Surface Superconductivity in Ni₅₀Mn₃₆Sn₁₄ Heusler Alloy

The changes in the magnetizations of Ni₅₀Mn₃₆Sn₁₄ Heusler alloy (L₂₁-NiMnSn-HA) with an antiferromagnetic interfacial exchange coupling have been investigated by using the effective field theory as a function of temperature and external magnetic field. It is shown that the Ni component is antiferromagnetic and its other components are ferromagnetic while the L₂₁-NiMnSn-HA has a ferrimagnetic ordering below T_C. And, they are paramagnetic above T_C. The L₂₁-NiMnSn-HA and its Ni component have the triple hysteresis loops below T* and the other components have only single hysteresis loop below T_C. It is found that they exhibit type II/1 superconductivity at higher external magnetic field below T* and type II/2 superconductivity at lower external magnetic field above T*. Moreover, it is suggested that the surface superconductivity emerges below the conversion temperature of T^∗≈ 0.74 and can be associated with the triple hysteresis loops which occur in the systems with an antiferromagnetic interfacial exchange coupling. And, the vortex state terminates at T_V ≈ 4.37 for the Ni component of the L₂₁-NiMnSn-HA. The H_C-T phase diagrams of the L₂₁-NiMnSn-HA and its Ni component are in qualitatively good agreement with the experimental and theoretical findings.