Hidden constant in the anomalous Hall effect of high-purity magnet MnSi

Measurements of the Hall conductivity in MnSi can provide incisive tests of theories of the anomalous Hall (AH) effect, because both the mean free path and magnetoresistance (MR) are unusually large for a ferromagnet. The large MR provides an accurate way to separate the AH conductivity sigma(A)(xy) from the ordinary Hall conductivity sigma(N)(xy). Below the Curie temperature T-C, sigma(A)(xy) is linearly proportional to M (magnetization) with a proportionality constant S-H that is independent of both T and H. In particular, S-H remains a constant while sigma(N)(xy) changes by a factor of 100 between 5 K and T-C. We discuss implications of the hidden constancy in S-H.