Recent progress in magneto-optics and research on its application
Various kinds of magneto-optical properties are investigated in rare earth orthochromites. From an analysis of Cr³⁺ exciton absorption in RCrO₃ (R=Tb, Dy, and Ho), it is unferred that these compounds exhibit an anomalous spin-reorientation in a magnetic field along the b axis, where the weak ferroma...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2002
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irk-123456789-1302372018-02-10T03:03:23Z Recent progress in magneto-optics and research on its application Norimichi Kojima Kuniro Tsushima Обзор Various kinds of magneto-optical properties are investigated in rare earth orthochromites. From an analysis of Cr³⁺ exciton absorption in RCrO₃ (R=Tb, Dy, and Ho), it is unferred that these compounds exhibit an anomalous spin-reorientation in a magnetic field along the b axis, where the weak ferromagnetic moment of the Cr³⁺ spins rotates in the ac plane perpendicular to the b axis. In these compounds, when the R³⁺ spin configuration is disordered, an anomalous satellite band appears on the lower-energy side of the Cr³⁺ exciton absorption, which is associated with the breakdown of the k=0 selection rule due to the disorder of the R³⁺ spin configuration. In YbCrO₃, various kinds of cooperative excitations, such as a Cr³⁺ exciton coupled with an Yb³⁺ magnon and a Cr³⁺−Yb³⁺ exciton molecule, which are induced by the antisymmetric exchange interaction between the Cr³⁺ and Yb³⁺ spins, appear in the visible region. The propagation of these cooperative excitations depends strongly on the spin structure and the external magnetic field. In ErCrO₃, a photo-induced spin-reorientation takes place within 50 μs after the photo-irradiation corresponding to the ⁴A2g→²Eg transition of Cr³⁺, and it returns to the initial spin configuration in about 400 ms. This phenomenon is detected in the time-resolved Er³⁺ absorption spectra corresponding to the ⁴I₁₅/₂→⁴I₉/₂ transition. Finally, we briefly review the recent frontier research on applications developed mainly in Japan. 2002 Article Recent progress in magneto-optics and research on its application / Norimichi Kojima, Kuniro Tsushima // Физика низких температур. — 2002. — Т. 28, № 7. — С. 677-690. — Бібліогр.: 49 назв. — англ. 0132-6414 PACS: 78.20.Ls http://dspace.nbuv.gov.ua/handle/123456789/130237 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Обзор Обзор Norimichi Kojima Kuniro Tsushima Recent progress in magneto-optics and research on its application Физика низких температур |
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Various kinds of magneto-optical properties are investigated in rare earth orthochromites. From an analysis of Cr³⁺ exciton absorption in RCrO₃ (R=Tb, Dy, and Ho), it is unferred that these compounds exhibit an anomalous spin-reorientation in a magnetic field along the b axis, where the weak ferromagnetic moment of the Cr³⁺ spins rotates in the ac plane perpendicular to the b axis. In these compounds, when the R³⁺ spin configuration is disordered, an anomalous satellite band appears on the lower-energy side of the Cr³⁺ exciton absorption, which is associated with the breakdown of the k=0 selection rule due to the disorder of the R³⁺ spin configuration. In YbCrO₃, various kinds of cooperative excitations, such as a Cr³⁺ exciton coupled with an Yb³⁺ magnon and a Cr³⁺−Yb³⁺ exciton molecule, which are induced by the antisymmetric exchange interaction between the Cr³⁺ and Yb³⁺ spins, appear in the visible region. The propagation of these cooperative excitations depends strongly on the spin structure and the external magnetic field. In ErCrO₃, a photo-induced spin-reorientation takes place within 50 μs after the photo-irradiation corresponding to the ⁴A2g→²Eg transition of Cr³⁺, and it returns to the initial spin configuration in about 400 ms. This phenomenon is detected in the time-resolved Er³⁺ absorption spectra corresponding to the ⁴I₁₅/₂→⁴I₉/₂ transition. Finally, we briefly review the recent frontier research on applications developed mainly in Japan. |
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Article |
| author |
Norimichi Kojima Kuniro Tsushima |
| author_facet |
Norimichi Kojima Kuniro Tsushima |
| author_sort |
Norimichi Kojima |
| title |
Recent progress in magneto-optics and research on its application |
| title_short |
Recent progress in magneto-optics and research on its application |
| title_full |
Recent progress in magneto-optics and research on its application |
| title_fullStr |
Recent progress in magneto-optics and research on its application |
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Recent progress in magneto-optics and research on its application |
| title_sort |
recent progress in magneto-optics and research on its application |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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2002 |
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Обзор |
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| citation_txt |
Recent progress in magneto-optics and research on its application / Norimichi Kojima, Kuniro Tsushima // Физика низких температур. — 2002. — Т. 28, № 7. — С. 677-690. — Бібліогр.: 49 назв. — англ. |
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Физика низких температур |
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2025-07-09T13:07:17Z |
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2025-07-09T13:07:17Z |
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1837174818860957696 |
| fulltext |
Fizika Nizkih Temperatur, 2002, v. 28, No. 7, p. 677–690
Recent progress in magneto-optics and
research on its application
(Review Article)
Norimichi Kojima
Graduate School of Arts and Sciences, The University of Komaba 3-8-1,
Meguro-ku, Tokyo 153-8902, Japan
E-mail: cnori@mail.ecc.u-tokyo.ac.jp
Kuniro Tsushima
Kyushu Institute of Information Sciences, saifu 6-3-1, Dazaifu, Fukuoka 818-0117, Japan
Received April 10, 2002
We have investigated various kinds of magneto-optical properties for rare earth
orthochromites. In RCrO3(R = Tb, Dy and Ho), from the analysis of Cr3+ exciton absorption,
it was elucidated that these compounds exhibit an anomalous spin-reorientation under the
magnetic field along the b axis, where the weak ferromagnetic moment of the Cr3+ spins ro-
tates in the ac plane perpendicular to the b axis. In these compounds, when the R3+ spin con-
figuration is disordered, an anomalous satellite band appears in the lower energy side of the
Cr3+ exciton absorption, which is associated with the breakdown of k = 0 selection rule caused
by the disorder of R3+ spin configuration. In YbCrO3 , various kinds of cooperative excitations
such as Cr3+ exciton coupled with Yb3+ magnon and Cr3+–Yb3+ exciton molecule appear in the
visible region, which are induced by the antisymmetric exchange interaction between the Cr3+
and Yb3+ spins. The propagation of these cooperative excitations strongly depends on the spin
structure and the external magnetic field. In ErCrO3 , a photo-induced spin-reorientation takes
place within 50 µs after the photo-irradiation corresponding to the 4A2g �2Eg transition of Cr3+,
and it returns to the initial spin configuration in about 400 ms. This phenomenon was detected by
the time-resolved Er3+ absorption spectra corresponding to the 4I15/2 �4I9/2 transition. Finally,
we briefly review the recent frontier research on application mainly developed in Japan.
