Capabilities of Compact High-Frequency EPR/ESE/ODMR Spectrometers Based on a Series of Microwave Bridges and a Cryogen-Free Magneto-optical Cryostat

A magnetic resonance spectrometer operating at several fixed frequencies in the millimeter range is designed on the basis of a new generation of microwave bridges and a cryogen-free magneto-optical cryogenic system. The spectrometer allows measurements of EPR in continuous wave and pulse (free-induction decay, FID and electron spin echo, ESE) modes, photo-EPR, and optically detected magnetic resonance (ODMR) in a wide range of temperatures (1.5–300 K) and magnetic fields (up to 7 T with the ability to invert the field). It is based on a line of unified microwave bridges with a powerful oscillator and a superheterodyne receiver. Currently, they operate in W and D bands (at 94 and 130 GHz, respectively), but the frequency bands can be extended. In addition to highly stable fixed-frequency oscillators with narrow spectrum (better than − 100 dBc at 10 kHz offset at 94 GHz), they have variable frequency oscillators, which simplify tuning of the microwave circuit with a resonator and allow operation with frequency modulation when using a non-resonant microwave system. In the latter, it is possible to study large samples and quickly change the operating frequency of the spectrometer, simply replacing the microwave bridge. For all frequencies, the spectrometer uses common hardware and original software. The performance of the spectrometer at 94 GHz and 130 GHz was tested in measurements of CW-EPR, ESE and ODMR of NV defects in diamond single crystals, Tb 3+ and Ce 3+ ions in yttrium aluminum garnet crystals, nitrogen donors, and V 3+ ions in 6H-SiC.