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Terahertz spintronic magnetometer (TSM). (a) Setup schematic for the TSM demonstrating a simplified approach for ultrafast magnetization detection. (b)-(e) Hysteresis measurement on Quartz/Co 40 Fe 40 B 20 (1.8 nm)/Pt(2 nm). (b) and (c) Comparing hysteresis measurement from heterostructure when the magnetic field is applied perpendicular to FM easy axis (EA). (b) M-H hysteresis measured from commercial VSM system. (c) THz hysteresis measured from TSM. (d) and (e) Comparing hysteresis measurement from heterostructure when the magnetic field is applied parallel to FM easy axis (EA). (d) M-H hysteresis measured from the commercial VSM system. (e) THz hysteresis measured from the TSM system. (Coercivity is denoted in green color.)
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A ferromagnetic metal consists of localized electrons and conduction electrons coupled through strong exchange interaction. Together, these localized electrons contribute to the magnetization of the system, while conduction electrons lead to the formation of spin and charge current. Femtosecond out of equilibrium photoexcitation of ferromagnetic th...
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... unlike the VSM technique that highly depends on the subtraction of diamagnetic signals from substrate and probe, the proposed terahertz spintronic magnetometer (TSM) provides a unique addition to the state-of-the-art in-plane magnetization detection for ultrafast thin film characterization and an enthralling ultrafast toolkit to probe the magnetic phases of matter. Figure 1(a) illustrates the schematic for a simplified terahertz spintronic magnetometer where femtosecond laser pulses illuminate the spintronic sample. The resultant terahertz emission is electrooptically detected using ZnTe nonlinear crystal (refer to Sec. ...
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... used for the measurements, as shown in Fig. 1. In addition to the close match, a slight variation in the coercivity with respect to VSM can also be accounted to the precise magnetic field calibration settings and different lab environments. With this well-proven empirical relation, we understand the microscopic mechanism that leads to such behavior. Phenomenologically, the ...
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Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. For heavy-metal/ferromagnet systems, harmonic characte...
Citations
... Efforts to enhance the performance of these emitters have primarily centered on optimizing attributes such as bandwidth and amplitude. While the amplitude of THz pulses can be manipulated through material choice and varying pump amplitudes [5][6][7][8][9], understanding and controlling the bandwidth of the THz generated from STE, which largely depends on spin current generation dynamics, remains challenging [10]. ...
The rise of spintronic terahertz (THz) emitters has captured considerable attention owing to their potential for delivering high-intensity broadband THz pulses. The quest for enhancing their performance has largely concentrated on two key aspects: the out-coupling of THz radiation and its generation within the material system. A thorough understanding of THz generation and its interaction with photoexcited, out-of-equilibrium materials is pivotal for achieving further progress. Ultrafast optical excitations elicit a range of dynamic responses within the picosecond timescale, aligning with the THz frequency range. Although existing methodologies, such as optical-pump THz-probe experiments, offer valuable insights, the intricate details of spin current generation in spintronic THz emitters, including the possible delay between energy rise and spin current generation, continue to be somewhat obscure. To address this gap, we theoretically propose two approaches to extract the subpicosecond timescales involved in the complex process of converting a laser excitation to a THz pulse in spintronics THz emitter use. In doing so we leverage the interference of the THz-probe pulse interacting with time-varying material properties and the THz pulse generated by the spin-to-charge conversion process. To describe such processes and theoretically support the experimental approaches we suggest, we introduce the perturbative transfer matrix method to take into account both the interaction of a THz probe with a multilayer undergoing subpicosecond dynamics and the production of THz radiation within some of the layers.
... 12 Very recently, Agarwal and colleagues proposed an approach to record the magnetic hysteresis loop by sweeping the THz pulse amplitude as a function of the magnetic field. 21,22 By doing so, they demonstrated a THz spintronic magnetometer capable of recording the hysteresis loop without contributions from the diamagnetic substrate. However, the optimal acquisition parameters for THz magnetometry and an enhanced signal-to-noise ratio of the hysteresis loops based on an ac modulation of the biasing magnetic field have not been demonstrated until now. ...
