Unusual size-dependent magnetic anisotropy in Co nanomagnets made from self-organized fast laser processing*

Unusual size-dependent magnetic anisotropy has been observed in hemispherical polycrystalline Co nanomagnet on SiO$_{2}$ substrates produced by fast pulsed-laser-induced self-organization. The magnetic states of these particles have been characterized by using magnetic force microscopy (MFM) and hysteresis measurements. The results for single domain particles up to a diameter of 180 nm, the magnetization direction of smaller sized particles tends to be in-plane, while the larger particles tend ot have out-of-plane orientation. This finding is not consistent with shape anisotropy which predicts a size-independent in-plane alignment. Microstructural analysis revealed that particles had a granular microstructure with the grain size increasing with particle size. This unusual behavior has been attributed to large residual tensile strain in the hemispherical nanoparticles due to the large heating/cooling rates ($\sim$10$^{10}$ K/s) under ns laser self-organization, the large thermal expansion mismatch and the negative magnetostrictive constant for polycrystalline Co.