Intermartensitic transitions in Ni-Mn-Fe-Cu-Ga Heusler alloys

A series of Fe doped Ni₂Mn_0.75-xFe_xCu _0.25Ga Heusler alloys have been investigated by means of x-ray diffraction, magnetizations, thermal expansion, and electrical resistivity measurements. In Ni₂Mn_0.75Cu_0.25Ga, martensitic and ferromagnetic transitions occur at the same temperature. Partial substitution of Mn by Fe results in a decrease of the martensitic transition temperature, T_M, and an increase of the ferromagnetic transition temperature, T_C, resulting in separation of the two transitions. In addition to the martensitic transition, complete thermoelastic intermartensitic transformations have been observed in the Fe doped Ni₂Mn _0.75-xFe_xCu_0.25Ga samples with x>0.04. An unusual transition is observed in the alloy with x = 0.04. The magnetization curve as a function of increasing temperature shows only one first-order transition in the temperature range 5-400K, which is identified as a typical coupled magnetostructural martensitic transformation. The magnetization curve as a function of decreasing temperature shows three different transitions, which are characterized as the ferromagnetic transition, the martensitic transition and the intermartensitic transition.