MAGNETIC-PROPERTIES, MOSSBAUER, AND SPECIFIC-HEAT STUDIES OF RBA2FE3O8(R=Y,EU) COMPOUNDS

Complete replacement of Cu by Fe in RBa2CU307 leads to RBa2Fe3O8 (R=Y and Eu). We investigated YBa2Fe3O8 and EuBa2Fe3O8 by several complementary experimental techniques. These compounds crystallize in a tetragonal structure with the configuration of the atoms in the unit cell very similar to that in the superconductor YBa2CU307. Magnetic-susceptibility and Mossbauer-spectroscopy measurements of Fe-57 and Eu-151 in RBa2Fe3O8 (R=Y and Eu) at temperatures from 4.2 to 800 K have been performed. In both compounds the iron moments order antiferromagnetically at the same Noel temperature T(N) approximately 700 K. Iri YBa2Fe3O8 the Mossbauer spectra reveal two inequivalent iron sites, probably corresponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (distorted octahedral oxygen coordination). The quadrupole splitting of Fe(2) is zero. Specific-heat (C(p)) measurements at different magnetic fields have been carried out between 1.5 and 30 K. The C(p)(T,H) curves can be resolved into a contribution of the form C(e)(T)=gamma*T with finite gamma* and a lattice contribution that consists of Debye terms. The pronounced high gamma*=10. 5 mJ/mol K2 for YBa2Fe3O8 is discussed.