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Fluctuating local moments, itinerant electrons and the magnetocaloric effect: the compositional hypersensitivity of FeRh

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 $T_t =$ 495K and a maximum isothermal entropy change in 2 Tesla of $|ΔS|= 21.1$ J~K$^{-1}$~kg$^{-1}$. A large (40\%) component of $|ΔS|$ 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 290K. This hypersensitivity explains the narrow compositional range of the transition and impurity doping effects.