  The layered perovskite YBaCuFeO5 (YBCFO) is able to adopt chiral magnetic
order up to unexpectedly high temperatures, paving the way to strong
magnetoelectric coupling at room temperature. In this perovskite A-site cations
are fully ordered whereas the occupancy of the B-sites strongly depend on the
preparation process. Though the structure is not geometrically frustrated, the
presence of partial Fe3+/Cu2+ disorder at the B-sites produces magnetic
frustration. In an effort to increase the spin-orbit coupling in the system, we
have synthesized and studied YBaCuFe0.95Mn0.05O5 in single crystal form, where
the highly symmetric Fe3+ ions (3d5) are partially substituted with Jahn-Teller
active 3d4 Mn3+ ions. We report the structural and magnetic properties of a
highly ordered single crystal of this layered perovskite, which are presented
in comparison with a polycrystalline specimen (three times more disordered).
Single-crystal neutron diffraction measurements reveal two collinear magnetic
phases and the lack of incommensurate spiral order. The magnetic phases and
transitions found in the crystal grown by the traveling solvent floating zone
(TSFZ) method are fully described and analyzed in the light of its high level
of Fe/Cu cationic order (~90%).
