Detection of the magnetlzation reversal of individual interacting single-domain particles within co-cr columnar thin-films

Abstract

The fundamental “Barkhausen” noise generated by the magnetization reversal of individual particles within a particulate magnetic medium has been observed using the anomalous Hall effect (AHE) as a strikingly sensitive magnetization probe. This is the first time the reversal of individual interacting single or nearly single domain particles has been detected. The jumps correspond to magnetic switching volumes of ~3 x 10–15cm3with moments around 10–12emu. The magnetization in a Co-Cr thin film, a columnar perpendicular magnetic recording medium, is determined by passing a current through the film and measuring the AHE voltage induced within the sample itself. Hall samples as small as 0.6µ and containing only a few hundred columns have been made via microlithography. For these samples, the voltage change due to the reversal of a single column can be readily detected, with a system sensitivity of 4 x 10–14emu. The spatial dependence of the measurement response over the surface of the Hall geometry has been determined by both calculation and experiment so that the magnitude of magnetization changes detected by jumps in the AHE voltage can be adjusted for geometric effects. Barkhausen noise,1i.e., step-like changes in the magnetization of continuous magnetic media, is generated by the stick and slip motion of domain walls when a magnetic field is applied. Magnetic switching in particulate magnetic media may also be discontinuous, when the magnetization changes by discrete reversals of individual particles. While the wall motions of Barkhausen noise cause moment changes around 10–6emu, the single domain particles2of a particulate medium have volumes of less than 10–15cm3and moments of less than 10–12emu. Thus the magnetization jumps due to the reversal of individual particles within a particulate medium have never been previously detected. The sensitivity of a commercial SQUID magnetometer is ~10-8 emu, approximately six orders of magnitude poorer than that desired for our experiments. Wouri and Judy3 proposed that the necessary sensitivity could be obtained by using the anomalous Hall effect as a probe of the sample magnetization in a perpendicularly-magnetized, particulate magnetic thin film. When a current is passed through such a film, the anomalous Hall effect4(AHE) generates a voltage, V, across the film proportional to the perpendicular component of the magnetization and the width of the film w. Since number of particles, N, is proportional to the area of the sample or w2, the voltage per particle, V/N increases as 1/w if the sample is made smaller. The abrupt reversal of a single particle will produce a jump in the anomalous Hall voltage equal to V/N. Wouri and Judy made Co-Cr Hall samples on a five micron scale but were unable to detect voltage jumps within their electronic resolution. We have repeated this type of experiment using improved electronics and with Co-Cr Hall structures as small as 0.7 µ x 0.7 µ. Under these conditions the change in voltage ΔV, due to the reversal of a single 800 A diameter particle is ~300 nV which is approximately fifty times our electronic noise level. We have observed distinct steps in the magnetization during a hysteresis cycle which we interpret as due to the reversal of individual particles. © 1988 IEEE