Ga ion irradiation induced spin reorientations in Co films

Some years ago, it has been demonstrated that after submitting Pt/Co/Pt films to increasing ion irradiation dose, one observes a decrease of: (i) perpendicular magnetic anisotropy (up to reach a spin reorientation transition (SRT) towards an in-plane magnetisation state); (ii) coercive field; (iii) Curie temperature (see e.g. [1-3]). In thicker Co films, by increasing the Ga ion dose, two successive inverse SRTs take place [4].
Pt(4.5nm)/Co(2.6nm)/Pt(3.5nm) films were deposited by sputtering on a sapphire substrate. 1mm wide strip regions were irradiated by 30 keV Ga ions under different doses D ranging from 2x10¹³ to 3x10¹⁴ Ga ions/cm². Samples were studied by polar Kerr PMOKE (out-of-plane magnetization component sensitive), magneto-optical microscopy and local scanning magnetometry using a focused light beam. In-plane magnetized domains were studied using longitudinal magneto-optical Kerr effect, LMOKE. Magnetic Force Microscopy (MFM) was used to image domain patterns at higher spatial resolution. TRIDYN simulations were performed to estimate irradiation driven changes of the in-depth ion distributions. The irradiation driven SRTs are illustrated in Fig.1. Magnetization lies in-plane in the non-irradiated (NI) region. When increasing D, two successive SRTs undergo between planar to perpendicular magnetization states, and again back to a planar state (through a canted magnetization state, as shown from the magnetization curve obtained for D=3x10¹⁴ Ga ions/cm²).

This work was partly supported by the EU-“Research Infrastructures Transnational Access” program “Center for Application of Ion Beams in Materials Research” under contract no. 025646 and DAAD.

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