Strain-induced ferromagnetism enhancement in Co:ZnO films

The structural and ferromagnetic properties of Zn0.95Co0.05O films grown on Si and LiNbO3 (LNO) substrates have been studied as a function of thickness (15–900 nm). The structural characterizations indicate that the c-axis lattice constant and Co–O bond length slightly decrease with the increase in film thickness, implying the progressive relaxation of the tensile strain. The magnetic measurements show that a larger strain can result in an enhancement of room temperature ferromagnetism. The thinnest films (15 nm) with the largest lattice strains possess the highest saturated magnetic moments, i.e., 5.52 and 2.96μB/Co in Co:ZnO/LNO and Co:ZnO/Si films, respectively. As the film becomes thicker, the saturated ferromagnetism rapidly decreases, which is about two orders of magnitude smaller than that of the 15-nm-thick film when its thickness is 900 nm. The enhancement of ferromagnetism in Co:ZnO films originates from the combination of enlarged Co–O bond length and increased defect amount induced by strain.