Jumps in Magnetization and Hall Current in a Hard Magnet

In general, electronic motion in a crystal possesses time-reversal symmetry. One cannot tell if a movie of the conduction electrons is running backwards. However, in a ferromagnet, this time-reversal "symmetry" is no longer valid. Because the spins are all aligned in a magnet, the time-reversed movie looks very different. A major consequence is the appearance of a sideways current called the anomalous Hall effect (AHE). There is intense interest in the AHE current because it reveals important aspects of electronic behavior in spin-aligned materials. However, it is often difficult to separate from the ordinary Hall current (OHC). Recently, PCCM researchers discovered an unusual ferromagnet Fe_1/4TaS₂ in which the spins (magnetization M) reverse direction very abruptly as a function of the magnetic field H (Fig. a, [1]). The sharp reversal causes an abrupt jump in the Hall current, which allows the AHE current to be measured directly (Fig. b). The clean measurement of the AHE over a broad range of temperatures T reveals a previously unsuspected scaling relationship with the resistivity [2].