Magnetic structure of Tb_0.5Y_1.5BaNiO₅: relevance to multiferroicity

The multiferroic materials continue to be attractive due to their relevance to potential applications such as spintronics, read and write memory devices etc. Recently, we have shown that the compound, Tb₂BaNiO₅, exhibits spin-driven multiferroicity and largest magnetodielectric coupling within the prototype Haldane spin-chain R₂BaNiO₅ (R = rare-earth ion) family. Magnetic structure of Tb₂BaNiO₅ is made up of mutually canted collinear Ni and collinear Tb magnetic sublattices below 63 K down to lowest measured temperature (2 K). From detailed neutron powder diffraction (NPD) measurements, it was observed that there is a sudden increase in the (Tb)4f - (Ni)3d relative canting angle below (T_N2=) 25 K, and this is correlated to the onset of spontaneous electric polarization setting in below this temperature. This means that the relative canting angle has to cross a certain threshold for the system to show multiferroicity, at least in this case. This result further strengthens the idea of the 'critical canting angle' in such family of multiferroic compounds. (author)