Mechanism of magnetization process of island-like L1₀ FePt films

L1₀ FePt films composed of separated island-like particles with a size distribution were epitaxially deposited on MgO (0 0 1) substrate at 780 °C. The most probable size of particle at nominal thickness t _N=25 nm is 10 times as big as that at t_N=5 nm.Most of the as-deposited particles are single-domain at t_N=5 nm, and the majority are multi-domain at t_N=25 nm.Out-of-plane coercivity is ∼50 kOe at t_N=25 nm.A peculiar asymmetric initial magnetization process against the direction of applied perpendicular field was imaged by an in-field magnetic force microscope (MFM). At +6 kOe, a few particles were kept as multi-domain structures as long as the majority became saturated; however, they became saturated only at -1 kOe. High coercivity is attributed to a wide distribution of nucleation field H_N ranging from 20 to 70 kOe.