Features of the pulsed magnetization switching in a high-coercivity material based on $\varepsilon$-Fe$_{2}$O$_{3}$ nanoparticles

The magnetic structure of the $\varepsilon$-Fe$_{2}$O$_{3}$ iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to $\sim$ 150 K. As the temperature decreases, $\varepsilon$-Fe$_{2}$O$_{3}$ 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 $\varepsilon$-Fe$_{2}$O$_{3}$ 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 SiO$_2$ with the $\varepsilon$-Fe$_{2}$O$_{3}$ nanoparticles embedded in its pores was examined. The magnetic hysteresis loops under dynamic magnetization switching were measured using pulsed magnetic fields $H_{\operatorname{max}}$ 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 $\sim$ 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 $\varepsilon$-Fe$_{2}$O$_{3}$ 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 $H_{\operatorname{max}}$. Such a behavior atypical of systems of ferrimagnetic particles is caused already by the dynamic spin processes inside the $\varepsilon$-Fe$_{2}$O$_{3}$ particles during fast magnetization switching.