Isotope-driven perturbation of the ground-state properties in (Sm, Gd)-Sr manganites near the half-doped hole concentration

We report on the effect of isotopic exchange O-16-->O-18 on the magnetic and transport properties of the ground state in (Gd0.5Sm0.5)(0.55)Sr0.45MnO3. The ground state of the O-16 sample is a ferromagnetic metal. Isotopic exchange drives that state into a antiferromagnetic insulator. Moderate applied magnetic fields have little effect on both ground states. However, higher fields can restore the ferromagnetic metallic properties of the O-18 sample. The high-temperature phase at that concentration shows a competition between different ordered phases as evident from the existence of a different scale transition temperature T*>T-C at which the bulk of the samples transforms into a disordered paramagnetic phase.