Compositional dependence of magnetic state and thermodynamic properties of Ni50Mn50-xSnx alloys

The compositional dependence of magnetic, martensitic and caloric properties of Ni 50 Mn 50- x Sn x shape memory alloys has been investigated. It has been shown that an increase of Sn content weakens the long-range antiferromagnetic (AFM) order observed for x < 8 and decreases the Néel temperature. For 10 ≤ x ≤ 12.5, this gives rise to low-temperature short-range ordered magnetic states inherent to systems with competing spin-exchange interactions. Further increase of Sn content induces the ferromagnetic (FM) ordering and elevates the Curie temperature. Theoretical estimations of Néel temperatures qualitatively agree with neutron diffraction data. Based on the obtained results, a temperature – composition phase diagram was constructed, and six thermodynamic states were identified. Low-temperature specific-heat measurements show linear composition trends in the weakly magnetic martensitic phase ( x ≤ 11.5): the electronic coefficient γ increases with Sn content, while the Debye temperature T D decreases. In ferromagnetic austenite ( x ≥ 17), T D remains nearly composition-independent, but γ decreases with Sn content, although its maximum value is still much higher than in martensite. A notable magnetic contribution to the specific heat has been observed for x = 17, suggesting additional mechanisms influencing the electronic density of states in the austenite of Ni 50 Mn 50- x Sn x alloys.