  We report a pervasive phenomenon of gradual emergence of negative
magnetization in typical $R_{1-x}D_x$MnO$_3$ orthorhombic perovskite manganites
in nano form (with $R^{3+}$ being a trivalent high-magnetic-moment ion from the
heavier rare-earth elements Gd, Tb, Dy and $D^{2+}$ refers to divalent alkaline
elements Sr, Ca, etc.), while the bulk form manifests no negative
magnetization. Extensive magnetization studies have been carried out in
Gd$_{1-x}$Ca$_x$MnO$_3$ manganites around half doping. We demonstrate
experimentally that particle size reduction in nano-form manganites enhances
the propensity of the system to exhibit negative magnetization and provide a
theoretical explanation for the phenomenon. To test the universality of our
findings, we have extended the measurement to Dy$_{0.6}$Ca$_{0.4}$MnO$_3$ and
obtained similar results.
