  High critical-temperature superconductivity and large (colossal)
magnetoresistances are two important electronic conducting phenomena found in
transition metal oxides. High-Tc materials have applications such as
superconducting magnets for MRI and NMR, and magnetoresistive materials may
find use in magnetic sensors and spintronic devices. Here we report chemical
doping studies of RuSr2(R2-xCex)Cu2O10-d ruthenocuprates which show that a
single oxide system can be tuned between superconductivity at high hole dopings
and varied magnetoresistive properties at low doping levels. A robust variation
of negative magnetoresistance with hole concentration is found in the
RuSr2R1.8-xY0.2CexCu2O10-d series, while RuSr2R1.1Ce0.9Cu2O10-d materials show
an unprecedented crossover from negative to positive magnetoresistance with
rare earth (R) ion radius.
