Posted by Steve Simon on March 9, 2014, 7:41 pm, in reply to "5.7 d first subpoint"
Once you have decided that M wants to be nonzero, you have to figure out what keeps it from being maximal. You derive some energy and expand in powers of M. At lowest nontrivial order you get
E = f M^2 + ...
So when f is negative, M wants to be nonzero (this is the stoner criterion). However, if you want to figure out how big M actually gets, you need to expand to next nontrivial order in M. In this case, you have
E = f M^2 + b M^4
and even when f is negative, you will still find a small M is the lowest energy solution.
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