  The layered perovskite YBaCuFeO5 (YBCFO) is considered one of the best
candidates to high-temperature chiral multiferroics with strong magnetoelectric
coupling. In RBaCuFeO5 perovskites (R: rare-earth or Y) A-site cations are
fully ordered whereas their magnetic properties strongly depend on the
preparation process. They exhibit partial cationic disorder at the B-site that
generates a magnetic spiral stabilized through directionally assisted long
range coupling between canted locally frustrated spins. Moreover the
orientation of its magnetic spiral can be critical for the magnetoelectric
response of this chiral magnetic oxide. We have synthesized and studied
YBaCuFe1-xMnxO5 samples doped with Mn, with the aim of increasing spin-orbit
coupling effects, and found that the overall Fe/Cu cation disorder at the
B-sites can be increased by doping without changing the sample preparation
process. In YBaCuFe1-xMnxO5 samples prepared under the same conditions, the T-x
magnetic phase diagram have been constructed in the range 10K-500K combining
magnetometry, X-ray and neutron powder diffraction measurements. The tilting
angles of the spins in the collinear, {\theta}col , and spiral phases,
{\theta}spiral, barely vary with temperature. In the collinear phase
{\theta}col is also independent of the Mn content. In contrast, the presence of
Mn produces a progressive reorientation of the plane of the magnetic helix in
the incommensurate phase, capable to transform the helicoidal spin ordering
into a cycloidal one, which may critically determine the ferroelectric and
magnetoelectric behavior in these compounds. Some of the observations are of
interest for engineering and developing this family of potential
high-temperature multiferroics.
