Ultrasensitive Technique for Microwave Susceptibility Determination Down to 10−5

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Abstract
An ultrasensitive technique has been developed which allows the measurement of the microwave susceptibility components down to levels on the order of 10−5 on small spherical samples of ferrite materials. The novel feature is a feedback scheme to lock the signal klystron onto the 3 dB points of the high‐Q cylindrical activity containing the sample. By beating the signal klystron against a crystal‐oscillator‐stabilized reference klystron, the frequency width of the cavity at half‐power can be measured with an error less than a few hundred hertz, leading to the extremely high resolution. The most useful application of the technique to date is the study of microwave relaxation in the form of an effective linewidth several thousand oersteds away from ferromagnetic resonance.