Oscillatory behavior of spin current transmission in epitaxial NiO(001) films with Pt underlayer thickness

We observe a non-trivial oscillatory behavior in spin current transmission through antiferromagnetic epitaxial NiO(001) films, realized in Pt/NiO/CoFeB trilayers by varying the thickness of the Pt underlayer (tPt). To examine the influence of epitaxy on spin transport characteristics, we prepare both (001)-epitaxial and polycrystalline Pt/NiO heterostructures, and evaluate the spin current transmitted through the NiO layer via measurements of the thermo-spin effects. In the epitaxial samples, the spin current transmission efficiency in NiO, evaluated from the thermo-spin-effects-induced temperature amplitude, exhibits several peaks at NiO thicknesses (tNiO) of 10 and 22 nm. Strikingly, for tNiO = 10 nm, the transmission efficiency as a function of tPt exhibits an oscillation-like dependence with a period of 8–10 nm, which does not appear in polycrystalline counterparts. These results indicate that the spin current transmission in epitaxial NiO is strongly modulated by the underlying epitaxial Pt layer, despite the conventional role of Pt only as a spin current source. Our findings provide experimental insights into the complex and still unresolved mechanisms governing spin current transmission in NiO.