PACS: 78.20.Ls
1. Introduction
It is our very great honor and a great pleasure to
be able to submit our special review paper to cele-
brate and dedicate to Prof. V.V. Eremenko for his
70th birth. One of the authors (K.T.) has known
him since 1960’s and soon later both of us had a
mutual opportunity to meet and discuss, for in-
stance, at the time of the first and the second inter-
national symposium on magneto-optics, each as an
organizer in Kyoto, 1987 [1], and in Kharkov, 1991
[2], respectively. We had also a frequent communi-
cation through long years, sometimes including his
visit to Japan with his son. Every our communica-
tion with each other is an unforgettable remem-
brance.
More than 150 years ago, M. Faraday discovered
that, when linearly polarized light propagates
through a flint glass under an applied magnetic
field, its plane of polarization is rotated. Since
Faraday’s original discovery, magneto-optics has
become a highly fascinating field of research, which
is of a great importance from the viewpoint of basic
science and application. Especially, the mag-
neto-optics and spectroscopy of magnetically or-
dered materials have actively been developing since
1960s. Recently, Eremenko et al. [3], Zvezdin and
Kotov [4], and Sugano and Kojima [5] have edited
the books in which recent fascinating topics of
magneto-optical properties are described.
In magnetically ordered materials, various kinds
of magneto-optical properties appear as the syner-
© Norimichi Kojima and Kuniro Tsushima, 2002
gistic effect between optical properties and mag-
netic properties. The magnetic interaction between
elementary excitations such as excitons and
magnons enables them to combine with one an-
other, forming new and more complex excitations
such as exciton–magnon transitions, which are ob-
served in the visible region. Moreover, both the
magnetic interaction and spin structure have effects
on the propagation, the energy position and the
shape of elementary excitations. Therefore, the
analysis of magneto-optical properties is one of the
most powerful tools to investigate the spin configu-
rations and magnetic phase transitions.
In this paper, we describe various kinds of mag-
neto-optical properties of rare earth orthochromites
and review the recent frontier research on applica-
tion. In Sec. 2, we report an anomalous field-in-
duced spin-reorientation in RCrO3(R = Tb, Dy and
Ho) by the analysis of Cr3+ exciton absorption.
Moreover, we describe the breakdown of k = 0 se-
lection rule for the Cr3+ exciton absorption in
RCrO3(R = Dy and Ho) induced by the disorder
of the R3+ spin configuration. In Sec. 3, we de-
scribe various types of cooperative excitations in
YbCrO3 induced by the Cr3+–Yb3+ exchange inter-
action. Moreover, we describe that the Cr3+–Yb3+
antisymmetric exchange interaction depends on the
dynamics of their cooperative excitations. In Sec. 4,
after the survey of the recent progress of photo-in-
duced magnetism, we describe our pioneering work
of the photo-induced spin-reorientation in ErCrO3.
In Sec. 5, we survey the recent frontier research on
application especially developed in Japan.
2. Optical investigation of various magnetic
phase transition
It is well known that electronic excitations of
several electronvolts in magnetic insulators can be
regarded as transitions within an incomplete d or f
shell of a single magnetic ion. Such a localized exci-
tation cannot, however, be an eigenstate of the
crystal, and excitation migrates on magnetic ions as
an excitation wave called as Frenkel exciton. This
property is reflected in the energy dispersion and
the Davydov splitting. In magnetically ordered
state, the analysis of Frenkel exciton is one of the
most powerful methods to elucidate the spin confi-
guration and magnetic phase transitions, because
the magnetic symmetry change associated with
phase transition is directly reflected in the selection
rule of the polarized exciton lines.
In this Section, we elucidate an anomalous-type
of spin-reorientation in rare earth orthochromites,
RCrO3(R = Ho, Dy, and Tb), by the analysis of
Cr3+ exciton lines.
RCrO3 has an orthorhombically distorted
perovskite structure belonging to the space group
Pbnm (D h2
16) [6]. The unit cell contains four mole-
cules as shown in Fig. 1. The Cr3+ spins order spon-
taneously at Néel temperature (TN1) and these
compounds generally exhibit a weak ferromagnetic
moment. At the second Néel temperature (TN2),
the R3+ spins begin to reorder antiferromagne-
tically. The allowed spin configurations for the
Cr3+ and R3+ sites at TN1< T< TN2 are shown in
Fig. 2. The allowed spin configurations are denoted
as �1(AxGyCz;Cz
R), �2(FxCyGz;Fx
RCy
R), and
�4(GxAyFz;Fz
R) in Bertaut notation [7].
678
Norimichi Kojima and Kuniro Tsushima
Fig. 1. Unit cell of RCrO3. The Cr3+ and R3+ positions
are indicated with numbers 1–4 and 5–8, respectively.
Fig. 2. Spin configuration of RCrO3 at T T TN N2 1� � .
2.1. Field induced spin-reorientation in HoCrO3
It is well known that RCrO3 and RFeO3 exhibit
various types of field induced spin reorientation
where the weak ferromagnetic moment becomes
parallel to the direction of the applied magnetic
field. In the cases of H0 || a and c, the spin reorien-
tations of � �� �1 4, � �2 and � �� �1 2, � �4 have
been reported for various RCrO3 and FeCrO3 . In
the case of H0 || b, on the other hand, any spin re-
orientation is not expected, because any spin con-
figuration with a weak ferromagnetic moment of
the Cr3+ spins along the b axis can not be allowed
in the absence of an external magnetic field. How-
ever, Courths et al. and we have observed an
abrupt spectral change in HoCrO3 at 1.8 K with
the magnetic field H0 � 20 kOe along the b axis,
and they have suggested that this spectral change
at 20 kOe is interpreted as a spin reorientation
where the weak ferromagnetic moment of the Cr3+
spins rotates in the ac plane perpendicular to the b
axis [8,9].
In HoCrO3 , the Cr3+ spins are antiferromag-
netically ordered below TN1 = 140 K with a weak
ferromagnetic moment as �2(FxCyGz;Fx
RCy
R)
[10]. Any additional transition has not been ob-
served down to 1.5 K. As shown in Fig. 2, in the
case of �
2
spin configuration, the Cr3+ sublattice
magnetic moments and the Cr3+ net magnetic mo-
ment lie along the c axis and the a axis, respec-
tively. The anisotropy axes of the Ho3+ ions lie in
the ab plane at about �65� from the a axis [10], and
the g values along the a, b and c axes are 7.3, 15.7,
and 0, respectively [8], where the magnitude of the
effective spin of Ho3+ is 1/2. Therefore, the Ho3+
spins are strictly confined in the ab plane.
