Nature Physics 3, 650–654 (2007).

The authors wish to point out that in the above article, the coefficients α and β as defined in equation (1) and used in equation (2) are a factor of two larger than in the usual convention (see, for example, ref. 1). Correspondingly, the quoted numbers for α and β for sample 3 should be divided by two when being compared with other work. This does not affect other parts of the paper; in particular, all reported effective-magnetic-field values remain the same.