# What is crystal field anisotropy or effect ? It forces the magnetic moment to point in particular local direction..

Can you give a basic explanation of what is crystal field anisotropy ?

What is the reason to arise ?

In spin ice it forces the dipoles to point in the local 111 direction.

For partially filled rare earth atoms hund rule requires S and L max. This leaves (2s +1)(2l+1) degeneracy which is partially lifted from the LS coupling. When inserted in anisotropic field inside crystal the expectation value , or mean value of L is 0 = 0 and the L is quenched leaving only S so is forced to point in some local direction, but this is if the field removes the deneracy ?

Is it this or it is much more complicated ?

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The $p$, $d$ and $f$ orbitals are only degenerate in a central potential. If you make the potential non-central, for example by putting the atom in an external magnetic field the energy levels are no longer degenerate. Putting the atom into a crystal is just another way of creating an non-central potential (though due to electostatic forces rather than magnetic). This is perhaps more clearly seen in the ligand field splitting you get when complexing your atom with suitable ligands.