Anisotropy-graded FePt films induced by ion irradiation

FePt nanoparticle samples, in the face centered tetragonal (fct) L10 chemically ordered phase, have attracted much attention for their potential application as ultrahigh density magnetic data storage media, since they exhibit [1] a high bulk magnetocrystalline anisotropy, Ku ~6.6×10^7 ergs/cm^3. Such a high anisotropy permits to maintain the thermal stability even using very small grains, which are necessary in order to increase the signal-to-noise ratio. However, the magnetic field required to switch the magnetization of one individual L10 FePt bit, in bit patterned media at density 1 Tbit/inch^2, was estimated [2] to be about 110 kOe, which exceeds the maximum available write field [3] ( ~ 20 kOe) provided by a magnetic recording head. In order to reduce the switching fields, different strategies have been proposed, such as those based upon tilted magnetic media, exchange coupled composite media, and graded anisotropy films. In this work, we investigate the properties of a continuously graded anisotropy film obtained by ion irradiation of a single continuous sample of L10 FePt [4]. The FePt films, 15 nm thick, have been irradiated by a hot cathode Ar+ sputtering gun, at an incidence angle theta=45° and 85° from sample normal. Ion irradiation turns the L10, chemically ordered, magnetically hard phase, into the A1, chemically disordered, magnetically soft phase, 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 layers have been modified. The static magnetic properties have been studied by alternating gradient force magnetometer (AGFM). Before irradiation, the out-of-plane loop presents a square shape and an appreciable coercive field, while the in-plane loop has an S-shape with a low remanence. These are the characteristics of a hard magnetic material whose magnetization is preferentially oriented perpendicular to the film plane, and they persist even when the sample is irradiated at large incidence angle, theta= 85°. In contrast, after irradiation at 45°, the sample presents a considerable increase of the in-plane remanence, while the out-of-plane loop, with reduced remanence and coercive field, assumes a shape characteristic of an exchange-spring material. Brillouin light scattering (BLS), from thermally excited spin waves, has been exploited to study the dynamical magnetic properties. Figure 1 shows the comparison between the measured (symbols) and the calculated (curves) spin wave frequencies versus the intensity H of the in-plane applied field, for both films irradiated at theta= 85° and 45°. It is clearly seen that, as H tends to 0, both the samples exhibit a spin-wave frequency gap, whose value decreases for the softer specimen. The corresponding theoretical frequencies have been obtained [5] as follows. A discrete nonlinear mapping was first used to determine the non uniform profile of the magnetization along the film normal; then the frequencies of the spin wave excitations with respect to the ground state were calculated linearizing a system of 2N Landau-Lifshitz equations of motion, where N is the number of atomic layers. The two FePt irradiated samples were simulated by a model with N = 39 parallel ferromagnetic atomic layers. The Ns topmost layers were assumed to be magnetically soft, with saturation magnetization Ms = 800 emu/cm^3 and anisotropy constant K?,s = 5×10^5 erg/cm^3, while the Nh innermost layers were assumed to be hard, with Mh=760 emu/cm^3 and K?,h = 1.15×10^7 erg/cm^3. For the Ng intervening layers, both the saturation magnetization and the anisotropy constant were assumed to interpolate linearly between the respective values in the magnetically soft and hard regions. In Fig.2 we report the ground state configurations calculated using the above material parameters. It can be seen that the Ar+ irradiation causes a profound change in the magnetic ground state. In the sample irradiated at theta= 45°, the magnetization gradually rotates from nearly perpendicular to the surface plane in the lowermost layers to nearly parallel to the surface plane in the topmost layers. On the contrary, in the sample irradiated at 85°, for small values of the external applied field the magnetization is almost perpendicular to the surface plane, due to the large out-of-plane anisotropy. This implies a high anisotropy-energy cost and an overall increase of the spin wave frequency. In conclusion, we showed that by means of Ar+ irradiation, it is possible to obtain anisotropy-graded systems, whose magnetic properties can be manipulated changing the irradiation incidence angle. In particular, we observed a zero-field frequency tunability which may have large applications for microwave magnetic devices.REFERENCES[1] X. W. Wu, K. Y. Guslienko, R. W. Chantrell, and D. Weller, Appl. Phys. Lett. 82, 3475 (2003).[2] P. Krone, D. Makarov, T. Schrefl, and M. Albrecht, Appl. Phys. Lett. 97, 082501 (2010).[3] S. Batra, J. D. Hannay, H. Zhou, and J. S. Goldberg, IEEE Trans. Magn. 40, 319 (2004).[4] A. di Bona, P. Luches, F. Albertini, F. Casoli, P. Lupo, L. Nasi, S. D'Addato, G. C. Gazzadi, S. Valeri, Acta Mater. 61, 4840 (2013).[5] S. Tacchi, T. N. Anh Nguyen, G. Carlotti, G. Gubbiotti, M. Madami, R. K. Dumas, J. W. Lau, J. Akerman, A. Rettori, and M. G. Pini, Phys. Rev. B 87, 144426 (2013).