Ultrathin magnetic films are often used as a model system for investigating low dimensional and interfacial effects. Both, surface and volume anisotropy can favor in-plane or out-of-plane magnetization. The easy axis of magnetization can change from in-plane to out-of-plane when decreasing the thickness of the magnetic layer at a fixed temperature, or upon decreasing the temperature at a fixed layer thickness. A prominent example for such a Spin Reorientation Transition (SRT) is an ultrathin Co film, sandwiched between Au layers. Our previous work showed the SRT at the room temperature with decreasing the Au-cap thickness, linked with an 38% enhancement of the Co orbital moment and the hybridization of the Co 3d and Au 5d bands, resulting in a charge transfer from Co to Au.
Present study shows the evolution of the SRT with decreasing temperature, giving evidence for the influence of the Au/Co interface versus the interior of the Co film in the SRT process. X-ray magnetic circular dichroism (XMCD) and soft x-ray resonant reflectivity (SXRR) measurements were performed at MAX-lab, Lund on a Au/Co/Au trilayer. XMCD experiments provide quantitatively both the in-plane and out-of-plane magnetic response across the SRT temperature, and assess the functional onset of the magnetization reorientation versus temperature. The temperature dependent spin and orbital moments were determined by means of the magneto-optical sum rules. A set of element-specific hysteresis loops based on the SXRR signal allows us to quantify the onset of the SRT at the photon penetration depth. On the other hand, the XMCD spectral areas are used to enhance the interface contrast. Comparison of the data from both methods indicates for the first time that with decreasing temperature from the in-plane phase the Co at the Co/Au interface exhibits the spin-reorientation at higher temperatures than the interior of the Co film, i.e. the SRT starts at the interface.