Development of an ultra-low temperature multi-frequency ESR apparatus

An ultra-low temperature multi-frequency electron spin resonance (ESR) apparatus has been developed for use at a temperature down to about 200 mK by utilizing a ³He-⁴He compact dilution refrigerator and a vector network analyzer. The microwave frequency can be varied between 30 GHz and 700 GHz almost continuously. Either of the two kinds of copper blocks with a sample holder for typical two configurations, namely Faraday and Voigt configurations, is attached to a stainless steel block neighboring to a mixing chamber which provides ultra-low temperarures down to 160 mK. A magnetic field up to 16 T is produced with a superconducting magnet (14 T at 4.2 K and 16 T at 2.2 K). Thus, we have a potential to perform ESR measurements in a very wide frequency-field window at ultra-low temperatures.