LAUSR

Abstract Ferromagnetic shape memory Ni–Mn-Ga alloys exhibit magnetostructural coupling during the Curie points, which may alter their magnetocaloric effect (MCE). Here MCE of Ni–Mn-Ga alloy microwires are tuned by Fe-doping. The chemical ordering annealed Ni49·6Mn25.5Ga22.6Fe2.3 (Fe2), Ni49·6Mn25Ga21Fe4.4 (Fe4) and Ni48Mn26Ga19.5Fe6.5 (Fe6) microwires have diameters of ~30–50 μm and grain sizes of ~2–5 μm. The magnetic entropy changes (ΔSm) of the microwires increase with external magnetic fields, then reach a positive peak and finally decrease to negative peak at a high field. The direct (negative ΔSm, DMCE) and inverse (positive ΔSm, IMCE) MCEs are attributed to microscopic and mesoscopic couplings in microwires. In addition, the specific features of MCEs are also controlled by the magnetization difference (ΔM) between martensite and austenite phases. Interestingly, the DMCE and IMCE are component dependent. At high ΔM, the linearly decreasing region is dominant and Fe4 and Fe6 microwires produce a negative ΔSm at high magnetic fields due to the microscopic coupling with free electron valence ratio e/a = 7.692 and 7.725. On the other hand, the ΔM is small in Fe2 microwire, which has a positive ΔSm because mesoscopic coupling is dominant with e/a = 7.607; the ΔSm is highest when a single variant martensite occurs, then decreases linearly with increasing magnetic fields. The transition from positive to negative values in ΔSm with the applied fields is related to the counterbalance of mesoscopic and microscopic couplings at e/a = 7.66.