Multi-frequency ESR using a microcantilever in the millimeter wave region

Low-temperature magnetic properties of condensed matter system is investigated microscopically with high-frequency electron spin resonance (ESR). Experiments are carried out conventionally with a transmission method, but its sensitivity is not often sufficient to detect ESR signals of systems with small number of spins such as newly synthesized microcrystals. To attain better sensitivity, we focus on a novel technique of multi-frequency ESR system utilizing a microcantilever. In this method, ESR signal is detected as a torque change associated with the absorption of electromagnetic wave. Its sensitivity is greatly increased due to mechanical resonance when the modulation frequency of electromagnetic wave coincides with the eigenfrequency of the cantilever. In this study, we have succeeded in mechanical detection of ESR signals in the millimeter wave region up to 240 GHz. The achieved signal-to-noise ratio was greater than 10³ for 1-μg sample of Co-Tutton salt, corresponding to the spin sensitivity of 10⁹ spins/G.