Work On Resonance Phenomena
Zavoisky started systematic studies on interaction of electromagnetic waves with matter in 1933. He formed a group of talented experimentalists and theoretists of various background, which included Boris Kozyrev, A. V. Nesmelov and later Semen Altshuler. He also visited several laboratories in major Russian cities and found that the experimental techniques in this research field were undeveloped. He was particularly dissatisfied with poor detection sensitivity and spent much effort on improving it using better detectors and electronic circuitry.
Zavoisky was much interested in the pioneering results obtained by Isidor Isaac Rabi in 1938 on interaction of molecular beams with electromagnetic waves in a static magnetic field, that is nuclear magnetic resonance (NMR). Eight years later, in 1946, Felix Bloch and Edward Mills Purcell refined the technique for use on liquids and solids, for which they shared the Nobel Prize in physics in 1952. Zavoisky too tried to detect NMR in solids and liquids around 1940–1941. He had a sensitive enough detection system and managed to obtain the resonance signals. However, the strict requirement for the spatial homogeneity of the magnetic field were likely not met. The signals were unstable and poorly reproducible and thus were discarded. The work was interrupted by the World War II and had not been resumed.
Instead, starting from 1943, Zavoisky focused on electron paramagnetic resonance (EPR), which is much less demanding for the homogeneity of magnetic field. On the other hand, it requires much more sensitive detection electronics, but Zavoisky was well prepared in this area. In particular he had replaced the calorimetric (thermal) detection of C. J. Gorter by a much more sensitive electronic technique of grid current. A further improvement was addition of a small AC magnetic field to the main static magnetic field. This dramatically increased the detection sensitivity and allowed easy amplification of the resonance signal and outputing it directly to an oscilloscope. In 1944, EPR signals were detected in several salts, including hydrous copper chloride (CuCl2·2H2O), copper sulfate and manganese sulfate. The results were revolutionary and were first not accepted even by the Soviet scientists (including Pyotr Kapitsa). The doubts were dispersed when Zavoisky visited Moscow, assembled an EPR spectrometer from scratch and reproduced his results there. In 1945, Zavoisky defended his habilitation on the phenomenon of electron paramagnetic resonance.
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