Magnetic and structural investigation of Fe50Co50 thin films across the transition from in-plane to out-of-plane anisotropy

Thin Fe50Co50 (FeCo) layers have recently attracted great attention due to their high saturation magnetization, spin polarization factor and possible application in magnetic devices, showing perpendicular magnetization, as well [1]. In thin FeCo films, the growth process highly affects the features of the magnetization reversal process as compressive or tensile stress may induce an additional source of magnetic anisotropy [2]. In detail, due to their high positive magnetostrictive coefficients, 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 on Si substrates in Ar atmosphere and covered by a 5 nm thick Cr overlayer to prevent oxidation. Their thickness, t, ranged from 5 nm up to 100 nm as in this range we expect to find a compressive stress [3]. Room temperature magnetization (M) data were collected using a SQUID magnetometer and a MOKE apparatus, using both an in-plane and a out-of-plane magnetic field (H). Magnetoresistance (MR) measurements were collected using the Van der Pauw method with an in-plane H and an in-plane current both parallel and perpendicular to H. The structural characterization was performed with a high resolution X-Ray diffractometer; the samples profile was investigated through an optical profilometer having a 1 nm vertical resolution. For small values of t, the shape of the in-plane M loops is squared and the coercive field increases with t for 5 nm < t < 15 nm. This result is possibly ascribable to the increase in grain size induced by the thickness increase. 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 techniques 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 [4]. Those results will be discussed and compared with the structural ones, in order to investigate the interplay between the developing anisotropy and the stacking of the FeCo films and to estimate the strength of the compressive stress. [1] N. Miyamoto et al., J. Appl. Phys. 107 (2010) 09CT19. [2] W. Yu et al., J. Appl. Phys. 99 (2006) 08B706. [3] J. McCord et al., J. Magn. Magn. Mater. 271 (2004) 46. [4] G. Ausanio et al., Thin Solid Films 519 (2011) 5420.