Magnetic viscosity of single domain magnetite particles

The acquisition of viscous magnetization has been measured on a single domain magnetite sample in the temperature range 80 to 330 K and in applied fields up to 1540 Oe. Based on Néel's single domain theory, we have calculated the field and temperature dependence of viscous acquisition by numerical two-dimensional integration, using experimentally determined volume and switching field distributions. Predicted and experimental viscosity coefficients do not match, with especially strong discrepancies below the Verwey transition. The experimental acquisition coefficient is larger below the transition than at room temperature, even though the corresponding switching field distributions are nearly identical. Thus we cannot explain single domain viscosity by switching field and volume distributions alone, and we suspect that stress plays a prominent role.