  Multiferroic properties of orthorhombic HoMnO3 (Pbnm space group) are
significantly modified by epitaxial compressive strain along the a-axis. We are
able to focus on the effect of strain solely along the a-axis by using an YAlO3
(010) substrate, which has only a small lattice mismatch with HoMnO3 along the
other in-plane direction (the c-axis). Multiferroic properties of strained and
relaxed HoMnO3 thin films are compared with those reported for bulk, and are
found to differ widely. A relaxed film exhibits bulk-like properties such as a
ferroelectric transition temperature of 25 K and an incommensurate
antiferromagnetic order below 39 K, with an ordering wave vector of (0 qb 0)
with qb ~ 0.41 at 10 K. A strained film becomes ferroelectric already at 37.5 K
and has an incommensurate magnetic order with qb ~ 0.49 at 10 K.
