  Combining topological insulators (TIs) and magnetic materials in
heterostructures is crucial for advancing spin-based electronics. Magnetic
insulators (MIs) can be deposited on TIs using the spin-spray process, which is
a unique non-vacuum, low-temperature growth process. TIs have highly reactive
surfaces that oxidize upon exposure to atmosphere, making it challenging to
grow spin-spray ferrites on TIs. In this work, it is demonstrated that a thin
titanium capping layer on TI, followed by oxidation in atmosphere to produce a
thin TiOx interfacial layer, protects the TI surface, without significantly
compromising spin transport from the magnetic material across the TiOx to the
TI surface states. First, it was demonstrated that in Bi2Te3/TiOx/Ni80Fe20
heterostructures that TiOx provided an excellent barrier against diffusion of
magnetic species, yet maintained a large spin-pumping effect. Second, the TiOx
was also used as a protective capping layer on Bi2Te3, followed by the
spin-spray growth of the MI, NixZnyFe2O4 (NZFO). For the thinnest TiOx
barriers, Bi2Te3/TiOx/NZFO samples had AFM disordered interfacial layer because
of diffusion. With increasing TiOx barrier thickness, the diffusion was
reduced, but still maintained strong interfacial spin-pumping interaction.
These experimental results demonstrate a novel method of low-temperature growth
of magnetic insulators on TIs enabled by interface engineering.
