First-Order Magnetic Phase Transitions in Fe

The first observation of a first-order phase transition in the magnetization of Fe in an applied field is reported. The samples were epitaxially grown (110) Fe films on GaAs. Mean-field theory, with the inclusion of uniaxial anisotropy and the constraint that $\overrightarrow{\mathrm{M}}$ lie in the plane of the film, gives excellent agreement with the observed transition. The relationship of this transition to the predictions of the three-state Potts model for cubic ferromagnets is discussed.