Origin of ferromagnetism in Zn1-xCoxO from magnetization and spin-dependent magnetoresistance measurements

In order to elucidate the nature of ferromagnetic signatures observed in (Zn,Co)O, we have examined experimentally and theoretically magnetic properties and spin-dependent quantum localization effects that control low-temperature magnetoresistance. Our findings, together with a through structural characterization, substantiate the model assigning spontaneous magnetization of (Zn,Co)O to uncompensated spins at the surface of antiferromagnetic nanocrystal of Co-rich wurtzite (Zn,Co)O. The model explains a large anisotropy observed in both magnetization and magnetoresistance in terms of spin Hamiltonian of Co ions in the crystal field of the wurtzite lattice.