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Figures and Tables
Figure 5.1 (a) PXRD pattern and Rietveld analysis of ε-Al0.47Fe1.53O3. Red marks, black
line, and gray line are the observed pattern, calculated pattern, and their difference,
respectively. Green tick marks are the calculated positions of the Bragg peaks. Inset is the
crystal structure viewed from the a-axis direction. (b) Magnetic hysteresis loop of
ε-Al0.47Fe1.53O3 at 300 K. [© 2016 IEEE. Adapted with permission from IEEE Magn. Lett.,
7, 5506704 (2016).]
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Figure 5.2 Diagram of the terahertz-time domain spectroscopy (THz-TDS)
measurement system. [© 2016 IEEE. Adapted with permission from IEEE Magn. Lett.,
7, 5506704 (2016).]
103
THz-field (a.u.)
reference
(without pellet sample)
d = 1.130 mm
d = 2.339 mm
d = 3.549 mm
20
40
60
80
100
120
Time (ps)
Figure 5.3 Temporal waveforms of the input THz pulse light and the transmitted THz
light from the ε-Al0.47Fe1.53O3 pellet samples. [© 2016 IEEE. Adapted with permission
from IEEE Magn. Lett., 7, 5506704 (2016).]
104
30
Absorption (dB)
25
20
15
10
60
80
100
120
140
160
Frequency (GHz)
Figure 5.4 Absorption spectra of ε-Al0.47Fe1.53O3 pellet samples for d = 1.130 mm
(purple), 2.339 mm (blue), and 3.549 mm (green). [© 2016 IEEE. Adapted with
permission from IEEE Magn. Lett., 7, 5506704 (2016).]
105
(a)
30
d = 1.130 mm
Absorption (dB)
25
20
15
10
(b)
60
80
100
120
Frequency (GHz)
140
160
30
d = 2.339 mm
Absorption (dB)
25
20
15
10
(c)
60
80
100
120
Frequency (GHz)
140
160
30
d = 3.549 mm
Absorption (dB)
25
20
15
10
60
80
100
120
Frequency (GHz)
140
160
Figure 5.5 Spectral fitting of the absorption of ε-Al0.47Fe1.53O3 pellet samples for (a) d
= 1.130 mm, (b) 2.339 mm, and (c) 3.549 mm. Red open circles and solid lines indicate
the observed spectra and fitted spectra, respectively.
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25
Absorption (dB)
25
Absorption (dB)
20
20
15
10
15
at 100 GHz
0.0 0.5 1.0 1.5 2.0 2.5
d100 % (mm)
10
60
80
100
120
140
160
Frequency (GHz)
Figure 5.6 Analyzed absorption spectra of ε-Al0.47Fe1.53O3 pellet samples for d = 1.130
mm (purple), 2.339 mm (blue), and 3.549 mm (green). Inset figure shows the absorption
intensity at 100 GHz versus sample thickness converted to 100% filling ratio. [© 2016
IEEE. Adapted with permission from IEEE Magn. Lett., 7, 5506704 (2016).]
107
(a)
Ellipticity
-1
(b)
60
80
100
120
Frequency (GHz)
140
60
80
100
120
Frequency (GHz)
140
Rotation angle (deg.)
40
20
-20
-40
Figure 5.7 Frequency dependence of the (a) ellipticity and (b) rotation angle of the
magnetized ε-Al0.47Fe1.53O3 pellet-form samples (d = 2.339 mm). Blue and red lines
denote the results measured by irradiating from the N-pole direction and the S-pole
direction, respectively. [© 2016 IEEE. Adapted with permission from IEEE Magn. Lett.,
7, 5506704 (2016).]
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Figure 5.8 (a) Overview of ε-Fe2O3/SiO2 film fabrication and photograph of the film
sample. (b) Observed (gray crosses), calculated (black line), and difference (gray line)
patterns of the XRD pattern with Rietveld analysis of the ε-Fe2O3/SiO2 film. Additionally,
the calculated positions of the Bragg reflections (black bars) and crystal structure (inset)
are shown. [Reproduced from AIP Adv., 7, 056218 (2017), with the permission of AIP
Publishing.]
