Multiscale origin of the magnetocaloric effect in Ni-Mn-Ga shape-memory alloys

Abstract

We have analyzed magnetization measurements in a series of composition-related Ni-Mn-Ga shape-memory alloys. It is shown that the magnetocaloric effect in the vicinity of the martensitic transition mainly originates from two different contributions: ͑i͒ magnetostructural coupling on the mesoscopic scale between the magnetic moments and the martensitic variants, which is also responsible for the magnetic shape-memory effect and ͑ii͒ the microscopic spin-phonon coupling which gives rise to the shift of the transition temperature with the applied magnetic field. The relative importance of these two contributions has been shown to vary with composition, which is suitably expressed through the average number of valence electrons per atom e/a. In alloys with a large difference between the Curie and martensitic transition temperatures (e/aӍ7.5), mesoscopic coupling is dominant and a negative giant magnetocaloric effect ͑increase of temperature by adiabatic demagnetization͒ is induced at moderate applied fields. In contrast, in alloys when these temperatures are very close to one another (e/aӍ7.7), the microscopic coupling is the most relevant contribution and gives rise to a positive giant effect.

Large negative magnetoresistance in a ferromagnetic shape memory alloy: Ni2+xMn1−xGa

“…[11,12] For example, varying x from 0 to 0.2 causes T C to decrease from 376 to 325 K and T M to increase from 210 to 325 K. Giant-magnetocaloric effect has been observed in samples where T C and T M are equal. [8,13] In this paper, we report 5% negative magnetoresistance (MR) at 8 T in bulk Ni 2+x Mn 1−x Ga polycrystals at room temperature. To the best of our knowledge, there are no MR studies in literature till date on well characterized bulk Ni 2+x Mn 1−x Ga samples as functions of composition and temperature.…”

Magnetic properties and magnetic entropy change in Heusler alloys Ni50Mn35−x Cu x Sn15

“…It has been demonstrated that the MCE effect strongly depends on the nature of magnetic transitions in the magnetic materials. A number of compounds such as Ga 5 Si 2 Ge 2 [1], LaFe 13−x Si x [2,3], MnAs 1−x Sb x [4] and MnFeP 1−x As x [5], Ni-Mn-Ga [6][7][8][9][10][11], exhibit a large MCE because of the first-order nature of their magnetic transition. Among those materials, an attractive candidate is the Mn-based Heusler alloys.…”

Exchange Bias and Inverse Magnetocaloric Effect in Co and Mn Co-Doped Ni2MnGa Shape Memory Alloy

“…The most striking features of the ΔS M are the large and positive entropy change within the MT temperature range. The positive sign of ΔS M exists even under high magnetic field, showing the mechanism involved is different from that of Co-free Ni-Mn-Ga alloys which is related to the coupling between the martensitic and magnetic domains [4]. The modification of magnetic order through magnetic-field-induced reverse MT from partially AFM martensite to FM austenite is responsible for the IMCE in the present alloy.…”

“…The maximum MCE was obtained when structural and magnetic transition temperatures overlap each other (T M = T c ) [3]. Furthermore, an extrinsic inverse MCE (IMCE, namely, positive ΔS M ) phenomenon has also been reported in several Ni-Mn-Ga alloys under application of low magnetic field, arising from the coupling between the martensitic and magnetic domains at the mesoscale [4], i.e., magnetization change due to martensite reorientation associated with the high magnetic anisotropy. However, this process cannot result in a true ΔS M since it is originated from the field dependent change of magnetic order parameter [7].…”

“…Stoichiometric Ni 2 MnGa is a full Heusler alloy that undergoes two separate transitions on cooling: (i) a magnetic transition (paramagnetic-ferromagnetic (FM) transition in the austenite) at T c = 376 K, and (ii) a structural transition (martensitic transformation (MT) from austenite to martensite) at temperatures close to the MT [2][3][4][5][6]. By varying the composition, the transition temperature can be altered within a broad temperature range.…”