Rotatable magnetic anisotropy in anisotropy-graded FePt films induced by ion irradiation at low incidence angle

Alternating gradient force magnetometry (AGFM), Brillouin light scattering (BLS) from thermally excited spin waves, and magnetic force microscopy (MFM) have been used to investigate the static and dynamic magnetic properties of a 15-nm-thick FePt film after Ar+ irradiation, performed at two different incidence angles, 85o and 45o, with respect to the film normal. Ion irradiation turns the L10, chemically ordered, magnetically hard phase, into the A1, chemically disordered, magnetically soft phase [1], inducing an anisotropy gradient along the film normal. On changing the ion incidence angle, the ion penetration depth and the relative thicknesses of the soft and hard phases have been modified. We found that the sample irradiated at 45o is characterized by a profound softening of the perpendicular magnetic anisotropy near the surface plane. Moreover, BLS measurements, performed at remanence as a function of the in-plane rotation angle, indicate the presence of a rotatable magnetic anisotropy [2]: namely, an anisotropy whose direction can only be rotated by changing the saturating field direction. The presence of this anisotropy can be ascribed to the competition between the moderate perpendicular magnetic anisotropy and the easy-plane shape energy. Finally, from the fit of the BLS frequency versus the intensity of the in-plane field H, the effective rotatable anisotropy field Hrot was estimated to be Hrot<=510 Oe for H<=2 kOe. The tunability of the zero-field frequency, obtained by Ar+ irradiation varying the incidence angle of the ion beam, may have large applications for microwave magnetic devices. [1] A. di Bona et al., Acta Materialia 61 , 4840 (2013). [2] L. M. Alvarez-Prado et al., Phys. Rev. B 56 , 3306 (1997).