Exploring Critical Synthetic Parameters for Nanoscale ε-Fe₂O₃ and Their Influence on Magnetic Behaviors

Abstract

An intermediate polymorph of iron oxide, ε-Fe2O3, has attracted significant attention due to its giant coercive field (H c) and potential applications in high-frequency millimeter-wave absorption and high-density magnetic recording. However, the fabrication of ε-Fe2O3 with high phase purity is still a challenge due to complicated synthetic procedures and a large variety of reaction parameters. Here, we have identified critical reaction parameters to improve the phase purity of ε-Fe2O3, and the effects of all possible reaction parameters have been tested through systematic studies. A combination of structural and magnetic characterization techniques provides us with an accurate and reliable phase purity analysis of the ε-Fe2O3 phase. Specifically, we observed that (1) the reaction temperature and time and (2) the addition of Ba are critical parameters to improve the phase purity. We identified the optimal conditions that maximize the coercivity and phase purity, giving insight into the effects of each parameter on the γ- to ε- to α-phase-transition pathway. We obtained nearly single-phase ε-Fe2O3 (∼87 wt % with ∼13 wt % α-phase) with a large coercivity of H c = 20.6 kOe, enabling us to obtain pure ε-Fe2O3 by a simple magnetic separation protocol, and tuned the H c of the ε-Fe2O3 nanoparticles in the range of 4.0–20.6 kOe by controlling the reaction parameters. Furthermore, the structural properties of the resulting ε-Fe2O3 nanoparticles are confirmed by characterizing their chemical and magnetic properties using X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements.

Technological Peculiarities of Epsilon Ferrite Epitaxial Stabilization by PLD

“…Among numerous iron oxide polymorphs, the most intriguing one is the metastable orthorhombic ε-Fe 2 O 3 phase, which does not exist in the bulk form. Epsilon ferrite is a ferrimagnet with a huge magnetocrystalline anisotropy that is responsible for the values of coercive force exceeding 2 T. Having attracted a lot of attention during the past few years [5][6][7], epsilon ferrite in the form of nanofilms [8][9][10] and nanoparticles [11][12][13][14] was shown to exhibit a complicated magnetic structure with four magnetic sub-lattices and room-temperature (RT) multiferroic behavior [15], which has not been observed in other simple metal oxides. Its large magnetic coercivity and proven magneto-electric coupling make this material ideal for the creation of novel oxide-based ferroic-on-semiconductor devices for spintronic applications, including low-power consumption magnetic media storage devices [16].…”

Simple Fabrication of ϵ‐Fe2O3 Nanoparticles Containing Silica Monoliths with Enhanced Coercivity

“…[24,25] It could also be shown that the addition of alkaline earth metal ions, such as Ca 2 + , Sr 2 + or Ba 2 + , in the synthesis medium favors the stabilization of the ɛ-Fe 2 O 3 phase and induces a morphology change of the formed nanoparticles from spherical to rodlike. [19,[26][27][28][29] Besides the difficulties to obtain the pure ɛ-Fe 2 O 3 phase, the crystal structure is not solely responsible for the magnetic properties. The magnetic behavior also depends on the particle size.…”

“…However, it is also possible to directly use γ‐Fe 2 O 3 nanoparticles and thermally treat them, inducing their transformation into the ϵ‐Fe 2 O 3 phase [24,25] . It could also be shown that the addition of alkaline earth metal ions, such as Ca 2+ , Sr 2+ or Ba 2+ , in the synthesis medium favors the stabilization of the ϵ‐Fe 2 O 3 phase and induces a morphology change of the formed nanoparticles from spherical to rod‐like [19,26–29] …”