Spintronic terahertz (THz) emitters have been shown to be a cost-efficient source for use in time-domain THz spectroscopy. The use of external magnetic fields to control the polarity of the THz emission provides an opportunity to measure the magnetization of spintronic materials as well as shaping THz emission. Here, we demonstrate an efficient method of measuring magnetic hysteresis with material sensitivity and speed several orders of magnitude greater than typical magnetometry methods. In addition, we utilize the rapid control of material magnetization for lock-in detection in time-domain THz spectroscopy of spintronic emitters. The ability to rapidly control and measure the material magnetization on very small volumes provides an opportunity to study magnetic hetero-structures with sub-micron spatial resolution.
... 30,31 The THz emission from them can be tuned in properties 32,33 and optimized with external electric 34 and magnetic fields. [35][36][37] These low-cost devices are feasible for large-scale production 23 and easily integrable into miniatured devices, 38,39 enabling near-field microscopy and imaging applications 40,41 and similar other high end applications. [42][43][44][45][46][47] Unlike the nonlinear crystals that are used for broadband and highpower THz radiation generation, the nanometric thick spintronic emitters are free from the need of the critical phase matching conditions. ...
... THz emission from the respective heterostructures can be a direct tool to understand those states. 36,37,99 Agarwal et al. 37 have studied the impact of femtosecond laser pulse-induced ultrafast nonlinear photothermal effect on the spin current generation in the FM layer of an USTE. THz hysteresis loops, i.e., magnitude of the THz signal with varying applied magnetic fields, were recorded at different fluence values of the excitation pulses. ...
Ultrafast optically excited ferromagnetic (FM)/nonmagnetic (NM) multilayer heterostructures have been demonstrated recently as efficient, high-power, and broadband sources of terahertz (THz) electromagnetic radiation. Since these spintronic THz emitters exploit the conversion from ultrafast spin to charge current, either in bulk or at the interface, the THz pulses inhere all the characteristics of the involved mechanisms and dynamics associated with spin-charge interconversion processes. Deconvolving the same requires meticulous and careful experimentation and analysis. In this article, we review the current state-of-the-art in this field and provide a perspective on the emerging phenomena, which are prospering as new research avenues and demonstrate application potential for futuristic THz technologies. In the process of developing efficient spintronic THz emitters by optimizing various conditions including those with material parameters and excitation light, it turns out that THz emission spectroscopy itself can be a unique experimental tool for probing microscopic dynamical magnetic and spintronic effects, induced by femtosecond laser pulse excitation, in a noncontact and noninvasive manner. Several breakthroughs can be listed from the literature in this regard from the last decade. Just recently, ultrafast orbitronics is another dimension that is taking shape and will impact the field immensely. A fair account to this topic is also presented in the article.
... In the above equations, the denotes the amplitude of the THz pulse, which is known to scale proportionally with the (refer to Supplementary Methods section for THz-H hysteresis measurement details) 46 . However, equation (2) we focus on the transition region in white, as shown in Figure 2(b). ...
The broken inversion symmetry at the ferromagnet (FM)/heavy-metal (HM) interface leads to spin-dependent degeneracy of the energy band, forming spin-polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into two-dimensional charge current through the inverse Rashba-Edelstein effect. Exploring and assessing this spin-to-charge conversion (SCC) phenomenon at the FM/HM interface could offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. We utilize spintronic heterostructures as a platform to measure the spin-to-charge conversion (SCC) experienced by photoexcited spin currents. These heterostructures emit terahertz electric field when illuminated by femtosecond laser pulses, enabling us to quantitatively assess the ultrafast SCC process. Our results demonstrate a robust interfacial spin-to-charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface itself, separate from the bulk heavy-metal (HM) region. Moreover, the iSCC at the NiFe/Ru interface is discovered to be approximately 27% of the strength observed in the highest spin-Hall conducting heavy-metal, Pt. Our results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.
