  In this investigation, Gd and Mn co-doped
Bi$_{0.85}$Gd$_{0.15}$Fe$_{1-x}$Mn$_x$O$_3$ (x=0.0-0.15) nanoparticles have
been prepared to report the influence of co-substitution on their structural,
optical, magnetic and electrical properties. Due to simultaneous substitution
of Gd and Mn in BiFeO$_3$, the crystal structure has been modified from
rhombohedral (R3c) to orthorhombic (Pn21a) and the Fe-O-Fe bond angle and Fe-O
bond length have been changed. For Mn doping up to 10 % in
Bi$_{0.85}$Gd$_{0.15}$Fe$_{1-x}$Mn$_x$O$_3$ nanoparticles, the saturation
magnetization (Ms) has been enhanced significantly, however, for a further
increase of doping up to 15 %, the Ms has started to reduce again. The
co-substitution of Gd and Mn in BiFeO$_3$ nanoparticles also demonstrates a
strong reduction in the optical band gap energy and electrical resistivity
compared to that of undoped BiFeO$_3$.
