The magnetic susceptibility of LiFePO4 and LiCoPO4 has been measured. The results, when interpreted on a simple molecular field theory, explain why antiferromagnetic resonance at millimeter wavelengths was not observed with magnetic fields of up to 50 kG. ?? 1970.
Creer, J. G., & Troup, G. J. (1970). The magnetic susceptibility of LiFePO4 and LiCoPO4. Physics Letters A, 32(6), 439–440. https://doi.org/10.1016/0375-9601(70)90044-7