We describe an ab initio disordered local moment theory for materials with quenched static compositional disorder traversing first-order magnetic phase transitions. It accounts quantitatively for metamagnetic changes and the magnetocaloric effect. For perfect stoichiometric B2-ordered FeRh, we calculate the transition temperature of the ferromagnetic-antiferromagnetic transition to be $T_t=$ 495 K and a maximum isothermal entropy change in 2 T of $\left|\Delta S\right|=21.1$ J K${}^{-1}$ kg${}^{-1}$. A large (40%) component of $\left|\Delta S\right|$ is electronic. The transition results from a fine balance of competing electronic effects which is disturbed by small compositional changes; e.g., swapping just 2% Fe of “defects” onto the Rh sublattice makes $T_t$ drop by 290 K. This hypersensitivity explains the narrow compositional range of the transition and impurity doping effects.

• Received 3 January 2014
• Revised 10 February 2014

DOI:https://doi.org/10.1103/PhysRevB.89.054427

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©2014 American Physical Society