Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.