  Breaking the time-reversal symmetry of a topological insulator (TI) by
ferromagnetism can induce exotic magnetoelectric phenomena such as quantized
anomalous Hall (QAH) effect. Experimental observation of QAH effect in a
magnetically doped TI requires ferromagnetism not relying on the charge
carriers. We have realized the ferromagnetism independent of both polarity and
density of carriers in Cr-doped BixSb2-xTe3 thin films grown by molecular beam
epitaxy. Meanwhile, the anomalous Hall effect is found significantly enhanced
with decreasing carrier density, with the anomalous Hall angle reaching
unusually large value 0.2 and the zero field Hall resistance reaching one
quarter of the quantum resistance (h/e2), indicating the approaching of the QAH
regime. The work paves the way to ultimately realize QAH effect and other
unique magnetoelectric phenomena in TIs.
