Spintronic THz emitters based on transition metals and semi-metals/Pt multilayers

Abstract

Spintronic terahertz (THz) emitters based on the inverse spin Hall effect in ferromagnetic/heavy metal (FM/HM) heterostructures have become important sources for THz pulse generation. The design, materials, and control of these interfaces at the nanometer level have become vital to engineer their THz emission properties. In this work, we present studies of the optimization of such structures through a multi-pronged approach, taking advantage of material and interface engineering to enhance THz spintronic emission. This includes the application of multi-stacks of HM/FM junctions and their application to trilayer structures, the use of spin-sinks to simultaneously enhance the THz emitted fields and reduce the use of thick Pt layers to reduce optical absorption, and the use of semi-metals to increase the spin polarization and, thus, THz emission. Through these approaches, significant enhancements of the THz field can be achieved. Importantly, taking into account the optical absorption permits to elucidate novel phenomena such as the relation between the spin diffusion length and the spin-sink using THz spectroscopy, as well as possibly distinguishing between self- and interface-spin-to-charge conversion in semi-metals.

Spin‐Momentum Locking and Ultrafast Spin‐Charge Conversion in Ultrathin Epitaxial Bi1 − xSbx Topological Insulator

“…The net charge current would drop to zero, as $$\tanh(\frac{{t}_{\text{TI}}}{{\lambda}_{\text{sf}}})\tanh(\frac{{t}_{\text{TI}}}{2{\lambda}_{\text{sf}}})\propto \frac{{t}_{\text{TI}}^{2}}{2{\lambda}_{\text{sf}}^{2}}$$ [ 43 ] for small TI thicknesses t TI when considering the multiple spin current reflections at the TI interfaces. [ 40 ] This is in contrast with the thickness‐independent SCC observed here, in the ultrathin limit. We thus anticipate that the bulk states are hardly involved, and that the surface states at the FM/TI interface are mainly responsible for a net charge current through IREE.…”

“…η THz represents the spin‐injection and conversion efficiencies and is obtained after having withdrawn the NIR and THz absorptions in the heterostructure following the procedure proposed in refs. [ 38 , 39 , 40 ] (Section S3C , Supporting Information). Strikingly, η THz for BiSb remains constant for the whole thickness series where the BiSb bandgap widens dramatically by several hundreds of meV as the thickness is reduced, as calculated previously.…”

Spintronics Based Terahertz Sources

“…Co 40 Fe 40 B 20 is slightly higher than other FM metals, such as Fe, Co, Fe-Co alloys, and Ni 81 Fe 19 , while the efficiency of pure Ni-based STEs is rather low. Recent works [57][58][59] have also investigated STEs based on half-metallic FM materials, such as Heusler alloys, which are believed to have large spin polarization. The experimental results show that the spin-current generation efficiency of half-metallic FMs in STEs can reach that of typical 3D-transition-metal FMs, such as Fe, but not be higher.…”

Quantifying Spin‐Charge Conversion Mechanisms for THz Emission in Magnetic Multilayers

“…For instance, THz emission by ISHE has been reported for ferromagnetic/heavy‐metal heterostructures, [ 22 ] ferromagnet/semiconductor heterostructures, [ 23 ] and transition‐metal/Pt multilayers. [ 24 ] THz emission by IREE has been reported for Fe/Ag/Bi film [ 17 ] and CoFeB/Ag/Bi film. [ 18 ] THz emission by ISOT has also been reported for Co/Pt‐based multilayer with space inversion symmetry broken.…”