We report the broadband terahertz (THz) radiation in the metallic ferromagnetic (FM) heterostructures, upon irradiation of a femtosecond laser pulse at room temperature. The origin of THz generation from FM heterostructures can be interpreted using two terms: the transient demagnetization (a local modification of spin order of the FM metal) and electricdipole radiation resulting from a non-local spin current pulses. Here, we show that the THz emission is dominated by the photo-excited transient charge current, which is converted from the spin current with inverse spin Hall effect. We tailor the metallic heterostructures with different non-magnetic thin layer (Pd or Ru) and FM materials (CoFeB or CoFe), to shape the THz transients. Moreover, we find that a saturation effect of THz radiation for CoFeB/Pd is less compared to CoFeB/Ru. THz emission spectroscopy can be used to qualitatively visualize the spin accumulation in the heterostructures.

The ultrashort laser excitation of nonmagnetic/ferromagnetic/nonmagnetic (NM/FM/NM) heterostructures is intensively studied as a terahertz (THz) emitter. Herein, the combined spintronic and photonic ultrathin metal heterostructures are exploited to realize actively modulated THz radiation. It is demonstrated that the THz radiation can be mediated coherently through the charge current induced by the inverse spin Hall effect (ISHE) and the built‐in transient current quasi‐simultaneously created within the striped NM/FM/NM heterostructures. The waveforms of THz radiation can be shaped by the charge current confinement effect, including the amplitude modulation and the center‐frequency shift. These findings can be utilized for device‐oriented opto‐spintronics and also extend the established THz emitter to THz modulator.

We report the broadband terahertz (THz) radiation in ferromagnetic half-metallic Heusler alloy CoMnSn thin film upon the irradiation of a femtosecond laser pulse at room temperature. The magnetic-, sample symmetry-, and pump fluence-dependent THz emission reveals that the THz radiation is originated from the magnetic-dipole radiation, i.e., the light-induced subpicosecond demagnetization. In addition, by optical pump-THz probe spectroscopy, we found that the photoexcited increase of the scattering rate of hot carriers thereby leads to the photoinduced negative THz conductivity in CoMnSn thin film.

We report the broadband emission of terahertz (THz) pulse in metallic patterned ferromagnetic heterostructures CoFeB/Pt based on inverse spin-Hall effect, by illuminating a train of linearly polarized 120-fs-wide laser pulses at 800 nm. The spatial-temporal distribution of charge currents by changing the length of the subwavelength rectangular metal blocks allows for not only effectively controlling the magnitude, but subtly tuning the center frequency and bandwidth of the emitted THz pulses. Our results will open new avenues for the study of modulated spintronic-based THz emitters.

The development of efficient terahertz (THz) radiation sources is driven by the scientific and technological applications. To date, as far as the radiation of THz pulses is concerned, the widely used methods are biased semiconductor, electro-optical crystal and air plasma, which are excited separately by femtosecond laser pulses. The mechanisms involved in these THz sources are photo-carrier acceleration, second order nonlinear effect, and plasma oscillations, respectively. Here, we report the generation of coherent THz radiation in the designed ferromagnetic/non-magnetic metallic W/CoFeB/Pt and Ta/CoFeB/Pt trilayers on SiO2 substrates, excited separately by ultrafast laser pulses. The transient THz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the sample. We present the THz radiation results of the bilayers, CoFeB/W, CoFeB/Pt and CoFeB/Ta, which are comprised of the trilayer heterostructures used in our experiments. We find that all experimental results are in good agreement with the results from the inversed spin-Hall effect (ISHE) mechanism. Owing to the ISHE, the transient spin current converts into a transient transverse charge current, which launches the THz electromagnetic wave. In our experiments, W or Ta has an opposite spin Hall angle to Pt. Therefore, the amplitude of the THz emission can be increased by a constructive superposition of two charge currents in metallic layers. Our results indicate that the peak-values of the THz radiation covering the 0-2.5 THz range from W/CoFeB/Pt and Ta/CoFeB/Pt are stronger than that from 0.5 mm thick ZnTe (110) crystal, under very similar excitation conditions. Finally, we investigate the dependence of peak-to-peak values for two different heterostructures on the pump fluence. The saturations of THz pulse at pump fluences of~0.47 mJ/cm2 and~0.61 mJ/cm2 are found for W/CoFeB/Pt and Ta/CoFeB/Pt heterostructures, respectively. The saturation can be generally attributed to the spin accumulation effect and laser-induced thermal effect. Our results indicate that the spin accumulation effect, by which the density of spin-polarized electrons is restricted in a non-magnetic metallic layer, is slightly less pronounced for Ta/CoFeB/Pt system at high fluences. Our findings provide a new pathway for fabricating the spintronic THz emitter, which is comparable to the conventional nonlinear optical crystals.

Recently, ferromagnetic (FM) heterostructures have been demonstrated to emit THz radiation induced by femtosecond laser pulses. In such spintronic THz emitters, an in-plane external magnetic field is technological required to assist the orientations of the magnetization, which could be an obstacle for practical micro-electrooptic applications. Here, as a proof-of-concept, THz generation is based on the inverse spin Hall effect of the FM/Ta within a magnetic tunneling junction (MTJ) without external magnetic field. Furthermore, THz emission can be engineered by modulating of the tunneling conductivity within a photoexcited MTJ as the relative magnetizations of two FM layers change their alignments.

In this work, by using terahertz time-domain spectroscopy (THz-TDS), we investigate the temperature range of the SRT, and the resonant frequency and relaxation time of the THz spin waves in Sm x Dy 1−x FeO 3 single crystal. We show that the resonant frequency of the FM mode (measured at 40 K) increases linearly with the Sm dopant concentration within the range from x = 0.5 to 0.7. The temperature-and dopant-induced changes of the magnetic anisotropy of Fe 3+ ions are accessed by the resonant frequency shifts. Upon cooling, the lifetime of oscillations in the AFM mode increases exponentially and can be subtly tuned by varying the Sm dopant. These results lead to an improved understanding of dopant-tuned spin wave dynamics and magnetoanisotropy parameter in rare-earth orthoferrites.

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