Tunneling magnetoresistance studies of Sr$_3$Ru$_2$O$_7$

Recent work has supported the existence a new type of field- tuned quantum phase transition (QPT) in the double layered ruthenate Sr$_3$Ru$_2$O$_7$. To further probe the physical properties near this QPT, we have performed planar tunneling measurements on Sr$_3$Ru$_2$O$_7$ single crystals. Our previously reported work revealed an unusual oscillation in tunneling magnetoresistance. We here report further characterization of this new phenomenon, showing that the oscillation has a systematic dependence on the tunnel barrier, temperature, and the field orientation. The oscillation pattern is identical even for different barrier materials (such as Al$_2 $O$_3$ and SiO), but is only prominent when the junction resistance is between roughly 15$\Omega$ and 1k$\Omega$. The oscillation shows a field orientation dependence for $H$//$c$ and $H$//$ab$, both in its pattern and its temperature dependence. The oscillation frequency for $H$//$ab$ appears to be smaller than that for $H$//$c$. We discuss possible origins of this unusual oscillation phenomenon in light of recent bulk measurements on Sr$_3$Ru$_2$O$_7$.