In standard electron paramagnetic resonance (EPR) spectroscopy, the frequency of an experiment is set and the spectrum is acquired using magnetic field as the independent variable. There are cases in which it is desirable instead to fix the field and tune the frequency such as when studying avoided level crossings. We have designed and tested an adjustable frequency and variable coupling EPR probe with loop-gap resonators (LGRs) that works at a temperature down to 1.8 K. The frequency is tuned by adjusting the height of a dielectric piece of sapphire inserted into the gap of an LGR; coupling of the microwave antenna is varied with the height of antenna above the LGR. Both coupling antenna and dielectric are located within the cryogenic sample chamber, but their motion is controlled with external micrometers located outside the cryostat. The frequency of the LGR can be adjusted by more than 1 GHz. To cover a wide range of frequencies, different LGRs can be designed to cover frequencies up to X-band. We demonstrate the operation of our probe by mapping out avoided crossings for the Ni$_4$ single-molecule magnet to determine the tunnel splittings with high precision.
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