  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 (delT = 7.5 K) was also studied.
