Magnetization detecting electron paramagnetic resonance spectroscopy using a dc-SQUID directly coupled to an electron spin ensemble*

Electron parametric resonance (EPR) spectroscopy is one of the most widely-used tool to characterize materials containing unpaired electrons. In the case of conventional EPR spectrometers, the resonance is detected as a change of microwave transmittance of a cavity. In our method, on the other hand, magnetization of the sample induced by the resonance is detected by a direct current superconducting quantum interference device (dc-SQUID) magnetometer, which is bonded to the sample. Here, we report detection of electron spin polarization and EPR spectroscopy using a micrometer-sized dc-SQUID magnetometer. We measure temperature and in-plane magnetic field dependence of spin polarization ratio and it has good agreement to the hyperbolic tangent law. We also successfully demonstrate EPR spectroscopy by applying a continuous microwave signal to the sample with a on-chip microstrip. We estimate the sensing volume and the minimum distinguishable number of electron spins to be $\sim$ $10^{-10}$ cm$^{3}$ ($\sim$ 0.1 pl) and $\sim$ $10^6$, respectively. This result paves the way towards realizing highly sensitive EPR spectroscopy in nanometer-sized area.