Asymmetric initial magnetization process of elongated particles in nucleation-type L1₀ FePt films

Following the sequence of applied magnetic field H=0→+6 kOe→-6 kOe along easy axis, an asymmetric initial magnetization process of multi-domain particles against the direction of H was observed by using an in-field magnetic force microscopy in nucleation-type L1₀ FePt films with nominal thickness t_N≥25 nm. The initial magnetizations increased quickly due to domain wall movement. At H=+6 kOe, a fraction of particles kept multi-domain configurations though the majority became single domain successively with the increase of H. However, almost all these multi-domain particles became single domain at lower opposite field H=-2 kOe. The saturation magnetizations of particles are hard to reverse because of high nucleation field. This asymmetric behavior is likely to be caused by the intrinsic asymmetry of energy barrier against the movement/wiping out of domain walls due to the geometry of particle, and can be degenerated by the irregularity of particle shape.