Magnetoresistance plays an essential role in materials research as well as in the technology of magnetic field sensors and electronic devices. One of the new magnetoresistance phenomena that has attracted the attention of the scientific community is Spin-Hall magnetoresistance [1]. Thanks to a microscopic mechanism based on different spin interactions with the magnetic layer after a change of magnetic field [2], it is observed in bilayers of heavy metal (such as Pt or W) and a wide range of magnetic materials, ranging from ferromagnetic metals (Co, CoFeB, Py. ..)[3], ferrimagnets (YIG, GcIG...)[1], antiferromagnets (NiO, Mn3Sn, Cr2O3)[4] up to altermagnetic candidates (α-Fe2O3 (hematite) [5], Ba2CoGeO7).

Microscopically, spin-Hall magnetoresistance is based on spin interaction with the magnetic film through spin-orbit torque, spin current transfer and magnon excitation[2,6]. However, the behaviour of these channels on ultra-short timescales has been the subject of recent debate, and therefore the behaviour of SMR at high frequencies (100 GHz-30 THz) has remained questionable.

This talk is dedicated to our recent experiments on the THz behaviour of SMRs and their comparison with theoretical models. In the first part we will discuss SMR from different perspectives - from basic models [2] to advanced models of ultrafast response [6]. In the second part I will present experimental results of the THz response of the model system CFB/Pt and the contribution of electron and magnon transport. The last part is devoted to THz SMR in YIG/Pt and discussion of magnon transport at THz frequencies. The obtained results show significantly different behaviour between YIG/Pt and CoFeB/Pt, pointing to our different nature of electron and magnon behaviour at THz frequencies.

References:
[1] Vlietstra, N., Shan, J., Castel, V., Ben Youssef, J., Bauer, G. E. W., & Van Wees, B. J. Exchange magnetic field torques in YIG/Pt bilayers observed by the spin-Hall magnetoresistance. Applied Physics Letters, 103(3)(2013).
[2] Chen, Y. T., Takahashi, S., Nakayama, H., Althammer, M., Goennenwein, S. T., Saitoh, E., & Bauer, G. E.. Theory of spin Hall magnetoresistance. Physical Review B—Condensed Matter and Materials Physics, 87(14), 144411 (2013)
[3] Kim, J., Sheng, P., Takahashi, S., Mitani, S., & Hayashi, M. Spin Hall magnetoresistance in metallic bilayers. Physical review letters, 116(9), 097201 (2016).
[4] Geprägs, S. et al. Spin Hall magnetoresistance in antiferromagnetic insulators. Journal of Applied Physics 127 ,24(2020).
[5] Fischer, J. et al. Large spin Hall magnetoresistance in antiferromagnetic α-Fe 2 O 3/Pt heterostructures. Physical Review Applied 13, 014019 (2020).
[6] Reiss, D. A., Kampfrath, T., & Brouwer, P. W. Theory of spin-Hall magnetoresistance in the ac terahertz regime. Physical Review B, 104(2), 024415(2021).