Time-dependent magnetization in epitaxial hard magnetic thin films

Abstract

Time-dependent magnetic behaviour at room temperature is investigated in highly oriented epitaxial hard magnetic thin films. Single-layer films with nominal composition PrCo5 and PrCo7 were studied, along with two SmCo5/Fe/SmCo5 trilayer films, with Fe layer thicknesses of 11 and 16 nm. The films were prepared using pulsed laser deposition on Cr buffered MgO (110) substrates. For the trilayer system, judicious choice of the thickness of the hard (SmCo5) and soft (Fe) magnetic layers results in an exchange-coupled two-phase magnet. All the thin films exhibit switching behaviour, i.e. a near square hysteresis loop, and the details of this behaviour are explored through both measurement of the hysteresis loop, at very slow field sweep rates, and the irreversible susceptibility, χirr. Measurements of the sweep rate dependence of the intrinsic coercivity, and magnetic viscosity on the major hysteresis loop were performed. For each film, three techniques were used to determine the fluctuation field, Hf: from the sweep rate data; from the magnetization relaxation data using the waiting time method; and from S/χirr, where S is the magnetic viscosity coefficient. Differences in the value of Hf determined using the three different techniques, and the equivalence of the three techniques, are discussed. Further insights into the time-dependent behaviour of the magnetization are gained from observations of the spontaneous remagnetization, following dc demagnetization. The study shows the thin films to be of high-stability. Furthermore, it supports the view that magnetization reversal in these materials is most likely an incoherent process, and not a process involving the uniform reversal of a grain. The model of weak pinning provides a useful framework for understanding magnetization processes.