Micromagnetic modeling of Bloch Walls with Néel Caps in Magnetite

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Abstract
Two‐dimensional micromagnetic modeling of 180° domain walls in magnetite predicts that at depths >0.07 micro;m below the surface, the walls are Bloch‐like, with spins rotating in the plane of the wall in order to eliminate magnetic poles on the wall and reduce demagnetizing energy, Ed. The Bloch wall width is ≈ 0.16 µm, in agreement with magnetic force microscope (MFM) data. Near a crystal boundary, the walls are Néel‐like: spins rotate in the plane of the surface in order to eliminate surface poles. This Néel cap is ≈ 0.3 µm wide and is offset with respect to the underlying Bloch wall. The Néel cap is narrower and shallower in magnetite (relative to Bloch wall width) than in iron, mainly because Ed is not as overwhelmingly important compared to other energies in magnetite as it is in iron. As a consequence of the smaller Néel cap in magnetite, the surface field produced by a domain wall is due mainly to the underlying Bloch wall, with only a minor contribution from the Néel cap. This prediction is consistent with MFM imaging of domain walls on free surfaces of magnetite crystals.

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