Finite-temperature magnetism of disordered Fe-Co alloys

The magnetic properties of disordered Fe-Co alloys have been investigated for both bcc and fcc structures using the finite-temperature theory of local-environment effects. The calculated magnetization, Curie temperature, effective Bohr magneton number, Weiss constant, and high-field susceptibility are shown to explain the concentration dependence of the experimental data. It is found that fcc Fe-Co alloys show ferromagnetic instability at 50 at. % Fe and form a spin-glass state after the disappearance of ferromagnetism at $c^{\mathrm{*}}$=78 at. % Fe. This is caused by the nonlinear magnetic coupling between Fe local moments and the local-environment effect on the amplitudes of Fe local moments. Strong local-environment effects around $c^{\mathrm{*}}$ are examined in detail with a prediction of the internal-field distribution for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$.