  The laminar perovskite Ca3Ru2O7 naturally forms ferromagnetic double-layers
of alternating moment directions, as in the spin-valve superlattices. The
mechanism of huge magnetoresistive effect in the material has been
controversial due to a lack of clear understanding of various magnetic phases
and phase-transitions. In this neutron diffraction study in a magnetic field,
we identify four different magnetic phases in Ca3Ru2O7 and determine all
first-order and second-order phase transitions between them. The spin-valve
mechanism then readily explains the dominant magnetoresistive effect in
Ca3Ru2O7.
