Critical behavior of the surface of FeBO3 single crystals

The behavior of the surface and near-surface layers of macroscopic FeBO3 single crystals is studied over the temperature range from 291 K to Neel temperature (T-N) using depth-selective conversion-electron Mossbauer spectroscopy. Three different phases or states, namely, an antiferromagnetically ordered phase (similar to the crystal bulk state), a surface phase, and a transition layer between them coexist near the Neel point in a surface layer similar to 500 nm thick. The critical parameters found for the bulk phase agree well with the theoretical critical index nu(th) congruent to 0.63 predicted by the 3D Ising model. As the crystal surface is approached, the critical parameter beta increases to 0.51(2) but remains smaller than the value of beta = 0.8 for the surface of a semi-infinite Heisenberg model. Therefore, the effective dimensionality of the system, being equal to 3 in the bulk, decreases at the crystal surface. (C) 2000 MAIK "Nauka/Interperiodica".