Features of the Pulsed Magnetization Switching in a High-Coercivity Material Based on epsilon-Fe2O3 Nanoparticles

The magnetic structure of the epsilon-Fe2O3 iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to similar to 150 K. As the temperature decreases, epsilon-Fe2O3 undergoes a magnetic transition accompanied by a significant decrease in the coercivity H-c and, in the low-temperature range, the compound has a complex incommensurate magnetic structure. We experimentally investigated the dynamic magnetization switching of the epsilon-Fe2O3 nanoparticles with an average size of 8 nm in the temperature range of 80-300 K, which covers different types of the magnetic structure of this iron oxide. A bulk material consisting of xerogel SiO2 with the epsilon-Fe2O3 nanoparticles embedded in its pores was examined. The magnetic hysteresis loops under dynamic magnetization switching were measured using pulsed magnetic fields H-max of up to 130 kOe by discharging a capacitor bank through a solenoid. The coercivity H-c upon the dynamic magnetization switching noticeably exceeds the H-c value under the quasi-static conditions. This is caused by the superparamagnetic relaxation of magnetic moments of particles upon the pulsed magnetization switching. In the range from room temperature to similar to 150 K, the external field variation rate dH/dt is the main parameter that determines the behavior of the coercivity under the dynamic magnetization switching. It is the behavior that is expected for a system of single-domain ferro- and ferrimagnetic particles. Under external conditions (at a temperature of 80 K) when the epsilon-Fe2O3 magnetic structure is incommensurate, the coercivity during the pulsed magnetization switching depends already on the parameter dH/dt and is determined, to a great extent, by the maximum applied field H-max. Such a behavior atypical of systems of ferrimagnetic particles is caused already by the dynamic spin processes inside the epsilon-Fe2O3 particles during fast magnetization switching.