  Electrical manipulation of magnetism via spin-orbit torques (SOTs) promises
efficient spintronic devices. In systems comprising magnetic insulators and
heavy metals, SOTs have started to be investigated only recently, especially in
systems with interfacial Dzyaloshinskii-Moriya interaction (iDMI). Here, we
quantitatively study the SOT efficiency and iDMI in a series of gadolinium
gallium garnet (GGG) / thulium iron garnet (TmIG) / platinum (Pt)
heterostructures with varying TmIG and Pt thicknesses. We find that the
non-monotonic SOT efficiency as a function of the magnetic layer thickness is
not consistent with the 1/thickness dependence expected from a simple
interfacial SOT mechanism. Moreover, considering the insulating nature of TmIG,
our results cannot be explained by the SOT mechanism established for metallic
magnets where the transverse charge spin current can inject and dephase in the
magnetic layers. Rather we can explain this non-monotonic behavior by a model
based on the interfacial spin mixing conductance that is affected by the
thickness-dependent exchange splitting energy by determining the phase
difference of the reflected spin-up and spin-down electrons at the TmIG / Pt
interface. By studying the Pt thickness dependence, we find that the effective
DMI for GGG / TmIG / Pt does not depend on the Pt thickness, which indicates
that the GGG / TmIG interface is the source of the iDMI in this system. Our
work demonstrates that SOT and DMI can originate from two different interfaces,
which enables independent optimization of DMI and SOT for advanced chiral
spintronics with low damping magnetic insulators.
