A systematic study of the magnetocaloric effect of a Ni51Mn33.4In15.6 Heusler alloy converted to nanoparticles via high energy ball-milling technique in the temperature range of 270 to 310 K has been performed. The properties of the particles were characterized by x-ray diffraction, electron microscopy, and magnetometer techniques. Isothermal magnetic field variation of magnetization exhibits field hysteresis in bulk Ni51Mn33.4In15.6 alloy across the martensitic transition which significantly lessened in the nanoparticles. The magnetocaloric effects of the bulk and nanoparticle samples were measured both with direct method, through our state of the art direct test bed apparatus with controllability over the applied fields and temperatures, as well as an indirect method through Maxwell and thermodynamic equations. In direct measurements, nanoparticle sample’s critical temperature decreased by 6 K, but its magnetocaloric effect enhanced by 17% over the bulk counterpart. Additionally, when comparing the direct and indirect magnetocaloric curves, the direct method showed 14% less adiabatic temperature change in the bulk and 5% less adiabatic temperature change in the nanostructured sample.

Heusler alloys feature both conventional and inverse magnetocaloric effects near room temperature as they undergo two different transitions. In this paper, new data are presented and analyzed and a new mechanism to explain the complex hysteretic behavior of a Ni48Co2Mn35In13Ga2 Heusler alloy is developed. This mechanism explains isothermal loops near room temperature. The various descriptions and classifications of these transitions, however, is not critical to this analysis.

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The NiMnInSi Heusler alloy family is analyzed, and a self-similarity based method is used to analyze the first-order transition of Ni-Mn Heusler alloys. This method is appropriate to determine magnetic characteristics of the Magnetocaloric Effect providing that there is sufficient separation in their phase transition temperatures. A temperature scaling methodology is used to model the cluster compositions in the mixed-state regions where two stable magnetic states co-exist. The various descriptions and classifications of these transitions, however, are not critical to this analysis.