109
Figure 5.9 (a) Cross section SEM image, (inset) illustration, and (b) TEM image of εFe2O3/SiO2 film on a quartz substrate. [Reproduced from AIP Adv., 7, 056218 (2017),
with the permission of AIP Publishing.]
110
(a)
100
Transmittance (%)
80
60
40
20
400
500
600
700
800
700
800
Wavelength (nm)
(b)
0.30
0.25
Absorbance
0.20
0.15
0.10
0.05
400
500
600
Wavelength (nm)
Figure 5.10 UV-vis spectra of the ε-Fe2O3/SiO2 film. (a) Transmittance and (b)
absorbance plotted as functions of wavelength. [Reproduced from AIP Adv., 7, 056218
(2017), with the permission of AIP Publishing.]
111
(a)
Faraday rotation (degree)
0.010
0.005
0.000
-0.005
-0.010
400
500
600
700
800
Wavelength (nm)
(b)
0.03
Faraday ellipticity (degree)
0.02
0.01
-0.01
-0.02
-0.03
-15
-10
-5
10
15
External field (kOe)
Figure 5.11 (a) Faraday rotation angle spectra for the ε-Fe2O3/SiO2 film. (b) Faraday
ellipticity of the ε-Fe2O3/SiO2 film as a function of external field at 390 nm. Black line is
to highlight the shape. [Reproduced from AIP Adv., 7, 056218 (2017), with the permission
of AIP Publishing.]
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Chapter 6
Summary and perspective
The present doctoral thesis focuses on the spectroscopic study of ε-Fe2O3 and εMxFe2−xO3 in the range from millimeter-waves to terahertz-waves. Chapter 1 shows the
background knowledge concerning ε-Fe2O3, which exhibits a huge coercive field at room
temperature. This is an exceptionally large value among metal oxide based magnets, and
this huge coercivity also enables millimeter-wave absorption at a very high frequency due
to zero-field ferromagnetic resonance, so-called natural resonance. Furthermore, several
metal-substituted series, ε-MxFe2−xO3 (M = In, Ga, Al), have been reported, and the
magnetic properties and millimeter-wave absorption properties can be widely controlled
by metal-substitution. Therefore, this material is expected to be used in future magnetic
recordings and electromagnetic wave absorbers. In order to optimize the performance to
meet the needs, it is important to understand the fundamental properties of the material
from both experimental and theoretical approaches, which had still be underway.
Chapter 2 reports a mesoscopic single crystal bar magnet composed of ε-Fe2O3. The
atomic movements of ε-Fe2O3 were calculated by phonon mode calculations. The lowest
frequency phonon mode of 2.51 THz in ε-Fe2O3 shows a movement of the Fe atoms
oscillating along the crystallographic a-axis, which is the growth direction of the bar
magnet. The simulated IR spectrum from the phonon mode calculations showed good
correspondence with the experimentally obtained far-IR spectrum. The lowest frequency
absorption peak was observed at 2.62 THz, corresponding to the A1 symmetry phonon
mode at 2.51 THz.
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In chapter 3, first-principles phonon mode calculations of ε-Ga0.5Fe1.5O3 were
carried out, which showed 117 optical phonon modes (fundamental vibrations) with
symmetries of A1, A2, B1, and B2, ranging from 2.63 THz to 21.76 THz. The movements
of Fe and Ga contribute to the phonon modes in the lower energy region, while the
movements of O contribute to the phonon modes in the higher energy region. Far- and
mid-IR and Raman spectroscopic measurements confirmed that the calculated spectra
agree well with the observed spectra. Additionally, a crystallographically oriented
magnetic film was prepared, which showed a rectangular magnetic hysteresis loop with
a coercive field of 9.7 kOe.
