Large exchange bias in Mn-Ni-Sn Heusler alloys: Role of cluster spin glass state

We report a combined experimental and theoretical study of the exchange bias (EB), structural and magnetic properties of manganese-rich Mn50Ni41.5+xSn8.5-x (x = 0, 0.75, and 1) Heusler alloys. All these alloys crystallize in a tetragonal structure at room temperature. Ab initio simulation using density functional theory (DFT) on off-stoichiometric Mn50Ni42.1875Sn7.8125 alloy (very close to the actual composition) predicts the tetragonal structure with the ferrimagnetic arrangement as the ground state, in good agreement with the experimental results. These alloys are found to exhibit a large exchange bias at low temperatures, with a maximum EB field of 7.1 kOe at 2 K for x = 1, which is significantly larger than that reported for any Mn-Ni-based Heusler systems. Frequency dependence of spin freezing temperature obeying the scaling law confirms the presence of a cluster spin glass (CSG)-like state in these alloys. The latter is also confirmed from the memory and aging effect studies in zero-field cooling and field-cooled protocols for these alloys. The large EB is attributed to the strong exchange coupling between the CSG clusters embedded in an otherwise ferrimagnetic matrix. The exchange mechanism behind the large EB effect is discussed in the light of varying bond lengths between different magnetic pairs, and hence the magnetic coupling, due to the large tetragonal distortion. The present experimental findings are found to be in good agreement with the DFT results.