Account/Review for Masterpiece Materials with Functional Excellence
Optical Properties of Epsilon Iron Oxide Nanoparticles
in the Millimeter- and Terahertz-Wave Regions
Hiroko Tokoro,*
1,2
Koji Nakabayashi,
2
Shuntaro Nagashima,
1
Qinyu Song,
2
Marie Yoshikiyo,
2
and Shin-ichi Ohkoshi*
2
1
Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
2
Department of Chemistry, School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: tokoro@ims.tsukuba.ac.jp (H. Tokoro), ohkoshi@chem.s.u-tokyo.ac.jp (S. Ohkoshi)
Received: November 12, 2021; Accepted: February 1, 2022; Web Released: February 8, 2022
Shin-ichi Ohkoshi
Shin-ichi Ohkoshi earned his PhD in 1995 from Tohoku University. In 2003, he became Associate Professor
at the Department of Applied Chemistry, School of Engineering, The University of Tokyo. Since 2006, he has
been a Professor at the Department of Chemistry, School of Science, The University of Tokyo. He also
serves as the Vice Dean of the School of Science, Special Adviser to the President at The University of
Tokyo, and the Director of the CNRS International Associated Laboratory. Shin-ichi Ohkoshi received the
Chemical Society of Japan (CSJ) Award in 2019 and the Humboldt Research Award in 2020.
Abstract
Epsilon iron oxide (ε-Fe
2
O
3
) is attracting global attention as
a magnetic material with a large magnetic anisotropy. In this
article, the optical properties of ε-Fe
2
O
3
nanoparticles and the
metal-substituted series of ε-M
x
Fe
2¹x
O
3
(M = Ga, In, and Al)
are studied over a wide frequency range from the millimeter-
wave to terahertz-wave region, 30 GHz30 THz, using tera-
hertz time-domain, far-infrared, and Raman spectroscopies. To
understand the spectroscopic data, first-principles calculations
of the electronic structure and phonon modes are performed.
First, an ε-Fe
2
O
3
bar magnet is introduced and its atomic move-
ments are calculated by phonon mode calculations. Second, the
phonon modes of Ga-substituted ε-Fe
2
O
3
are calculated. Far-
IR, mid-IR, and Raman spectroscopies confirm that the calcu-
lated and observed spectra show good agreement. Third, the
influences of In-substitution on the crystal structure, magnetic
properties, and millimeter-wave absorption are described. In
high-frequency millimeter-wave absorption due to magnon, the
resonance frequency decreased with In-substitution. Finally,
the millimeter-wave absorption property of ε-Al
x
Fe
2¹x
O
3
is
described. An absorption peak due to the natural resonance
occurs at 100 GHz. The rotation data of the transmitted milli-
meter wave are determined by millimeter-wavepolarization-
plane measurements.
Keywords: Nanomagnet j Epsilon iron oxide j
Millimeter-wave absorption
1. Introduction
Ferrite magnets were discovered in the 7th century BC.
Today, they are ubiquitous due to the abundance ofiron. Ferrite
magnets come in many forms such as Fe
2
O
3
, Fe
3
O
4
, and
BaFe
12
O
19
. Ferrites are applied indiverse industrial products
such as motors, electromagnetic wave absorbers, magnetic
recording media, Faraday isolators, and generators.
19
Faraday
isolators and electromagnetic (EM) wave absorbers are repre-
sentative applications from the viewpoint of magneto-optical
functionalities offerrites. The former, which is based on the
magneto-optical effects in the UVvisibleinfrared region, uses
yttrium iron garnet or bismuth-substituted yttrium iron garnet,
while the latter uses Fe/C composites or barium ferrite and
operates in the MHzGHz region to avoid EM interference
with electronic devices.
Recently, a ferrite material, which has a high transparency in
the visible region and exhibits EM wave absorption properties
in the high-frequency millimeter-wave region, has drawn much
attention. This materialis called epsilon iron oxide (ε-Fe
2
O
3
),
which is a type offerric oxide. Ferric oxide is composed of
trivalent iron cations Fe
3+
and oxygen anions O
2¹
(i.e., Fe
2
O
3
).
Document type: Review/Account
538 | Bull. Chem. Soc. Jpn. 2022, 95, 538–552 | doi:10.1246/bcsj.20210406 © 2022 The Chemical Society of Japan