Tuneable magnetic patterning of paramagnetic Fe_(60)Al_(40) (at. %) by consecutive ion irradiation through pre-lithographed shadow masks

摘要

Arrays of ferromagnetic circular dots (with diameters ranging from 225 to 420 nm) have been prepared at the surface of atomically ordered paramagnetic Fe_(60)Al_(40) (at. %) sheets by means of ion irradiation through prelithographed poly (methyl methacrylate) (PMMA) masks. The cumulative effects of consecutive ion irradiation (using Ar~+ ions at 1.2 × 10~(14) ions/cm~2 with 10, 13, 16, 19 and 22 keV incident energies) on the properties of the patterned dots have been investigated. A progressive increase in the overall magneto-optical Kerr signal is observed for increasingly larger irradiation energies, an effect which is ascribed to accumulation of atomic disorder. Conversely, the coercivity, H_c, shows a maximum after irradiating at 16-19 keV and it decreases for larger irradiation energies. Such a decrease in H_c is ascribed to the formation of vortex states during magnetization reversal, in agreement with results obtained from micromagnetic simulations. At the same time, the PMMA layer, with an initial thickness of 90 nm, becomes progressively thinned during the successive irradiation processes. After irradiation at 22 keV, the remaining PMMA layer is too thin to stop the incoming ions and, consequently, ferromagnetism starts to be generated underneath the nominally masked areas. These experimental results are in agreement with calculations using the Monte-Carlo simulation Stopping Range of Ions in Matter software, which show that for exceedingly thin PMMA layers Ar~+ ions can reach the Fe_(60)Al_(40) layer despite the presence of the mask.