Nucleation field, reversal mechanism and coercivity paradox in two-phased magnetic nanosystem

The magnetic reversal mechanism has been determined within a micromagnetic model reliably for a two-phased magnetic nanosystem, with the formulae for nucleation fields derived analytically. It is found that the nucleation field HN decreases uniformly as the size of the soft phase Ls increases whereas it increases with the size of the hard phase Lh. The analysis shows that whilst the effect of Lh could be ignored in most cases, where the nucleation field is dominated by the Ls and the calculation could be significantly simplified, the overly simple inverse square law between HN and the soft layer thickness Ls adopted by the previous researchers is not a good approximation. While nucleation is the beginning of the magnetic reversal, pinning is the dominant coercivity mechanism in both two-phased and single-phased magnetic materials, where the crystalline defects exist. Comparison with the experimental data confirms this conclusion, indicating that Brown’s paradox results from the much lower effective anisotropy in both single-phased and composite materials, as speculated in the literature.