Stress induced switch from in-plane to out-of-plane anisotropy in Fe50Co50 thin films

Thin Fe50Co50 (FeCo) layers have recently attracted great attention due to their high saturation magnetization, spin polarization factor [1] and possible application in magnetic devices, showing perpendicular magnetization, as well [2]. In thin FeCo films, the growth process highly affects the features of the magnetization reversal process. Indeed, due to their high positive magnetostrictive coefficients, the presence of compressive or tensile stress may induce an additional source of magnetic anisotropy, thus affecting the magnetic structure of the layer [3]. In particular, the presence of a compressive stress my favour an out-of-plane orientation of the magnetization. In this paper, we present FeCo layers grown by dc-magnetron sputtering in Ar atmosphere on Si substrates. Their thickness, t, ranged from 5 nm up to 100 nm as in this range we expect to find compressive stress within the samples [4]. The layers were covered with a 5 nm Cu overlayer to protect them from oxidation. Room temperature magnetization (M) measurements were performed using a SQUID magnetometer with a magnetic field (H) applied in the plane of the film. MOKE date were collected at room temperature both with in-plane and out-of-plane H. Magnetoresistive (MR) measurements were collected using the Van der Pauw method with H parallel to the plane of the sample and an in-plane current parallel and perpendicular to H. For small values of t, the shape of the in-plane M loops is squared and the coercive field increases with t for t up to 15 nm. This increase is possibly ascribable to the change in grain size. For higher values of t, coercivity smoothly decreases and the shape of the loops changes; in detail for t > 20 nm the approach to saturation is slower and the shape of the whole loop gets less and less squared. MR data indicate that, for t up to 20 nm the MR effect is anisotropic in character, its intensity is small (~ 0.02 %), and saturation fields are in agreement with those measured from M loops. For t > 20 nm the MR effect becomes almost isotropic, its intensity increases of about one order of magnitude and the approach to saturation gets slower. Therefore, both measurement methods suggest that the magnetization reorientation process changes for t > 20 nm, and the results are in agreement with the progressive development of an out-of-plane easy axis. Those results will be discussed and compared with the MOKE ones, where for t > 20 nm a two-phase magnetic behaviour is observed. [1] L. Platt et al, J. Appl. Phys. 88 (2000) 2058. [2] N. Miyamoto et al., J. Appl. Phys. 107 (2010) 09CT19. [3] W. Yu et al., J. Appl. Phys. 99 (2006) 08B706. [4] J. McCord et al., J. Magn. Magn. Mater. 271 (2004) 46.