Ferromagnetic clusters and the spin-state transition in clusters of La0.5Sr0.5Co1-xRuxO3

La0.5Sr0.5Co1-xRuxO3 system has been studied by the measurements of XRD, magnetic, electrical transport, and magnetoresistance properties. Magnetic measurements indicate that most Co3+ and Co4+ ions are stabilized in intermediate spin IS state, while partial Co ions are converted into HS state. The FM state resistivity behavior for x=0, 0.025, 0.05 can be fitted using the formula ρ(T)=ρ0+ρ1T9/2+ρ2T2, while the insulating behavior for x=0.1, 0.2, 0.3 obeys the VRH model. Electronic phase separation ferromagnetic-cluster model and the quantum interference effects are used to explain the MR behavior. Meanwhile, in this system, we find that the magnetic field has two competing contributions to the MR. One is the suppression of spin-disorder scattering, which result in a negative magnetoresistance. The others are both the spin-state transition of Co3+ in clusters and quantum interference effects, which are enhanced by an applied field and produce a positive magnetoresistance. This result is supported by the low-temperature positive MR for x=0.025 and 0.05. The ferromagnetic-cluster model is also supported by the shift of the low-temperature peak in χ″(T), which is corresponding to the freezing of the ferromagnetic clusters, indicating the decrease of the size of clusters with increasing x.