Curie Temperature and Hopkinson Effect in Twin Roller Melt Spun ${\hbox {Ni}}_{2}{\hbox {MnGa}}$ Shape Memory Alloys

The temperature dependence of the magnetic polarization near the Curie temperature T_C in Ni₂MnGa stoichiometric alloys, directly processed from the melt in a twin-roller melt-spinning device, is investigated. The effect of the solidification rate on the Hopkinson peak detected is evaluated in samples quenched at three different tangential wheel speeds of 10, 15, and 20 m/s. The resulting microstructures were previously characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and by transmission electron microscopy (TEM). EDS results indicated that all the alloys have the composition Ni₂MnGa; at room temperature and above this temperature, a cubic L2₁ ferromagnetic ordered austenitic phase is observed. The Curie temperatures and the magnitude of the Hopkinson effect are estimated from the magnetic polarization versus temperature curves measured in a Faraday balance, in the range 300 K-400 K. As expected for samples with identical composition, the Curie temperatures remain insensitive to the processing route. At low fields (10 mT), the magnitude of the Hopkinson effect is larger in samples quenched at lower rates and it practically vanishes in all the alloys for applied fields near 100 mT.