Enhancement of magnetic ordering temperature in iron substituted ytterbium manganate (YbMn_1-xFe_xO₃)

Oxides of the type YbMn_1-xFe_xO₃; x≤0.3 showing multiferroic behavior have been synthesized by the solid state route. These oxides crystallize in the hexagonal structure known for the parent YbMnO₃ with the c/a ratio increasing with Fe substitution. The distortion of the MnO₅ polyhedra (tbp) decreases and the Mn-O-Mn bonds in the a-b plane become shorter with Fe-substitution. Magnetic ordering is observed from the low temperature neutron diffraction study. The compounds were found to be antiferromagnetic and the ordering temperature T_N increased from 82 K for pure YbMnO₃ to 95 K for YbMn_0.7Fe_0.3O₃. Variable temperature dielectric measurements (15-110 K) show an anomaly in the dielectric constant at temperatures close to the antiferromagnetic ordering temperature for all the compositions, showing a unique correlation between the magnetic and electric field. The increase in the ordering temperature in YbMn_1-xFe_xO₃ is explained on the basis of increase in covalence of Mn/Fe-O-Mn/Fe bonds (shorter) with iron substitution.