaller temperature dependence of the observed NMR parameters, when compared to that of the susceptibility above 45% Mn, is speculatively explained by the magnetization of Mn atoms determined by local Mn concentration, in reasonable agreement with the results. A Curie-Weiss component of the susceptibility above 40% Mn may have its origin in an itinerant magnetism based on the Anderson-Wolff band model as elaborated on by Schrieffer and co-workers. Strong field-dependent increases in the susceptibility below 85°K are observed in alloys above 35% Mn, but the modest observed changes in the NMR parameters through this transition suggest that these magnetic phenomena originate in a small fraction of the sample volume. The effects of the nuclear quadrupole interaction, and of Knight-shift in-homogeneity, on the ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ NMR are discussed in detail. The NMR in three $\sigma$-phase samples was observable for both nuclear species down to 1.4°K, and the susceptibility is only moderately temperature dependent; no evidence of magnetic ordering was found.