Terahertz Spectral Signatures of Ultrafast Spin Transports in Ferromagnetic Heusler Alloy

Abstract

Although the Co‐based Heusler compounds are predicted to be half‐metals, their sub‐picosecond demagnetization dynamics upon laser excitation show a transition‐metal‐like behavior. Any possible role of ultrafast nonlocal spin transport on the ultrafast demagnetization of half‐metallic Heusler compounds has only been inferred indirectly by time‐resolved magneto‐optical Kerr effect. Here, an ultrafast optically driven spin current traveling from Co2FeSi half metal into an adjacent Pt layer by using terahertz (THz) emission time‐domain spectroscopy (TDS) is demonstrated. By varying the magnetic field and excitation symmetry, the sizable THz generation from Co2FeSi/Pt stack arises from optically induced spin transport across the Co2FeSi/Pt interface in conjunction with spin‐to‐charge current conversion, via the inverse spin Hall effect (ISHE). The chemical ordering, interface roughness and magnetization of the samples become vital to engineer the THz emission properties. The amplitude of THz emission is comparable to that of CoFeB/Pt stack, thereby indicating Co2FeSi as an efficient ultrafast light‐driven spin current injector. Finally, by varying the pumping energy, the contributions are distinguished to the ultrafast spin current from thermalization and optical excitation of majority spins in Heusler alloy. This work illustrates THz emission TDS as a powerful tool to investigate the ultrafast spintronic properties of Heusler alloy/normal‐metal bilayers.

Temperature-Dependent Spin-to-Charge Conversion and Efficient Manipulation of Elliptical THz Waves in Bi2Te3/TbFeCo Heterostructures

“…Spintronic THz emitters have proved to be highly efficient sources of THz radiation with low-cost, broad bandwidth, and compact size − in contrast to photoconductive antennas and electro-optical crystals, which offer promising applications in various scenarios such as wireless communications, biomedical detections, and coherent spin resonance. − In traditional spintronic THz devices composed of ferromagnetic (FM) and nonmagnetic (NM) layers, the inverse spin Hall effect (ISHE) in NM is exploited to transform the photoinduced spin current into the ultrafast charge current. − Thus, heavy metals, acting as NM, like platinum and tungsten with strong spin–orbit coupling, are preferred in generating significant THz electric field for commercial use. Recently, topological insulators (TIs) have been anticipated to be a productive medium for spin-to-charge conversion (SCC) where the strong spin–orbit interaction gives rise to the topologically protected surface states. − The spin-momentum locking property of the surface states has been proved to exert large spin–orbit torques on the adjacent FM layer, leading to the detection of considerable charge current in the electric transport measurements. − As a reciprocal phenomenon, the illumination of femtosecond laser pulses on the FM layer produces a nonequilibrium spin accumulation at the interface due to different densities of states and band velocities of majority and minority spin electrons.…”

Comprehensive study of the ultrafast photoexcited carrier dynamics in Sb2Te3–GeTe superlattices

“…[25][26][27] On one hand, since the nonequilibrium distribution of photoexcited carriers causes the transient currents and subsequently generates the THz radiation, [28][29][30] THz emission spectroscopy has been proven to be a good method for investigating the transient carrier transport properties in symmetry-broken materials, such as ferromagnetic superlattices, transition metal dichalcogenides, strongly correlated and topological materials. [31][32][33][34] On the other hand, transient reflectance spectroscopy is a powerful technique to investigate bandgap renormalization, Pauli blocking and free carriers' absorption, providing an understanding of excited-state carrier dynamics in topological materials. [35,36] In this paper, we combine THz emission spectroscopy and transient reflectance spectroscopy to investigate the ultrafast current and carrier dynamics in Sb 2 Te 3 -GeTe superlattices.…”

High Efficiency and Flexible Modulation of Spintronic Terahertz Emitters in Synthetic Antiferromagnets

“…The ISHE can be expressed as j c = θ SH · j s × M , where j c denotes the charge current density, j s is the spin current density, and M is the magnetization vector. The parameter of the spin Hall angle θ SH reflects the spin-to-charge conversion efficiency of the NM layer, which is related to the spin–orbit coupling interaction. − Apparently, the NM layer plays a key role in the laser-excited THz emission process, and materials with a high θ SH are expected. Various NM layer materials with strong spin–orbit coupling, such as nonmagnetic heavy metals, − antiferromagnets, − two-dimensional materials, − and topological insulators, − have been experimentally demonstrated to be able to generate strong THz signals. − The THz emission process in STEs is complex and involves various successive effects including spin generation, spin diffusion, spin decoherence, and spin–charge conversion.…”