  We have successfully induced multiferroic behavior in the A-type
antiferromagnet SmMnO3 by the substitution of Y at the Sm site. A magnetic
transition develops at 24 K for Sm1-xYxMnO3 (x=0.4,0.5) which is not present in
the parent compound. This transition coincides with the onset of electric
order, with an electric polarization measured along the c axis. It is proposed
that the effect of Y doping is to bring about a subtle distortion of the MnO6
octahedra, causing a magnetic ordering of the Mn3+ moments similar to that
reported for the well studied multiferroic TbMnO3. Following on from our
previous study on polycrystalline samples, we present measurements of the
magnetic and electric properties of single crystal Sm0.6Y0.4MnO3 and
Sm0.5Y0.5MnO3. The data are summarized in a phase diagram for each of the
principal crystallographic axes for the x=0.5 compound. Powder neutron
diffraction experiments on SmMnO3 and Sm0.6Y0.4MnO3 show that the Y
substitution causes a change in the Mn-O-Mn bond angle towards the value found
for TbMnO3. The magnetic structure of Sm0.6Y0.4MnO3 has been shown to consist
of two phases: a sinusoidal ordering of the Mn3+ moments below 50 K and a
cycloidal ordering below 27 K. The cycloidal ordering occurs at the same
temperature as the previously observed ferroelectric polarization, suggesting a
similar multiferroic mechanism to that found in TbMnO3.
