Microstructure and magnetic properties of as-cast Ni2MnGa alloys processed by twin roller melt spinning

G. POZO LÓPEZ; A.M. CONDÓ; R.N. GIORDANO; S.E. URRETA; N. HABERKORN; E. WINKLER; L.M. FABIETTI

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2013 vol. 335 p. 75 - 85

0304-8853

The magnetism and the microstructure of Ni2MnGa alloys, processed for the first time by twin roller melt spinning at tangential wheel speeds of 10m/s(V10), 15m/s(V15) and 20m/s(V20) are investigated. At room temperature, the major phase in the as-cast alloys is the cubic L21 Ni2MnGa ordered austenitic phase, with a lattice parameter only approx. 0.1% larger than the tabulated value. The order domain size in the austenitic phase decreases from (40 +/- 1) nm in samples V10 to (19 +/- 1) nm in V20. Mn(S,Se) small precipitates are also found uniformly embedded in the ribbons with mean size of (26 +/- 2) nm (V10) and (7 +/- 2) nm (V20), exhibiting a definite orientation relation with the austenitic matrix, (1 0 0)P // (1 0 0)a. The as-cast alloys transform to an intermediate cubic phase I at about 220-230 K depending on the quenching rate and to a martensitic phase at about 130K. The high temperature austenitic phase and the low temperature martensitic phase are ferromagnetic; in both cases the saturation polarization is lower in samples quenched at higher rates. The demagnetization curves measured from saturation in the martensitic state show two marked steps: a first one for positive fields in V15 and V10 (approx. 46 mT) and a larger second one for relatively large inverse fields (approx. 130-250 mT) in all the samples. These steps are likely to arise from a demagnetization mechanism involving a field induced twin boundary motion in the few martensite variants selected by the crystallographic texture and the quenched stresses in the ribbons.