Dielectric Ceramic EPR Resonators for Low Temperature Spectroscopy at X-band Frequencies

Abstract

The performance of new dielectric ceramic resonators (DRs) for continuous wave (cw) X-band electron paramagnetic resonance (EPR) spectroscopy is investigated at room temperature and low temperatures (77, 6 K). The DRs with high dielectric constants of about $\varepsilon _r = 80$, featuring low residual paramagnetic impurities, have been developed and produced on the basis of barium lanthanide titanates solid solutions with the general formula Ba$_{6-x}$Ln$_{8+2x/3}$Ti$_{18}$O$_{54}$ (Ln = Sm, Nd) that demonstrate at once low dielectric losses in the microwave range ($\tan \delta = 7$–14 × 10⁻⁴ at 10 GHz) and appropriate temperature stability of the dielectric constant ($\tau _\varepsilon = \pm$5 ppm/K). They were optimized for samples with small dimensions and can be used in commercial Oxford instruments flow cryostats if the coupling is done via cavity resonators. We found a maximal EPR signal enhancement by a factor up to 74 at 6 K. The increases of quality and filling factors as well as that of the microwave (mw) $B_1$-field in the resonator setup are discussed in dependence on temperature. We show that the absolute sensitivity gain must be related to that increase in the mw field and the remaining relative gain of the SNR is about 18 for small samples. The developed DRs have shown a good potential in EPR application as reliable and easy-to-use components allowing research of thin films and in particular small crystalline structures.