  Magnetic and/or dielectric behaviors have been studied for YBaCuFeO5,
LuBaCuFeO5 and TmBaCuFeO5, which are known as members of oxygen-defect ordered
perovskite systems RBaCuFeO5 (R=lanthanides Ln and other trivalent elements)
and have two magnetic transitions: The magnetic structure of the high
temperature(T) ordered phase is basically antiferromagnetic, and with
decreasing T, a modulated magnetic component superposed on the
antiferromagnetic moments appears at the second transition, where the
ferroelectricity also appears (multiferroic). Here, we have found that all the
systems studied here exhibit multiferroic behavior in the low T phase, and that
TmBaCuFeO5 becomes multiferroic at a temperature as high as the melting point
of ice, which is, to our knowledge, the highest value ever found in zero
magnetic field.
