Evolution of phase transformation and magnetic properties with Fe content in Ni55−xFexMn20Ga25 Heusler alloys

Abstract

A series of Ni55−xFexMn20Ga25 (0 ⩽ x ⩽ 5) Heusler alloys was prepared to investigate their phase transitions and magnetic properties. At room temperature, these alloys present various crystal structures, and the unit cell volume enlarges with increase of Fe content in both austenite and martensite. Multiple magneto-structural transformations were observed in the parent alloy (x = 0). In the process of cooling, it undergoes martensitic transformation (MT) from L21-type paramagnetic austenite to L10-type ferromagnetic martensite, accompanying an intermartensitic transformation (IMT, 7M → L10). By establishing a detailed phase diagram, we found that both MT and IMT shift to lower temperature simultaneously, while the ferromagnetic (FM) transition of austenite moves to higher temperature as Fe increases. With the further increase of Fe content beyond a critical value, both the IMT and the FM transitions split off from MT, and the former follows with the transforming sequence of 7M → 5M. Based on the experimental data, some key magnetic parameters have been obtained in this system. The calculated magnetocrystalline anisotropy constant () of martensite quickly increases as Fe increases, and then it almost reaches a saturated value (~5.5 × 105 J m−3) for the alloys with x > 3. However, the spontaneous magnetic moment () attains a peak value of about 4.2 μB/f.u. in the alloy with x = 4, which is not consistent with the linear increasing of effective magnetic moment (). Further magnetic measurements with hydrostatic pressure indicate that such a discrepancy could be ascribed to the competition between the magnetic exchange interaction and the volume change of unit cell governed by the dopant Fe content.