Spin polarization of Zn1−xCoxO probed by magnetoresistance

The spin polarization of Zn0.32Co0.68O1−v (v means oxygen vacancies) concentrated magnetic semiconductor (CMS) films was extracted from measurements of tunneling magnetoresistance (TMR), and spin-dependent variable range hopping, respectively. A TMR ratio of 19.1% was observed at 2 K in Co/ZnO/Zn0.32Co0.68O1−v magnetic tunnel junctions, which gives a low limit of the spin polarization of 25% in the Zn0.32Co0.68O1−v CMS. The TMR decreases with increasing temperature and bias voltage mainly due to the tunneling via localized impurity states in the barrier. By contrast, the spin polarization of the Zn0.32Co0.68O1−v CMS was estimated to be 36.1% by spin-dependent variable range hopping.