We report the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained LaCoO3 (LCO) thin film, which is a ferromagnetic insulator with a Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependence of the SMR. These unusual behaviors indicate that a low-dimensional magnetic system forms at the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. Transmission electron microscopy analysis of the heterostructure reveals that an ultrathin Co-rich layer forms at the LCO surface upon deposition of Pt, which is separated from the rest of the LCO film by a ∼1-nm La/O-rich layer, thus supporting the presence of a low-dimensional ferromagnetic system. To explain the magnetoresistance measurements, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the spin-spin correlation functions and microscopic quantities describing the magnetism at the interface. Comparisons between theory and experiment confirm the existence of a low-dimensional Heisenberg ferromagnet at the interface. Our results pave the way for exploring complex magnetic textures of insulating films by simple transport measurements.

This work was supported by the European Union 7th Framework Programme under the Marie Curie Actions (607904- 13-SPINOGRAPH), the European Research Council (257654-SPINTROS), and the European Regional Development Fund (ERDF), by the Spanish Ministry of Economy, Industry and Competitiveness (MAT2015-65159-R, FIS2014-55987-P, MAT2013-46593-C6-4-P, MAT2016-80762-R and FIS2017-82804-P), by the Basque Government (UPV/EHU Project IT-756-13 and IT-621-13), by the Regional Council of Gipuzkoa (100/16), and by the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016–2019, ED431G/09).