  The orthorhombic Haldane spin chain compound Tb2BaNiO5 (Neel order, TN1= 63
K) has been shown to be an exotic multiferroic system below (TN2) 25 K due to
various fascinating features, pointing to a strong potential for the
advancement of concepts in this field. In particular, the rare-earth ions play
a direct decisive role unlike in many other well known multiferroic materials
and there appears to be a critical canting angle, developing below TN2,
subtended by Tb 4f and Ni 3d moments to trigger this cross coupling phenomenon.
However, for a small replacement of Sr for Ba, viz. in Tb2Ba0.9Sr0.1NiO5,
ferroelectricity was reported to get destroyed, but retaining magnetic features
at (TN1) 55 K and (TN2) 14 K. In this article, we address the origin of
suppression of multiferrocity in this Sr doped system through neutron
diffraction studies and density functional theory calculations. We find that,
unlike in Tb2BaNiO5, there is no pronounced change in the relative canting
angle of the magnetic moments around TN2 and that the absolute value of this
parameter down to 2 K fails to exceed the critical value noted for the parent,
thereby explaining the origin of destruction of magnetoelectric coupling in the
Sr doped material. This finding renders strong support to the proposal of
possible existence of critical canting angle, at least in some cases, to induce
multiferroicity, apart from serving as a route to engineer multiferroic
materials for applications.
