Stabilization of the ferromagnetic metallic state in rare earth-doped La0.49X0.01Ca0.50MnO3+δ (X=Nd, Sm, Gd and Yb)

We report the effects of disorder induced by a small amount of substitution of the smaller cations (Nd, Sm, Gd and Yb) for La in the La0.50Ca0.50MnO3+δ   system. With decreasing size of the dopant, the ferromagnetic and metallic state is stabilized while the AFM and insulating behaviour is completely eliminating. The magnetic moment below TcTc increases in general, with decreasing dopant size. The behaviour is interpreted in terms of the destabilization of the charge ordering (CO) due to the disorder induced by the size mismatch of the cations. Our data support the view that close to the COI–FM phase boundary, the effect of disorder is to weaken the CO that is more sensitive to disorder, whereas it leaves the more robust double exchange relatively unaffected, thereby extending the region in phase space where the FM phase is stable.