[Wien] Why the antiferromagnetic structure of Cr is still cubic

[Stefaan Cottenier]

> According to what your are saying , the real magnetic structures for
> Chromide is tetragonal and orthorhombic and not cubic as it's mentionned
> in the userguide.
>
> I think the cubic magnetic structure fo Cr is given just to simply the
> calculation for the beginners of the antiferromagnetic calculation.

The deviation from cubic symmetry is very, very small, and for most 
purposes it is perfectly fine to treat Cr as cubic.

Moreover, spin-orbit coupling is needed to allow the magnetic moments to 
break the cubic symmetry to tetragonal. Spin-orbit coupling is a very 
small effect for a light element as Cr. And in order to get the 
reduction to orthorhombic symmetry, the spin-density wave in Cr is must 
be taken into account (its wave vector is transversal to the magnetic 
moments between 123 K and 311 K, whence the orthorhombic symmetry -- it 
is parallel to the magnetic moments between 0 K and 123 K, that's why a 
tetragonal symmetry is possible).

When Cr is approximated as an antiferromagnetic crystal without 
spin-density wave and without spin-orbit coupling, its ground state is 
exactly cubic.

Stefaan
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