ANOMALOUS REORIENTATION PHASE-TRANSITION OF THE MAGNETIZATION IN FCT NI CU(001)/

An unusual change of direction of the magnetization (M) in Ni/Cu(001) monolayers from in-plane at low temperature to perpendicular at high t emperature is measured by in situ ferromagnetic resonance and the magn eto-optic Kerr effect. Two boundaries for the appearance of the perpen dicular component and for the vanishing of the parallel component of t he magnetization are determined as a function of temperature and film thickness. In between, stable intermediate directions of M are measure d. The reorientation occurs with a continuous rotation of M, similar t o a second-order phase transition. It is shown that the temperature-de pendent volume contribution to the magnetic anisotropy must be include d in the interpretation of the spin reorientation transition.