University of Groningen
A terahertz view on magnetization dynamics
Awari, Nilesh
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Awari, N. (2019). A terahertz view on magnetization dynamics. University of Groningen.
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A Terahertz View On Magnetization
Dynamics
Zernike Institute PhD thesis series 2019-03 ISSN: 1570-1530
ISBN: 978-94-034-1301-3 (printed version) ISBN: 978-94-034-1300-6 (electronic version)
The work presented in this thesis was performed in the Optical Condensed Matter Physics group at the Zernike Institute for Advanced Materials of the University of Groningen, The Netherlands and at Helmholtz Zentrum Dres-den Rossendorf, DresDres-den, Germany.
Cover design by Nilesh Awari Printed by Gildeprint
A Terahertz View On Magnetization
Dynamics
PhD thesis
to obtain the degree of PhD at the
University of Groningen on the authority of the Rector Magnificus Prof. E. Sterken
and in accordance with
the decision by the College of Deans. This thesis will be defended in public on
Friday 18 January 2019 at 14.30 hours
by
Nilesh Awari
born on 28 September 1987 in Sangamner, India
Supervisor Prof. T. Banerjee Co-supervisors Dr. M. Gensch Dr. R. I. Tobey Assessment committee Prof. B. Koopmans Prof. M. M¨unzenberg Prof. L.J.A. Koster
Contents
List of Figures vii
List of Tables ix
1 Introduction 1
1.1 Outline of the thesis . . . 3
1.2 Bibliography . . . 4
2 Introduction to Magnetism 7 2.1 Origin of magnetism and magnetic properties . . . 8
2.2 Magnetic properties of materials . . . 10
2.3 Ultra-fast magnetization dynamics . . . 13
2.4 Bibliography . . . 19 3 Experimental Techniques 23 3.1 THz emission spectroscopy . . . 24 3.1.1 Electro-Optic Sampling . . . 25 3.2 Magneto-optic effect . . . 26 3.2.1 Faraday effect . . . 27
3.2.2 Magneto-optical Kerr effect (MOKE) . . . 28
3.3 Light sources . . . 29
3.3.1 Near infra-red (NIR) femtosecond laser sources . . . 29
3.3.2 Laser-based THz light sources . . . 30
3.3.3 TELBE . . . 32
3.4 Bibliography . . . 34
4 Narrow-band Tunable THz Emission from Ferrimagnetic Mn3-XGa Thin Films 41 4.1 Introduction . . . 42
4.2 Experimental details . . . 43
4.3 Results & Discussion . . . 48
4.3.1 Effect of Mn content on THz emission from Mn3-XGa . . . 50
4.3.2 Effect of laser power on THz emission from Mn3-XGa . . . 52
4.3.3 Effect of temperature on THz emission from Mn3-XGa . . . 54
4.3.4 Field dispersion for Mn3-XGa . . . 55
4.3.5 Thickness dependence of THz emission from Mn3-XGa . . . 57
4.4 Conclusion & Outlook . . . 58
Contents CONTENTS
4.5 Bibliography . . . 60
5 THz-Induced Demagnetization: Case of CoFeB 65 5.1 Introduction . . . 66
5.2 Experimental details . . . 68
5.3 Results & Discussion . . . 72
5.4 Conclusion & Outlook . . . 79
5.5 Bibliography . . . 80
6 THz-Driven Spin Excitation in High Magnetic Fields: Case of NiO 83 6.1 Introduction . . . 84
6.2 Experimental details . . . 85
6.3 Results & Discussion . . . 89
6.3.1 Temperature dependence of AFM mode . . . 89
6.3.2 Field dependence of AFM mode . . . 91
6.4 Conclusion & Outlook . . . 95
6.5 Bibliography . . . 96 Summary 99 Samenvatting 101 Acknowledgements 103 Publications 107 Curriculum Vitae 111 vi
List of Figures
1.1 Areal density growth of HDD devices as a function of time. . . 2
2.1 Different types of magnetic ordering present in materials. . . 10
2.2 Properties of a typical ferromagnet. . . 12
2.3 Susceptibility as a function of temperature for different magnetic ordering. 13 2.4 Schematic of the magnetic precession. . . 15
2.5 Schematic of time scales involved in laser driven excitation of magnetic materials. . . 17
2.6 Effect of femtosecond laser excitation on magnetic materials. . . 18
3.1 Schematic of the electro-optic set-up. . . 26
3.2 Schematic of the Faraday set-up. . . 28
