  During the past years there has been renewed interest in the wide-bandgap
II-VI semiconductor ZnO, triggered by promising prospects for spintronic
applications. First, ferromagnetism was predicted for dilute magnetic doping.
In comprehensive investigation of ZnO:Co thin films based on the combined
measurement of macroscopic and microscopic properties, we find no evidence for
carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically
excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when
phase separation or defect formation occurs, due to nanometer-sized metallic
precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit
indication of ferromagnetism.
  Second, its small spin-orbit coupling and correspondingly large spin
coherence length makes ZnO suitable for transporting or manipulating spins in
spintronic devices. From optical pump/optical probe experiments, we find a spin
dephasing time of the order of 15 ns at low temperatures which we attribute to
electrons bound to Al donors. In all-electrical magnetotransport measurements,
we successfully create and detect a spin-polarized ensemble of electrons and
transport this spin information across several nanometers. We derive a spin
lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding
well to results from an electrical pump/optical probe experiment.
