  We observe an unusual behavior of the spin Hall magnetoresistance (SMR)
measured in a Pt ultra-thin film deposited on a ferromagnetic insulator, which
is a tensile-strained LaCoO3 (LCO) thin film with the 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 results indicate that a novel low-dimensional magnetic system forms
on the surface of LCO and that the Pt/LCO interface decouples magnetically from
the rest of the LCO thin film. To explain the experiment, we revisit the
derivation of the SMR corrections and relate the spin-mixing conductances to
the microscopic quantities describing the magnetism at the interface. Our
results can be used as a technique to probe quantum magnetism on the surface of
a magnetic insulator.
