Pressure-induced anomalies in the magnetic transitions of the exotic multiferroic material Tb 2 BaNiO 5

We have studied the influence of external pressure up to 1 GPa on the magnetic transitions of the orthorhombic Haldane-spin chain compound ${\mathrm{Tb}}_{2}{\mathrm{BaNiO}}_{5}$, an exotic multiferroic material. This parent compound is known to undergo N\'eel ordering at ${T}_{N1}=63\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and another magnetic transition at ${T}_{N2}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ 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 ${T}_{N2}$, which is supported by the fact that there is a metamagnetic transition below ${T}_{N2}$ only (for 5 K, at \ensuremath{\sim}60 kOe). We report here that, with the application of external pressure, there is an upward shift of ${T}_{N1}$, while ${T}_{N2}$ 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 ${T}_{N2}$ 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 ${T}_{N1}$ enhancement), whereas the chemical pressure reduces ${T}_{N1}$, suggesting an important role of the changes in local hybridization induced by doping on magnetism in this material.