16 research outputs found
Combined effect of magnetic field and hydrostatic pressure on the phase transitions exhibited by Ni-Mn-In metamagnetic shape memory alloy
We present a systematic study of the magnetostructural and magnetic transitions in the prototype metamagnetic shape memory alloy Ni50Mn34.5In15.5 under hydrostatic pressure and combined pressure and magnetic field. Pressure extends the area of stability of the antiferromagnetic martensitic phase. At low magnetic field the pressure derivatives of the Curie temperatures of austenite, TCA, and martensite, TCM, show opposite signs. This fact is described in the framework of the Landau thermodynamic model as arising from a weak long-range antiferromagnetic state of martensite. Two volume magnetoelastic constants were estimated using the experimental values of the pressure derivatives of TCA and TCM. A correlation between the signs of the pressure shifts of TCA, and TCM and the distance between Mn-Mn nearest neighbours is established, which matches the empirical Castelliz-Kanomata diagram. The entropy change at martensitic transformation (MT), ?SMT, grows up when the MT temperature, TM, is approaching TCA under the influence of pressure, but under constant non-zero pressure this dependence is inverse.The financial supports from Ministry of Science, Innovations and Universities (projects MAT2017-83631-C3-3-R and RTI2018-094683-B-C53-54) and from the Basque Government Department of Education (project IT1245-19) are greatly acknowledged
Influence of Fe doping and magnetic field on martensitic transition in Ni–Mn–Sn melt-spun ribbons
"Mn-rich Ni-Mn-Sn metamagnetic shape memory alloys exhibiting magnetostructural transformation are of a great potential as the base materials for solid-state refrigeration. With the aim of fine tuning of the transformation characteristics and improving functional properties, in the present work we have fabricated polycrystalline Ni50-xFexMn40Sn10 (x = 0, 2, 4, 6, 8 at.%) melt-spun ribbons, starting from the base alloy with x = 0, which is weakly magnetic in both austenitic and martensitic phases. By exploring martensitic transformation (MT) and magnetic behaviors as a function of Fe doping and magnetic field, we have found that Fe and/or magnetic field reduce the MT temperature and Curie temperature of austenite phase, becoming closer to each other as the Fe-content increases, accompanied by an increase of the magnetic moment of austenite, magnetization jump at MT, transformation volume, and magnetic contribution, Delta S-M, to the total entropy change at MT. The ribbons present moderate values of Delta S-M equal to 11 J kg(-11)K(-1) at 5 T for x = 8, moderate thermal hysteresis (10-14 K) nearly independent of Fe doping or magnetic field, and adjustable structural and magnetic transition temperatures close to room temperature. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys
Structural and magnetic transition temperatures of
ferromagnetic shape memory alloys present a strong dependence on
slight departures from the stoichiometry, as does the mobility of
twin boundaries responsible for the large magnetic field induced
strains. In this work we study four non stoichiometric Ni-Mn-Ga
polycrystalline alloys with compositions of 43–52 at.% nickel,
excess manganese and deficient in gallium, and a single crystal of
composition Ni52Mn26Ga22. Those compounds are of
technical interest due to the observed large room temperature
magnetic field induced strains. Calorimetric and magnetic
measurements determined the martensitic transition and Curie
temperatures of the alloys (AS = 331 K and
TCurie = 366 K for 52 at.% nickel alloy).
Nickel defective alloys present a martensitic transition region
broader than excess nickel ones. Neutron powder diffraction analysis
confirmed orthorhombic martensitic structures for nickel defective
alloys, and tetragonal for excess nickel ones. In the 52 atomic %
nickel alloys case the crystallographic structure of the martensitic
phase was also obtained on a single crystal with the same
composition, trained to get a single variant in agreement with
determined in the powder sample
Transformation behavior of Ni–Mn–Ga in the low-temperature limit
The magnetic, magnetocaloric and thermal characteristics have been studied in a Ni50.3Mn20.8Ga27.6V1.3 ferromagnetic shape memory alloy (FSMA) transforming martensitically at around 40 K. The alloy shows first a transformation from austenite to an intermediate phase and then a partial transformation to an orthorhombic martensite, all the phases being ferromagnetically ordered. The thermomagnetization dependences enabled observation of the magnetocaloric effect in the vicinity of the martensitic transformation (MT). The Debye temperature and the density of states at the Fermi level are equal to theta(D) = (276 +/- 4) K and 1.3 states = atom eV, respectively, and scarcely dependent on the magnetic field. The MT exhibited by Ni-Mn-Ga FSMAs at very low temperatures is distinctive in the sense that it is accompanied by a hardly detectable entropy change as a sign of a small driving force. The enhanced stability of the cubic phase and the low driving force of the MT stem from the reduced density of states near the Fermi level
Probing structural and magnetic instabilites and hysteresis in Heuslers by Density Functional Theory calculations
Martensitic transformations of rapidly quenched and less rapidly cooled Heusler alloys of type Ni–Mn–X with X = Ga, In, and Sn are investigated by ab initio calculatioms. For the rapidly cooled alloys, we obtain the magnetocaloric properties near the magnetocaloric transition. For the less rapidly quenched alloys these magnetocaloric properties start to change considerably, each alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic Ni–Mn–X and tetragonal NiMn