Figure 3 shows the behavior of the energies of
the absorption spectra around 13700 cm−1 for
HoCrO3 in the magnetic field H0 b at 1.5 K. In
this energy region, four sharp lines (R1, R2, R3,
and R4) of magnetic dipole character are observed.
The four magnetic dipole lines are assigned to the
Davydov-split components of the Cr3+ exciton cor-
responding to the transition from the lowest
substate of 4A2g to the lowest substate of 2Eg . As
shown in Fig. 3, when the magnetic field is applied
along the b axis, the R1-4 lines split into two pairs
of R1, R2 and R3, R4 in the range of 10 to 20 kOe,
and at about 20 kOe, an abrupt spectral change oc-
curs. As the magnetic field increases from 19.5 to
20.5 kOe, the R lines for the lower magnetic field
phase vanish, while those for the higher magnetic
field phase grow up. The discontinuous variation of
the energies of R lines and the existence of both
phases in a small field region (
1 kOe) around the
critical field Hc (
20 kOe) indicate that this phase
transition is of first order. The energies, polariza-
tions and intensities of the R lines show notable
changes at this phase transition, which suggests the
occurrence of the spin reorientation of Cr3+. Above
20 kOe, the energies of R lines remain unchanged,
which indicates that the antiferromagnetic axis in
the higher magnetic field phase is perpendicular to
the b axis. Therefore, it is obvious that the spin
configurations of the Cr3+ spins below and above
Hc are �2 (FxCyGz) and ���Gx AyFz), respectively.
The remarkable splitting of the R lines in the ex-
ternal magnetic field region between 10 kOe and
20 kOe are explained as follows. The molecular
field HB (|| c) on the Si �1
Cr spin induced by the
Cr3+– Ho3+ exchange interaction is expressed as,
� �g H D S D S
D S D S
c B y x y x
x y x y
Cr Ho Ho
Ho Ho
� � � �
� � ��
�
4
4
5 7
5 7
~ ~
~ ~
� �
�
�
(1)
and that on the Si �3
Cr spin is expressed as
� �g H D S D S
D S D S
c B y x y x
x y x y
Cr Ho Ho
Ho Ho
� � � �
� � ��
�
4
4
5 7
5 7
~ ~
~ ~
� �
�
�
(2)
679
Recent progress in magneto-optics and research on its application
Fig. 3. Magnetic field dependence of the lowest energy
region of the 4A2g � 2Eg transition of Cr3+ in HoCrO3 .
E and H denote the electric and magnetic vectors of the
incident light, respectively. Solid and open circles: ex-
perimental points. Broken lines: calculated energy shifts
of the R lines corresponding to the transition from the
lowest substate of 4A2g to the lowest substate of 2Eg of
Cr3+ by setting ~ ~ .D Dx x� � � 12 cm�� and ~ ~
D Dy y� � =
= 1.2 cm�� .
where, ~Dx , ~ �Dx , ~Dy , and ~ �Dy denote the antisym-
metric exhange interaction constants between Cr3+
and Ho3+. In the absence of external magnetic
field, the molecuar fields HB on the four inequivalent
are Cr3+ spins due to the Cr Ho3 3� �� interaction are
equal because of S j S jjx jx
Ho Ho( , ) ( , )� � �5 6 7 8
and S j S jjy jy
Ho Ho( , ) ( , )� � � �5 6 7 8 . However,
when the magnetic field is applied along the b axis,
S jj
Ho( , )� 7 8 change and then the molecular
fields on the Si �12,
Cr spins due to the Cr3+ � Ho3+ in-
teraction become different from those on the Si �3 4,
Cr
spins, which causes the remarkable splitting of the
R lines in the magnetic field region between 10 and
20 kOe. The broken lines in Fig. 3 show the calcu-
lated energy shifts of the R lines with H0 || b, by
setting ~Dx= ~ �Dx= 1.2 cm–1 and ~Dy= ~ �Dy= 1.2 cm–1.
In this calculation, we assumed that the molecular
field for the 2Eg state is equal to that for the
ground state. As shown in Fig. 3, the behavior of
the R lines in the magnetic field region H0 < Hc
is well reproduced in the calculation.
In order to confirm the above mentioned result,
we have investigated the behavior of the 5I8 � 5S2
transition of Ho3+ in the magnetic field H0 || b at
T= 1.5 K, which is shown in Fig. 4. At T = 1.5 K,
only the transitions from the lowest energy level I
of the ground multiplet (5I8) to the five singlet
states of 5S2 labeled as Ia , Ib , Ic , Id, and Ie in
order of increasing energy can be observed. As
shown in Fig. 4, when an external magnetic field is
applied up to Hc, the Ia�Ie lines split and the split-
ting increases with increasing magnetic field. How-
ever, the splitting vanishes at Hc. If the spin con-
figuration of Cr3+ is �2(FxCyGz), the effective
field on the Ho3+ site with S Sy
Ho � is dif-
ferent from that with S Sy
Ho � � when the e{xternal
field is applied along the b axis, which causes the
so-called sublattice splitting. Then, the splitting
of Ia � Ie lines in the magnetic field region of
H0 � Hc is interpreted as the sublattice splitting of
the 5I8 � 5S2 transition.
In the case of H>Hc, from the analysis of Cr3+
exciton lines, the spin configuration is considered to be
�4 (GxAyFz), where the molecular field on the Ho3+
spin becomes parallel to the c axis and its value should
be nearly zero because gc
Ho is negligibly small. There-
fore, in the higher magnetic field phase (H0 > Hc),
the Ho3+ spins are arrayed only by the external mag-
netic field (H0 || b), and the Zeeman energies upon the
four Ho3+ sites are equal. Thus, the sublattice splitting
of Ia – Ie lines corresponding to the 5I8 � 5S2 transi-
tion of Ho3+ vanish above Hc (= 20 kOe). Thus, we
can determine the spin configurations of HoCrO3 in
various magnetic fields of H0 || b, which is schemati-
cally shown in Fig. 5.
2.2. Field induced spin-reorientation in TbCrO3
The Cr3+ spins in TbCrO3 are antiferromagne-
tically ordered below TN1 = 167 K with a weak ferro-
magnetic moment as �2(FxCyGz;Fx
RCy
R) [11]. The
Tb3+ spins are antiferromagnetically ordered below
TN2 = 3.1 K, and the spin configuration below TN2 is
denoted as �25(FxCyGz: Fx
RCy
R; Gx
RAy
R) [11]. The
easy axes of Tb3+ spins lie in the ab plane at about
680
Norimichi Kojima and Kuniro Tsushima
Fig. 5. Spin configurations of HoCrO3 in various mag-
netic fields at 1.5 K. The magnetic field H0 || b.
