  Patterned arrays of ferromagnetic nanoparticles of Co, Ni, and Fe_{\text{50}}
Co_{\text{50}} have been synthesized from their ultrathin metal films on
SiO_{\text{2}} substrate by nanosecond laser-induced self-organization. The
morphology, nanostructure, and magnetic behavior of the nanoparticle arrays
were investigated by a combination of electron, atomic force, 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 direction of the Co and Ni
nanoparticles was predominantly out-of-plane while those for the
Fe_{\text{50}}Co_{\text{50}} alloy was in the plane of the substrate. This
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 the magnetostriction coefficient is negative for
polycrystalline Co and Ni, and positive for Fe_{\text{50}}Co_{\text{50}}, the
tensile residual strain forces the magnetization direction of the negative
magnetostriction materials out-of-plane and the positive magnetostriction
materials in-plane. This demonstrates a cost-effective non-epitaxial technique
for the fabrication of patterned arrays of magnetic nanoparticles with tailored
magnetization orientations.