In chapter 4, the influences of indium substitution on the crystal structure, magnetic
properties, and millimeter-wave absorption for spherical ε-InxFe2−xO3 nanoparticles are
described. For nanoparticles prepared by a reverse-micelle and sol-gel combination
technique, In3+ selectively occupies Fe3+ at the largely distorted B site. As the Insubstitution increased, the coercive field was found to decrease from 21.9 kOe (x = 0) to
5.9 kOe (x = 0.18). In high-frequency millimeter-wave absorption, the resonance
frequency decreased with In-substitution because the nonmagnetic In3+ substitutes for
Fe3+ at B sites, which is considered to be an important site contributing to the magnetic
anisotropy of the material. In the field of electromagnetic wave absorbing material, εInxFe2−xO3 has potential in future millimeter-wave wireless communications. The
resonance frequency of the sample for x = 0.09 is particularly interesting because it
corresponds to the 140-GHz window of air in wireless communications.
In chapter 5, ε-Al0.47Fe1.53O3 was prepared and millimeter-wave absorption property
was measured using THz-TDS. An absorption peak due to natural resonance was
observed at 100 GHz with an intensity of 10 dB (90%) per 1 mm. In addition, millimeter-
114
wave polarization plane measurement of the magnetized ε-Al0.47Fe1.53O3 pellet was
carried out to obtain the rotation data of the transmitted millimeter-wave. Since εAl0.47Fe1.53O3 is eco-friendly and has high durability, a series of Al-substituted ε-Fe2O3 is
expected to be used as millimeter-wave absorbers or isolators.
As a perspective of the present thesis, spectroscopic study of ε-Fe2O3 in the
frequency region between terahertz-waves and millimeter-waves is an interesting
research direction. Since the millimeter-wave absorption of ε-Fe2O3 due to k = 0 magnon
(0.18 THz) and the optical phonon due to lattice vibration (2.54 THz) are energetically
close, ε-Fe2O3 is a suitable material for future investigation of the phonon-magnon
coupling effect. I would like to plan measurements to observe such coupling effects.
Another direction of future research is the investigation of ε-Fe2O3 for a new methodology
of optomagnetic recording. Considering the fact that ε-Fe2O3 is commercialized as a
material for magnetic recording and also for high-frequency millimeter-wave absorbers,
research development could be further extended by adding the characteristic of
millimeter-wave absorption to the magnetic recording technology. Currently, magnetic
recording tapes and hard disc drives are facing the “trilemma of magnetic recording,” a
common issue in the magnetic recording industry. In order to increase the recording
density to store the increasing amount of information, there are three important aspects:
signal to noise (S/N) ratio, thermal stability, and writability. To increase the S/N ratio, the
magnetic particle size must be downsized, but small magnetic particles lose thermal
stability. To maintain thermal stability while downsizing, the magnetic anisotropy of the
material must be increased. Then, however, the writing head cannot record with the
current magnetic field. This trade-off between the three aspects is the “trilemma of
magnetic recording.” To overcome this challenge, several approaches have been proposed,
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e.g., heat-assisted magnetic recording (HAMR). The idea of HAMR is to heat the
magnetic media close to Curie temperature so that the magnetization could be flipped by
a small magnetic field. In the case of ε-Fe2O3, precession or tilting of the magnetization
can be triggered by irradiating millimeter-waves. This phenomenon indicates that there is
a possibility to inverse the magnetic pole direction by irradiating intense millimeter-wave.
Ohkoshi, et al. has recently proposed a new recording method of “focused millimeterwave assisted magnetic recording (F-MIMR)”. To test this methodology, Ohkoshi and his
colleagues prepared magnetic films using epsilon iron oxide and irradiated an intense
millimeter-wave focused by a metal ring, under an external magnetic field slightly weaker
than the coercive field. Magnetic force microscopy (MFM) measurement indicated a
magnetic pole flip of the epsilon iron oxide film, proving the concept of this new
recording method that could contribute to raising the magnetic recording density in the
big data era. Further investigations will be done to introduce this concept to actual
applications.
116
List of publications related to the thesis
1.
“Mesoscopic bar magnet based on ε-Fe2O3 hard ferrite”
S. Ohkoshi, A. Namai, T. Yamaoka, M. Yoshikiyo, K. Imoto, T. Nasu, S. Anan, Y.
Umeta, K. Nakagawa, and H. Tokoro
Scientific Reports, 6, 27212/1–10 (2016).
2.
