I would say the hyperfine field decreases with increasing temperature. I’m thinking this has to do with the cubic symmetry we discussed some weeks ago.
We saw that if there was cubic symmetry the orbital and spin dipolar contributions for the hyperfine magnetic field vanish. The temperature will not change the chemical symmetry, but it does for the magnetic symmetry?
For higher temperature, the magnetic moments are more randomly distributed. Whereas for situations closer to absolute zero, the system approaches its true groundstate with all magnetic moments aligned, breaking therefore cubic symmetry. This then results in the orbital and spin dipolar contributions to ‘vanish less’ and thus a larger hf magnetic field.