  We have studied the influence of external pressure up to 1 GPa on the
magnetic transitions of the orthorhombic Haldane-spin chain compound Tb2BaNiO5
an exotic multiferroic material. This parent compound is known to undergo
N\'eel ordering at TN1= 63 K and another magnetic transition at TN2= 25K at
which ferroelectricity sets in, however, without any change in the magnetic
symmetry, but with only a sharp change in the canting angle of Tb 4f and Ni 3d
magnetic moments. There is a subtle difference in the antiferromagnetic state
above and below TN2, which is supported by the fact that there is a
metamagnetic transition below TN2only (for 5 K, at about 60 kOe). We report
here that, with the application of external pressure, there is an upward shift
of TN1, while TN2 shifts towards lower temperatures. It is interesting that the
two magnetic transitions in the same compound behave differently under pressure
and the opposite behavior at TN2 is attributed to local distortion leading to
ferroelectricity. The results are augmented by temperature dependent x-ray
diffraction and positive chemical pressure studies. The chemical pressure
caused by the isoelectronic doping at Ba site by Sr reduces both the transition
temperatures. Clearly, the external pressure favors antiferromagnetic coupling
(that is, leading to TN1 enhancement), whereas the chemical pressure reduces
TN1, suggesting important role of the changes in local hybridization induced by
doping on magnetism in this material.
