1. Ashida lab. Onishi Yohei
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, R 2009
Ashida lab.
Onishi Yohei

2. Contents Introduction Multiferroic Ba2Mg2Fe12O22 Motivation THz-TDS
electromagnon
Motivation
THz-TDS
Experimental Result
Summary

3. Introduction E P Ferroelectric Electric polarization Electric field
電気分極
Electric field E ＋ ―
Electric field E
Electric polarization is controlled by the electric field. P
There is the flash memory as an example of applying this character. ―
０　　　　　１
not charged
charged
information

4. Introduction M H Ferromagnetic Magnetization Magnetic field磁化
Magnetic field H
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　　　　　１ up
down
information

5. 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)
Electric field E
Magnetic field H
Electric polarization P
電気分極
Magnetization M 磁化
magnetoelectric (ME) effect
（電気磁気効果）
In the meaning of material with two ferroic order , it is called Multiferroics.
The physics phenomenon that cannot be expected "Control of an electric polarization by the magnetic field" and "Control of the magnetization by the electric field" becomes possible.

6. 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.

7. Ba2Mg2Fe12O22 The sample has ferrimagnetism at the room temperature.
A weak magnetic field cause metamagnetic transition at the low temperature. M H
Ferrimagnetism
hexagonal ferrite
metamagnetism
六方晶フェライト
フェリ磁性
メタ磁性

8. Ba2Mg2Fe12O22 Direction of magnetic-field H is b.
Metamagnetic transition is generated in the sample around 0.12 T.
Hysteresis occur in this material. Ｈ

9. Ba2Mg2Fe12O22 O Fe Mg Ba
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* Ｈ

10. Ba2Mg2Fe12O22
conic type cycloid c
metamagnetic transition H
below 195K
below 50K
Electric polarization is caused external magnetic field for diagonal one.

11. 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

12. 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.

13. 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).

14. 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.
Pump beam
Delay　stage
Probe beam
THz-TDS
fs pulse laser
THz emitter
THz detector
sample

15. Derivation of complex refractive index
Transmission Fresnel constants
Fourier transformed spectrum
Complex transmittance d
Sample

16. Derivation of complex refractive index
phase modulation 位相
change of amplitude
(absorption)

17. Experimental setup
The temperature and the external magnetic field of the sample can be changed.

18. 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

19. 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].

20. 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.

21. 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).

22. 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.

23. 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