Absence of ferromagnetism in transition metal (Co, Ni and Cu) doped ZnO films

Structural, optical and magnetic properties of oriented Zn-0.95 TM0.05O (TM = Co, Ni, Cu) films prepared on amorphous quartz substrate using RF magnetron sputtering have been studied. X-ray diffraction patterns show that all films are single phase and are preferably oriented in (002) direction. An analysis of transmission spectra shows that the optical band gap is reduced for all doped ZnO films with respect to that of bulk ZnO single crystal. With increase in thickness of Cu doped ZnO films, the band gap reduces further (3.00 eV for 200 nm film and 2.86 eV for 400 nm film). A strong absorption is observed at 665 nm for Co doped ZnO film due to transitions of tetrahedrally coordinated Co2+ from (4)A(2) (F-4) -> E-2 (( 2)G). A pre-edge peak observed in TM K-edge XANES spectra of each film confirms that TM ions get substituted in Zn site of ZnO hexagonal wurtzite structure. No room temperature ferromagnetism is observed in any of doped films.