Magnetic properties, Mössbauer, and specific-heat studies of R${\mathrm{Ba}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ (R=Y,Eu) compounds

Complete replacement of Cu by Fe in R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ leads to R${\mathrm{Ba}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ (R=Y and Eu). We investigated ${\mathrm{YBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ and ${\mathrm{EuBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Magnetic-susceptibility and Mössbauer-spectroscopy measurements of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ and ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ in R${\mathrm{Ba}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ (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 Néel temperature ${\mathit{T}}_{\mathit{N}}$∼700 K. In ${\mathrm{YBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ the Mössbauer 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 (${\mathit{C}}_{\mathit{p}}$) measurements at different magnetic fields have been carried out between 1.5 and 30 K. The ${\mathit{C}}_{\mathit{p}}$(T,H) curves can be resolved into a contribution of the form ${\mathit{C}}_{\mathit{e}}$(T)=${\gamma }^{\mathrm{*}}$T with finite ${\gamma }^{\mathrm{*}}$ and a lattice contribution that consists of Debye terms. The pronounced high ${\gamma }^{\mathrm{*}}$=10.5 mJ/mol ${\mathrm{K}}^2$ for ${\mathrm{YBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ is discussed.