Surface magnetism of Al-substituted Sr-M-type hexagonal ferrites

The surface and bulk magnetic structures of Sr-M-type single-crystal hexagonal ferrites (with the chemical formula SrFe_12?x Al_xO₁₉) have been directly compared by simultaneous gamma, x-ray, and electron Mössbauer spectroscopy. It was found, that when the magnetic lattice of Sr-M hexagonal ferrites is slightly diluted by diamagnetic Al ions, namely, for x=1.8 (SrFe_10.2Al_1.8O₁₉), a ～300-nm thick macroscopic anisotropic layer forms on the crystal surface, wherein iron-ion magnetic moments are oriented differently from those in the bulk of the sample. The reason for the onset of a noncollinear magnetic structure in the surface layer of SrFe_10.2Al_1.8O₁₉ crystals is the additional lowering of the exchange interaction energy caused by the presence of such a “defect” as the surface. Thus an anisotropic surface layer predicted theoretically by L. Néel in 1954 has been detected in ferromagnetic crystals.