Magnetic viscosity in recording media

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Abstract
Assuming a continuous distribution of relaxation times or frequencies responsible for magnetic aftereffect, it is shown that the inverse Laplace transform of the relaxing magnetization M(t) yields the function g(f)/f, where g(f) is the distribution of relaxation frequencies. Published data for carbonyl iron is fitted by the form M(T)/M0=(1+t/t0)−n, with n=1.5 and t0 =26 ms. The corresponding distribution of relaxation frequencies is g(f)∼(ft0)n exp(−ft0). Using a simple, noninteracting particle model of a recording medium based upon the switching field distribution of the material, we have calculated the change in magnetization (decay) between 100 and 1000 s after applying a reversing field. A Lorentzian switching field distribution 20 Oe wide and centered at 200 Oe was assumed. The decay over this time span versus applied field is determined for various temperatures between 6 and 300 K. As the temperature is reduced, the field for peak decay increases, approaching the assumed value of Hc (200 Oe) as the temperature approaches zero, and the height of the peak is simultaneously reduced. The heights of these peaks versus temperature approximate a T0.5 variation.