Antiferromagnetism and bulk spin valve effect in Ca$_{3}$(Ru$_{1-x}$Ti$_{x}$)$_{2}$O$_{7}$

Ca$_{3}$Ru$_{2}$O$_{7}$ has generated growing interest. It shows an antiferromagnetic (AFM) transition at $T_{N}=56$ K, followed by a metal-insulator (MI) transition at $T_{MI}=48$ K $\left[1\right]$. Giant magnetoresistance (GMR) across its metamagnetic transition is also observed. We have determined the magnetic structures of Ca$_{3}$Ru$_{2}$O$_{7}$ under magnetic fields using neutron scattering $\left[2\right]$. Our results demonstrate that the GMR in this material originates from a bulk spin-valve effect, and clarify the origin for the puzzling observation that the GMR occurs under easy axis field alignment, while a colossal magnetoresistance appears with hard axis field alignment $\left[1\right]$. In addition, we have studied the effect of Ti doping on Ca$_{3}$Ru$_{2}$O$_{7}$. We find that Ti doping dramatically affects both the AFM and the MI transition by shifting them to much higher temperatures, e.g. $T_{N}=114$ K and $T_{MI}=107$ K for 10\% Ti doping. We will discuss possible origins for this remarkable doping effect. This work is supported by the NSF under grant DMR-0645305, the DOE under DE-FG02-07ER46358.\\ $\left[1\right]$ G.Cao \textit{et al}., Phys. Rev. Lett. \textbf{78}, 1751 (1997)\\ $\left[2\right]$ Wei Bao \textit{et al}., Phys. Rev. Lett. \textbf{100}, 247203 (2008)