Ultrafast Magnetization Dynamics for Magnetic Recording Materials Design In heat-assisted magnetic recording (HAMR), the magnetization of recording media is reversed under transient laser heating. Therefore, not only conventional magnetic properties such as magnetic anisotropy and coercivity, but also dynamic magnetic properties that determine how rapidly magnetization responds and relaxes are important. In particular, L10-FePt, a key material for magnetic recording media, exhibits large perpendicular magnetic anisotropy, resulting in magnetization precession frequencies exceeding 100 GHz. This makes it difficult to evaluate its dynamic properties using conventional microwave-based ferromagnetic resonance techniques. We therefore employ the time-resolved magneto-optical Kerr effect (TRMOKE) using femtosecond laser pulses to directly observe ultrafast magnetization dynamics in FePt and quantitatively evaluate material parameters such as magnetic damping. Our research has progressed from continuous FePt films to nanogranular FePt films that more closely resemble practical HAMR recording media. Since the recording media are heated to high temperatures during the recording process, we developed a unique experimental setup at NIMS for high-temperature TRMOKE measurements. This enables us to experimentally evaluate dynamic magnetic properties at elevated temperatures and obtain materials information for understanding magnetization reversal processes. More recently, we have expanded our research from the evaluation of dynamic magnetic properties to the control of magnetic damping through microstructural engineering and elemental addition. Although the methods for engineering the damping constant in L10-FePt have not been well established, we demonstrated that the damping constant can be significantly reduced by controlling its microstructure. We also revealed that the damping constant at elevated temperatures can be enhanced by introducing rare-earth elements. Our research thus progresses from the observation of ultrafast magnetization dynamics to the quantitative evaluation of dynamic magnetic properties and their application to materials design. In addition to conventional magnetic properties such as magnetization, coercivity, and magnetic anisotropy, we aim to clarify how dynamic magnetic properties, including magnetic damping, influence the performance of spintronic devices and magnetic recording media, and to establish new guidelines for the development of next-generation magnetic recording materials.

Ultrafast spin current transport and spintronic terahertz emitter Femtosecond laser pulses generate ultrafast spin current in ferromagnetic metal thin films, which are converted into transient charge currents through the inverse spin Hall effect in adjacent nonmagnetic heavy-metal layers, resulting in the emission of terahertz (THz) waves into free space. This phenomenon has attracted considerable attention as a new THz radiation source known as a spintronic THz emitter. To improve the emission efficiency, it is important to understand ultrafast spin-current generation and transport, as well as the electromagnetic response in the THz frequency range, from a materials design perspective. Our research therefore focuses on the electronic structures and transport properties of magnetic materials to develop highly efficient spintronic THz emitters. In CoFeB-based magnetic thin films, we investigated the effects of composition and annealing on THz emission properties and identified spin polarization in the Δ bands, electrical resistivity, and the electron mean free path as important material parameters. Higher electrical resistivity is favorable for efficient THz emission. However, an increase in resistivity generally accompanies a reduction in the electron mean free path, resulting in a trade-off with ultrafast spin transport in ferromagnetic metal layers. To address this trade-off, we focused on half-metallic materials with high spin polarization and extended our research to THz emitters using the Heusler alloy Co₂MnSi. We demonstrated that THz emission from Co₂MnSi/Pt increased with improved chemical ordering and exhibited a higher emission intensity than CoFe/Pt. Our research aims to identify the material parameters governing ultrafast spin transport and THz emission and to develop efficient THz generation and control technologies through engineering of electronic structures, composition, crystal structures, and interfaces. Furthermore, we employ THz time-domain spectroscopy (THz-TDS) for evaluating spin transport properties in new materials and exploring ultrafast control of spin currents, extending its application to broader materials research.