Observation and Control of Magnetic Order Dynamics by Terahertz Magnetic Nearfield (Springer Theses)

This book explicates the optical controls of antiferromagnetic spins by intense terahertz (THz) electromagnetic waves. The book comprises two key components: (1) the experimental demonstration of the enhancement of a THz magnetic field using a split-ring resonator (SRR) and (2) the control of the direction of magnetization by using the enhanced THz magnetic field to break the symmetry of optically-induced phase transition. These make up the first step leading to future spintronics devices. In the beginning of the book, the author reviews the basics of the ultrafast laser and nonlinear optical techniques as well as the previously achieved experiments to control spin dynamics by THz magnetic fields. In this context, a new experimental protocol is described, in which electron spins in a ferromagnetic material are redirected at the unprecedented level in cooperation with the enhanced THz magnetic field. Subsequently, the author demonstrates that the THz magnetic field is significantly amplified as a nearfield around the SRR structured metamaterial, which is implemented by measuring spin precession in a solid. At the end, the author presents the key experiment in which the amplified THz magnetic nearfield is applied to the weak ferromagnet ErFeO3 along with the femtosecond near-infrared pulse, demonstrating the successful control of symmetry breaking of the spin system due to coherent control of the optically-induced spin reorientation phase transition pathways. The comprehensive introductory review in this book allows readers to overview state-of-the-art terahertz spectroscopic techniques. In addition, the skillful description of the experiments is highly informative for readers in ultrafast magnonics, ultrafast optics, terahertz technology and plasmonic science. Supervisor’s Foreword 6 Peer-Reviewed Papers 8 Conference Contributions 8 Acknowledgements 9 Contents 10 1 Introduction 13 1.1 Terahertz (THz) Control of Magnetic System 13 1.1.1 THz Spectroscopy 13 1.1.2 Usage of Intense THz Electric Fields 14 1.1.3 Control of Spin Dynamics with THz Magnetic Fields 14 1.1.4 Spin Reorientation Phase Transition (SRPT) 15 1.2 Field Enhancement Using Metallic Subwavelength Structures 17 1.2.1 Plasmonic Enhancement in Metallic Structures and Metamaterials 17 1.2.2 Plasmonic Techniques in the THz Region 17 1.2.3 Split-Ring Resonator as a Tool for THz Magnetic Field Enhancement 18 1.3 Purpose of This Thesis and Outline 18 References 19 2 Background 24 2.1 Rare-earth Orthoferrites (RFeO3) 24 2.1.1 Basic Properties of Orthoferrite 24 2.1.2 Spin Configuration 25 2.1.3 Free-Energy Description of the Rotation-Type SRPT 26 2.1.4 Magnetic Resonance Modes 29 2.1.5 Temperature Dependence of FM Mode Resonance 30 2.2 Generation and Detection of THz Pulses 30 2.2.1 THz Time-Domain Spectroscopy (THz-TDS) 31 2.2.2 Generation of THz Waves via Optical Rectification 32 2.2.3 Generation of Intense THz Waves by Tilted Wave Front Technique 33 2.2.4 Detection of Magnetization Dynamics by Magnetooptical (MO) Effects 36 2.2.5 Detection of THz Waves by Electro-Optic (EO) Sampling 38 2.3 THz Magnetic Nearfields in Split-Ring Resonator (SRR) 40 2.3.1 Electric Excitation of Magnetic Resonance Mode in SRR 41 2.3.2 Usage of Field Enhancement Effects in THz-SRR 42 2.3.3 Coupling of SRR and THz Spin Resonance Mediated by Magnetic Nearfield 42 References 44 3 Resonant Enhancement of Spin Precession by SRR-induced Magnetic Nearfields and Interactive Energy Transfer 46 3.1 Sample Fabrication 46 3.1.1 Fabrication of Single-Crystal ErFeO3 (001) Plate with Floating-Zone Method 46 3.1.2 Optical Characterization of ErFeO3 Sample 47 3.1.3 Fabrication of SRR Structures 49 3.2 Experiment Configuration and Measurement Setup 52 3.3 Result 1: Resonant Enhancement of FM Mode Precession 54 3.3.1 Original Spin Precession Dynamics of FM-Mode Without SRR 54 3.3.2 Temperature-Tuning of FM-Mode Frequency Around SRR Resonance 56 3.3.3 Coupled LLG-LCR Resonance Model 59 3.4 Result 2: Interactive Energy Transfer Between SRR and Spin 61 3.5 Chapter Summary 68 References 68 4 Control of Macroscopic Magnetic Order Dynamics Using SRR-Enhanced THz Magnetic Fields 70 4.1 Background: Controlling the Path of Phase Transition by the Coherent Spin Precession 70 4.2 Motivation 71 4.3 Experiment Setup/Sample Properties 72 4.3.1 Design of the SRR Structure 72 4.3.2 Experimental Setup and Sample 76 4.4 Results 79 4.4.1 Temperature-Dependence of the THz-Induced Spin Precession 79 4.4.2 Creation of Macroscopic Magnetization by SRPT 80 4.4.3 Estimation of Tilt Angle of Spins 83 4.4.4 Incident THz Amplitude-Dependence of the Created Magnetization 84 4.4.5 Temperature-Dependence of the Created Magnetization 87 4.4.6 Contribution of SRR Magnetic Fields on the Process of Macroscopic Magnetization Formation 91 4.5 Chapter Summary 93 References 94 5 Numerical Simulation of the Macroscopic Domain Formation 95 5.1 Simulation of Magnetization Dynamics Using LLG Equation and Free Energy Model 95 5.1.1 2-Spin Free-Energy Model 95 5.1.2 Equilibrium Spin States 96 5.1.3 Temperature-Dependence of Spin Resonance Frequency 97 5.1.4 Spin Precession Excited by SRR Magnetic Fields 98 5.1.5 Simulation of Ultrafast Heating Process 99 5.1.6 Simulation of dt-Waveform 100 5.2 Mechanism of Domain Creation by THz Magnetic Fields 104 5.2.1 Comparison of t- and Dt-Waveforms 104 5.2.2 Spin Dynamics During Potential Reshaping 106 5.3 Chapter Summary 111 References 112 6 Conclusion 113 6.1 Summary 113 6.2 Future Remarks 115 References 117 Appendix Relation Between the s- and m-parameters 118 A.1 Hamiltonian of SRR and Spin Systems Without Interaction 118 A.2 Inclusion of Interaction Term into Total Hamiltonian 120 A.3 Retrieving s and m from the New Hamiltonian 120 A.4 Remark 122