We demonstrate how exchange hardening can arise in a chemically-disordered solid solution from a first-principles statistical mechanics approach. A general mixed-basis chemical and magnetic cluster expansion has been developed, and applied to the Mn$_{1-x}$Fe$_x$Ru$_2$Sn Heusler alloy system; single-phase solid solutions between antiferromagnetic ch{MnRu2Sn} and ferromagnetic ch{FeRu2Sn} with disorder on the Mn/Fe sublattice that exhibit unexpected exchange hardening. Monte Carlo simulations applied to the cluster expansion are able to reproduce the experimentally measured magnetic transition temperatures and the bulk magnetization as a function of composition. The magnetic ordering around a site is shown to be dependent not only on bulk composition, but also on the identity of the site and the local composition around that site. The simulations predict that local antiferromagnetic orderings form inside a bulk ferromagnetic region at intermediate compositions that drives the exchange hardening. Furthermore, the antiferromagnetic regions disorder at a lower temperature than the ferromagnetic regions, providing an atomistic explanation for the experimentally-observed decrease in exchange hardening with increasing temperature. These effects occur on a length scale too small to be resolved with previously-used characterization techniques.
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