Size-induced metallic state in nanoparticles of ferromagnetic insulating Nd0.8Sr0.2MnO3

A detailed investigation of the electronic- and magneto-transport properties of Nd0.8Sr0.2MnO3 with the variation of grain size (down to 42 nm) is presented here. Interestingly, we observe that the ferromagnetic insulating state is suppressed and a metallic state is stabilized as the grain size of the sample is reduced. As a result, a metal-insulator transition is observed in this low doped manganite which is insulating in nature in its bulk form. Destabilization of polaronic order in the ferromagnetic insulating state due to enhanced surface disorder on grain size reduction is attributed to this effect. A phenomenological model is proposed to represent the concept of destabilization of polaron formation in the surface region of the nanograins. Resistivity and magnetoresistance data are carefully analysed employing different suitable models. Electrical third harmonic resistance is measured to directly probe the electrical nonlinearity in the samples.