Broadband ESR Spectroscopy With a Tunable Interferometer

Abstract

This paper presents a broadband and quantitative electron spin resonance (ESR) spectroscopy technique that is tunable over a wide frequency range. Different from the detection approaches in conventional ESR instrumentation, an interferometer-based method is used to remove radio frequency probing signals at the vector network analyzer (VNA) detector. As a result, high-resolution tuning enables highly sensitive ESR detection. A planar microwave resonator is designed at ~7.6 GHz and incorporated into the interferometer to measure $0.2~\mu \text{g}$ ( $2.8 \times 10^{14}$ spins) 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample at a signal-to-noise ratio (SNR) of ~121 at room temperature. The broadband ESR operation capability is investigated with a broadband meander microstrip line (MML) by measuring $6~\mu \text{g}$ ( $8.4 \times 10^{15}$ ) DPPH between 8 and 13 GHz at a ~194 SNR with 10 kHz VNA intermediate frequency. The obtained sensitivity is significantly higher than that of current broadband ESR methods. In both MR and MML cases, dispersion and absorption ESR signals at room temperature are simultaneously obtained. With MML structures, the quantification of permeability $\mu (\omega )=\mu ' (\omega ) -j\mu ''(\omega )$ is possible. Thus, the ESR techniques are also a highly sensitive ferromagnetic resonance method. More work is needed to further improve ESR sensitivity and quantification accuracy.