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

Abstract

Spintronic terahertz (THz) emitters based on synthetic antiferromagnets (SAFs) of FM 1 /Ru/FM 2 (FM: ferromagnet) have shown great potential for achieving coherent superposition and significant THz power enhancement due to antiparallel magnetization alignment. However, key issues regarding the effects of interlayer exchange coupling and net magnetization on THz emissions remain unclear, which will inevitably hinder the performance improvement and practical application of THz devices. In this work, we have investigated the femtosecond laser-induced THz emission in Pt (3)/CoFe (3)/Ru (t Ru = 0−3.5)/CoFe (t CoFe = 1.5−10)/Pt (3) (in units of nm) films with compensated and uncompensated magnetic moments. Antiferromagnetic (AF) coupling occurs in the Ru thickness ranges of 0.2−1.1 and 1.9−2.3 nm, with the first peak (t Ru = 0.4 nm) of the AF coupling field (H ex ) significantly higher than that of the second peak (2.0 nm). Rather high THz amplitude is found for the samples with strong AF coupling. Nevertheless, despite the same remanence ratio of zero, the THz amplitude for the symmetric SAF films declines significantly as the t Ru decreases from 0.8 to 0.4 nm, which is mainly ascribed to the noncolinear magnetization vectors due to the increased biquadratic coupling term. Specifically, we demonstrate that an asymmetric SAF structure with a dominant FM layer is more favored than the completely compensated one, which could generate significantly enhanced THz electric field with well-controlled polarity and intensity. In addition, as the temperature decreases, the THz emission intensity increases for the SAF samples of t Ru = 0.9 nm with negligible biquadratic coupling, which is contrary to the decreasing trend of the t Ru = 0.4 nm sample and has been attributed to the greatly enhanced H ex .

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.…”