Thermal relaxation in the strong-demagnetizing limit

We have developed a new technique that simulates the thermal magnetization decay (magnetic viscosity) within bit transitions that have a strong demagnetizing field H_d approximately equal to the remanent coercivity of the recording medium. This technique uses a vibrating-sample magnetometer to measure the viscosity while allowing the applied field to decay logarithmically at a rate consistent with the measured magnetization decay. The decaying applied field simulates the decaying H_d of a broadening bit transition. The magnetic decay measured using this technique remains logarithmic in time, but is almost a factor of 4 smaller than that measured using a constant applied field. These results imply that the standard viscosity measurement is inappropriate when estimating the amount of thermal broadening due to H _d within a transition. We have also developed a model of the magnetization decay in a large decaying H_d using a modified Arrhenius-Neel rate equation with a single energy barrier that has explicit dependence on the magnetization. The model predicts logarithmic magnetization decay consistent with that observed experimentally and also indicates that the viscosity measured using this technique may be a measure of the average energy barrier preventing thermal switching