Two-dimensional paramagnetic-ferromagnetic phase transition and magnetic anisotropy in Co(110) epitaxial nanoparticle arrays

The coverage-dependent magnetic response and anisotropy of epitaxial Co/Cu(110) nanoparticle arrays were studied. A paramagnetic-ferromagnetic phase transition is found to occur at a critical thickness $d_C=4.6\mathrm{}\pm 1.15\mathrm{ML}.$ In the vicinity of $d_C,$ the paramagnetic susceptibility χ follows a power-law behavior. The observed magnetic phase transition is due to the two-dimensional percolation of epitaxial Co particles with a critical exponent $\gamma =2.39\mathrm{}\pm \mathrm{0.08.}$ At the very onset of the long-range ferromagnetic order, a uniaxial anisotropy induced by the nanoscale Co particles was observed.