Spin Valve Effect and Magnetoresistivity in Single Crystalline Ca₃Ru₂O₇

The laminar perovskite Ca₃Ru₂O₇ naturally forms ferromagnetic double layers of alternating moment directions, as in the spin-valve superlattices. The mechanism of the 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.