Fast and Slow Magnetization Processes in Magnetic Recording Media
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FAST AND SLOW MAGNETIZATION PROCESSES IN MAGNETIC RECORDING MEDIA J. Zhou, R. Skomski, S. Michalski, R. D. Kirby, and D. J. Sellmyer Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588 ABSTRACT Information loss due to thermal activation is a major concern in ultrahigh-density magnetic recording media. The usually considered mechanism is thermally activated magnetization reversal over micromagnetic energy barriers. However, micromagnetic approaches ignore local anisotropy fluctuations, which translate into a time-dependent reduction of the remanent magnetization. The effect is negligibly small in macroscopic magnets but becomes important on a scale of a few nanometers. INTRODUCTION Magnetic recording has been a driving force in nanotechnology and electronics. The key advantage is the high storage density, corresponding to bit sizes much smaller than the wavelength of visible light. However, a fundamental bit-size limit is given by the thermal stability of the stored information [1]. In very small grains or particles, thermal activation leads to local magnetization reversal and to the decay of the stored information. In macroscopic magnets, there is a clear separation of time scales between fast atomic or intrinsic processes and slow extrinsic magnetization processes. Intrinsic properties can be treated by equilibrium statistical mechanics. For example, the local magnetic anisotropy K1(r) can be replaced by the time or ensemble average [1, 2]. By contrast, extrinsic properties are related to hysteresis and often far from equilibrium [3, 4]. The question arises how magnetic systems behave on a length scale of a very few nanometers, where intrinsic phenomena become important. This applies, for example, to single-phase particles, core-shell structures, and exchange-coupled hard-soft structures, which have attracted renewed attention [5, 6] as magnetic-recording materials. This paper starts with a brief analysis of magnetization modes in the structures and then outlines how fluctuations of intrinsic properties affect the time-dependence magnetic properties. NUCLEATION MODES IN COMPOSITE NANOPARTICLES
The simplest model of magnetization reversal, the Stoner-Wohlfarth model, describes uniformly magnetized small particles of volume V. In zero field, it yields the energy barrier V and a dynamics described by the Arrhenius law exp(-V/kBT). In two-phase particles (Fig. 1), magnetization inhomogenities are essential, because the soft phase switches earlier than the hard phase. Figures 1(b) shows the nucleation mode, that is, the magnetization deviation from M = Ms ez at the onset of magnetization reversal.
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Fig. 1. Two-phase nanoparticle consisting of hard (h) and soft (s) phases: (a) structure, (b) magnetization modes, and (c) two-particle model. In (b), a magnetization tail reaches into the hard phase and reduces the energy barrier of the particle. The model of Fig. 1(c) approximates the composite nanopartic
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