Magnetization Reversal in Uniaxial Films near to the Preferred Direction

Abstract

It is now well known that uniaxial thin magnetic films can be made which have almost ideal square hysteresis loops in the preferred direction and almost straight line hysteresis loops in the hard direction. For this type of film, which can be prepared by evaporation from ingots of approximately 81:19 NiFe alloy, it is found that the switching time constant of the reversal process which occurs when magnetic fields are applied exactly in the easy direction can be very large. The switching process occurring when fields are applied at angle 10° to the preferred direction has been studied in detail, both by a new pulse response technique and by observation of the domain structure revealed by use of the Kerr magneto-optic effect. In the latter experiment, short pulses have been applied to the specimen and photographs taken of the domain pattern after each pulse. It was found that the specimen breaks into small domains whose area increases after each pulse and that eventually the filmcould be completely reversed if the pulse amplitude was between certain critical values. Curves were plotted of the pulse amplitude against the proportion of the film switched found by measuring the reversed area in the photographs. It was shown that the process is very well approximated by an equation of the form Sw = τ(H − H0),which implies a viscous damped process, and there is a critical nucleation field, which is less than the field required to move the walls. In the pulse response experiments, the switching time has been derived from the response to a reset pulse which was applied after a variable number of set pulses of variable amplitude. It was found that in some films there can be a rapid switching process, which is complete after about 100 mμsec, followed by a slow process lasting up to 10 μsec under the same conditions. From the experiments performed, it has been possible to compute a switching time constant which can be very large in some films. This constant is found to be correlated with the ratio p = Hc/Hk, where Hc is the coercivity in the easy direction and Hk is the rotational coercivity. If the film obeyed the simple coherent rotational model the pratio would be 1, and films which have p = 0.9 have been found have Sw = 0.3×10−6 oe μsec, whereas for p = 0.3, Sw = 7×10−6 oe μsec.