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The large magnetocaloric effect (MCE), which accompanies the first order ferromagnetic/anti-ferromagnetic transition in CsCl-ordered Fe-Rh alloys, has been investigated by measurements in slowly cycled magnetic fields of up to 2 T in magnitude for a range of temperatures, 300K < T < 350K. A bulk sample with composition Fe 50.4 Rh 49.6 was used and the results were compared with those produced by the ab-initio density functional theory-based disordered local moment (DLM) theory of the MCE. The measurements revealed an irreversibility effect in which the temperature of the material did not return to its initial value following several cycles of the magnetic field. These observations were explained in the framework of the ab-initio theory for the first order transition in which the consequences of the incomplete long range compositional order and small compositional inhomogeneities of the sample were included. The mean value of the long range order parameter S used in the theoretical work was 0.985, close to the value obtained experimentally from XRD measurements. The sample inhomogeneities were modeled by regions in the sample having a distribution of S values with narrow half-width 0.004 about the mean value. The influence of such compositional disorder on both the transition temperature (323.5 K) and MCE adiabatic temperature change (∆Т = 7.5 K) was also studied.

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