Strain-Induced Tunable\nand Field-Free Spintronic THz\nEmitters Based on Flexible Substrate Heterostructures

Spintronic THz emitters have been widely studied due\nto their advantages\nof broadband frequency, high efficiency, and easy fabrication. The\nspintronic THz signal is proportional to the magnetization of the\nferromagnetic (FM) layer and requires an external magnetic field to\nmaximize the THz signal intensity. Recently, a field-free emitter\nwas designed based on a CoFeB/IrMn<sub>3</sub> heterostructure via\nthe exchange bias between two films. However, field-free spintronic\nTHz emitters based on a common FM/nonmagnetic metal structure are\nrare. Here, we fabricate a tunable and field-free THz emitter with\ngiant THz emission modulation ability based on a polyimide/CoFeB/Pt\nheterostructure. The THz emission can be modulated by changing the\ncurvature radius of the polyimide substrate. After the emitter is\nforward bent, the THz radiation without an external magnetic field\nis stronger than in the flat state with a field, which we attribute\nto in-plane magnetic anisotropy on the FM layer induced by the tensile\nstrain. When we curve the emitter backward, the THz intensity decreases\nsharply. Moreover, the modulation (Δ<i>S</i>/<i>S</i><sub>max</sub>) of the THz wave from the forward-curved\nand backward-curved emitter is over 95%, and the device shows good\ncyclic repeatability. We provide a novel way to design field-free\nspintronic THz sources, and flexible devices are promising for application\nin THz devices.