Multipurpose High-Frequency ESR Spectrometer for Condensed Matter Research

We describe a multifrequency quasi-optical electron spin resonance (ESR) spectrometer operating in the 75–225 GHz range and optimized at 222.4 GHz for general use in condensed matter physics and chemistry. The quasi-optical bridge detects the change of millimeter wave polarization at the ESR condition. A controllable reference arm maintains a millimeter wave bias at the detector. The sensitivity of 2 × 10¹⁰ spin/(G Hz^0.5), measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to that of commercial high-sensitive X-band spectrometers. The spectrometer is robust, easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.