Growth, thermal stability, and magnetic properties of Co films on Ni3Al(001)

Abstract

The growth, morphology, thermal stability, and the magnetic properties of cobalt deposited on a Ni3Al(001) surface were investigated by scanning tunneling microscopy (STM), low energy electron diffraction, Auger electron spectroscopy, and by the magneto-optical Kerr effect (MOKE). Cobalt grows pseudomorph on Ni3Al(001), showing flat islands by STM. Although the lattice mismatch between the substrate and the Co film is small, we observed that the layer by layer growth is only observed when depositing at higher temperatures. In situ MOKE measurements on Co coverages in the range of 0.55–12 monolayers (ML) were performed. Unexpectedly, cobalt films 0.77 ML thick show ferromagnetism already at room temperature. The easy axis of magnetization is oriented in the [110] direction and the hard axis in the [100] direction of the Ni3Al(001) surface, as expected for cubic Co. Out of plane is a hard direction of magnetization. After annealing between 540 and 700K an irreversible increase on the coercivity Hc of the films appears with respect to the as deposited films, and the saturation ellipticity shows no change. The increase of Hc is correlated with the coalescence of the Co islands observed after annealing by STM and the diffusion of impurities from the substrate into the Co film. During stepwise annealing, at about 550K, the room temperature value of Hc increases abruptly, indicating that 550K is the temperature needed to produce the coalescence of the Co islands. The Co film is stable on the Ni3Al(001) surface up to 750K. Beyond that temperature Co atoms start to diffuse into the substrate. At 1100K, Co disappears completely from the surface via diffusion into the Ni3Al substrate. The Curie temperature of the Co films is beyond 750K, the stability limit of the films.