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

Abstract 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.