sci.physics.electromag

Imagine I have an air-cored transformer. This has some frequency response
up to (say) 100MHz where it tails off.

Then I insert a ferrite core. This increases the mutual inductance so I
have a greater frequency response at low frequencies. But at high
frequencies the response is (probably) worse than the air-cored case.

What effect is causing this? Is it just as a result of adding the core that
I increase the inductance of the transformer so I can't get much current to
flow in the primary? Is it that hysteresis in the core is converting much
more energy into heat - but there's still primary current flowing? Is it
eddy currents (but air and ferrite are insulators)?

What I'm trying to work out is what's going on in the core. I have a core
with a given susceptibility. I apply a strong AC field (so we can neglect
inductance). What's the quantity that controls how much magnetisation there
is in the core at a given time? In other words, the magnetic domains inside
the material need time to align. In air this is minimal (there's nothing to
align, unless you count diamagnetism), while there quite a lot of alignment
to be done in a ferr[o|i]magnetic material. So what sets this time constant
and what is it called?

Thanks
Theo