Structure, Magnetocaloric Effect and Critical Behaviour in Ni50Mn30(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 $$ {T}_{\mathrm{c},1}^{\mathrm{A}}(Sn20)=333\ \mathrm{K} $$ . However, the Ni50Mn30In20 alloy exhibits a mixture of cubic L21 and B2 austenite structures having Curie temperatures of $$ {T}_{\mathrm{c},2}^{\mathrm{A}}(In20)=285\ \mathrm{K} $$ and $$ {T}_{\mathrm{c}}^{\ast }(In20)=330\ K $$ , 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 ( $$ \Delta {S}_{\mathrm{M}}^{\mathrm{max}}=2.43\frac{J}{\mathrm{kg}}.\mathrm{K} $$ ) is slightly higher than that of In20 ( $$ \Delta {S}_{\mathrm{M}}^{\mathrm{max}}=2.05\frac{J}{\mathrm{kg}}.K $$ ). The experimental results of entropy changes are in good agreement with those calculated using Landau theory.