Magnetic states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Abstract

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe12O19) are performed in the phase transition region around the Curie temperature (Tc). The method of simultaneous gamma, x-ray, and electron Mossbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe12O19 the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below Tc. It is established that the transition temperature Tc(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value Tc at the lower boundary of the “critical” surface layer. Therefore, near Tc a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Neel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.