Al and Ni co-doped ZnO films with room temperature ferromagnetism, low resistivity and high transparence

Zn0.91Al0.07Ni0.02O and Zn0.90Al0.05Ni0.05O films of about 250 nm thick were deposited on glass substrates at 300 K by co-sputtering with ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h and then cooled down to room temperature under a magnetic field of 4.8 × 104 A m−1 applied along the film plane. After this process the films showed room temperature ferromagnetism, a resistivity of about 2 × 10−3 Ω cm and an average transmittance of 75% in the visible wavelength range. The films have a wurtzite structure with the c-axis orientation in the film growing direction and consist of thin columnar grains perpendicular to the substrate. A temperature dependence of the resistivity from 2 K to 300 K reveals that the carrier transport mechanism is thermally activated band conduction above 150 K and Mottʹs variable range hopping below 70 K.