Unusual interlayer magnetic coupling in quasi 2-D heavy-mass nearly ferromagnetic state of (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$

Perovskite ruthenates exhibit a wide range of complex magnetic ground states. In this talk we focus on an unusual heavy-mass, nearly ferromagnetic state with an extremely large Wilson ratio (Z. Qu \textit{et al.}, Phys. Rev. B \textbf{78} R180407 (2008)). Despite considerable FM correlations, this state never develops long-range FM order, instead freezing into a cluster-spin-glass (CSG) state. We have further investigated this magnetic state through in-plane angular dependence of magnetoresistivity and magnetization on (Sr$_{0.62}$Ca$_{0.38})_{3}$Ru$_{2}$O$_{7}$. The in-plane magnetoresistivity $\rho_{ab}(\phi)$ at high magnetic fields reveals a change in anisotropy symmetry from 2-fold to 4-fold at the frozen temperature $T_{f}$ of the CSG phase, whereas inter-planar magnetoresistivity $\rho_{c}(\phi)$ at high fields only exhibits 4-fold symmetry. For low magnetic fields, both $\rho_{ab}$ and $\rho_{c}$ only exhibit anisotropy below $T_{f}$, also with 4-fold symmetry. Angle-dependent magnetization data reveal that at high field the anisotropy exhibits 8-fold symmetry for $T > T_{f}$. However, for $T < T_{f}$, an additional asymmetric 2-fold anisotropy develops. These results may indicate non-traditional interlayer magnetic coupling, one possible scenario involving perpendicular spin stacking between alternate layers.