Accurate calculation of the nucleation field and hysteresis loops in hard-soft multilayers

Abstract

In this paper, the nucleation field HN, hysteresis loops, and coercivity mechanism for hard/soft multilayers have been investigated within a self-contained micromagnetic model, which change with the strength of the interface exchange coupling Ji as well as the thickness of the hard and soft layers Lh and Ls. Analyses show that the nucleation field goes up as the interface exchange coupling increases. However, Ji has a significant effect on HN only when Ls is about 10% of the domain wall width of the soft phase while the thickness of the hard layer is larger than its domain wall width. The hysteresis loops of the Nd2Fe14B/α-Fe/Nd2Fe14B trilayers for various Ji have been calculated. The calculated pinning field decreases when Ji increases. It is found that, as Ls increases, the coercivity mechanism changes from pinning to nucleation. The change of exchange coupling at the interface influences the soft layer thickness, at which the coercivity starts to deviate from pinning. The critical thickness of the soft phase, at which the behavior of the hysteresis loop changes from the single-phased one to the two-phased one, decreases as Ji decreases, so that the “rigid composite magnet” appears only when Ls is very small for smaller Ji.