ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9 T superconducting magnet is described. The probehead contains a compact two-loop, one gap resonator and is embedded in the variable temperature insert of the magnet enabling measurements in the 0- 9 T magnetic field and 1.5-400 K\ temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the $g$ $\approx 2$ paramagnetic condition such as in strongly interacting spin systems. The low quality factor of the resonator allows time resolved experiments such as e.g. longitudinally detected ESR. We demonstrate the performance of the spectrometer on the MgB$_{2}$ superconductor and the RbC$_{60}$ conducting alkaline fulleride polymer.