Fig. 4. Magnetic field dependence of the 5I8 � 5S2 tran-
sition of Ho3+ in HoCrO3.
� 45° from the a axis. The g values along the a, b, and
c axes are 12.6, 12.6, and 0, respectively [12], where
the magnitude of the effective spin of Tb3+ is 1/2.
Therefore, the Tb3+ spins are strictly confined in the
ab plane.
Figure 6 shows the behavior of the Davy-
dov-split components of the Cr3+ exciton line corre-
sponding to the 4A2g � 2Eg transition in TbCrO3.
As shown in Fig. 6, when the magnetic field is ap-
plied along the b axis, the R1-4 lines split into two
pairs of R1, R2 and R3, R4 in the magnetic field re-
gion between 8 and 15 kOe. At about 15 kOe, the
energies, polarizations, and intensities of the R
lines show notable changes, which resembles
closely the behavior of the R lines in HoCrO3 at
the spin-reorientation from �2 to �4. Therefore, it is
obvious that the spin configurations of the Cr3+
spins in TbCrO3 below and above Hc(= 15 kOe)
are �2 (FxCyGz) and �4 (GxAyFz), respectively.
In the magnetic field region below Hc, with in-
creasing external field, the energy gravity of the R
lines drops around 8 kOe and then it remains un-
changed. The drop in the energy of R lines at about
8 kOe is attributed to the spin flip of the Tb6
3+ and
Tb7
3+ sites. Thus, we can determine the spin con-
figurations of TbCrO3 in various magnetic fields of
H0 || b, which is schematically shown in Fig. 7.
2.3. Field induced spin-reorientation in DyCrO3
The Cr3+ spins in DyCrO3 are antiferromag-
netically ordered below TN1 = 146 K with a weak
ferromagnetic moment as �2(FxCyGz; Fx
RCy
R)
[13]. The Dy3+ spins are antiferromagnetically
ordered below TN2 = 2.0 K, and the spin configura-
tion below TN2 is denoted as �25(FxCyGz: Fx
RCy
R;
Gx
RAy
R) [13]. The easy axes of Dy3+ spins lie in
the ab plane at about �60° from the a axis. The g
values along the a, b, and c axes are 6.0, 17.0, and
0, respectively [14], where the magnitude of the ef-
fective spin of Dy3+ is 1/2. Therefore, the Dy3+
spins are strictly confined in the ab plane.
Figure 8 shows the behavior of the Davydov-split
components of the Cr3+ exciton line corresponding
to the 4A2g � 2Eg transition in DyCrO3. When the
magnetic field is applied along the b axis, the ener-
gies, polarizations and intensities of the R lines dra-
matically change at about 2 kOe. Above 3 kOe,
681
Recent progress in magneto-optics and research on its application
Fig. 7. Spin configurations of TbCrO3 in various mag-
netic fields at 1.5 K. The magnetic field H0 || b.
Fig. 6. Magnetic field dependence of the R lines of
TbCrO3 . Broken lines are the guide for eye.
Fig. 8. Magnetic field dependence of the R lines of
DyCrO3 . Broken lines are the guide for eye.
those of the R lines remain unchanged. From the
analogy of the behavior of R lines in HoCrO3 and
TbCrO3, DyCrO3 exhibits the field induced spin re-
orientation from �2(FxCyGz) to �4(GxAyFz) at
H0(|| b) = 2 kOe.
2.4. Breakdown of the k = 0 selection rule of
Frenkel exciton
In the optical absorption corresponding to the
pure Frenkel exciton, only the zone-center exciton
is observable, because the magnitude of the propa-
gation vector of visible light is of the order of 10–3
reciprocal lattice vectors, which is usually called
k = 0 selection rule. However, in the cases of mixed
crystals or amorphous materials, it is predicted that
the deviation from a periodic structure causes the
breakdown of the k = 0 selection rule for the
exciton transition. Impurities, alloy and interface
roughness occurring on the length scale of unit cell
are the best known examples which provide an ap-
propriate momentum of elastic scattering. Thus, the
deviation from the virtual crystal causes the k �0
(nonvertical) transition through the medium of the
momentum of elastic scattering. In fact, the disor-
der-induced optical transitions for the mixed crys-
tals such as AgCl1-xBrx [15], GaAs1-xPx [16] and
AlxGa1-xAs [17] have characteristics which can be
associated with the breakdown of the k selection
rule for a periodic structure.
However, the breakdown of k = 0 selection rule
induced by the disorder of spin-configuration has
not yet been elucidated. In this Section, we report
the breakdown of k = 0 selection rule for the Cr3+
exciton absorption in RCrO3(R = Ho and Dy) in-
duced by the disorder of the R3+ spin configura-
tion.
Figure 9 shows the field dependence of the opti-
cal absorption spectra corresponding to the lowest
energy region of the 4A2g� 2Eg transition of Cr3+
in HoCrO3 [18]. The R2 and R3 lines are the
Davydov-split components of the pure Cr3+
exciton. When an external magnetic field is applied
along the b axis at 1.5 K, an anomalous satellite
band (R�) appears in the lower energy side of R
lines at about 7.5 kOe and its intensity passes
through a maximum at 16 kOe and then disappears
at Hc= 20 kOe. As mentioned already, when the ex-
ternal magnetic field is increased along the b axis of
HoCrO3, the sublattice magnetic moments, M7
Ho
and M8
Ho due to the S7
Ho and S8
Ho spins decrease
and vanish at about 16 kOe, and above 16 kOe they
grow gradually toward the direction of the applied
magnetic field. On the other hand, M5
Ho and M6
Ho
are saturated in the whole field range. From the be-
havior of the R� band and the spin configuration of
HoCrO3 in the magnetic field along the b axis, it is
obvious that the transition responsible for the R�
satellite band in HoCrO3 is allowed when the spin
configuration of the Ho3+ sites is disordered.
Figure 10 shows the field dependence of the op-
tical absorption spectra corresponding to the lowest
energy region of the 4A2g� 2Eg transition of Cr3+
in DyCrO3. The R1 line is the Davydov-split com-
ponent of the pure Cr3+ exciton. When an external
magnetic field is applied along the b axis at 1.7 K,
the intensity of the R� satellite band increases
abruptly at 1.5 kOe, and then decreases rapidly. As
shown in the inset of Fig. 10, when the external
magnetic field is applied along the b axis at 1.8 K,
the metamagnetic transition takes place at 1.5 kOe,
where the magnetic moments due to the S6
Dy and
S7
Dy spins are going to reverse their directions from
being antiparallel to parallel to the external mag-
netic field. Therefore, it is obvious that the transi-
tion responsible for the R′ satellite band in DyCrO3
is allowed when the spin configuration of the Dy3+ sites
is disordered.
