Terahertz Emission Modulation Caused by Ultrafast Breaking and Recovery of Exchange Bias

Abstract

Magnetic interfacial exchange bias, as a key control method for spintronic devices, remains unclear in terms of its ultrafast dynamic behavior and its role in regulating spintronic terahertz emissions. In this work, femtosecond optical pulses are used to excite ferromagnetic/antiferromagnetic bilayer films with interfacial exchange bias, and a significant modulation phenomenon of terahertz emission is observed by comparing samples with different magnetization pinning states induced by exchange bias. Through the measurement of dynamic hysteresis loops under femtosecond optical pulse excitation, it is confirmed that the optical pulse can rapidly break and then recover the exchange bias within the picosecond time scale. This transient reconstruction process of exchange bias effectively enhances the ultrafast spin precession signal at approximate to 2 THz, while suppressing the ultrafast demagnetization-related signal at approximate to 0.77 THz. By exploiting the difference in flip symmetry of the samples, this is found that the photo-introduced magnetization dynamics process dominated the modulation effect of the exchange bias on the two frequency bands. These results reveal that picosecond-scale transient exchange bias can regulate both the frequency content and coherence of spintronic terahertz emission, offering a pathway toward tunable terahertz spintronic sources.