Field-tuned collapse of an orbitally ordered and spin-polarized state: Colossal magnetoresistance in the bilayered ruthenate Ca 3 Ru 2 O 7

Ca3Ru2O7 with a Mott-like transition at 48 K and a Neel temperature at 56 K features different in-plane anisotropies of the magnetization and magnetoresistance. Applying a magnetic field along the magnetic easy- axis precipitates a spin-polarized state via a first-order metamagnetic transition, but does not lead to a full suppression of the Mott state, whereas applying a magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of an orbitally ordered and spin-polarized state. This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detrimental to the colossal magnetoresistence in this 4 d-based electron system. Evidence of a density wave is also presented. DOI: 10.1103/PhysRevB.69.014404 PACS number~s!: 75.47.Gk, 71.70.Ej The physics of magnetoresistance has generated enor- mous interest in recent years. While this quantum mechani- cal phenomenon is in general associated with the spin scat- tering process of conduction electrons, the origins of various kinds of magnetoresistance are vastly different. The giant magnetoresistance observed in magnetic metallic multiplayer structures can be qualitatively explained using the two cur- rent model, corresponding to up-spin and down-spin electrons. 1 Tunneling magnetoresistance, often seen in mag- netic tunnel junctions separated by an insulating spacer layer, is a consequence of spin-polarization. On the other hand, colossal magnetoresistance ~CMR!, seen only in the mixed- valence manganites so far, originates from a metal-insulator transition in the vicinity of the Curie temperature driven pri- marily by double exchange due to the hopping of eg elec- trons of Mn 31 ions and the Jahn-Teller effect. 2 The novelty of the bilayered Ca3Ru2O7 , as presented in this paper, is that the colossal magnetoresistivity is a result of the collapse of the orbitally ordered state that is realized by demolishing the spin-polarized state . This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detri- mental to the colossal magnetoresistence in this 4 d-based electron system. Indeed, the electron kinetic energy hinges on the spin-orbital-lattice coupling in such a way that apply- ing magnetic field, B, along the magnetic easy axis ~a axis! precipitates a spin-polarized state via a first-order metamag- netic transition, but does not lead to a full suppression of the Mott state, whereas applying B along the magnetic hard axis ~b axis! does, giving rise to a resistivity reduction of three orders of magnitude. Our previous work indicated the puz- zling anisotropic behavior observed in the field dependence