Units of susceptibility / exchange interaction

[Fellow-baka]

Dear Richard,
Thanks a lot for your hard work on Vampire.

• I'm wondering what are the units of "output:material-mean-susceptibility".
  I tried to perform basic Curie-Weiss fit of output and resulting moments (extracted from Curie constant) was not nearly close to expected value of "atomic-spin-moment" from .mat file.

• I know that this question was discussed quite a lot in google groups, but I still have some complications with defining of exchange interaction in .ucf files.
  Lets say I have a simple 3d AFM with exchange interactions known from inelastic neutron scattering experiments. These exchange interactions usually published in meV / link.
  Should I just convert meV -> J (by multiplication to 1.6021766*10^-22) or any additional normalization by number of interactions or unit cell volume should be performed?
  I tried to use direct conversion meV -> J and resulting Neel temperature was more then 10 times different from expected one.

Thanks in advance!

[richard-evans]

Hi, sorry for the slow reply! The units of susceptibility are 1/tesla. The history of exchange interactions, Bloch law etc is a bit of a mess - there is no correlation between the size of moment and Tc, nor between the shape of Ms(T) at low temperature and Tc - these are only determined by the exchange interaction.

For the neutron scattering, my understanding is that they presume some relation between low energy spin waves and Tc. My experience is that the values of exchange are far too low compared to the Heisenberg model. While you can convert the energies, Tc is going to be very low. However, qualitative values are probably still valid, and so you could increase the values proportionally to get a similar Tc as experiment. This is an interesting thing to compare, and I haven't done anything with this yet.