Inhomogeneous magnetic phases: a LOFF-like phase in Sr$_3$Ru$_2$O$_7$

The phase diagram of Sr$_3$Ru$_2$O$_7$ contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We show - through a Ginzburg- Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous - that a Stoner model with electronic band dispersion can reproduce this phase diagram and transport behaviour.