Structure, Magnetocaloric Effect and Critical Behaviour in Ni 50 Mn 30 (Sn, In) 20 Heusler Alloys

Abstract

A detailed investigation of structure, critical behaviour and magnetocaloric properties of Ni50Mn30Sn20 (Sn20) and Ni50Mn30In20 (In20) alloys has been investigated by means of X-ray diffraction and magnetic measurements. Ni50Mn30Sn20 alloy shows a cubic austenite L21 structure and undergoes a second order magnetic transition at a Curie temperature of TAc,1(Sn20)=333 K. However, the Ni50Mn30In20 alloy exhibits a mixture of cubic L21 and B2 austenite structures having Curie temperatures of TAc,2(In20)=285 K and T∗c(In20)=330 K, respectively. The modified Arrott plots, Kouvel-Fisher curves and critical isotherm analysis have been used to estimate the critical exponents (β, γ and δ) around the Curie temperature. For Sn20 alloy, the reliable exponents are consistent with the mean field model, revealing long-range ferromagnetic interactions. Nevertheless, the critical exponents of In20 alloy around 330 K cannot be arranged into any of the universality classes of well-known classical standard models. The maximum entropy change under 5 T of Sn20 (ΔSmaxM=2.43Jkg.K) is slightly higher than that of In20 (ΔSmaxM=2.05Jkg.K). The experimental results of entropy changes are in good agreement with those calculated using Landau theory