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Propagating spin waves excited by a micro-focused ultra-short pulse laser were investigated in Permalloy films with the thicknesses of 20, 50, and 100 nm by means of an all-optical space-and-time-resolved magneto-optical Kerr effect. Although various perpendicular standing spin-wave modes were excited, the propagating magneto-static surface spin wave was clearly detected in each film, where the group velocities of approximately 4, 8, and 12 km/s and the propagation lengths of 3.8, 6.6, and 13 ${\mu }\text{m}$ for the 20, 50, and 100 nm-thick films, respectively, were evaluated. The evaluated group velocities were consistent with the theoretical values, whereas the propagation lengths were smaller than the theoretical values with the Gilbert damping constant ${\alpha }$ of 0.008.

Laser pulse induced terahertz (THz) emission from Ta/${({\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x})}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1--0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

The spin injection efficiencies in epitaxial Co2MnSi (CMS)/Pt thin films were investigated in spin pumping configuration. The inverse spin Hall (effect) voltage significantly depends on the post-annealing temperature Ta of CMS, which shows one order larger values than those of Fe and CoFe at Ta = 400 °C. The effective spin mixing conductance derived by the analytical model reaches 6.5 × 1019 m−2. Our findings suggest that the Co-based Heusler alloys could be promising material candidates as the spin injection source with spin pumping.

Focused pulse-laser-induced propagating magnetostatic surface spin waves (MSSWs) were investigated using an all-optical space-time-resolved magneto-optical Kerr effect microscope in 20-nm-thick permalloy thin films to clarify reciprocity and symmetry of MSSW emission. The microscope setup was constructed with variable direction of the applied magnetic field, and the corresponding angular dependence was examined. MSSWs were reciprocally emitted from the laser spot, in contrast to the nonreciprocal emission obtained by the antenna method. Specifically, the observed excitation amplitude and phase were independent of the propagation and magnetization directions within experimental error. By transforming the data into wave number--frequency space, the MSSW dispersion relation was clearly identified within the wave number of 2--3 rad/$\ensuremath{\mu}\mathrm{m}$ and frequency of 10 GHz. These observations are consistent with the model of ultrafast modulation of out-of-plane shape magnetic anisotropy inside the focused laser spot. In addition to confirming the symmetric and reciprocal emission of laser-induced MSSWs, the study demonstrated that this technique can provide all-optical microscopic spectroscopy for MSSW in metals, such as Brillouin light-scattering technique.

We successfully synthesized the verdazyl-based salt (o-MePy-V)(TCNQ) [o-MePy-V = 3-(2-methylpyridyl)-1,5-diphenylverdazyl; TCNQ = 7,7,8,8-tetracyanoquinodimethane]. Ab initio molecular orbital calc...

Optical vortex, possessing an annular intensity profile and an orbital angular momentum (characterized by an integer termed a topological charge) associated with a helical wavefront, has attracted great attention for diverse applications due to its unique properties. In particular for terahertz (THz) frequency range, several approaches for THz vortex generation, including molded phase plates consisting of metal slit antennas, achromatic polarization elements and binary-diffractive optical elements, have been recently proposed, however, they are typically designed for a specific frequency. Here, we demonstrate highly intense broadband monocycle vortex generation near 0.6 THz by utilizing a polymeric Tsurupica spiral phase plate in combination with tilted-pulse-front optical rectification in a prism-cut LiNbO3 crystal. A maximum peak power of 2.3 MW was obtained for THz vortex output with an expected topological charge of 1.15. Furthermore, we applied the highly intense THz vortex beam for studying unique nonlinear behaviors in bilayer graphene towards the development of nonlinear super-resolution THz microscopy and imaging system.

Frequency doubling of optical vortices is demonstrated with an optical-optical efficiency exceeding 70%, using a spiral phase plate at a fundamental vortex energy of 10.6 mJ. Beam propagation of the doubled vortex output is also investigated both experimentally and theoretically. Spatial transforms in the output during propagation are observed.

Using a laterally configured structure, we demonstrate modulation of the magnetization dynamics of an epitaxial Heusler ferromagnet, Fe3Si, by nonlocally injecting pure spin currents. From the ferromagnetic resonance signals, the effective Gilbert damping constant ([Formula: see text]) of the epitaxial Fe3Si nanomagnet is estimated to be ∼0.003 at room temperature. By using the lateral and nonlocal spin injection, the [Formula: see text] value of the epitaxial Fe3Si nanomagnet can be modulated from 0.0034 to 0.0024. This study will open a way for control of the magnetization dynamics of high-performance ferromagnetic Heusler alloys due to pure spin currents even in laterally configured structures.

Coherent spin-wave generation by focused ultrashort laser pulse irradiation was investigated for a permalloy thin film at micrometer scale using an all-optical space and time-resolved magneto-optical Kerr effect. The spin-wave packet propagating perpendicular to magnetization direction was clearly observed, however that propagating parallel to the magnetization direction was not observed. The propagation length, group velocity, center frequency, and packet-width of the observed spin-wave packet were evaluated and quantitatively explained in terms of the propagation of a magneto-static spin-wave driven by ultrafast change of an out-of-plane demagnetization field induced by the focused-pulse laser.

We demonstrate highly intense, monocycle 0.6-terahertz (THz) vortex generation by utilizing a Tsurupica spiral phase plate in combination with a tilted-pulse front pumping optical rectification THz generator formed of a prism-cut LiNbO 3 crystal. A maximum THz vortex pulse energy of 2.3 μJ was obtained.

