1. Introduction1.1 Terahertz (THz) Control of a Magnetic System1.2 Field Enhancement Using Metallic Subwavelength Structures1.3 Purpose and Outline of This Thesis2. Technical Background2.1 Rare-Earth Orthoferrites (RFeO3)2.2 Generation and Detection of THz Pulses2.3 THz Magnetic Nearfields in a Split-Ring Resonator (SRR)3. Resonant Enhancement of Spin Precession by SRR-Induced Magnetic Nearfields and Interactive Energy Transfer3.1 Sample Fabrication3.2 Experiment Configuration and Measurement Setup3.3 Result 1: Resonant Enhancement of FM Mode Precession3.4 Result 2: Interactive Energy Transfer Between SRR and Spin3.5 Chapter Summary4. Control of Macroscopic Magnetic Order Dynamics Using SRR-Enhanced THz Magnetic Fields4.1 Background: Controlling the Path of Phase Transition by the Coherent Spin Precession4.2 Motivation4.3 Experiment Setup / Sample Properties4.4 Results4.5 Chapter Summary5. Numerical Simulation of the Macroscopic Domain Formation5.1 Brief Summary of Important Features Observed in Chapter 45.2 Simulation of Magnetization Dynamics Using LLG Equation and Free- Energy Model5.3 Mechanism of Domain Creation by THz Magnetic Fields5.4 Chapter Summary6. Conclusion6.1 Summary6.2 Future ProspectsAppendix. Relation Between the s- and m- Parameters