682
Norimichi Kojima and Kuniro Tsushima
Fig. 9. Behavior of the R lines and the R� band of
HoCrO3 under magnetic fields along the b axis.
The characteristic properties of the R� satellite
band in RCrO3(R = Ho and Dy) are summarized as
follows. 1. The energy of the R′ band is lower by
about 16 cm–1 than the average energy of the pure
Cr3+ exciton lines. 2. The dipole nature of the R�
band is poorly characterized, while that of the pure
Cr3+ exciton lines is explicitly magnetic. 3. The
field induced energy shift of the R′ band disagrees
with the sum of the energy shift of Cr3+ exciton
and the R3+ spin flip, which implies that the R�
band does not correspond to the Cr3+ exciton cou-
pled with R3+ spin flip. 4. The transition responsi-
ble for the R� band is allowed when the R3+ spin
configuration is disordered by temperature or exter-
nal magnetic field. 5. As the magnetic moment of
the R3+ spins approaching saturation, the R� band
disappears.
Now, we discuss the transition mechanism re-
sponsible for the R� band in RCrO3(R = Ho and
Dy). The Cr3+ exciton dispersion due to the spin
allowed transfer is expressed as,
E(k) = E0 + 2V11
a cos(ak
x
) + 2V11
b cos(bk
y
) �
� 8V13cos(ak
x
/2)cos(bk
y
/2)cos(ck
z
/2), (3)
where E0 is the relevant excitation energy for the
single Cr3+ ion, V11
a and V11
b represent the
intra-sublattice transfers along the a and b axes,
respectively, and V13 the spin allowed
inter-sublattice one. From the analysis of the ab-
sorption spectra corresponding to the Cr3+ exciton
coupled with R3+ magnon in RCrO3(R = Tm and
Yb), the Cr3+ exciton due to the 4A2g� 2Eg transi-
tion has a large negative dispersion whose values
for TmCrO3 and YbCrO3 are estimated at –14 cm–1
and –16 cm–1 [19,20], respectively. Therefore, it is
plausible that the sign and the magnitude of the
Cr3+ exciton dispersion in HoCrO3 and DyCrO3
are almost the same as those of the Cr3+ exciton
dispersion in TmCrO3 and YbCrO3. Figure 11,b
shows the energy dispersion of the Cr3+ exciton due
to the 4A2g � 2Eg transition in YbCrO3, where
V11
a = 3.0 cm–1 and V11
b = 1.0 cm–1. As shown in
Fig. 11, the energy position of the R� band in
RCrO3(R = Ho and Dy) agrees very closely with
that of the Cr3+ exciton at the Brillouin-zone
boundary, which implies that the R� band is as-
signed to the excitation of the pure Cr3+ exciton at
the zone boundary, which is caused by the disorder
of the R3+ spin configuration. However, in general,
the optical excitation of the pure Cr3+ exciton at
the zone boundary cannot be observed without the
breakdown of the k = 0 selection rule.
683
Recent progress in magneto-optics and research on its application
Fig. 11. (a) R lines and R� band of DyCrO3, (b) Cr3+
exciton dispersion in YbCrO3. The origin of the energy
is fixed at the average energy of the free Cr3+ exciton
lines.
Fig. 10. Behavior of the R lines and the R� band of
DyCrO3 under magnetic fields along the b axis
at 1.8 K. Inset shows the magnetization curve.
Thus, we arrives at the following conclusion.
The magneto-elastic effect due to the R3+ (R = Ho
and Dy) ion is extraordinarily large because of the
strong spin-orbit interaction. When the R3+ spins
are fluctuated by temperature or external magnetic
field, the disorder of the R3+ spin configuration de-
forms the periodic lattice potential. Through the
medium of the strong magneto-elastic effect, the
disorder of the R3+ spin configuration causes the
breakdown of the k = 0 selection rule of the Cr3+
exciton absorption and it is reflected in the appea-
rance of the R� satellite band in the lower energy
side of the free Cr3+ exciton.
3. Various cooperative excitations
In magnetically ordered materials, various types
of elementary excitations such as excitons and
magnons exist. The magnetic interaction between
these elementary excitations enables them to com-
bine with one another, forming cooperative excita-
tions such as exciton-magnon transition [21], which
are observed in the visible region. In this Section,
we describe two kinds of cooperative excitations in-
duced by 3d–4f exchange interaction in YbCrO3.
Figure 12 shows the optical absorption spectra of
YbCrO3 in the energy region from visible to
near-infrared at 1.5 K. The absorption spectra of
YbCrO3 in the energy region between 10000 cm�1
and 25000 cm�1 are assigned as shown in Fig. 12. In
this energy region, besides the elementary excita-
tions due to d–d and f–f transition and the coopera-
tive excitations induced by the 3d–3d and 4f–4f ex-
change interactions, various kinds of cooperative
excitations induced by the 3d–4f exchange interac-
tion are observed. The Yb3+ exciton–Cr3+ magnon
excitation is observed in the 2F7/2 � 2F5/2 transi-
tion of Yb3+ [22], and the Cr3+ exciton – Yb3+
magnon excitation is observed in the 4A2g � 2Eg
transition of Cr3+ [20]. Moreover, the Cr3+ exciton
–Yb3+ exciton excitation is observed in the higher
energy side of the 4A2g � 4T1g transition of Cr3+ [23].
Here, we briefly summarize the magnetic proper-
ties of YbCrO3. In YbCrO3, there are three types
of magnetic interactions, Cr3+�Cr3+, Cr3+�Yb3+,
and Yb3+�Yb3+, each of which generally consists of
the isotropic, and the anisotropic symmetric and
antisymmetric exchange interactions. Among vari-
ous RCrO3, YbCrO3 is an interesting one with a
strong anisotropic exchange interaction between
the Cr3+ and Yb3+ ions [24]. In the case of Yb3+ ion
having only one 4f hole, the 4f orbital is widely
spread, which is responsible for the strong 3d�4f
exchange interaction in YbCrO3. The strong
anisotropic exchange interaction between the Cr3+
and Yb3+ spins in YbCrO3 is able to induce various
kinds of cooperative excitations. The Cr3+ spins in
YbCrO3 are antiferromagnetically ordered below
TN1 = 118 K with a weak ferromagnetic moment as
�2(FxCyGz;Fx
RCy
R) [25]. The spontaneous mag-
netic moment of YbCrO3 crosses to zero at 16.5 K,
which reveals that the induced magnetic moment of
the Yb3+ spins couples antiparallel to the weak fer-
romagnetic moment of the Cr3+ spins [25].
