COMPARATIVE MOSSBAUER-SPECTROSCOPY AND NEUTRON-DIFFRACTION ANALYSIS IN YBA2(CU1-XFEX)(3)O-Y .2. MAGNETIC TRANSITIONS

The magnetic transitions in oxidized and reduced YBa2(Cu1-xFex)3O(y) compounds were studied by Mossbauer spectroscopy. The temperature dependences of the hyperfine magnetic fields H-hf at the Fe-57 nuclei were obtained and analyzed on the basis of some theoretical models to determine the type and dimensionality of the magnetic order for iron ions both in Cu2 sites and in Cu1 sites with different oxygen coordination. It was found in the reduced samples, that with temperature variation the competition between Cu1-Cu1 and Cu1-Cu2 exchange interactions can change the type and dimensionality of the magnetic order in a certain local site of the Cu1 sublattice, and a crossover effect can appear. In the oxidized superconducting samples, the 3D magnetic correlations, existing in the mainly two-dimensional magnetic system of the Cu1-O layers, imply the simultaneous participation of the superconducting electrons of the Cu2-O layers both in superconductivity and in the exchange interaction between neighboring magnetic Cu1-O layers. A comparative analysis of Mossbauer spectroscopy and neutron diffraction data is made.