3.3 Geometries for measurement of Kerr effect. . . 29
3.4 Schematic of the polar MOKE set-up. . . 30
3.5 Schematic of the optical rectification process for THz generation. . . 31
3.6 Electric field and power spectrum of LiNbO3 as a THz source. . . 32
3.7 Schematic representing the principle of superradiant process. . . 33
3.8 Maximum pulse energy observed at TELBE as a function of repetition rate, for a given THz frequency . . . 34
3.9 Frequency tunability of TELBE source. . . 35
4.1 Schematic of the THz emission spectroscopy set-up and sample geometry employed for the Mn3-XGa samples. . . 44
4.2 Schematic of the idealized crystal structure of Mn3Ga . . . 44
4.3 Schematic of the bilayer system in Mn3-XGa thin films . . . 46
4.4 Emitted THz wave-forms from Mn3-XGa thin films because of NIR laser irradiation . . . 49
4.5 Analysis of the THz emission measurements . . . 51
4.6 The 180◦ phase shift of FMR mode observed in Mn 3-XGa thin film. . . 51
4.7 Resonant THz excitation of the FMR mode in Mn3Ga thin film . . . 52
4.8 Laser power dependence of the emitted THz emission from Mn3-XGa thin films . . . 53
4.9 Temperature dependence of the emitted THz emission from Mn3-XGa thin films . . . 55
4.10 Schematic of the THz emission spectroscopy set-up and sample geometry employed with 10 T split coil magnet . . . 56
4.11 Field dispersion relation for ferromagnetic mode in Mn3-XGa thin films . . 56
4.12 THz emission from the films with island morphology . . . 57 vii
List of Figures LIST OF FIGURES
4.13 Thickness dependence of the emitted THz emission from Mn3-XGa thin
films . . . 59 4.14 Characterization of Mn3-XGa thin films for tunable, narrow band THz
source. . . 59 5.1 Experimental set-up used for narrow band THz pump MOKE probe
mea-surements. . . 69 5.2 Experimental geometry used in the experiments . . . 69 5.3 The electric field waveform of 0.5 THz used in the experiment . . . 70 5.4 Example showcasing the coherent and incoherent contributions of THz
induced magnetization dynamics in CoFeB . . . 70 5.5 Ultra-fast demagnetization observed in CoFeB at 0.5 THz pump . . . 71 5.6 Ultra-fast demagnetization observed in CoFeB thin films with THz pump
as a function of pump power . . . 73 5.7 Excitation of the FMR mode in CoFeB using THz as a pump. . . 73 5.8 Ultra-fast demagnetization observed in CoFeB thin films at 0.7 THz 1
THz pump . . . 74 5.9 Ultra-fast demagnetization observed in CoFeB thin films as a function of
the THz pump frequency . . . 75 5.10 Comparison of ultra-fast demagnetization observed in CoFeB thin films
at 0.7 THz pump, taken 6 months apart . . . 75 5.11 Effect of implantation on THz induced ultra-fast demagnetization
ob-served in CoFeB . . . 79 6.1 Illustration of the crystallographic and magnetic structure of NiO. . . 86 6.2 Sketch of the THz pump Faraday rotation probe technique used for NiO. 87 6.3 Electric field and power spectrum of the utilized THz radiation. . . 87 6.4 Illustration of the two distinct magnetic modes in antiferromangetic
res-onance. . . 88 6.5 Typical transient Faraday measurement for NiO obtained at 280 K. . . . 90 6.6 Temperature dependence of the magnon mode in NiO. . . 91 6.7 Field dispersion for magnon mode in NiO. . . 92 6.8 Theoretical calculation of Field dependence of the higher-energy spin
modes in NiO. . . 96
List of Tables
4.1 Ms from VSM [6, 8] and inferred values of 10Hkfrom dynamic THz
emis-sion measurements. THz emisemis-sion measurements have been performed in the presence of an external magnetic field of 400 mT and at a temperature of 19.5◦C. THz driven Faraday rotation measurements were performed with an external magnetic field of 200 mT. . . 53 5.1 A summary of the THz frequencies used in the THz pump Polar MOKE
experiments along with their peak electric field values. . . 71
