Change of coercivity mechanism with the soft film thickness in hard-soft trilayers

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Abstract Coercivity mechanism has been studied intensively and debated for many years in magnetic materials, particularly in permanent magnets where defects play important roles in adjusting the coercivity. Such a role can be manifested in hard/soft multilayers, where the soft layer can be taken as an enlarged soft defect, which normally exists in so-called single-phased permanent magnets. In this paper, hysteresis loops and spin distributions have been obtained based on both three-dimensional (3D) and one-dimensional (1D) micromagnetic methods for SmCo/Fe to reveal the in-depth coercivity mechanism. Two different geometric models have been constructed to mimic the experimental trilayers, in one of which a transition layer between hard and soft layers is adopted, where calculated nucleation fields and coercivity match very well with the experimental data. As the soft layer thickness increases, both nucleation and coercive fields reduce whilst the coercivity mechanism changes from nucleation to pinning. Such a pinning is inherently related to nucleation and has both attributes of traditional nucleation and pinning, called as a hybrid coercivity mechanism here. The thickness-dependent coercivity mechanism obtained in this work agrees perfectly with the experimental data, which is general for all hard/soft composites and can be extended to single-phased permanent magnets where defects are inevitable.