Magnetic and structural investigation of growth induced magnetic anisotropies in Fe₅₀Co₅₀ thin films

¹ Department of Physics and CNISM, University of Ferrara, via G. Saragat 1, Building C I–44122 Ferrara, Italy
² CNR-SPIN and Dept. of Physical Sciences, “Federico II” University of Naples, p.le V. Tecchio 80 I–80125 Naples, Italy
³ CIG nanoGUNE consolider, Tolosa Hiribidea, 76 E-20018 Donostia, San Sebastian, Spain

^a e-mail: tamisari@fe.infn.it

Abstract

In this paper, we investigate the magnetic properties of Fe₅₀ Co₅₀ polycrystalline thin films, grown by dc-magnetron sputtering, with thickness (t) ranging from 2.5 nm up to 100 nm. We focused on the magnetic properties of the samples to highlight the effects of possible intrinsic stress that may develop during growth, and their dependence on film thickness. Indeed, during film deposition, due to the growth technique and growth conditions, a metallic film may display an intrinsic compressive or tensile stress. In our case, due to the Fe₅₀Co₅₀ magnetolastic properties, this stress may in its turn promote the development of magnetic anisotropies. Samples magnetic properties were monitored with a SQUID magnetometer and a magneto–optic Kerr effect apparatus, using both an in–plane and an out–of–plane magnetic field. Magnetoresistance measurements were collected, as well, to further investigate the magnetic behavior of the samples. Indications about the presence of intrinsic stress were obtained accessing samples curvature with an optical profilometer. For t ≤ 20 nm, the shape of the in-plane magnetization loops is squared and coercivity increases with t, possibly due to fact that, for small t values, the grain size grows with t. The magnetoresistive response is anisotropic in character. For t > 20 nm, coercivity smoothly decreases, the approach to saturation gets slower and the shape of the whole loop gets less and less squared. The magnetoresistive effect becomes almost isotropic and its intensity increases of about one order of magnitude. These results suggest that the magnetization reorientation process changes for t > 20 nm, and are in agreement with the progressive development of an out-of-plane easy axis. This hypothesis is substantiated by profilometric analysis that reveals the presence of an in-plane compressive stress.