  The phase diagram of Sr3Ru2O7 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-Ferrell-Larkin-Ovchinnikov state. Based on a microscopic
theory of Stoner magnetism we derive a Ginzburg-Landau expansion where the
magnetisation transverse to the applied field can become spatially
inhomogeneous. We show that this reproduces the observed phase diagram of
Sr3Ru2O7.
