  Hexagonal Dy(0.5)Y(0.5)MnO3, a multiferroic rare-earth manganite with
geometrically frustrated antiferromagnetism, has been investigated with
single-crystal neutron diffraction measurements. Below 3.4 K magnetic order is
observed on both the Mn (antiferromagnetic) and Dy (ferrimagnetic) sublattices
that is identical to that of undiluted hexagonal DyMnO3 at low temperature. The
Mn moments undergo a spin reorientation transition between 3.4 K and 10 K, with
antiferromagnetic order of the Mn sublattice persisting up to 70 K; the
antiferromagnetic order in this phase is distinct from that observed in
undiluted (h)DyMnO3, yielding a qualitatively new phase diagram not seen in
other hexagonal rare-earth manganites. A magnetic field applied parallel to the
crystallographic c axis will drive a transition from the antiferromagnetic
phase into the low-temperature ferrimagnetic phase with little hysteresis.