... THz (0.1-30 THz) is a frequency range that is gaining popularity because of its enormous potential in basic process studies of materials as it is able to resonantly couple to conduction-electron transport, plasmons, excitons, phonons, or magnons [1][2][3]. It is also an effective tool in imaging, sensing for biomedical purposes, and security applications [2,4,5]. THz radiation can be generated using various methods, including photoconductive antennas and nonlinear optical crystals [2]. ...
Spintronic THz emitters (STE) are efficient THz sources constructed using thin heavy-metal (HM) and ferromagnetic-metal (FM) layers. To improve the performance of the STE, different structuring methods (trilayers, stacked bilayers) have been experimentally applied. A theoretical description of the overall THz emission process is necessary to optimize the efficiency of STE. In particular, geometry, composition, pump laser frequency, and spin diffusion will be significant in understanding the pathways for further research developments. This work will apply a generalized model based on a modified transfer matrix method. We will consider the spin generation and diffusion in the FM and HM layers and explain the spintronic THz emission process. This model is suitable for calculating emitted THz signal as a function of FM and HM thicknesses for different geometrical configurations. We will investigate a bilayer geometry as a test case, but the extension to a multi-layer configuration is straightforward. We will show how the different configurations of the sample will influence the THz emission amplitude.
... [2] The discovery of laser-induced ultrafast demagnetization provides a perfect platform to manipulate spins on a sub-picosecond timescale and opens up a promising route to developing future information technology at THz speed. [3][4][5] In the past two decades, THz electromagnetic spectrum technology has advanced in both basic and applied research fields. Compared with traditional light sources, THz pulses have the characteristics of transient, broadband, coherence, and low energy, which are crucial for exploring material structures. ...
Terahertz time-domain spectroscopy (THz-TDS) system, as a new means of spectral analysis and detection, plays an increasingly pivotal role in basic scientific research. However, owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning (ASOPS) system, which requires frequent calibration, we combined traditional THz-TDS and ASOPS systems to form a composite system and proposed an all fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses. Finally, the time-domain and frequency-domain spectra are obtained by using two discrete systems in the integrated systems. It is found that the full width at half maximum (FWHM) of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same, but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system, which provides a guarantee for the study of the transient characteristics of substances.
... [2][3][4] Recently, various groups have shown efficient generation of THz radiation by femtosecond laser excitation of ferromagnet/non-magnet structures. [5][6][7][8][9][10][11][12] Here, the non-magnetic material is a material with high spin-orbit coupling, such as a heavy metal (HM) or its alloys 13,14 or even a topological insulator. 15 While various groups have shown the emission of THz radiation in FM/AFM heretostructures, [16][17][18] THz emission from an AFM/HM system was recently demonstrated by Qui et al. 19 They showed the optical generation of ultrafast spin current from NiO by a highly non-linear optical effect. ...
Ultrafast generation of spin currents involving antiferromagnets is currently attracting tremendous interest. Here, we demonstrate broadband THz emission from a [111]-oriented NiO/Pt bilayer grown on MgO and Al 2 O 3 substrates. The NiO films are grown by pulsed laser deposition, whereas the Pt films are grown by magnetron sputtering. While we obtained epitaxial films on both substrates, NiO films on the Al 2 O 3 substrate showed the presence of crystalline twins. We show that the existence of crystalline twins reduces the THz amplitude by an order of magnitude while simultaneously dramatically changing the azimuthal dependency of the THz amplitude. The findings have significant implications for antiferromagnetic spintronics.
