Magnetic properties of perpendicularly orientated L1₀ FePt nanoparticles

L1₀ FePt films were deposited on MgO (001) substrates heated to 700°C by magnetron sputtering. Assisted by the misfit of lattice between film and substrate, strong (001) texture was formed. The film at nominal thickness t_N = 5 nm was composed of nanoparticles with a size of ~70 nm, and showed a high coercivity of ~105 kOe at 4.2 K. At t_N =~50 nm, as the film changed from discontinuous to continuous, the coercivity dropped about one order of magnitude. Micromagnetic simulation implies that the magnetization reversal is a vortex-like nuclear type. The ideal coercivity of a separated single-domain L1₀ FePt nanoparticle with a size of 70 nm× 70 nm× 5 nm is ~121 kOe. This tells us that the experimental coercivity has nearly reached the limit of ideal single crystalline nanoparticles.