Low-temperature hopping and absence of spin-dependent transport in single crystals of cobalt-doped ZnO

Long needle-shaped single crystals of Zn1-xCoxO were grown at low temperatures using a molten salt solvent technique, up to x=0.10. The conduction process at low temperatures is determined to be Mott variable range hopping. Both pristine and cobalt-doped crystals clearly exhibit a crossover from negative to positive magnetoresistance as the temperature is decreased. The positive magnetoresistance of the Zn1-xCoxO single crystals increases with increased Co concentration and reaches up to 20% at low temperatures (2.5 K) and high fields (> 1 T). Superconducting quantum interference device magnetometry confirms that the Zn1-xCoxO crystals are predominantly paramagnetic in nature and the magnetic response is independent of Co concentration. The results indicate that cobalt doping of single crystalline ZnO introduces localized electronic states and isolated Co2+ ions into the host matrix but that the magnetotransport and magnetic properties are decoupled.