  We report an unusual canted magnetism due to 3d and 4f electrons, occupying
two different crystallographic sites, with its consequence to electric dipole
order. This is based on neutron powder diffraction measurements on Tb2BaNiO5
(orthorhombic, Immm centrosymmetric space group), exhibiting Neel order below
(TN) 63 K, to understand multiferroic behavior below 25 K. The magnetic
structure is made up of Ni and Tb magnetic moments, which are found to be
mutually canted in the entire temperature range below TN, though collinearity
is seen within each sublattice, as known in the past. First-principles density
functional theory calculations (GCA plus SO and GCA plus U plus SO
approximations) support such a canted ground state. The intriguing finding,
being reported here, is that there is a sudden increase in this Tb-Ni relative
cantingle angle at the temperature (that is, at 25 K) at which spontaneous
electric polarization sets in, with bond distance and bong angle anomalies.
This finding emphasizes the need for a new spin-driven polarization mechanism,
that is, a critical canting angle coupled with exchangestriction, to induce
multiferroicity in magnetic insulators with canted spins.