3.1. Cooperative excitation between Cr3+
exciton and Yb3+ magnon
Figure 13 shows the absorption spectra corre-
sponding to the lowest energy region of the
4A2g� 2Eg transition of Cr3+ in YbCrO3. The R1
and R3 lines with magnetic dipole character are as-
signed to the Davydov-split components of the pure
Cr3+ exciton. In the neighborhood of the Cr3+
exciton lines, an electric dipole band (R� band) ap-
pears, which has an anomalous band shape with a
sharp cut-off at the lower energy side and fine
structure. The R� band corresponds to the coopera-
tive excitation of Cr3+ exciton and Yb3+ magnon.
The band width and the cut-off profile at the lower
energy side of the R� band could be reproduced
quantitatively by taking account of the negative
exciton dispersion of –16 cm–1 [20].
As shown in Fig. 13, when the external magnetic
field is applied along the a axis of YbCrO3 at
2.0 K, a sharp and strong peak (arrow in Fig. 13)
typical of bound state appears on the lowest energy
side of the R� band. At about 25 kOe, the bound
state grows most strongly. Above 30 kOe, the
bound state splits into several peaks and their in-
tensity decreases significantly. At about 68 kOe, a
684
Norimichi Kojima and Kuniro Tsushima
Fig. 12. Optical absorption spectra of YbCrO3 in the
energy region from visible to near-infrared.
dramatic spectral change occurs. The discontinuous
spectral change between 59 kOe and 68 kOe is due
to the metamagnetic phase transition (Hc) where
the weak ferromagnetic moment of the Cr3+ spins
reverses its direction from being antiparallel to pa-
rallel to the net magnetic moment of the Yb3+
spins. At this metamagnetic transition [26], the
profile of the R� band changes drastically and its
bound state disappears completely.
From the appearance of the bound state on the
lowest energy side of the R� band, we arrive at the
following concept. The Cr3+ exciton coupled with
Yb3+ magnon at the Brillouin-zone edge is localized
under the external magnetic field (H0 || a), while
the Cr3+ exciton coupled with Yb3+ magnon at any
point of the Brillouin-zone except the zone edge is
delocalized. At 25 kOe, the Cr3+ exciton coupled
with Yb3+ magnon at the Brillouin-zone edge is
most strongly localized, which is reflected in the
field dependent energy shift of the R� band. Since
the R� band is assigned to the cooperative excita-
tion between Cr3+ exciton and Yb3+ magnon, the
magnetic field dependent shift of the energy separa-
tion (�E (H0)) between the R� band and the Cr3+
exciton lines should be expressed as,
�E(H0) = �E
R�
(H0) – �E
R
(H0) =�EYb(H0), (4)
with
�E
R�
(H0) = E
R�
(H0)� E
R�
(H0 = 0),
�E
R
(H0) = E
R
(H0) �E
R
(H0 = 0),
�EYb(H0) = EYb(H0) � EYb(H0 = 0), (5)
where, ER�(H0) and ER(H0) denote the energy po-
sition of the lowest energy side of the R� band and
that of the average energy of the R lines under the
external magnetic field (H0 || a), respectively, and
EYb(H0) denotes the energy of the Yb3+ magnon
under the external magnetic field (H0 || a), which is
estimated from the analysis of the Yb3+ exciton –
Yb3+ magnon excitation appearing in the 2F7/2 �
2F5/2 transition of Yb3+[24].
Figure 14 shows the magnetic field dependence
of �ER�(H0) � �ER(H0) and �EYb(H0). The nega-
tive deviation of �ER�(H0)��ER(H0) from
�EYb(H0) is caused by the attractive force between
the Cr3+ exciton and the Yb3+ magnon. As shown
685
Recent progress in magneto-optics and research on its application
Fig. 13. Behavior of the R lines and the R � band of
YbCrO3 under magnetic fields along the a axis at
2.0 K. Arrow shows the bound state of the R � band.
Fig. 14. Comparison between experimentally obtained
values of �EYb(H0) and � � � �� �E H E HR R� �0 0 . The de-
scriptions of �EYb(H0), � ��E HR� 0 and � ��E HR 0 are
shown in the text.
in Fig. 14, the attractive force becomes to be stron-
gest at 25 kOe, where the Cr3+ exciton–Yb3+
magnon at the Brillouin-zone boundary is most
strongly localized. Therefore, the bound state of
the R′ band grows most strongly at 25 kOe.
In the case that the bound state begins to mi-
grate, the bound state distinguishes four Cr3+ sites
in the unit cell. Therefore, in the process of the
delocalization of the bound state, the bound state
exhibits the Davydov splitting. In fact, as shown in
Fig. 13, the bound state of the R� band splits above
30 kOe. Therefore, we arrived at the following con-
clusion. In the magnetic field region above 30 kOe,
the zone-edge Cr3+ exciton coupled with Yb3+
magnon begins to migrate, which reflects upon the
splitting of the bound state and the significant de-
crease in its intensity.
At the metamagnetic transition (Hc = 67 kOe),
the weak ferromagnetic moment of the Cr3+ spins
reverses its direction, where the antisymmetric ex-
change interaction between the Cr3+ and Yb3+
spins, D(SCr � SYb), changes discontinuously in its
sign and intensity. The change of the antisymmetric
exchange interaction between the Cr3+ and Yb3+
spins at Hc should be responsible for the drastic
change of the R� band. Above Hc, the shape of the
R� band resembles closely the density of states of
the Cr3+ exciton coupled with the Yb3+ magnon,
which implies that the cooperative excitation be-
haves as a two-particle continuous state above Hc.
Therefore, it is concluded that the Cr3+�Yb3+ anti-
symmetric exchange interaction between the Cr3+
exciton and the Yb3+ magnon is attractive below
Hc. On the contrary, its attractive force vanishes
above Hc.
3.2. Cooperative excitation between Cr3+
exciton and Yb3+ exciton
Figure 15 shows the absorption spectra in the
energy region between 23800 cm–1 and 24000 cm–1.
These spectra correspond to the cooperative transi-
tion consisting of the 4A2g � 2Eg transition of Cr3+
and the 2F7/2 � 2F5/2 transition of Yb3+ in
YbCrO3. The average energy of the R lines is
13695.6 cm–1. Since this Cr3+ exciton has a nega-
tive energy dispersion of –16 cm–1, the energy of
the Cr3+ exciton at the Brillouin-zone boundary is
estimated at 13679.6 cm–1. On the other hand, the
lowest energy of the 2F7/2 � 2F5/2 transition of
Yb3+ is 10164.6 cm–1[24]. The sum of the energies
of the Cr3+ exciton and the Yb3+ exciton at the
Brillouin-zone boundary is estimated at 23844.2
cm–1, which is almost equal to the energy of the A1
line (23836.0 cm–1) in Fig. 15. The A and B bands
are assigned as shown in the inset of Fig. 15.
