  We present a comprehensive exploration of loop-gap resonators (LGRs) for
electron spin resonance (ESR) studies, enabling investigations into the
hybridization of solid-state magnetic materials with microwave polariton modes.
The experimental setup, implemented in a Physical Property Measurement System
by Quantum Design, allows for ESR spectra at temperatures as low as 2 Kelvin.
The versatility of continuous wave ESR spectroscopy is demonstrated through
experiments on CuSO4.5H2O and MgCr2O4, showcasing the g-tensor and magnetic
susceptibilities of these materials. The study delves into the challenges of
fitting spectra under strong hybridization conditions and underscores the
significance of proper calibration and stabilization. The detailed guide
provided serves as a valuable resource for laboratories interested in exploring
hybrid quantum systems through microwave resonators.
