Large magnetocaloric effect and exchange bias properties in Mn-Ni-Co-Sn full Heusler alloy

We have studied the structural, magnetic, magnetocaloric properties and exchange bias (EB) of a Mn-rich Mn-Ni-Co-Sn Heusler alloy, by means of the combined experimental and theoretical investigations. The alloy possesses tetragonal structure at room temperature (RT), and exhibits a first order martensitic transition at around 335 K. It also shows a large magnetocaloric effect in the vicinity of martensitic transition, with a maximum isothermal magnetic entropy change of around 35 J/kg.K, which is significantly larger than that reported for many other Heusler systems. Frequency dependence of spin freezing temperature obeying the dynamical scaling and Vogel-Fulcher law, along with the memory effect confirms the presence of cluster spin glass (CSG) state at low temperatures in the alloy. It shows EB effect at low temperatures, with a maximum EB field of 0.099 Tesla at 2 K, after field cooling in presence of 1 Tesla. Strong exchange coupling among the CSG clusters embedded in a strong ferrimagnetic matrix is found to be responsible for the observed EB. Ab-initio calculations using density functional theory on a representative Mn50Ni35.94Co6.25Sn7.81 alloy (very close to our experimental composition) shows the tetragonal structure with the ferrimagnetic ordering as the ground state, which is in good agreement with the experimental results. Exchange mechanism behind the observed EB in the present alloy has been discussed in the light of varying interatomic distances between different atomic pairs, and hence the magnetic coupling.