Linewidth and Off-Resonance Loss in Polycrystalline Ferrites at Microwave Frequencies

The off‐resonance loss has been measured in polycrystalline $\mathrm{YIG}$ on samples with a controlled (and generally rather large) porosity. The results can conveniently be expressed in terms of an equivalent line‐width, which is a function of the dc magnetic field strength. It is defined as the width that an absorption peak with Lorentzian profile would have if the off‐resonance absorption of this reference profile at a given dc field matches the actually observed absorption. The equivalent linewidth of polycrystals is in most cases much smaller away from resonance than it is near resonance. For $\mathrm{YIG}$ the equivalent linewidth at X‐band is approx 10 Oe both at low and high dc fields but rises to 50–600 Oe (depending upon the porosity) in the vicinity of resonance. The experimental results are compared with theoretical predictions. based upon the assumption that the line broadening is primarily caused by inhomogeneity of the local saturation magnetization. It is shown that the linewidth, the magnetic hardness coefficient (which characterizes the approach to magnetic saturation at high field‐strengths), and the longitudinal susceptibility are intimately related.