Cantilever-detected high-frequency ESR measurement using a backward travelling wave oscillator

Our cantilever-detected electron spin resonance (ESR) technique is motivated for terahertz ESR spectroscopy of a tiny single crystal at low temperature. In this technique, ESR signal is detected as deflection of a sample-mounted cantilever, which is sensitively detected by built-in piezoresistors. So far, ESR detection at 315 GHz was succeeded using Gunn oscillator. In this study, we combine our ESR technique with a backward traveling wave oscillator (BWO), which can cover a wide frequency range 120-1200 GHz, to achieve better spectral resolution. Experiments were carried out at 4.2 K for a single crystal of Co Tutton salt with a newly constructed optical system. We successfully observed two ESR absorption lines in BWO frequencies up to 370 GHz. From multi-frequency measurements, the observed ESR lines shifted linearly with BWO frequency, being consistent with paramagnetic resonance. The estimated g values are g₁ = 3.00 and g₂ = 3.21. The spin sensitivity was estimated to ∼10¹² spins/gauss at 370 GHz.