Enhanced THz Emission From Ultrathin Ta/Fe/Pt Spintronic Trilayers

Terahertz (THz) spintronic emitters represent a novel class of heterostructures composed of ferromagnetic (FM) and non-magnetic (NM) metallic layers that strongly emit terahertz (THz) radiation upon femtosecond laser pulse excitation. The optimal geometric configuration to maximize the strength of the emission is currently considered a trilayer structure, NM₁/FM/NM₂, where the FM layer is confined between two NM layers with opposite spin Hall angles. To investigate this, ultrathin Ta/Fe/Pt trilayers are fabricated and their THz emission profiles are analyzed. These results show that the highest THz emission is achieved for the sample of Ta (1.5 nm)/Fe (2 nm)/Pt (2 nm), demonstrating a significant enhancement compared to standard FM/NM bilayers. Furthermore, the thickness dependence of the THz emission is modeled in Ta (t₁ nm)/Fe (2 nm)/Pt (t₂ nm), varying t₁ and t₂ from 1 nm to 3 nm. From this analysis, spin diffusion lengths of λ_Pt = 1.2 nm and λ_Ta = 0.85 nm are extracted. The structure–property relationship is assessed via transmission electron microscopy, revealing that an epitaxial single-crystalline Ta layer covers the MgO surface with Ta adopting a high-resistivity fcc allotropic phase with a lattice parameter of a = 0.436 nm. This phase, together with the prerequisite for low Ta+Pt thickness, emerges as a key factor in achieving high THz emission from trilayer structures.