R. J. Blume reported the first electron spin echo in 1958, which came from a solution of sodium in ammonia at its boiling point, -33.8˚C. A magnetic field of 0.62 mT was used requiring a frequency of 17.4 MHz. The first microwave electron spin echoes were reported in the same year by Gordon and Bowers using 23 GHz excitation of dopants in silicon.