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Description
Spin-Hall magnetoresistance (SMR) is a magnetoresistive effect observed in bilayers comprising a heavy metal (HM) and a magnetically ordered layer (M). It arises from the interaction between spin-Hall-effect-induced spin accumulation in the HM and the magnetic order in M, mediated by interfacial spin torque and spin transport through magnons or electrons. While SMR has been extensively studied at DC and GHz frequencies, its ultrafast dynamics and the underlying spin-sink channels remain poorly understood.
Here, we investigate SMR in YIG|Pt and CoFeB|Pt bilayers over a frequency range of 0.2–25 THz. Combining sensitive THz detection with high-frequency magnetic-field modulation, we observe a pronounced magnonic low-pass response in YIG|Pt, whereas SMR in CoFeB|Pt remains frequency independent up to 25 THz. Our modelling attributes this spectral filtering to the finite response time of the magnon chemical potential generated by the time-dependent interfacial spin accumulation. In contrast, the broadband response of CoFeB|Pt is enabled by fast and efficient electronic spin-sink channels.
These findings establish thermal magnon transport as a key factor governing the ultrafast SMR response. They further show that electronic spin relaxation can weaken the magnonic low-pass filtering by providing an additional, faster channel for spin dissipation. Metallic magnetic structures are therefore promising for broadband, high-frequency applications, while THz SMR spectroscopy in magnetic insulators offers a sensitive probe of magnon transport and relaxation.
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