Laser-induced magnetization precessional dynamics was investigated in epitaxial films of Mn$_3$Ge, which is a tetragonal Heusler-like nearly compensated ferrimagnet. The ferromagnetic resonance (FMR) mode was observed, the precession frequency for which exceeded 0.5 THz and originated from the large magnetic anisotropy field of approximately 200 kOe for this ferrimagnet. The effective damping constant was approximately 0.03. The corresponding effective Landau-Lifshitz constant of approximately 60 Mrad/s and is comparable to those of the similar Mn-Ga materials. The physical mechanisms for the Gilbert damping and for the laser-induced excitation of the FMR mode were also discussed in terms of the spin-orbit-induced damping and the laser-induced ultrafast modulation of the magnetic anisotropy, respectively., 3 figures

Perpendicular magnetic tunnel junctions (p-MTJs) with a MgO barrier and a 1-nm-thick MnGa electrode were investigated by inserting several monolayers (MLs) of Mn. The tunnel magnetoresistance (TMR) ratio systematically increased when increasing the Mn layer thickness with a maximum of 18 (38.4)% at 300 (5) K for a Mn layer thickness of 0.6–0.8 nm. This ratio is five times higher compared to that without the Mn layer. The perpendicular magnetic anisotropy (PMA) field and the PMA constant of the ultrathin MnGa layer also increased up to 62–90 kOe and 6.2–11.3 Merg/cm3, respectively, with an increase in the Mn interlayer thickness, even for the ultrathin regime of the MnGa layer. For p-MTJs showing a high TMR and PMA, electron microscopy indicated the presence of 3–4 MLs of Mn at the MnGa/MgO interface; thus, the Mn modification enhanced the TMR as well as improved the PMA. This may be a promising finding to develop a Mn-based free layer for spin-transfer-torque devices for high-recording-density magnetoresi...

Picosecond pulsed frequency-doubled optical vortices were generated using a pair of β-BaB2O4 crystals with their c axes inverted. This arrangement produced high-quality ultraviolet vortex output with low spatial separation of the phase singularity at a conversion efficiency of ∼40%. We also discuss the theoretical spatial form and beam propagation of the ultraviolet vortex output.

Laser-induced terahertz (THz) emission in Ta/CoFeB/MgO films with various thicknesses of CoFeB and for various annealing temperatures was studied. The THz emission intensity exhibited its maximum value for a CoFeB layer thickness of approximately 1.0 nm, and this maximum value was enhanced by 1.5 times by annealing at 300 °C. A correlation was found in the annealing dependence between saturation magnetization, conductivity, and THz emission intensity. The origin of the enhancement of THz emission intensity due to annealing was discussed in terms of the increase in the mean free path of majority spin hot electrons in the CoFeB layer with crystallization due to the annealing.

An all-optical time-resolved magneto-optical Kerr effect measurement of a micron-sized tunnel junction with a CoFeB electrode was performed. The femtosecond (fs) laser-induced magnetization precession was clearly observed at various magnetic field angles. The frequency f and relaxation time τ of the magnetization precession varied with the voltage applied via a MgO barrier. The precession dynamics were in accordance with Kittel's ferromagnetic resonance mode, and the voltage-induced changes in f and τ were well explained by the voltage-induced change in the perpendicular magnetic anisotropy of −36 fJ/Vm.

Current-induced magnetization switching is demonstrated in a micron sized Hall bar consisting of Pt-capped ultrathin ferrimagnetic MnGa films. The films showed a low magnetization M s ≃ 150 kA/m and a large perpendicular magnetic anisotropy (PMA) field T, indicating a PMA thickness t product mJ/m2, which is relatively larger than those reported for other material films with PMA. Magnetization switching induced by an in-plane electrical current was examined with the application of an in-plane magnetic field. The phase diagram of the switching current vs the in-plane magnetic field is qualitatively consistent with that of the torque due to the spin-Hall effect in the Pt layer.

Mn-based hard magnets are potentially suitable for advanced ultra-high frequency spintronics applications because they exhibit the Larmor precession of magnetization at THz wave-range frequencies with low magnetic relaxation. However, the low magnetic relaxation properties are not well understood; thus, a more detailed study is necessary. In this study, magnetization precessions in L10 Mn1.54Ga, D022 Mn2.12Ga, and C38 MnAlGe epitaxial films grown on MgO substrates were investigated using an all-optical pump-probe method under a magnetic field of ∼20 kOe. The coherent magnetization precessions at frequencies of more than 0.1, 0.2, and 0.3 THz for the C38 MnAlGe, L10 Mn1.54Ga, and D022 Mn2.12Ga films, respectively, were clearly measured. The effective damping constant for the C38 MnAlGe film was smaller than the previously reported value measured at 10 kOe by a factor of 2, whereas those showed a large angular dependence. The effective damping constants for the L10 Mn1.54Ga and D022 Mn2.12Ga films were inde...

The cause of the time scale for ultrafast demagnetization induced by irradiation using a fs pulse laser remains an open issue. Spin-flip mediated by electron–phonon scattering due to spin–orbit interactions is one major theory proposed to explain ultrafast demagnetization. Ultrafast demagnetization in Ni, FePd, and FePt films was investigated in order to systematically study the influence of heavy elements on the demagnetization time. The ultrafast demagnetization in these systems was analyzed using the microscopic three temperature model, which is a theory based on spin-flip mediated by electron–phonon scattering. The spin-flip probability () values for the Ni, FePd, and FePt films were evaluated in the low pump fluence regime. It was found that the value for the Ni film was larger than that for the FePd and FePt films. Thus, there is no correlation between the value and the spin–orbit coupling strength. Fast demagnetization of the FePd film was also observed due to large electron–phonon scattering. In addition, it was found that the values decreased with increasing magnetization quenching for all the films.