Cluster Spin Glass State and Observation of Giant Exchange Bias in Tetragonal Mn-Ni-Sn Heusler Alloys: Experiment and Theory

We have done a combined experimental and Density functional theoretical (DFT) study of the exchange bias (EB), structural and magnetic properties of Mn rich Mn₅₀Ni_41.5+xSn_8.5-x Heusler alloys. These alloys were synthesized experimentally by arc melting method, and found to possess the tetragonal structure at room temperature (RT) with increasing tetragonal distortion with x. DC magnetization (M vs. T) measurements have suggested that the martensitic transition occur above RT for all the compositions. A large splitting between zero field cooling (ZFC) and field cooled cooling (FCC) M vs. T curves is observed at low temperatures, which is a suggestive of the coexistence of AFM/FM or spin glass/FM or ferrimagnetic phases. DFT simulation on off-stoichiometric Mn₅₀Ni_42.1875Sn_7.8125 alloy (nearly exp. composition) predicts the tetragonal structure with the ferrimagnetic ground state, which is in good agreement with the experimental results. These alloys are found to exhibit giant exchange bias at low temperatures, with a maximum EB field of around 7100 Oe at 2 K for x=1, which is significantly larger than that reported for any other tetragonal Heusler systems. Frequency dependence of spin freezing temperature on scaling law through AC susceptibility measurements confirms the presence of cluster spin glass (CSG) like state in these alloys. CSG state has also been confirmed from the memory and aging effects recorded in various ZFC and FC protocols. Such a large EB is attributed to the strong exchange coupling between the CSG clusters embedded in an otherwise ferrimagnetic matrix. The exchange mechanism behind the EB effect has been verified in the light of varying bond lengths between different magnetic atoms and hence the magnetic coupling, due to the large tetragonal distortion. Thus, the present study provides not only an insight into the complex magnetism of the ground state of Mn rich tetragonal Heusler alloys but also facilitates in search of new Heusler systems in this family with enhanced physical properties, from fundamental as well as application point of view.