Recent advances in complex oxide heterostructures have realized extraordinary control over oxygen vacancies (VO), including strain-tuned VO order, and electric-field-controlled transformations between perovskite and VO-ordered structures. Perovskite cobaltites such as La1-xSrxCoO3-δ provide a prime example, recent work demonstrating that strain engineering of VO ordering induces large (∼107erg/cm3) perpendicular magnetic anisotropy. Here we show that VO-ordered epitaxial La0.5Sr0.5CoO3-δ films exhibit not only strong magnetic anisotropy, but also a giant form of anisotropic magnetoresistance (AMR). This has magnetic field, temperature, and angular dependencies in quantitative accord with conventional AMR, but with AMR ratios up to an extraordinary 40.3%, 20 times enhanced over bulk cobaltites, and ∼10-100 times larger than typical transition metals. This giant AMR has no strong dependence on heteroepitaxial strain (between -2.1% and +1.8%) or thickness, and is instead ascribed to symmetry lowering associated with VO ordering. The AMR ratios thus obtained are among the largest reported in the over 160-year history of this phenomenon, despite the absence of heavy elements.
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Acknowledgments. Work primarily supported by the U.S. Department of Energy through the University of Minnesota (UMN) Center for Quantum Materials, under Grant No. DE-SC-0016371. Parts of this work were carried out in the Characterization Facility, UMN, which receives partial support from the NSF MRSEC program. Electron microscopy was carried out at the Centro Nacional de Microscopía Electrónica (UCM, Spain). Financial support from the Spanish MINECO/FEDER through Grants No. MAT2015-066888-C3-3-R and No. RTI2018-097895-B-C43 is acknowledged by M.V. and M.C. We thank T. Birol for useful discussions.