Composition and temperature-dependent terahertz emission in ferrimagnetic Ta/Tb_x(FeCo)_1-x/Pt heterostructures

A spintronic terahertz (THz) emitter based on rare-earth-transition-metal ferrimagnetic alloys has drawn wide attention due to its prominent THz emission signals at a magnetic compensated state. Here, THz emission spectroscopy is performed in a layered ferrimagnetic/nonmagnetic structure of Ta/Tb_x(FeCo)_1-x/Pt to investigate the impact of magnetization and spin polarization on THz radiation. Composition and temperature-dependent THz emission signals are observed by an inverse spin Hall effect. The variation of THz amplitude approximately follows the in-plane magnetization in the composition range, yet a nonvanishing THz radiation at the compensation point indicates that net spin polarization dominates the emitted THz electric field rather than net magnetization in this two-sublattice system. A significant gap of 90 K between the compensation point of magnetic moment and spin polarization is found in a Tb_0.24(FeCo)_0.76 sample, which is a striking difference to that of 25 K in GdCo/Pt THz emitters. We attribute this anomalous gap to the strong intersublattice 3d-5d6s-4f exchange interaction of TbFeCo alloy; thus, the contribution of spin polarized Tb 4f electrons should be reconsidered after fs laser pulse excitation. These results gain insight into spin dynamics of complex multisublattice systems and pave the way for further optimized spintronic THz emitters to modulate THz radiation efficiently.