  A series of manganites Tb1-xHoxMnO3 (0<=x<=0.6) with orthorhombic structure
are synthesized and detailed investigations on their multiferroicity are
performed. Successive magnetic transitions upon temperature variation are
evidenced for all samples, and both the Mn3+ spiral spin ordering and
rare-earth spin ordering are suppressed with increasing x. Significant
enhancement of both the polarization and magnetoelectric response within
0.2<x<0.4 is observed, which may be ascribed to the competition possibly
existing between spiral and E-type spin orders. Theoretical calculation is
given based on two eg-orbital double-exchange model, and the result supports
the scenario of the multiferroic phase separation.
