Disorder driven crossover from superparamagnetism to an interacting-cluster state in Fe₂Ti_0.75Mn_0.25Sn Heusler alloy

Abstract

We report a comprehensive investigation of the structural and magnetic properties of Mn-substituted Fe₂TiSn Heusler alloy using powder neutron diffraction and magnetization measurements. Rietveld refinement of neutron diffraction confirms the cubic L2₁ structure with ∼9% B2-type antisite disorder between Fe and Ti/Mn sites. Magnetization measured as a function of temperature and magnetic field reveal the emergence of short-range ferromagnetic correlations below 325 K, arising from nanoscale ∼9.04 nm) ferromagnetic clusters. At high temperatures, these clusters exhibit superparamagnetic behavior, but on cooling, enhanced inter-cluster interactions drive a crossover to a correlated ferromagnetic-like state. The progression from superparamagnetism to interacting-cluster state correlates with antisite disorder and local lattice distortion, which tune both the cluster dimensions and its coupling strength. The results highlight the critical role of structural disorder in governing magnetic cluster dynamics in Fe-based Heusler alloys and establish a pathway to engineer collective spin states for spintronic and magnetic storage applications.