Anomalous reorientation phase transition of the magnetization in fct Ni/Cu(001)

An unusual change of direction of the magnetization $\mathrm{}\left(M\right)\mathrm{}$ in Ni/Cu(001) monolayers from in-plane at low temperature to perpendicular at high temperature is measured by in situ ferromagnetic resonance and the magneto-optic Kerr effect. Two boundaries for the appearance of the perpendicular component and for the vanishing of the parallel component of the magnetization are determined as a function of temperature and film thickness. In between, stable intermediate directions of $M$ are measured. The reorientation occurs with a continuous rotation of $M,$ similar to a second-order phase transition. It is shown that the temperature-dependent volume contribution to the magnetic anisotropy must be included in the interpretation of the spin reorientation transition.