Unconventionally Large Exchange Bias and Cluster Spin Glass State in Mn2(Ni,Sn) Heusler Alloys Melt Spun Ribbons

Here, we report the structural, microstructural, magnetic, and exchange bias (EB) properties of polycrystalline ribbons of Mn50Ni42.5–xSn7.5+x (x = 0 and 1) Heusler alloys. They exhibit monoclinic structure (7M) at room-temperature (RT). Both the compositions (x = 0, 1) exhibit first-order magneto-structural (martensitic) transition with the martensitic transition temperature (TM) of 389 K for x = 0 and 347 K for x = 1. These TM-values are lower than their bulk counterparts, which is possibly due to the decrease in grains size and atomic order in the later. They exhibit giant exchange bias (EB), with a maximum EB-field (HEB) of 10.2 kOe for x = 0 at 2 K, one of the highest values reported among Mn–Ni based Heusler systems in bulk, thin film, or ribbon form. Frequency dependent ac-susceptibility, scaling law, memory effect, and wait time dependent magnetization relaxation studies confirm the presence of the cluster spin glass (CSG) state. Enhanced exchange coupling between the CSG clusters embedded in a ferrimagnetic matrix is possibly responsible for the giant EB effect in these ribbons.