Nanosecond laser-induced synthesis of nanoparticles with tailorable magnetic anisotropy

Controlling the magnetic orientation of nanoparticle is important for many applications. Recently, it has been shown that single domain ferromagnetic hemispherical Co nanoparticles prepared by nanosecond laser-induced self-organization, show magnetic orientation that was related to the negative sign of the magnetostrictive coefficient lambda(S) [J. Appl. Phys. v103, p073902, 2008]. Here we have extented this work to the Fe50Co50 alloy, which has a positive lambda(S) and Ni, which has a negative lambda(S). Patterned arrays of ferromagnetic nanoparticles of Fe50Co50, Ni, (and Co) were synthesized from their ultrathin metal films on SiO2 substrate by nanosecond laser-induced self-organisation. The morphology, nanostructure, and magnetic behavior of the nanoparticle arrays were investigated by a combination of electron microscopy, atomic force microscopy, and magnetic force microscopy techniques. Transmission electron microscopy investigations revealed a granular polycrystalline nanostructure, with the number of grains inside the nanoparticle increasing with their diameter. Magnetic force measurements showed that the magnetization direct of the hemispherical Co and Ni nanoparticles was predominantly out-of-plane while those for the Fe50Co50 alloy was in the plane of the substrate. Finite element analysis was used to estimate the average residual strain in the nanoparticles, following laser processing. The difference in behavior is due to the dominating influence of magnetostrictive energy on the magnetization as a result of residual thermal strain following fast laser processing. Since lambda(S) is negative for polycrystalline Co and Ni, and positive for Fe50Co50, the tensile residual strain forces the magnetization direction to out-of-plane and in-plane, respectively. This work demonstrates a cost-effective non-epitaxial technique for the synthesis of magnetic nanoparticles with tailored magnetization orientations. (C) 2010 Elsevier B.V. All rights reserved.