image: The interfacial proximity effect increases the Curie temperature of the 4-layer Fe3GeTe2 from 140 K to 206 K in the (Fe3GeTe2/CrSb)3 superlattice. Subsequently, this study demonstrates that femtosecond laser pulses induce interfacial spin polarization in the superlattice, generating room-temperature terahertz spin currents.
Credit: ©Science China Press
The increasing demand for denser information storage and faster data processing has fuelled a keen interest in exploring spin currents up to terahertz frequencies. The emergent two-dimensional intrinsic magnetic materials offer a novel and highly controllable platform to access such femtosecond spin dynamics at atomic layer thickness. However, the practical application of van der Waals magnets is often limited by their low Curie temperatures.
This challenge is addressed by a collaborative team led by Prof. Dr. Xiaojun Wu (Beihang University) and Prof. Dr. Sheng Meng, Prof. Dr. Jimin Zhao, Prof. Dr. Wei He (Institute of Physics, Chinese Academy of Sciences) and Prof. Dr. Faxian Xiu (Fudan University). They found distinct mechanisms for ultrafast spin current generation below and above the Curie temperature. Below the Curie temperature, spin currents are driven by ultrafast demagnetization in the ferromagnetic phase, triggered by femtosecond optical pulses. Above the Curie temperature, however, the spin currents arise from a novel mechanism related to photoexcitation termed a laser-enhanced proximity effect.
The discovery of this effect was supported by theoretical simulations. Upon photoexcitation, the researchers observed a significant relative interlayer displacement of up to 1 Å between the Fe3GeTe2 and CrSb layers, accompanied by an enhancement of electron-electron exchange interactions at the interface. These interactions dominate the laser-induced spin dynamics in the superlattice, enabling spin current generation even above the Curie temperature. This understanding is corroborated by time-resolved magneto-optical Kerr effect measurements of the corresponding transient spin polarization.
“At first glance, our observation of spintronic terahertz emission in the (Fe3GeTe2/CrSb)3 superlattice at room temperature may be counter-intuitive, given the disappearance of ferromagnetic order in the superlattice above 206 K. However, femtosecond optical pulses substantially excite the spin polarization in a nonequilibrium manner, thus allowing the generation of ultrafast spin currents.” Prof. Dr. Xiaojun Wu says.
The team combined experimental and theoretical observations to outline the entire process following photoexcitation. The absorption of the 800-nm pump laser by the (Fe3GeTe2/CrSb)3 superlattice results in the shortening of the interlayer distance between the Fe3GeTe2 and CrSb layer in just a few hundred femtoseconds. This, in turn, amplifies the proximity effect or the interaction between the two materials sufficiently to cause the spin polarization of Fe3GeTe2 above Curie temperature. Meanwhile, the magnetic moment of CrSb reorients from out-of-plane to in-plane, polarizing the spin of Fe3GeTe2 in the in-plane direction.
Because the pump photon energy of 1.55 eV surpasses the optical gaps of the FGT and CrSb layers, photocarriers are simultaneously excited and polarized along the in-plane direction. The resulting spin-polarized current is subsequently injected into the CrSb layer and converted into the charge current through the spin-to-charge conversion effect, emitting terahertz radiation.
These findings present an innovative, magnetic-element-free route for generating ultrafast spin currents in two-dimensional limits, underscoring the significant potential of laser terahertz emission spectroscopy in investigating laser-induced extraordinary spin dynamics.
See the article:
Above Curie Temperature Ultrafast Terahertz Emission and Spin Current Generation in a Two-Dimensional Superlattice (Fe3GeTe2/CrSb)3
https://doi.org/10.1093/nsr/nwae447
Journal
National Science Review