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The effect of nanocrystallite size on crystal structure and magnetic properties of off-stoichiometric Ni_1.8Mn_1.2Sn Heusler alloy has been investigated. XRD (X-Ray Diffraction) and HRTEM (High-Resolution Transmission Electron Microscopy) analysis revealed the presence of both the full Heusler (Ni₂MnSn) L2₁ and inverse Heusler (Mn₂NiSn) XA crystal symmetries. Hence, the sample can be treated as a natural composite of both phases. However, the magnetic analysis reveals that the magnetization does not follow a linear relationship with the volume fraction of the individual phases. Rather, the antiferromagnetic coupling between the two phases may be responsible for lowering the net magnetization. On the other hand, magnetization value further decreases for ball-milled sample (nanocrystallite size). Critical magnetic behavior analysis reveals that the magnetic ordering in bulk sample follows mean field theory …