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The epsilon phase of Fe$_2$O$_3$ (ε-Fe$_2$O$_3$) presents captivating properties and it is receiving extraordinary attention due to its great application potentials. It stands out for its huge coercive field (up to 2 T at room temperature), millimeter-wave ferromagnetic resonance, remarkable non-linear magneto-optical effect, magneto-electric coupling [1], and room temperature ferroelectricity [2]. It has been much less studied than other iron (III) oxides because its formation requires special conditions [1].
ε-Fe$_2$O$_3$ presents a complex noncentrosymmetric structure (Pna2$_1$) with three distinct octahedral and one tetrahedral environments for Fe sites. We present a neutron-based investigation on the rich magnetic phase diagram and properties of geometrically frustrated ε-Fe$_2$O$_3$ nanoparticles. The nature of the incommensurate magnetic order, attributed by some authors to a spiral ground state [5], was investigated in zero and applied magnetic fields, and reinterpreted in the light of the models confronted to neutron data [6]. The study illustrates the interplay between the huge magnetic anisotropy, frustration and the stabilization of the super-hard ferrimagnetic phase in the 150-500 K interval.
[1] M. Gich et al., Nanotech. 17, 687 (2006).
[2] M. Gich et al., Adv. Mater. 26 (2014) 4645.
[3] J. A. Sans et al. Nature Comms (2018), 9, 4554.
[4] J.L. García-Muñoz et al., Chem. Mater. 29, 9705 (2017)
[5] Yu. V. Knyazev et al. Phys. Rev.B 101, 094408 (2020).
[6] A. Romaguera et al., submitted
