Magnetic properties of face-centered cubic Co films

We deposited face-centered cubic (FCC) Co films on glass substrates by sputtering. From Auger-depth profile analysis, we found that there is one CoO layer, about 13 Å thick, lying on the top surface of the Co film, and another CoO layer, about 37 Å thick, lying within the Co/glass interface. At room temperature, the thin CoO film is supposed to be paramagnetic. However, because of the proximity effect between CoO and Co, the CoO layer may become ferromagnetic, with saturation magnetization M_o. By fitting the saturation magnetization (M_s) data of the whole Co/CoO film as a function of (1/t_f), where t_f is the Co thickness, we can prove that the last conjecture is correct, and the M_o of the CoO layer is indeed not zero. Both the t_f dependence of the magnetostriction constant (λ_s) and the t_f dependence of the coercive field (H_c) show a two-region characteristic. The dividing line for the former quantity is at t_f = 88 Å, and for the latter it is at t_f = 120 Å. When crossing such a dividing line, there is a discontinuous jump in λ_s (or H_c). These phenomena occur because the lattice-strain and magnetoelastic effects within the CoO layer dominate the λ_s and the H_c behavior in ultra-thin (t_f < 88 Å) Co films. In this region, the roughness-to-thickness ratio (S_q/t_f) may also affect λ_s. Finally, there seems to be no connection between the grain size (D) and H_c.