Extraordinary enhancement of Néel transition temperature in nanoparticles of multiferroic tetragonal compositions of $\mathrm{(1 − x)BiFeO_{3}-xPbTiO_{3}}$ solid solutions

The Néel temperature (T$_N$) in conventional antiferromagnets (AFM) decreases with decreasing particle size. In contrast, we present here evidence for an exotic facet of multiferroicity, whereby one can raise the T$_{N}$ by more than 200 K by decreasing the particle size. We illustrate this by taking the example of a tetragonal composition with x = 0.5 in the solid solutions of (1-x)BiFeO$_{3}$-xPbTiO$_{3}$(BF-xPT). We attribute it to the increase in the strength of the superexchange interaction via a decrease in the ferroelectric distortion in nanocrystalline powder of BF-0.5PT. The BF-0.5PT nanoparticles also exhibit exchange bias effect due to AFM core-FM shell type magnetic nanostructure.