Magnetocaloric Effect and Critical Behavior of Alloys

This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of Ni₅₀Mn₃₇Sn₁₃ doped with 8% Ag, which were prepared by an arc-melting and rapidly quenched melt-spinning methods, respectively. Experimental results reveal that a partial replacement of Ag for Ni leads to stamping out the antiferromagnetic martensitic phase. This means that there is only the austenitic phase with a ferromagnetic-paramagnetic (FM-PM) phase-transition temperature of T_C ≈ 295 K. Detailed studies and analyses around the phase transition region prove both samples undergoing a second-order magnetic phase transition. Basing on magnetic field dependences of magnetization, we have determined the magnetic-entropy change (ΔS_m) of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change (|ΔS_max|) reaches values 0.54 and 0.69 J · kg^-1 · K^-1 for the alloy ingot and ribbon, respectively. Using Landau´s phase-transition theory, and careful analyses of the magnetic data around the FM-PM transition region, we have determined the critical parameters (T_C, β, γ, and δ) in the low field range (below 10 kOe) with T_C = 294.8 K, β = 0.469 ± 0.011, γ = 1.149 ± 0.060, and δ = 3.4 ± 0.1 for the alloy ingot, and with T_C = 294.4 K, β = 0.449 ± 0.005, γ = 1.319 ± 0.040, and δ = 3.9 ± 0.1 for the alloy ribbon. One can see that β values fall in between those expected for the 3-D Heisenberg model (β = 0.365) and mean-field theory (β = 0.5). This indicates a coexistence of short-range and long-range FM interactions in both the samples. The nature of changes in value related to the critical parameters and maximum ΔS_m is thoroughly discusse- by means of structural analyses.