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

Investigations of the magnetic state of a surface layer similar to 200 nm t hick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe 12O19) are performed in the phase transition region around the Curie temper ature (T-c). The method of simultaneous gamma, x-ray, and electron Mossbaue r 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 laye r similar to 200 nm thick to the paramagnetic state occurs at temperatures below T-c. It is established that the transition temperature T-c (L) of a t hin layer localized at depth L from the surface of the crystal increases wi th distance from the surface and reaches the value T-c at the lower boundar y of the "critical" surface layer. Therefore, near T-c a nonuniform state i n which the crystal is magnetically ordered in the bulk but disordered at t he 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. (C) 1999 American Institute of Phy sics. [S0021-3640(99)01101-9].