Negative magnetoresistance in quantized accumulation layers on ZnO surfaces

Results are presented of magnetoresistance in quantized accumulation layers on the ($000\overline{1}$) surface of ZnO. The measurements were performed at low temperatures $T(2-80\mathrm{} \mathrm{K})$ and very high surface electron densities [(0.7-7) × ${10}^{13}$ ${\mathrm{cm}}^{-2\mathrm{}}$]. The magnetoresistance was found to be negative over the entire studied range of magnetic induction $B$ (up to 60 kG). Its absolute magnitude increases quadratically with $\frac{B}{T}$ at low values of $\frac{B}{T}$ and tends to saturation at high $\frac{B}{T}$. A spin scattering model adapted to surface channels accounts very well for the experimental data. The main conclusion drawn is that scattering centers with giant magnetic moments (around 100 Bohr magnetons) must be present on the ZnO surface. Such giant moments may originate from ionic clusters in combination with the free electrons localized at and/or screening the charged clusters. A quantitative analysis of the data reveals also the presence of centers with much lower magnetic moments (around 10 Bohr magnetons).