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) in teraction parameters in similar to 200-nm thick surface layer and in the bu lk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe12O19 an d SrFe10.2Al1.8O19). The method used for the measurements is Mossbauer spec troscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF paramete rs in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magneti c fields, the fields at the nuclei of the iron ions located in a similar to 200-nm thick near-surface layer decrease with increasing temperature faste r than those of the ions in the bulk. The transition to paramagnetic state in a similar to 200-nm thick surface layer was found to occur 3 degrees bel ow 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 o ne moves away from the surface to reach T-c at the inner boundary of the su rface layer called critical. In the vicinity of T-c one observes a nonunifo rm state, with the crystal being magnetically ordered in the bulk but disor dered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near th e Curie (or Neel) temperature. (C) 1999 American Institute of Physics. [S10 63-7834(99)02003-1].