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 ₅₀Mn _50-xIn _x, Ni _50-xCo _xMn ₃₅In ₁₅, Ni ₅₀Mn _35-xCo _xIn ₁₅, Ni ₅₀Mn ₃₅In ₁₄Z (Z=Al, Ge), Ni ₅₀Mn ₃₅In _15-xSi _x, Ni _50-xCo _xMn _25yGa _25-y, and Ni _50-xCo _xMn _32-yFeyGa ₁₈. It was found that the magnetic entropy change, ΔS, 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 ΔH=5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni ₅₀Mn _50-xIn _x (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24 J/(kg K) was detected for ΔH=5 T at T=350 K. The variation in composition of Ni ₂MnGa 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 ₄₂Co ₈Mn _32-yFeyGa ₁₈ system. The adiabatic change of temperature (ΔT _ad) in the vicinity of TC and TM of Ni ₅₀Mn ₃₅In ₁₅ and Ni ₅₀Mn ₃₅In ₁₄Z (Z=Al, Ge) was found to be ΔT _ad=-2 K and 2 K for ΔH=1.8 T, respectively. It was observed that |ΔT _ad|≈1 K for ΔH=1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.