Unconventional exchange bias and enhanced spin pumping efficiency due to diluted magnetic oxide at the Co/ZnO interface

Exchange bias (EB) is normally created by the interfacial exchange coupling at a ferromagnetic/antiferromagnetic (FM/AFM) interface. FM/AFM interfaces have also been proved to perform enhanced spin angular momentum transfer efficiency in spin pumping (SP), compared with typical FM/nonmagnetic interfaces. Here, we report an unexpected EB and enhanced SP between a ferromagnet and semiconductor. Considerable EB has been observed in Co films grown on ZnO single crystal due to the interface antiferromagnetism of the Zn1−xCoxO (x depends on the Co solubility limit in ZnO) layer. Moreover, SP measurements demonstrate a giant spin pumping efficiency at the Co/ZnO interface with a bump (spin mixing conductance Geff↑↓= 28 nm−2) around the blocking temperature TB ∼ 75 K. The enhanced SP is further confirmed by inverse spin Hall effect measurements and the spin Hall angle θISHE of Zn1−xCoxO is estimated to be 0.011. The bound magnetic polarons with s–d exchange interaction between donor electrons and magnetic cation ions in Zn1−xCoxO play a key role in the formation of antiferromagnetism with giant Geff↑↓. Our work provides a new insight into spin physics at FM/semiconducting interfaces.