Nucleation size of hcp-CoPt dot arrays characterized by time dependence of coercivity

The magnetization reversal process for dot arrays is likely to start from a nucleation followed by propagation process. In this study, we estimated the nucleation diameter D_n for dot arrays made from thin hcp-CoPt perpendicular films (thickness δ=3 nm) and Co/Pt multilayered films (δ=9 nm), respectively. The dot diameter, D, was varied from 30 to 200 nm for CoPt dot arrays, and from 40 to 80 nm for Co/Pt dot arrays. The remanence coercivity was measured at measurement times t' 10³ s and 10 ^-5 s (pulse field), and defined as H_r and H _r^P. The energy barrier ΔE was evaluated by fitting H_r and H_r^P to Sharrock's equation. The value of D_n was estimated from ΔE, δ and the effective magnetic anisotropy of dot arrays including the demagnetizing energy due to the dot shape K_u^eff. D_n was independent of Din both series of dot arrays, and about 17 nm for CoPt dot arrays and about 11 nm for Co/Pt dot arrays. These values were close to both the grain size and the exchange length of these films.