  The Haldane spin-chain compound, Tb2BaNiO5, with two antiferromagnetic
transitions, one at T1=63K, and the other at T2=25K, has been recently shown by
us to be an exotic multiferroic below T2. Here, we report the results of our
investigations of Sr doping at the Ba site by magnetization, heat-capacity,
magnetoelectric (MDE), and pyrocurrent measurements. An intriguing finding,
which we stress, is that the ferroelectricity is lost even for a doping level
of 10 atomic percent, though magnetic ordering prevails. The doped specimens
however retain significant magnetodielectric behaviour, but with reduced
magnitudes and qualitative changes with respect to the behaviour of the parent
compound. This implies that ferroelectric order is also crucial for the
anomalously large MDE in the parent compound, in addition to the role of 4f
single-ion anisotropy.
