Ga+ ion irradiation-induced changes of magnetic ordering in Pt/Co(3nm)/Pt films

In Pt/Co(d)/Pt ultrathin film structures a spin reorientation transition (SRT) from out-of-plane to in-plane magnetization states is known to occur at a critical Co thickness d=dSRT ~ 1.8nm. For thinner Co thicknesses this SRT can also be forced by ion irradiation. In the present work we demonstrate that also the reverse SRT, i. e. from in-plane to out-of-plane, can be achieved by ion irradiation in certain cases. Therefore, easy-plane magnetized Pt/Co(3nm)/Pt samples were irradiated by 30 keV Ga+ ions with doses D ranging from 1014 to 5x1016 ions/cm2. The ion irradiation induced effects were investigated by polar magnetooptical microscopy, magnetometry and atomic/magnetic force microscopy. At low dose the reverse SRT is observed with a large out-of-plane magnetization component giving rise to a square perpendicular hysteresis loop. Sub-micrometer perpendicular magnetic domain structures either field-induced or in the demagnetized state confirm this new behavior. At higher dose, the easy magnetization direction is turned in-plane again. Topologically, a film swelling effect is first evidenced at low dose, but surface etching dominates at higher dose. Similar effects are also observed under quasi-uniform irradiation performed by scanning a focused ion beam (FIB) on the Pt/Co(3nm)/Pt structure. The results will be also compared to those reported earlier for d < dSRT, where ion irradiation leads to a reduction of the Curie temperature, coercivity and magnetic anisotropy [1,2].
This work was partially supported by EU MC TOK project NanomagLab No MTKD-CT-2004-003177 and EU-RITA program “Centre for Application of Ion Beams in Materials Research” under contract no. 025646. J.J. has benefited of an EC Marie-Curie fellowship (contract MEST CT 2004-51437).

[1] J. Ferré, J.-P. Jamet, in Handbook of Magnetism and Advanced Magnetic Materials, H. Kronmüller, S. Parkin (eds), (2007) John Wiley & Sons, Ltd.
[2] J. Fassbender, J. McCord, J. Magn. Magn. Mat. 320 (2008) 579.