Direct generation of nanoscale ferromagnetic dots by selective ion irradiation of paramagnetic FeAl alloys

Fe60Al40 (at. %) alloys show an interesting combination of magnetic and structural properties, where atomically ordered Fe60Al40 is paramagnetic at room temperature, whereas disordered Fe60Al40 becomes ferromagnetic [1]. The transformation from paramagnetic B2-phase to the ferromagnetic, A2-phase can be accomplished by means of homogeneous ion irradiation procedures. Furthermore, local ion irradiation procedures (i.e., focused ion beam or ion irradiation through masks) have been also used in order to fabricate periodic arrays of sub-50 nm ferromagnetic structures embedded in a paramagnetic matrix. Due to the low fluences used, this method does not induce any roughening of the surface, leading to topographically featureless dots. The fabricated entities exhibit a range of magnetic properties depending on the size and shape, which were investigated by means of magneto-optical Kerr effect magnetometry, while the local character of the induced ferromagnetism was examined by magnetic force microscopy. Interestingly, when the patterned sheets are annealed at sufficiently high temperatures, the ferromagnetic properties are removed due to the annealing-induced atomic reordering. Hence, these methods may lead to a novel type of patterned recording media free from tribological and exchange coupling effects. Moreover, these approaches can be easily extrapolated to a variety of other systems exhibiting disorder-induced magnetism.

[1] J. Nogués et al., Phys Rev. B Phys. Rev. B 74, 024407 (2006)