... The jtot( ) pulse consists of a positive peak followed by a negative one, which changes sign around 1.4 ps after onset. To gain more insight into our data, jtot( ) is fitted by a current profile [58,59], as follows: Here, , and , are the rise and decay time constants of ji=c,res, respectively. Furthermore, and t0,i are the amplitude and time at peak of the current profile, respectively. ...
... The j tot (t) pulse consists of a positive peak followed by a negative one, which changes sign around 1.4 ps after onset. To gain more insight into our data, j tot (t) is fitted by a current profile [58,59], as follows: ...
Due to its high sensitivity and because it does not rely on the magneto-optical response, terahertz (THz) emission spectroscopy has been used as a powerful time-resolved tool for investigating ultrafast demagnetization and spin current dynamics in nanometer-thick ferromagnetic (FM)/heavy metal (HM) heterostructures. Here, by changing the order of the conductive HM coating on the FM nanometer film, the dominant electric dipole contribution to the laser-induced THz radiation can be unraveled from the ultrafast magnetic dipole. Furthermore, to take charge equilibration into account, we separate the femtosecond laser-induced spin-to-charge converted current and the instantaneous discharging current within the illuminated area. The THz emission spectroscopy gives us direct information into the coupled spin and charge dynamics during the first moments of the light–matter interaction. Our results also open up new perspectives to manipulate and optimize the ultrafast charge current for promising high-performance and broadband THz radiation.
... At the same time, the spins superdiffuse semi-ballistically, forming direct channels of spin-transport between the FM and HM layers [8][9][10][11]. The phenomena has led to the development of a range of applications such as broadband terahertz sources [12][13][14][15], ultrafast spin-transfer-torque driven logic devices [16], terahertz (THz) magnetometry [17][18][19], spin-resolved electron spectroscopy [20], ballistic electron emission microscopy [21], next-generation data-processors [22][23][24][25][26], and ultrafast spin injection in semiconductors and topological insulators [27][28][29][30][31]. Theoretical calculations attributed to the superdiffusion predicted the scattering processes within the FM and HM to cause the flow of hot electrons both from the FM to HM and HM to FM [8,32], leading to enhanced ultrafast demagnetization of FM [32,33]. However, the underlying processes of spintronic terahertz emission have so far discussed only the primary photoexcitation of the FM layer, assuming unidirectional carrier flow from FM to HM. ...
... In principle, when the spins from FM superdiffuse into the HM layer, they undergo an inverse spin Hall effect (ISHE) and generate a transient ultrafast charge current, , where = × /| | [14], is the spin Hall angle of the HM layer, and is the magnetization vector of the FM layer. As a result, the heterostructure emits terahertz electric field according to ( ) = ( )/ [12][13][14]36] and terahertz pulse amplitude scales with the ultrafast charge current amplitude, given by, ( ) ∝ ∝ [17,37]. Besides, within such a heterostructure, the terahertz field is also produced from minor processes that do not involve superdiffusion of spin current across the FM/HM interface. ...
Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures have attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of ultrafast spin current, which is largely attributed to the direct photoexcitation of the FM layer. However, we reveal that during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. The results also emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin-transport mechanism.
... Agarwal et al. [93] observed the linear relationship between the STE-generated THz ...
... Agarwal et al. [93] observed the linear relationship between the STE-generated THz pulse amplitude and the magnetization of the FM layer in the STE. Therefore, the hysteresis loop of the FM layer can be obtained by measuring the change in the THz amplitude with the magnetic field. ...
Terahertz (THz) sources, covering a range from about 0.1 to 10 THz, are key devices for applying terahertz technology. Spintronics-based THz sources, with the advantages of low cost, ultra-broadband, high efficiency, and tunable polarization, have attracted a great deal of attention recently. This paper reviews the emission mechanism, experimental implementation, performance optimization, manipulation, and applications of spintronic THz sources. The recent advances and existing problems in spintronic THz sources are fully present and discussed. This review is expected to be an introduction of spintronic terahertz sources for novices in this field, as well as a comprehensive reference for experienced researchers.
