MICROMAGNETIC MODELING OF BLOCH WALLS WITH NEEL CAPS IN MAGNETITE

Two-dimensional micromagnetic modeling of 180 degrees domain walls in magnetite predicts that at depths >0.07 mu m below the surface, the wa lls are Bloch-like, with spins rotating in the plane of the wall in or der to eliminate magnetic poles on the wall and reduce demagnetizing e nergy, E(d) The Bloch wall width is approximate to 0.16 mu m, in agree ment with magnetic force microscope (MFM) data. Near a crystal boundar y, the walls are Neel-like: spins rotate in the plane of the surface i n order to eliminate surface poles. This Neel cap is approximate to 0. 3 mu m wide and is offset with respect to the underlying Bloch wall. T he Neel cap is narrower and shallower in magnetite (relative td Bloch wall width) than in iron, mainly because E(d) is not as overwhelmingly important compared to other energies in magnetite as it is in iron. A s a consequence of the smaller Neel cap in magnetite, the surface fiel d produced by a domain waif is due mainly to the underlying Bloch wall , with only a minor contribution from the Neel cap. This prediction is consistent with MFM imaging of domain walls on free surfaces of magne tite crystals.