Figure 16 shows the behavior of the A and B
bands under the magnetic field along the a axis. As
shown in Fig. 16, at the metamagnetic transition
Hc(= 67 kOe), the profile of the A band drastically
changes. In particular, above Hc, a sharp and
686
Norimichi Kojima and Kuniro Tsushima
Fig. 15. Optical absorption spectra of YbCrO3 at 1.5 K.
E and H denote the electric and magnetic vectors of the
incident light, respectively.
Fig. 16. Magnetic field dependence of the A and B
bands of YbCrO3 at 4.2 K.
strong peak typical of bound state appears on the
low-frequency edge of the A band. Since the energy
dispersion of the Cr3+ exciton corresponding to the
4A2g � 2Eg transition is –16 cm–1, the low-fre-
quency edge of the A band corresponds to the Cr3+�
Yb3+ exciton molecule at the Brillouin-zone bounda-
ry. From the appearance of the bound state on the
low-frequency edge of the A band above Hc, it is
considered that the Cr3+�Yb3+ exciton molecule at
the Brillouin-zone boundary is strongly localized
above Hc.
As mentioned already, the Cr3+�Yb3+ antisymmetric
exchange interaction, D(SCr � SYb), in YbCrO3 cre-
ates various cooperative excitations between the
Cr3+ and Yb3+ elementary excitations. In the case of
Cr3+ exciton�Yb3+ exciton system, the Cr3+�Yb3+
antisymmetric exchange interaction acts as a strong
attractive force above Hc. Contrary to this, it acts
on the Cr3+ exciton–Yb3+ magnon system as a
strong attractive force below Hc. From this result,
the sign of the antisymmetric exchange interaction
between the Cr3+ exciton (2Eg) and the Yb3+
exciton (2F5/2) is presumed to be different from
that between the Cr3+ exciton (2Eg) and the Yb3+
magnon (2F7/2).
4. Photo-induced magnetism
One of the recent topics of solid state physics is
photo-induced transformation of electronic and mag-
netic state of matters. The magnetic states of several
compounds such as [Fe(ptz)6](BF4)2 (ptz = 1-propyl-
tetrazole)[27],K0.4Co1.3[Fe(CN)6]�5H2O[28],(In,Mn)As/
GaSb [29], have been revealed to be transformed to another
states by photo-irradiation. In the case of [Fe(ptz)6](BF4)2,
the ground state of Fe(II) is converted between the
low-spin state (t2
6, S = 0) and the high spin state
(t2
4e2, S = 2) by the photo-irradiation corresponding
to the d–d transition [27,30]. In the case of
K0.4Co1.3[Fe(CN)6]�5H2O [28], this compound can
be switched reversibly back and forth between
ferrimagnetism and paramagnetism by the photo-ir-
radiation corresponding to the charge transfer tran-
sition between the Co and Fe sites. In a novel
III–V-based magnetic semiconductor heterostruc-
ture p-(In,Mn)As/GaSb grown by molecular beam
epitaxy, the ferromagnetic order is induced by
photo-generated carriers [29].
These studies on photo-induced magnetism are
creating a new field of solid state physics. In con-
nection with photo-induced magnetism, pioneering
works have been done by Tsushima et al. [31,32].
Kovalenko et al. [33,34] and Golovenchitz et al.
[35]. For instance, Kovalenko et al. have found
that the linearly polarized illumination of yttrium
iron garnet, Y3Fe5-xSixO12, results in a spin-reori-
entation transition as a result of the photo-induced
change of the crystalline magnetic anisotropy
[33,34]. On the other hand, Tsushima et al. have
observed a photo-induced spin-reorientation transi-
tion from the antiferromagnetic to the weak-ferro-
magnetic spin structure of ErCrO3 by using a
time-resolved spectroscopic method [31,32]. In this
Section, we describe the photo-induced spin-reori-
entation from �1(AxGyCz;Cz
R) to �4(GxAyFz;Fz
R)
for ErCrO3.
Now, we briefly summarize the magnetic proper-
ties of ErCrO3. ErCrO3 is magnetically ordered be-
low TN1 = 133 K with a weak-ferromagnetic struc-
ture denoted as �
�
(GxAyFz;Fz
R) [36]. At 9.8 K,
ErCrO3 exhibits the temperature induced spin-reori-
entation from �4(GxAyFz;Fz
R) to �1(AxGyCz;Cz
R),
where the ferromagnetic moment disappears. Below
9.8 K, the �4 phase can be recovered by applying a
small external magnetic field (H0< 1.5 kOe) along
the c axis [37,38].
In order to generate the photo-induced spin-reori-
entation, we used a Q-switched ruby laser (6943 Å,
25 ns in half width and output power of a few mJ)
as a photo-irradiation source. The absorption spec-
trum of Er3+ in ErCrO3 was observed at 1.8 K to de-
tect the photo-induced spin-reorientation transition
from �1(AxGyCz;Cz
R) to �4(GxAyFz;Fz
R). Figure 17,a
shows the energy levels and the selection rule for the
electric dipole transitions between the 4I15/2 (de-
noted I� and I��) ground state and the 4I9/2 (denoted
b� and b�) excited state of Er3+ in ErCrO3. As shown
in the figure, the absorption spectrum for this transi-
tion is composed of four lines, which we label I�b� ,
I�b�, I�b� and I�b�. In the �1(AxGyCz;Cz
R) spin con-
figuration, the I�b� and I�b� absorption lines are su-
perimposed because �E(I)
�E(b)
10 K. The four
lines, however, are well resolved in the �4(GxAyFz;Fz
R)
spin configuration. For the case of E � c, the absorption
lines, I�b� and I�b�, are observed at 8049 Å associated with
the �1(AxGyCz;Cz
R) spin configuration. In the
�4(GxAyFz;Fz
R) spin configuration, these absorption
lines are observed at 8045 and 8054 Å. Figure 17,b
shows the time-resolved Ib absorption spectra after ini-
tial laser excitation at 7 K. It can be seen that ErCrO3
undergoes a phase transition from �1(AxGyCz;Cz
R) to
�4(GxAyFz;Fz
R) within 50 �s after the photo-irradi-
ation, and that it returns to the �1(AxGyCz;Cz
R)
phase in about 400 ms after the irradiation.
The idea of photo-induced spin-reorientation
transition has been raised from the following argu-
ment. In many RCrO3 and RFeO3, the direction of
the easy axis of magnetization easily changes as the
temperature changes or an external magnetic field
687
Recent progress in magneto-optics and research on its application
is applied. It is expected that the critical temperature
of spin-reorientation is changed when some of the
magnetic ions are substituted by magnetic impurities.
