Magnetic-Field-Dependent THz Emission of Spintronic TbFe/Pt Layers

We measure the THz emission of a layered spintronic system based on platinum (Pt) and terbium-iron (Tb Fei(1-x)) alloys for the entire range of Tb content (0 <= x <= 1) under different external applied magnetic fields. We find that the THz emission amplitude closely follows the in-plane magnetization. Deviations occur when the ferrimagnetic TbFe layer changes from an in-plane to an out-of-plane easy axis at x = 0.2, and in the medium composition range x = 0.45-0.55, where Tb magnetic moments dominate the total magnetic moment. The increasing influence of Tb also leads to an inverted THz amplitude for samples with comparable Fe and Tb contents. The THz emission is highest for TbxFe1-x/Pt samples with small amounts of Tb (x = 0.03-0.15) due their reduced electrical conductivity compared to pure Fe/Pt and strongly decreases with increasing Tb content by 2 orders of magnitude. Our systematic study paves the way for designing optimized spintronic THz emitters and demonstrates that transient THz spectroscopy is a powerful tool to gain insight into complex magnetic systems.