Exchange Coupling Of Nanometer Two-phase Composite Permanent Magnetic Materials Micromagnetic Study

The composite nano-magnet is a new type of permanent magnetic material with the hard and soft phases exchange-coupled in nano-scale. It exhibits excellent magnetic properties with the hard phase to provide high coercivity and the soft phase to provide high saturation magnetization. The theoretic and experimental study of composite nano-magnets , especially the coercivity , is a hot topic in magnetism. In this thesis, the reversal mechanism of the oriented and unoriented two-phase multilayers has been investigated in detail based on Brown's micromagnetics. The results have been extended to nano composite magnets(bulk materials) with the dominant coercivity mechanism obtained.The reversal process of oriented nanometer two-phase multilayers includes three steps:(Ι) the nucleation of the domain wall; (ΙΙ) the propagation of the domain wall;(ΙΙΙ)the complete reversal of the magnetization vector. As the thickness of the soft phase increases , the shape of the hysteresis loop changes gradually from rectangle to Z-shape, the coercivity of systems becomes more and more small,and the multilayers change from completely exchange-coupled state to decoupled state. In the meantime,the dominant coercivity mechanism changes from nucleation to pinning.The reversal of unoriented two-phase multilayers depends onβ, the angle between the easy axis of the soft phase and the applied field. The reversal pocess forβ=90°is similar to that forβ=0°( the oriented case),but the nucleation field is much smaller while the pinning field is larger than the parallel case. For 0°<β<90°,the magnets couldn't be saturated at the finite applied field , and the nucleation of the domain wall doesn't occur.The reversal process only includes two steps, i.e., steps of (ΙΙ) and(ΙΙΙ).The formulae of the nucleation field for bulk composite permanent magnets have been derived with the anisotropy of the soft phase incorporated in our calculation. The dependence of the nucleation field on the soft grain size and magnetic parameters has been discussed. As the critical size at which the coercivity mechanism changes from nucleation to pinning is very small, we demonstrated that the main coercivity mechanism in the exchange-coupled permanent magnets is pinning rather than nucleation. Such a pinning is different from traditional nucleation and pinning and has both attributes of them, which is called as self-pinning. This nucleation-pinning duality helps to solve the long-lasting disputes on the coercivity mechanism in permanent /composite magnets.