Room-temperature ferromagnetism and the scaling relation between magnetization and average granule size in nanocrystalline Zn/ZnO core-shell structures prepared by sputtering

Ferromagnetism is found in nanocrystalline Zn/ZnO core-shell structures prepared by sputtering pure Zn with subsequent oxidation. The saturation magnetization (M-S) of the passivated ZnO shells increases with decrease in average particle size (d). The Curie temperature of the samples is above 400 degrees C. It is found that the ferromagnetism has a close relationship with point defects in ZnO shells. The maximum magnetization is estimated to be 28 emu cm(-3) (i.e. 0.14 mu B per unit cell) at 300 K, which is over three orders of magnitude larger than that of undoped ZnO nanoparticles or nanorods (10(-3)-10(-2) emu cm(-3)). More importantly, there is a scaling relation of M-S proportional to 1/d(n) (n = 5.20 +/- 0.20) for samples with d <= 76 nm despite substantial differences in the particle size and shape. The results suggest that defects at the interface of the Zn/ZnO heterostructure make the main magnetic contributions.