Recently, ferromagnetic-heterostructure spintronic terahertz (THz) emitters have been recognized as one of the most promising candidates for next-generation THz sources, owing to their peculiarities of high efficiency, high stability, low cost, ultrabroad bandwidth, controllable polarization, and high scalability. Despite the substantial efforts, they rely on external magnetic fields to initiate the spin-to-charge conversion, which hitherto greatly limits their proliferation as practical devices. Here, a unique antiferromagnetic-ferromagnetic (IrMn3 |Co20 Fe60 B20 ) heterostructure is innovated, and it is demonstrated that it can efficiently generate THz radiation without any external magnetic field. It is assigned to the exchange bias or interfacial exchange coupling effect and enhanced anisotropy. By precisely balancing the exchange bias effect and enhanced THz radiation efficiency, an optimized 5.6 nm-thick IrMn3 |Co20 Fe60 B20 |W trilayer heterostructure is successfully realized, yielding an intensity surpassing that of Pt|Co20 Fe60 B20 |W. Moreover, the intensity of THz emission is further boosted by togethering the trilayer sample and bilayer sample. Besides, the THz polarization may be flexibly controlled by rotating the sample azimuthal angle, manifesting sophisticated active THz field manipulation capability. The field-free coherent THz emission that is demonstrated here shines light on the development of spintronic THz optoelectronic devices.
Keywords: THz spin currents; heterostructures; terahertz emitters.
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