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Re: Mechanism Underlying Ferromagnetism



I think one of the problems here is understanding that there are *two*
kinds of exchange interactions going on. The one usually called an
exchange interaction involves adjacent magnetic *atoms or ions*, not
electrons *per se*. This interaction may produce either parallel
(ferromagnetic) or antiparallel (antiferromagnetic) or even spiral
(also called antiferromagnetic) or mixed (ferrimagnetic) ordering
below the Curie point. What happens all depends on the details. The *a
priori* explanation of magnetic ordering is not always easy, so the
theory is somewhat, er, inelegant and it does not lend itself well to
oversimplification. Thus teaching this sort of detail to students
below the graduate level is probably less productive than teaching
other worthwhile material, for example GR. I had GR left out of my
curriculum and magnetic structures put into it when I was an
undergraduate, and I never did "understand" the latter (I still don't
know the former, and I'm told it is conceptually easier). I just took
a look at my ancient textbook (Kittel 2d ed.) and I still find it
obscure.

The second kind of "exchange", which is not conventionally called that,
is what causes atoms or ions to be magnetic. This is the interaction
among the electrons within each atom or ion. The two interactions are
often not independent (especially when the d-electrons of iron group
metals are involved), further complicating the task of simplifying an
explanation.

Leigh

(who successfully derived the dispersion relation for ferromagnetic
spin waves at his qualifying exam oral - from scratch, no memory)