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Theoretical Analysis of Magnetic Properties of ε-Fe2O3

International Symposium on Computics: Quantum Simulation and Design (ISC-QSD), TheUniversity of Tokyo

2014年12月3日(水)
(2014.12.03 更新)

Daisuke Hirai, Shinji Tsuneyuki, and Yoshihiro Gohda

概要/Abstract

Recent developments of chemical synthesis have enabled us to make ε-Fe2O3 in the form ofa nanoparticle, showing the largest coercivity among all the metallic oxides [1,2]. Therefore, it isexpected that a superparamagnetic limit can be overcome using this material. However, theorigin of high coercivity remains unclear. To understand this mechanism is crucial not only forbasic sicence, but also for designing novel hard magnetic materials. In this study, we performed first-principles simulations to understand the magnetic propertiesof ε-Fe2O3, on the basis of density functional theory. We used the OpenMX code [3] tocalculate the electronic structure. First, we found that zero magnetization appears in pristine ε-Fe2O3. This is in contrat to finite magnetization observed in several experiments, where thevalues of magnetization are 3-6 emu/g [1,2]. We also caluculated the magnetocrystallineanisotropy energy (MAE) based on second-order perturbation theory for spin-orbit interaction[4]. The calculated MAE is 71.5 kJ/m3, which is smaller than experimental values of 200-500kJ/m3 [5]. Next, we examined the effect of an oxygen vacancy on the magnetic properties. It is shownthat the introduction of a specific vacancy site among six nonequivalent oxygen sites leads tofinite magnetization with the value of 2 emu/g when a single oxygen vacancy is introduced perunit cell. This structure is the most stable among six possible structures and the formationenergy are 3.26 eV. The MAE is 352 kJ/m3, which is about 5 times larger that that of thepristine case. To clarify this MAE enhancement, we decomposed the MAE into each Fe site andorbital. As a result, we found a large contribution from the nearest Fe site from the vacancysite, which is absent in the pristine case. We also elucidated that the spin-orbit interactionbetween occupied dx2-y2↓ and unoccupied dxy↓ significantly increases the MAE due to theintroduction of the oxygen vacancy. We will give more details in the presentation.1. S. Sakurai et al., J. Am. Chem. Soc. 131, 18299 (2009).2. S.-I. Ohkoshi et al., Bull. Chem. Soc. Jpn. 86, 897 (2013).3. www.openmx-square.org4. Z. Torbatian et al., Appl. Phys. Lett. 104, 242403 (2014).5. J. Tucek et al., Chem. Mater. 22, 6491 (2010).



研究活動

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触媒・電池元素戦略研究拠点 (京都大学)
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東工大元素戦略拠点 (東京工業大学)
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構造材料元素戦略研究拠点 (京都大学)
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高効率モーター用 磁性材料技術研究組合