A study on spin mechanics phenomena in spin-lattice coupled systems

概要

（別紙様式５）

論

文

内

容

要

旨
(NO．1)

氏

名

有沢 洋希

提出年

令和 4 年

学位論文の

Study on spin mechanics phenomena in spin-lattice coupled systems

題

(磁気-歪み結合系におけるスピンメカニクス現象に関する研究)

目

論 文 目 次
1

Research background

5

1.1 Introduction and research motivation…..…….……………………………………….................. 5
1.2 Structure of thesis …….………….………………………………………………............................ 6
1.3 Spin dynamics………….………….……………………………………………................................ 7
1.3.1 Conduction-electron spin current…….………………………………………….................. 7
1.3.2 Spin Hall effect................................................................................................................. 8
1.3.3 LLG equation.................................................................................................................... 9
1.3.4 Spin transfer torque ....................................................................................................... 10
1.4 Lattice dynamics.......................................................................................................................11
1.4.1 Mechanics of elasticity.................................................................................................... 11
1.4.2 Mechanical motion of mechanical oscillator ……...........................................................19
1.5 Spin-lattice coupled dynamics................................................................................................. 21
1.5.1 Magnetoelastic energy ................................................................................................... 21
1.5.2 Magnetostriction ............................................................................................................ 24
1.5.3 Morphotropic phase transition....................................................................................... 26
2

Experimental methods

39

2.1 Physical properties of samples .............................................................................................. 39
2.1.1 Ferrimagnetic insulatorY3Fe5O12 ................................................................................. 39
2.1.2 Ferromagnet Tb0.3Dy0.7Fe2 ............................................................................................ 40
2.2 Sample fabrication ................................................................................................................. 42
2.2.1 Focused ion bean method ….......................................................................................... 42
2.2.2 Electroplating method ................................................................................................... 42
2.3 Mechanical response measurement with laser Doppler vibrometer.................................... 44
2.3.1 Laser Doppler vibrometry ............................................................................................. 44
2.3.2 Measurement system .................................................................................................... 45
2.4 Ultrafast spin/lattice dynamics measurement with femtosecond laser................................47
2.4.1 Time-resolved magneto-optical Kerr effect detection....................................................47
2.4.2 Time-resolved reflectivity detection ...............................................................................49

（別紙様式５）

3

Magnetomechanical coupling in magnetic micro mechanical oscillator

(NO. 2)

50

3.1 Fabrication of YIG micro bridge by FIB method ...................................................................50
3.1.1 Fabrication process of YIG micro bridge........................................................................51
3.1.2 Evaluation of sample damage by Ga+ ion beam irradiation .........................................51
3.2 Mechanical property of YIG micro bridge...............................................................................53
3.2.1 Mechanical vibration spectrum .....................................................................................53
3.2.2 Spatial profile of mechanical vibration..........................................................................55
3.3 Magnetic property of YIG micro bridge ..................................................................................56
3.4 Magnetic field dependence of mechanical vibration spectrum..............................................57
3.5 Theoretical analysis .................................................................................................................59
3.5.1 Change in Young’s modulus due to magnetization rotation .........................................59
3.5.2 Field dependence of resonance frequency......................................................................63
3.5.3 Physical origin ................................................................................................................64
3.6 Summary...................................................................................................................................65
4

Spin current volume effect in spin-lattice coupled system

66

4.1 Spin current volume effect ......................................................................................................66
4.2 Evaluation of film properties...................................................................................................68
4.2.1 Chemical composition ....................................................................................................69
4.2.2 Magnetization process ...................................................................................................70
4.2.3 Magnetostriction ............................................................................................................71
4.2.4 Resistivity........................................................................................................................71
4.3 Spin-current mechanical response in Tb0.3Dy0.7Fe2 ...............................................................72
4.3.1 Mechanical vibration spectrum .....................................................................................72
4.3.2 Field dependence of observed mechanical signal ..........................................................73
4.3.3 Paramagnetic metal dependence .................................................................................. 75
4.3.4 Field angle dependence ................................................................................................. 75
4.4 Theoretical calculation of spin current volume effect ............................................................76
4.4.1 Modulation of magnetization angle................................................................................77
4.4.2 Modulation of thermal fluctuations of magnetization...................................................79
4.4.3 Spin current volume effect by magnetization fluctuation modulation........................ 84
4.5 Origin of observed mechanical signal .................................................................................... 86
5

Ultrafast spin-lattice dynamics in magnetostrictive material

88

5.1 Excitation and detection of spin-lattice dynamics by femtosecond laser ..............................88
5.2 Magnetization process of Tb0.3Dy0.7Fe2 film............................................................................89
5.3 Ultrafast magnetization dynamics ..........................................................................................90
5.3.1 Temporal change in magneto-optical Kerr signal..........................................................90

（別紙様式５）

(NO. 3)

5.3.2 Magnetic field dependence..............................................................................................91
5.3.3 Sample composition dependence ....................................................................................92
5.4 Ultrafast strain dynamics........................................................................................................93
5.4.1 Temporal change in reflectivity.......................................................................................93
5.4.2 Magnetic field dependence ..............................................................................................94
5.4.3 Thickness dependence .....................................................................................................95
5.4.4 Theoretical analysis.........................................................................................................96
5.5 Summary...................................................................................................................................96
6

Conclusion

98

7

References

100

8

Acknowledgement

107

9

List of publications and presentations

111

（別紙様式５）

(NO. 4)

The field of spin mechanics is a research area that explores the cross-correlation between spin
angular momentum and mechanical motion in a solid. Spin angular momentum, lattice motion, and
mechanical motion interact with each other via magnetoelastic coupling, and spin mechanical
coupling is realized. While extensive studies on spin mechanics phenomena have been carried out
due to their high applicability for magnetic control of mechanical motions, further experimental
elucidation of the basic physics is still indispensable. Behind this situation, there is an issue that
the detection of the spin mechanical coupling is difficult due to the weak magnetoelastic coupling in
a solid. If this issue is overcome, it should be possible to investigate the basic spin mechanical
phenomena.
The purpose of the present research is to explore spin mechanical coupling phenomena and to
clarify their physical mechanism. For this aim we performed three experiments in spin-lattice
coupled systems. ...