Temperature and magnetic field dependent martensite transformation in Al doped Ni-Mn-Sn disorder alloys and its effects on magnetoresistance and magnetocaloric effect near room temperature

Ferromagnetic shape memory alloys Ni₄₅Mn₄₄Sn_11-xAl_x (x = 1, 2) were prepared by arc melting technique. The martensite transition, which is first order in nature, has been clearly observed through magnetic, differential scanning calorimetry and electronic transport measurements. Magnetic path dependent very large magnetic entropy change (19 Jkg⁻¹K⁻¹ and 14 Jkg⁻¹K⁻¹ obtained from decreasing and increasing field magnetization data, respectively) due to sharp increase of magnetization from weak magnetic martensite phase to ferromagnetic austenite phase has been observed for Ni₄₅Mn₄₄Sn₁₀Al₁ alloy. However, the observed large magnetic entropy change in Ni₄₅Mn₄₄Sn₉Al₂ alloy (20.6 Jkg⁻¹K⁻¹ and 19.2 Jkg⁻¹K⁻¹ obtained from decreasing and increasing field magnetization data, respectively) are found to be not much dependent on magnetic path. Thermal path dependent large negative magneto-resistance (∼ −15% and −24% at 288(C) K and 288(W) K, respectively, for 8 T applied magnetic field), due to difference in austenite and martensite phase fraction, has been found inside the martensite transition regime for Ni₄₅Mn₄₄Sn₁₀Al₁ alloy. From calorimertic measurement, the lattice entropy change for Ni₄₅Mn₄₄Sn₁₀Al₁ alloy due to the martensite transition has been observed to be nearly 55 Jkg⁻¹K⁻¹. The maximum adiabatic temperature change for Ni₄₅Mn₄₄Sn₁₀Al₁ and Ni₄₅Mn₄₄Sn₉Al₂ alloy are found to be −2.7 K and −1.2 K, respectively, for the change of magnetic field of 5 T. Magnetic field dependent isothermal resistivity does not change much outside the martensite transition temperature. Large decrease in resistivity by applied magnetic field has been observed inside martensite transition because of field induced conversion of martensite to austenite phase. The decreasing field isothermal magnetization and resistivity data do not follow increasing field data for having more austenite phase fraction than the field increasing path. (paper)