For example, YFe1-xCox/2Tix/2O3 (x= 0.003) un-
dergoes the spin-reorientation from �2(FxCyGz) to
�
4
(GxAyFz) at 247 K, while YFeO3 does not show
any spin-reorientation [39]. The magnetic anisotropy
of YFeO3 influenced by small amount of Co2+ is re-
sponsible for the appearance of spin-reorientation in-
YFe1-xCox/2Tix/2O3(x�� ��
� ���
. Therefore, the
photo-irradiation is regarded as a transient substi-
tution of the magnetic ions by magnetic impurities,
which presumably induces the phase transition
when the sample is kept near the critical tempera-
ture. However, in general, both of thermo-magnetic
effect and transient impurity effect are induced by
laser irradiation. The transient impurity effect is
justified if the phase transition is induced before
any non-radiative and radiative decay of the
photo-excited state occurs. Since the life time of
the 2Eg state of Cr3+ is quite long (several ms), the
photo-irradiation corresponding to the 4A2g �2Eg
transition of Cr3+ in ErCrO3 is the most effective
to generate the photo-induced spin-reorientation
transition. In order to further prove the transient
impurity effect, the time-resolved spectroscopic
measurement by using ultrashort laser (femto-
second laser) will be indispensable.
Finally, in connection with photo-induced magne-
tism, it should be noted that in some crystals of the
family of antiferromagnetic garnets Ca3Mn2Ge3O12
etc., photo-induced phenomena have been discovered
by Eremenko, Gnatchenko et al. [41– 44]. Photo-ir-
radiation with visible light results in arising of
long-lived changes: linear birefringence, magnetic
moment in the antiferromagnetic state, augmenta-
tion of optical absorption coefficient. The revealed
photo-induced changes of optical and magnetic
properties in antiferromagnetic garnets persist for a
long time after switching off illumination. The dis-
covery of sufficiently great photo-induced changes
of refractive index and absorption coefficient in
antiferromagnetic garnets will create a new field of
optical and magneto-optical recording.
5. Recent frontier research on application
We shall describe briefly about a recent research
on application of magneto-optical materials using
their unique magneto-optical non-reciprocity. The
most advantageous uniqueness of magneto-optical
materials lies in a fact that a propagation of the
light in those magnetic materials is antisymmetric
(non-reciprocal) to an inversion of time.
One is on highly bismuth-substituted rare-earth
iron garnets in a near-infrared wave-length region
for optical isolators and circulators [45,46]. The
other is on a diluted magnetic semiconductors, such
as CdMnTe in a more shorter wave-length region
for integrated magneto-optical waveguides [47]. A
schematic figure of an achieved integrated mag-
neto-optical guide on GaAs substrate by Zaets and
Ando is shown in Fig. 18 [48].
Finally, in addition to the above, a so-called
magneto-optical recording has become a real device
for a rewritable high density non-volatile memory.
It shows more development year by year. The most
recent progress in the magneto-optical recording for
688
Norimichi Kojima and Kuniro Tsushima
Fig. 17. (a) Schematic energy level diagram for the
4I15/2(I) �4I9/2(b) transitions of ErCrO3. Also shown
are optical transitions and their polarization. (b) Time-re-
solved Ib absorption spectra after initial laser excitation.
more high density recording, including super-reso-
lution limit recording density smaller than a do-
main size has been reviewed in a recent book by
Kaneko [49].
6. Conclusion
We have investigated various kinds of mag-
neto-optical properties for rare earth orthochromites.
In RCrO3(R = Tb, Dy and Ho), from the analysis of
Cr3+ exciton absorption, we have elucidated that
these compounds exhibit an anomalous spin-reorien-
tation under the magnetic field along the b axis,
where the weak ferromagnetic moment of the Cr3+
spins rotates in the ac plane perpendicular to the b
axis. It is quite difficult to elucidate the microscopic
mechanism of this phase transition by means of
magnetization measurement.
In RCrO3(R= Dy and Ho), we have elucidated
the breakdown of the k = 0 selection rule for the
Cr3+ exciton absorption induced by the disorder of
the R3+ spin configuration. The magneto-elastic ef-
fect due to the R3+(R = Dy and Ho) ion is extraor-
dinarily large because of the strong spin-orbit inter-
action. Through the medium of this strong mag-
neto-elastic effect, the disorder of the R3+ spin con-
figuration causes the breakdown of the k=0 selection
rule of the Cr3+ exciton absorption, which is re-
flected in the appearance of the anomalous satellite
band (R�) in the lower energy side of the free Cr3+
exciton absorption.
In YbCrO3, we have observed various kinds of
cooperative excitations such as Cr3+ exciton cou-
pled with Yb3+ magnon, and Cr3+�Yb3+ exciton
molecule in the visible region, which are induced
by the antisymmetric exchange interaction between
the Cr3+ and Yb3+ spins. In these cooperative excita-
tions, the Cr3+�Yb3+ antisymmetric exchange inter-
action acts as a strong attractive force, which is re-
sponsible for the appearance of the bound state in
the lower frequency edge in the cooperative excita-
tions.
In ErCrO3, a photo-induced spin-reorientation
from �1 to �4 takes place within 50 �s after the
photo-irradiation corresponding to the 4A2g � 2Eg
transition of Cr3+, and it returns to the initial spin
configuration in about 400 ms. This phenomenon
was detected by the time-resolved Er3+ absorption
spectra corresponding to the 4I15/2� 4I9/2 transi-
tion. The photo-irradiation is regarded as a tran-
sient substitution of the magnetic ions by magnetic
impurities, which presumably induces the phase
transition. In order to further prove the transient
impurity effect, the time-resolved spectroscopic
measurement by using ultrashort laser will be indis-
pensable.
Finally, in connection with the recent topics of
magneto-optics such as optical isolator, integrated
magneto-optical waveguide and magneto-optical re-
cording, we briefly reviewed the recent frontier re-
search on application mainly developed in Japan.
Acknowledgments
The authors would like to thank many scientists
who have given us an opportunity and collaborated
with us for a long time of our recent work. Some of
them are S. Sugano, I. Tsujikawa, K. Aoyagi,
H. Kamimura, W. M. Yen, T. Tamaki, N. Wata-
nabe, J. Dillon Jr., K. V. Rao, P. L. Richards,
M. Pardavi-Horvath, and H. Szymczak
V. V. Eremenko is undoubtedly to be specially
cited as an exclusively intimate physicist among
those. The authors also would like to express to
thank many other younger research colleagues for a
pleasant collaboration in many subjects in many oc-
casions.
This work was partly supported by the Science
and Technical Research Laboratories of Japan
broadcasting Corporation, and a Grant-in Aid for
Science Research from the Ministry of Education,
Science, Sports and Culture.
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