Spehro Pefhany said:
On N49 anyway (Tc> 240°C), the mu goes _up_ almost monotonically from
room temperature to about 225°C, after which it plummets. There's a
little dip between 80°C and 140°C, but nothing significant. A bit
surprising.
Actually, mu always goes up with temperature. Stochastic resonance --
think of it as white noise dithering the magnetic domains' hysteresis.
At low temperature, the domains just stick around. The B-H loop is wider
(I think... I'd like to see some data though), and around B=0, it's much
flatter (low initial permeability). When looking at a B-H curve, you have
to remember it's not actually a nice sigmoidal loop: real materials often
have pronounced flat spots around the axis ("butterfly curve"), which
manufacturers typically ignore.
At higher temperatures, thermal energy scatters the magnetic domains,
raising the ground state energy level, reducing hysteresis but leaving
less room for magnetic energy. You can still force the domains into
alignment, but you don't get as much flux from doing it -- so, saturation
Bmax drops linearly as Tc is approached, hitting zero at Tc.
Presumably, materials with a gradual, progressive curve are a blend of
many Tc's, and materials with two spikes are mainly two Tc's. And
accordingly, Bmax would drop off in a sum-of-parameters slope. I don't
know if this is a correct interpretation or if there's more subtle physics
going on.
Tim
