Electric-dipole-active magnetic resonance in the

Electric-dipole-active magnetic
resonance in the conical-spin
magnet Ba2Mg2Fe12O22
N. Kida, D. Okuyama, S. Ishiwata, Y. Taguchi, R. Shimano,K. Iwasa, T. Arima, and Y. Tokura
PHYSICAL REVIEW B 80, 220406R 2009
Ashida lab.
Onishi Yohei
Contents
• Introduction
Multiferroic
Ba2Mg2Fe12O22
electromagnon
• Motivation
THz-TDS
• Experimental Result
• Summary
Introduction
Ferroelectric
P
E
Electric polarization
Electric field
電気分極
Electric field
Electric polarization is
controlled by the electric field.
E
＋―
P
＋
• There is the flash memory as an example of applying this character.
―
not charged
charged
０
１
information
―
Introduction
Ferromagnetic
M
H
Magnetization
Magnetic field
磁化
magnetic field
H
Magnetization is controlled by the
magnetic field.
M
• There is the Hard disk drive as an example of applying this character.
０
up
down
１
information
Multiferroic
• There is the material with the character of both ferromagnetic and
ferroelectric.
• In the meaning of material with two ferroic order , it is called Multiferroics.
• In multiferroic materials, "Control of an electric polarization by the
magnetic field" and "Control of the magnetization by the electric field"
becomes possible.(magnetoelectric effect)
E
magnetoelectric (ME) effect
（電気磁気効果）
P
Electric field
Electric polarization
電気分極
H
M
Magnetic field
Magnetization
磁化
Multiferroic
• The application to the memory material using two order parameters
(magnetization and electric polarization) is expected.
• But, the relation between ferroelectric and ferromagnetism is small in
conventional solid. So, there is no application example using
magnetoelectric effect now.
• Recently, the new material in which ferroelectric is invented by the spin
order.
• A huge electric magnetic effect is observed in new material.
Ba2Mg2Fe12O22
O
Fe
Mg
• The sample has ferrimagnetism at the
room temperature.
• A weak magnetic field cause
metamagnetic transition at the low
temperature.
Ba
M
Ferrimagnetism
フェリ磁性
H
metamagnetism
メタ磁性
hexagonal ferrite
六方晶フェライト
Ba2Mg2Fe12O22
• Direction of magnetic-field H is b.
• Metamagnetic transition is generated in
the sample around 0.12 T.
• Hysteresis occur in this material.
O
Fe
Mg
Ba
Ｈ
Ba2Mg2Fe12O22
• About 100μC/m2 electric polarization
appears along with the metamagnetic
transition.
• The direction of an electric polarization
reverses along with the scanning of the
magnetic field.
Direction of a*
O
Fe
Mg
Ba
Ｈ
Ba2Mg2Fe12O22
conic type cycloid
c
metamagnetic
transition
H
below 195K
below 50K
• Electric polarization is caused external
magnetic field for diagonal one.
Electromagnon
A little gap of the direction between adjoined spins is caused. The gap
spreads wave-like in the entire crystal . (spin wave)
The quasiparticle that quantizes the spin wave is called magnon.
When the vibration frequency is assumed to be ν as well as the case of the
photon, the energy of magnon is given by hν.
transmission of spin wave
Electromagnon
Magnon can be excited only by the magnetic field in conventional solid.
but
There are magnon that can be excited by electric field in Multiferroic materials.
Magnetic excitation (spin wave excitation) by electric field of light (terahertz
wave).
Such excitation is called electromagnon.
It is known that Ba2Mg2Fe12O22 has magnetic excitation about 2.8 meV by
inelastic neutron scattering.
Motivation
The multiferroic material with ferromagnetism and
ferroelectric controls the magnetization by the electric field,
and enables the polarization by the magnetic field to be
controlled.
• The relation of the spin and strong dielectric in the
multiferroic material is clarified.
• We search for spin wave excited by electric field
element(electromagnon).
Terahertz time-domain spectroscopy
(THz-TDS)
• THz-TDS enables us to observe a waveform ”E(t)” directly.
• Information on both amplitude and phase are directly obtained.
Probe beam
fs pulse laser
Pump beam
sample
Delay stage
THz emitter
~
~
Er   Es  
THz-TDS
THz detector
Derivation of complex refractive
index
complexrefractiveindex： n~( )  n( )  i ( )
Transmission Fresnel constants
Fourier transformed spectrum
~
Er ()  Er () expr ()
~
Es ()  Es () exps ()
Complex transmittance
Sample
d
Ereference (t )
Esample (t )
n~( )  n  i
Derivation of complex refractive index
Complex refractive index
phase modulation
位相
change of amplitude
(absorption)
complexdielectricconstant： ~( )  1 ( )  i 2 ( )
1 ( )  n 2   2 ,  2 ( )  2n　com plexelectrical　conductivity :  ( )    ( )  i 2 ( )
  ( )  0 2 ,  2 ( )  0 (1  1)
Experimental setup
The temperature and the external magnetic field of the sample can be changed.
Light-polarization dependence
• Real and imaginary parts of the complex dielectric
constant spectra measured at 5K in zero H .
• Sharp resonance around 2.8meV is observed,
when electric-field E and magnetic-field H
polarizations of light were set parallel to [001] and
[100].
• When the direction of E is rotated , signature of
the resonance around 2.8 meV is disappear and
another resonance around 8 meV is observed .
• The sample has anisotropy
for the electric field of light.
c
Temperature dependence
• We measured the T dependence in zero H .
• The conspicuous thermochromism is observed
with the evolution of the conical-spin order below
50 K.
• ε１ around the resonance is strongly modified by T.
• Figure(a) show the spectral weight of the 2.8 meV
peak.
• These experimental results ensure the magnetic
nature of the electric-dipole-active resonance with
E[001].
Magnetic-field dependence
• ε１ around the resonance is strongly modified by H.
• Figure(d) is the spectral weight of the 2.8 meV peak.
• We found the conspicuous magnetochromism at
terahertz frequencies arising from a remarkable
change in the electric-dipole-active magnetic
resonance around this critical temperature 53.5K by
an application of the external H.
Magnetic-field dependence
• We compare ε2 in H applied H along [001] (Fig.(c)) with that in H along
[100](Fig.(f)) at the lowest temperature 5 K.
• Figure(c) show the steep enhancement of the resonance as well as the case
at 53.5K.
• A tiny effect of H on the resonance is observed in Fig(f).
Summary
• Sharp electric-dipole-active magnetic resonance
(spin wave
excitation(electromagnon)) at terahertz frequencies is identified
in the ordered conical-spin phase of Ba2Mg2Fe12O22.
• From the crystallographic orientation and the polarized light
dependency , this absorption is observed only in a specific
direction of the crystal axis.
• Even if a voluntary polarization is caused by the external
magnetizing field, this sharp absorption is unaffected.
• The observed gigantic magnetochromism yields a concept for
future terahertz devices such as a tunable terahertz color filter
controlled by H.
My work
• Resently , the new material (Sr3CO2Fe24O41)that
has electromagnon at room temperature is
discovered by Kimura lab in Osaka university.
• I try to observe electromagnon in this material.
• In this material , various applications are
expected. Because it is possible to use it at the
room temperature .
Sr3CO2Fe24O41

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