Interplay of Magnetic Dilution and Lattice Expansion in Cu-Substituted Ni-Mn-Sn Ribbons: A Combined Heisenberg and Landau Phase Transition Model

Journal of Superconductivity and Novel Magnetism, vol.39, pp.1-9, 2026 (SCI-Expanded)

In this study, we present a dual-framework theoretical model that combines a microscopic Heisenberg Hamiltonian

and a macroscopic Landau free energy expansion to clarify the magneto-structural phase transitions in Cu-substituted

Ni50−xCuxMn38Sn12+B(3% at.) ribbons. Experimental results show a marked suppression of the martensitic transformation

temperature with Cu substitution, despite only minimal lattice expansion (Δa/a ≈ 0.15%). To address this discrepancy,

our model demonstrates that the shift in transition temperatures is primarily caused by magnetic dilution rather than purely

distance-dependent RKKY interactions. Using an inverse Mean Field Theory approach, we quantitatively demonstrate a

systematic reduction in the effective nearest-neighbour exchange integral ( J1) from 12.0 meV to 6.0 meV, resulting from

the disruption of exchange paths by non-magnetic Cu (3d10). Additionally, the macroscopic Landau framework reveals

that Cu substitution weakens the magneto-structural coupling term (λQ2M2), significantly lowering the activation energy

barrier for the phase transition. At the microscopic level, the presence of fully filled Cu-3d states leads to “orbital dilution”

near the Fermi level, suppressing d-d orbital hybridization and alleviating the band-Jahn-Teller instability. This unified

model successfully accounts for the reduction in thermal hysteresis, the persistence of field-induced metamagnetic behaviour,

and the enhanced low-field actuation capabilities in heavily Cu-substituted Heusler alloys.