The Schottky anomaly is well-studied, but it exists in ideal systems without interactions. What is the dependence of specific heat on temperature, if the spin-spin interaction is significant? I've found only some fragmented information on it.
See: Tang J. 2015: JFS Special Issue: 75 Years of Advancing Food Science, and Preparing for the Next 75 Unlocking Potentials of Microwaves for Food Safety and Quality, Journal of Food Science; Vol. 80, Nr. 8, 2015, E1776
I expect that extensive information can be found in the book: A.Abragam, M.Goldman. Nuclear Magnetism: Order & Disorder (Russian electronic version exists) and in textbooks on electronic paramagnetic resonance and dynamic nuclear polarization.
Prof. E.K.Henner (Perm State University) can give additional information. Something you can find in his and my articles, placed at my page, see attached links as well.
With best regards,
F.S.Dzheparov.
Article Magnetic resonance in magnetically dilute solids at low temperatures
Article Magnetic Resonance in Magnetically Diluted Solids at Low Temperatures