Quasicoherent nucleation mode in two-phase nanomagnets

Magnetization processes in advanced magnetic nanostructures are investigate d. For the case of spherical soft or semihard grains surrounded by a very h ard matrix a bulging nucleation mode is discovered. The bulging mode exhibi ts the radial angular symmetry of the coherent mode, but it is incoherent d ue to its radial variation. The radial dependence of the bulging mode is ob tained by solving a spherical Bessel equation which is subject to appropria te boundary conditions. In contrast to the coherent mode, the bulging mode yields a nucleation-field coercivity which depends on the exchange stiffnes s and on the size of the grain. There is a critical grain radius 7.869 root A/mu(0)M(s)(2) above which the bulging mode is replaced by a modified curl ing mode. The nucleation modes realized in nanostructures affect the demagn etizing-field corrections necessary to account for the external shape of ma gnetic samples. Since strong but short-range exchange and weak but long-ran ge magnetostatic interactions compete on nanostructural length scales, the sample-shape dependence of the hysteresis loops cannot be mapped onto a pur ely magnetostatic demagnetizing factor. [S0163-1829(99)01734-8].