Effective-Mass Splitting in a Magnetized Metal

The (density-of-states) effective masses at the two Fermi surfaces of a magnetized, free-electron metal are calculated as a function of fractional spin polarization $P$. Coulomb interactions cause the minority-spin mass to exceed the majority-spin mass. The effective-mass splitting is linear in $P$, for small $P$, and approaches 20% at $P=1\mathrm{}$. It is concluded that the smallness of exchange and correlation corrections to $m^{*}$ for an ideal metal is accidental. For real band structures they may be of greater significance.