We report the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained ${\mathrm{LaCoO}}_3$ (LCO) thin film, which is a ferromagnetic insulator with a Curie temperature $T_{\mathrm{c}}=85\mathrm{K}$. The SMR displays a strong magnetic-field dependence below $T_{\mathrm{c}}$, 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 $T_{\mathrm{c}}$ and remains measurable even above $T_{\mathrm{c}}$. 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.

• Received 29 May 2018
• Revised 13 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.180401