High temperature ferromagnetism in NbCo2 nanoparticles - Bulk magnetization and hyperfine field measurements

We report the observation of high temperature ferromagnetism in nanoparticles of NbCo2, prepared by high speed ball milling. The magnetic properties of the bulk and noncrystalline samples have been studied through measurements of bulk magnetization and hyperfine field of Rh-100, implanted into the sample. Compared to the low, temperature independent magnetic susceptibility (chi similar to 10(-5) emu/gm) observed for bulk NbCo2, the nanoparticle samples exhibit very high value of magnetic susceptibility (chi similar to 10(-3) emu/gm), whose temperature dependence indicates possible ferromagnetic ordering near or above 300 K. The saturation magnetic moment obtained from isothermal magnetization (M vs H) data, however, turns out to be quite small (0.10 mu(B)). On the other hand, hyperfine field measurements performed for Rh-100 probe using time differential perturbed angular correlation technique indicate strong paramagnetic behavior of nano-NbCo2. We believe that ferromagnetism observed in nano-NbCo2 most probably arise from a small fraction of surface atoms, while the bulk of the material remains paramagnetic.