The manifestation of surface and size effects in the magnetic properties of $\varepsilon$-Fe$_2$O$_3$ nanoparticles. (Brief overview)

Polymorphic modification of iron oxide, known as $\varepsilon$-Fe$_2$O$_3$, exists only in the form of nanoparticles with characteristic sizes up to several tens of nanometers. Particles of these sizes exhibit a large coercive force, about 20 kOe at room temperature. In the temperature range of 80–150 K, a magnetic transition occurs in $\varepsilon$-Fe$_2$O$_3$, accompanied by a sharp decrease in the coercive force. At the same time, there are significant differences in the magnetic behavior of “large” ($\sim$20 nm) particles and ultra-small particles (up to 6 nm). A number of experimental facts indicate the manifestation of size effects that lead to a change in the magnetic structure in particles of these sizes. In addition, a surface effect is also manifested for such particles – a significant contribution to the magnetic behavior is governed by surface magnetic anisotropy. In this paper, a brief review of the manifestation of these size and surface effects in the magnetic properties of $\varepsilon$-Fe$_2$O$_3$ nanoparticles is carried out.