Ferrimagnetic thin film systems for spintronic THz emitters

THz radiation in the frequency range from 0.3 to 30 THz bridges the gap between electronic and optical frequencies. It has been used for probing and driving fundamental resonances in gaseous, liquid, and solid materials. However, it is still challenging to generate broadband THz radiation with sufficient power in a convenient way. Recently, a new type of an efficient THz emitter has been discovered, which is based on the inverse spin Hall effect [1,2]. These “spintronic” THz emitters typically consist of thin ferromagnetic (FM)/nonmagnetic metal bilayers which need to be excited by fs laser pulses.
In previous studies we have shown that bilayers consisting of ferrimagnetic (FI) Tb(Gd)-Fe alloys and Pt can be employed as well as efficient spintronic THz emitters [3,4] . We find that the THz emission amplitude closely follows the in-plane magnetization of the Fe sublattice.
In a further study, we have utilized the magnetic compensation temperature of a FI layer to control the THz emission solely by temperature [5]. This is enabled by coupling two ferrimagnetic layers and depending on the relative alignment of the Fe moments in the two layers, the spintronic emitter system can be either in a high- or in a low-amplitude THz emitting state.
These studies open a route for a controllable and efficient type of spintronic terahertz emitters enabled by the ferrimagnetic properties of rare earth-3d transition metal alloys.

[1] T. Kampfrath et al., Nature Nanotechnology 8, 256 (2013).
[2] T. Seifert et al., Nature Photonics 10, 483 (2016).
[3] R. Schneider et al., ACS Photonics 5, 3936 (2018).
[4] R. Schneider et al., Appl. Phys. Lett. 115, 152401 (2019).
[5] M. Fix et al., Appl. Phys. Lett. 116, 012402 (2020).