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https://doi.org/10.3938/NPSM.68.819
Short Review on Fe$_2$O$_3$ Polymorphs and $\varepsilon$-Fe$_2$O$_3$ Thin Films
New Phys.: Sae Mulli 2018; 68: 819~828
Published online August 31, 2018;  https://doi.org/10.3938/NPSM.68.819
© 2018 New Physics: Sae Mulli.

Heon-Jung KIM*

Department of Materials-Energy Science and Engineering, Daegu University, Gyeongsan 38453, Korea
Correspondence to: hjkim76@daegu.ac.kr
Received June 7, 2018; Accepted June 7, 2018.
cc This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
This review summarizes the features of the four Fe$_2$O$_3$ polymorphs and discusses thin-film synthesis of $\varepsilon$-Fe$_2$O$_3$. First, emphasis is given to the discussion on their structural and magnetic properties. Among these four polymorphs, we review the structure and the magnetic properties of $\varepsilon$-Fe$_2$O$_3$ in depth. Before thin film synthesis of $\varepsilon$-Fe$_2$O$_3$ is introduced, the method to synthesize $\varepsilon$-Fe$_2$O$_3$ nanoparticles or powders and the polymorphic transition between $\gamma$-Fe$_2$O$_3$ and $\alpha$-Fe$_2$O$_3$ are briefly examined. $\varepsilon$-Fe$_2$O$_3$ has nonzero orbital angular momentum, a large magnetic anisotropy and a coercive field of 2 T at room temperature. These features are due to the pronounced distortion of the oxygen octahedron surrounding Fe$^{3+}$ ion, resulting in a strong hybridization between the Fe 3d orbitals and the O 2p orbitals. Even though $\varepsilon$-Fe$_2$O$_3$ has potential to be applied in next-generation magnetic devices, its thin-film synthesis has been hampered due to its phase instability. This review briefly presents a solution to this problem. Finally, we discuss the mechanism for the growth of the $\alpha$-Fe$_2$O$_3$ second phase in $\varepsilon$-Fe$_2$O$_3$ thin films. 
PACS numbers: 75.47.Lx, 77.55.Nv, 64.60.-i, 64.60.My
Keywords: Iron oxide, Polymorph, $\varepsilon$-Fe$_2$O$_3$, Pulsed laser deposition, Polymorphous transition


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