Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni 50 Mn 50− x In x , Ni 50− x Co x Mn 35 In 15 , Ni 50 Mn 35− x Co x In 15 , Ni 50 Mn 35 In 14 Z (Z=Al, Ge), Ni 50 Mn 35 In 15− x Si x , Ni 50− x Co x Mn 25+ y Ga 25− y , and Ni 50– x Co x Mn 32− y FeyGa 18 . It was found that the magnetic entropy change, Δ S , associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5) J/(kg K) for a magnetic field change Δ H =5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni 50 Mn 50− x In x ( x =13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition Δ S =24 J/(kg K) was detected for Δ H =5 T at T =350 K. The variation in composition of Ni 2 MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni 42 Co 8 Mn 32− y FeyGa 18 system. The adiabatic change of temperature (Δ T ad ) in the vicinity of TC and TM of Ni 50 Mn 35 In 15 and Ni 50 Mn 35 In 14 Z (Z=Al, Ge) was found to be Δ T ad =−2 K and 2 K for Δ H =1.8 T, respectively. It was observed that |Δ T ad |≈1 K for Δ H =1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.