“Phonon-mode calculation, far- and mid-infrared, and Raman spectra of an εGa0.5Fe1.5O3 magnet”
S. Ohkoshi, M. Yoshikiyo, Y. Umeta, M. Komine, R. Fujiwara, H. Tokoro, K. Chiba,
T. Soejima, A. Namai, Y. Miyamoto, and T. Nasu
J. Phys. Chem. C, 121, 5812–5819 (2017).
3.
“Highly oriented magnetic film composed of Ga-substituted ε-iron oxide and the
angular dependence of the magnetic hysteresis loops”
M. Yoshikiyo, A. Namai, K. Imoto, H. Tokoro, and S. Ohkoshi
Eur. J. Inorg. Chem., 847–851 (2018).
4.
“High-frequency millimeter wave absorption of indium-substituted ε-Fe2O3
spherical nanoparticles”
M. Yoshikiyo, A. Namai, M. Nakajima, K. Yamaguchi, T. Suemoto, and S. Ohkoshi
J. Appl. Phys., 115, 172613/1–5 (2014).
5. “Millimeter wave rotation in ε-Al0.47Fe1.53O3 at one hundred gigahertz”
A. Namai, M. Yoshikiyo, and S. Ohkoshi
IEEE Magn. Lett., 7, 5506704/1–4 (2016).
6. “Magnetic glass-film based on single-nanosize ε-Fe2O3 nanoparticles”
M. Yoshikiyo, A. Namai, K. Nakagawa, and S. Ohkoshi
AIP Adv., 7, 056218/1–6 (2017).
117
Acknowledgements
I would like to express my deepest gratitude to my supervisor, Prof. Shin-ichi
Ohkoshi for leading my way into this research on ε-Fe2O3. He has looked after my work
with tender care and has given me so many advices and suggestions. It is greatly owing
to his heartfelt guidance that I am here right now, continuing my career as a researcher in
this laboratory. He has given me an invaluable chance and a constant sense of security to
take on the challenge in this field. I am always stimulated by Prof. Ohkoshi’s pure
curiosity and inquisitive mind toward science, and doing research under his supervision
is continuing to teach me the great pleasure of doing research. The more I dig into the
research, the deeper I understand the significance of our work on ε-Fe2O3 and other novel
functional materials developed in the group. Especially, ε-Fe2O3 is an amazing material
with extraordinary physical properties, and I am very grateful to be able to take part in
this research. In addition to the fundamental aspect of research on ε-Fe2O3, I learned from
Prof. Ohkoshi the great joy of working on the development toward industrial applications.
Thanks to the chance of participating in the collaboration research, my network of
researchers have greatly expanded, and I am stimulated by each member working in
different fields. I cannot thank Prof. Ohkoshi enough for this opportunity.
I would also like to give my great thanks to Dr. Asuka Namai for all of the guidance
in the research. From the very beginning, when I first started research in Ohkoshi lab for
bachelor thesis, she has taught me the basic techniques and the attitude toward research.
She has cared for not only my research work but also for my daily life in the group. Even
when I am feeling down, she would encourage me and support me to bring myself up.
Her presence and working with her has encouraged me many times, and she has always
118
given me great comfort. This work could not have been done without her support.
I am also thankful to Dr. Koji Nakabayashi, Dr. Kosuke Nakagawa, and Dr. Kenta
Imoto of Ohkoshi lab, and Prof. Hiroko Tokoro of Tsukuba University for their scientific
guidance and numerous support for this research. They have also greatly supported me as
a staff member of the research group.
I would like to give my thanks to Dr. Kouji Chiba of Molsis Inc. for the attentive
guidance in the first-principles calculations. I would also like to thank Prof. Tohru
Suemoto, Prof. Makoto Nakajima, and Dr. Keita Yamaguchi for their support with
terahertz time-domain spectroscopy measurements. I would like to thank Mr. Takeo
Soejima for the support with Raman measurements. I am grateful to Mr. Yoshida, and Mr.
Miyazaki of DOWA Electronics Materials Co,. Ltd. for the support with sample
preparation and for valuable discussions. I am also thankful to all of the Ohkoshi lab
members and graduates for their kind support and cooperation.
Last but not least, I would like to thank my family for their heartfelt support.
Marie Yoshikiyo
November 2020
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