We describe a fully broadband approach for electron spin resonance (ESR) experiments, where it is possible to tune not only the magnetic field but also the frequency continuously over wide ranges. Here, a metallic coplanar transmission line acts as compact and versatile microwave probe that can easily be implemented in different cryogenic setups. We perform ESR measurements at frequencies between 0.1 and 67 GHz and at temperatures between 50 mK and room temperature. Three different types of samples (Cr3+ ions in ruby, organic radicals of the nitronyl-nitroxide family, and the doped semiconductor Si:P) represent different possible fields of application for the technique. We demonstrate that an extremely large phase space in temperature, magnetic field, and frequency for ESR measurements, substantially exceeding the range of conventional ESR setups, is accessible with metallic coplanar lines.
The design of the experimental insert allows a film orientation parallel to the magnetic field. We cannot exclude a misalignment of the field but we assume a tilting out of plane of less than 2°.
Due to frequency-dependent attenuation in the coaxial cables and in the coplanar line, the transmission decreases with increasing frequency. Therefore, the input power was swept linearly from −20 dBm to 10 dBm for NITPhOMe and ruby. The temperature-dependent measurements on NITPhOMe were performed at a constant input power of Pin = −10dBm. For the doped Si:P, a constant value of −20 dBm was used.
These horizontal lines stem from undesired resonant modes within the sample box or the overall microwave assembly. In particular, thick ruby sample fills a substantial fraction of the sample box volume and thus can interact with such parasitic microwave box modes, leading to anticrossings in Fig. 4.