Local Coercivity at X-rays Nanobeam Irradiated Regions in Amorphous Fe80B20 stripes

Abstract

The modification in magnetic films of the local magnetic properties is a fruitful strategy to achieve or optimize application required functionalities as it was shown on the development of spin waves phase shifters [1] and that of magnetic field sensors [2]. This work focuses on the use of synchrotron nanobeams to explore the possibility of modifying, at length scale of the beam size, the magnetic anisotropy of materials suitable for implementing spin waves guides. Our samples were amorphous Fe80B20 stripes (15 nm thick, 700 µm long and 15 µm wide) that were prepared using UV lithography from films grown by using pulsed laser ablation deposition. Irradiation, at the ID16B-ESRF, was performed by using a 56 nm x 56 nm spot size, an energy maximum at E = 17.5 keV and a flux of 2.21 x 1011 photons/s. Our characterization of the irradiation outcome, Figure 1, revealed that the local coercive force, measured by means of a Kerr effect device at points in between different irradiated regions (IR), in the same stripe and separated by 100 µm, increased with the IR width (varied by scanning in different areas the nanobeam). That increase can be understood by recalling that the reversal mechanisms of our stripes corresponds to the wall propagation/pinning type [3]. The wall involved in the reversal gets pinned when it finds a region at which it lowers its energy (as, for instance, a reduced anisotropy region). The depinning field (that is, the local coercivity) is related both to the local anisotropy decrease and to the domain wall width that is, approximately, 350 nm [3]. When the width of the IR is lower than that of the domain wall the local coercivity can be approximated to the local anisotropy reduction induced by the irradiation (local amorphous relaxation) multiplied by the ratio of the IR width to the wall one. Differently, when the wall is narrower than the IR the coercivity gets almost constant and measures, to a first approach, the difference in anisotropy energy between the irradiated and non-irradiated zones. We conclude about the need of implementing IR widths larger than ca. 500 nm in order to adequately stabilize a propagating wall.