From a historical perspective, the discovery of nuclear magnetic resonance probably had the greatest impact on the use of ferrites at microwave frequencies. The phenomenon of magnetic resonance was predicted around the early 1930s. As microwave sources became available during the 1940s, there were many efforts then to observe electron magnetic resonance as well as nuclear magnetic resonance. Perhaps, the development of the radar had
something to do with these efforts. Microwave sources were available during and after World War II. Gorter mentioned to the author in 1970 that he planned to measure electron paramagnetic resonance on a magnetic sulﬁde compound very early in the development of paramagnetic salts, but unfortunately the linewidth was as broad as the ‘‘Mississippi River’’ to have made a meaningful measurement. However, the ﬁrst veriﬁed experiment on magnetic resonance in which nuclear spins precessed about the magnetic ﬁeld was conceived in 1947 using standard resonant R-L-C tank circuits, as shown in Figure 2.1. Tap water from the Charles River in Boston, MA was used for testing and
absorption was monitored by adjusting the two tank circuits as in an electrical bridge arrangement in which the voltages at the output of the two tank circuits were nulled out at frequencies well above and below the magnetic resonance frequency. The bridge technique was extended later at microwave frequencies in which electron spins precessed at resonance.