Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy and magnetic susceptibility experiments have been used to characterize strain coupling phenomena associated with structural and magnetic properties of the shape-memory Heusler alloy series Ni\(_{50+x}\)Mn\(_{25-x}\)Ga\(_{25}\) (\(x=0\), 2.5, 5.0, and 7.5). All samples exhibit a martensitic transformation at temperature \(T_M\) and ferromagnetic ordering at temperature \(T_C\), while the pure end member (\(x=0\)) also has a premartensitic transition at \(T_{PM}\), giving four different scenarios: \(T_C>T_{PM}>T_M\), \(T_C>T_M\) without premartensitic transition, \(T_C\approx T_M\), and \(T_C