A comparison study of the entropy changes in materials with and without short-range magnetic order

The effects of short-range magnetic order (SRMO) on magnetic entropy changes have been studied for the compounds Sm 0.55 Sr 0.45 MnO 3 , Eu 0.55 Sr 0.45 MnO 3 and the rare-earth metal Gd. Ferromagnetic clusters composed of ∼8 Mn ions are found to be formed in the paramagnetic phase of the manganites based on the analysis of susceptibility. As a result, the entropy change for a field variation of 0–5 T is ∼6 J kg −1 K −1 for Sm 0.55 Sr 0.45 MnO 3 and ∼7 J kg −1 K −1 for Eu 0.55 Sr 0.45 MnO 3 , well below the values expected without considering the effect of SRMO. (The maximum entropy change will be ∼56 J kg −1 K −1 for both compounds.) Assuming the presence of sizable magnetic clusters, the field-induced entropy change, not only its peak value but also its temperature dependence, can be well reproduced by the mean field theory. The average cluster size is found to be m ≈8, in agreement with the result obtained from a Curie–Weiss analysis of the susceptibility. The presence of SRMO reduces the entropy change by ∼80%. In contrast, the mean field theory gives a good description of the entropy change in Gd, for which no significant SRMO is observed. The present work reveals the important role of SRMO in modifying the magnetocaloric properties of the magnetic materials.