We study theoretically the role of quenched magnetic disorder at the surface of a topological insulator by numerical simulation and scaling analysis. It is found that all the surface states are localized while the transverse conductivity is quantized to be ±2he2 as long as the Fermi energy is within the bulk gap. This greatly facilitates the realization of the topological magnetoelectric effect proposed by Qi {\it et al.} (Phys. Rev. B\textbf{78}, 195424 (2008)) with the surface magnetization direction being controlled by the simultaneous application of magnetic and electric fields .
@article{arxiv.1012.3804,
title = {Surface quantized anomalous Hall current and magneto-electric effect in magnetically disordered topological insulators},
author = {Kentaro Nomura and Naoto Nagaosa},
journal= {arXiv preprint arXiv:1012.3804},
year = {2011}
}