Second- and first-order phase transitions in the magnetic reorientation of ultrathin Fe on Gd

Ultrathin Fe films grown on the Gd(0001) surface exhibit a magnetic in-plane to out-of-plane spin-reorientation transition with increasing temperature. We provide evidence that this reorientation transition is accomplished in two steps. The first step, at low temperature, is a continuous reorientation of the surface moment from in-plane to canted out-of-plane with a corresponding peak in the susceptibility. The second step is a thermally irreversible rotation from this canted direction to perpendicular to the film plane. These two steps therefore have the characteristic signatures of second- and first-order phase transitions, respectively.