The basic concept for the effective linewidth originated from the idea that microwave losses in ferrite materials can be field dependent. In this way, the relaxation rate can be measured for magnetic fields away from ferromagnetic resonance (FMR). The effective linewidth technique involves the measurement of cavity Q-value and resonance frequency versus external static magnetic field for a transmission cavity containing the material of interest. In conjunction with perturbation theory and with the analysis of the dynamic magnetic response of the material, these data are used to obtain the real and imaginary part of the microwave susceptibility as a function of static field. The susceptibility data are then related to the relaxation rate through the application of an appropriate model for the microwave response. The effective linewidth is simply this relaxation rate expressed in field units.
The above diagram shows the effective linewidth spectrometer with a high-Q (around 22000) cavity that operates at 10 GHz. A spherical sample is mounted at the center of the cavity. The system instrumentation includes a synthesized sweeper 8341D as microwave source, a scalar network analyzer 8575D to detect the microwave signal through the cavity, the magnet power supply and magnet for static magnetic field generation, a Gaussmeter DTM-151 for the measurement of the static field, and a temperature sensor and a digital multimeter 3478A for the measurement of cavity temperature.