Surface magnetism of Sc-substituted Ba-M hexaferrites

Abstract

A technique of simultaneous gamma-ray, x-ray, and electron Mossbauer spectroscopy is used to study the magnetic structure of the surface layer with direct comparison to the magnetic structure inside single crystal samples of hexagonal Ba-M ferrites, in which part of the iron ions have been replaced by diamagnetic Sc ions (chemical formula BaFe12−δScδO9). It is found that when the diamagnetic Sc ions are introduced into the crystal lattice of BaFe12−δScδO19 at concentrations (x=0.4 and 0.6) far below the level at which the collinear magnetic structure inside the sample is destroyed, a macroscopic layer of thickness ∼300 nm develops on the surface, in which the magnetic moments of the iron ions are oriented noncollinearly with respect to the moments inside the sample. The deviation 〈θ〉 of the magnetic moments in BaFe11.6Sc0.4O19 was 10° ± 62° for x=0.4, and when the Sc concentration was raised to 0.6, the angle 〈θ〉 increased to 17° ± 62°. The noncollinear magnetic structure in the surface layer in these crystals develops because of further reduction in the energy of the exchange interactions owing to the presence of a “defect,” such as the surface. For the first time, therefore, an anisotropic surface layer whose magnetic properties differ from those in the interior of a sample has been observed experimentally in ferromagnetic crystals, as predicted by Neel [L. Neel, Phys. Radium. 15, 225 (1954)].