Thanks, Tim, that helps a lot! For what you've said, if I were to apply a
mass-spring analogy, the electrons would end up as the mass and the DC field
strength would correspond to the spring constant.

I'll read up at the link you provided. It would appear that with no DC field
applied, you'd get the lowest resonant frequency... at until not too long ago,
that appeared to be in the ballpark of 1GHz. Now it's not uncommon to see
500MHz YIG filters... I wonder what changed?

The application I had in the back of my mind was for nice, sharp filters in
the high VHF/low UHF range. I saw a circulator recently for 432MHz and, while
it was large at about 4" square (but flat -- only about 1/4" thick), it still
struck me as a pretty nice alternative to trying to build a transformer-based
isolator at such frequencies. Hence the thought of perhaps being able to use
YIG filters down there as well...

---Joel