Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel Cucr 2 Se

In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question whether JH is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4−xBrx, the resistivity ρ (at low temperature) may be increased 1000 fold by varying x(Br), without degrading the M. We show that JH/E (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.