Microstrip Resonators and Broadband Lines for X-band EPR Spectroscopy of Molecular Nanomagnets

We present a practical setup to perform continuous-wave X-band electron paramagnetic resonance spectroscopy by using planar microstrip lines and general purpose instrumentation. We fabricated Ag/alumina and Nb/sapphire microstrip resonators and transmission lines and compared their performance down to 2 K and under applied magnetic field. We used these devices to study single crystals of molecular Cr$$_{3}$$3 nanomagnets. By means of X-band planar resonators we measured angle-dependent spectra at fixed frequency, while broadband transmission lines were used to measure continuous wave spectra with varying frequency in the range 2-25 GHz. The spectra acquired at low temperatures allowed to extract the essential parameters of the low-lying energy levels of Cr$$_{3}$$3 and demonstrate that this method is particularly suitable to study small crystals of molecular nanomagnets.