Spin-polarized transport current in n-type codoped ZnO thin films measured by Andreev spectroscopy

We use point-contact Andreev-reflection measurements to determine the spin polarization of the transport current in pulse laser deposited thin films of ZnO with 1% Al and with and without 2% Mn. Only films with Mn are ferromagnetic and show spin polarization of the transport current of up to 55 {+-} 0.5% at 4.2 K, in sharp contrast to measurements of the nonmagnetic films without Mn where the polarization is consistent with zero. Our results imply strongly that ferromagnetism in these Al-doped ZnO films requires the presence of Mn.