Microstructural Evolution on Annealing and its Correlation to Structural and Magnetic Properties of Melt-Spun Ni50Mn29Ga21Ribbons

Polycrystalline Ni(50)Mn(29)Ga(21)thin ribbons were prepared by melt-spinning and the effect of annealing temperatures on the microstructural evolution, phase transformations, crystal structure, and the magnetic properties of the ribbons was systematically investigated. Different annealing temperatures from 473 to 1100 K were used to alter the degree of quenched-in disorder gradually, which led to evolution of microstructure from cellular to well-defined grain structure. The crystal structure was found to be 7 M monoclinic in ribbons annealed at 773 K or below, while a minority phase with 5 M tetragonal structure is found to co-exist on annealing at 1100 K. The as-spun ribbon exhibit superparamagnetic behavior whereas the sample annealed at 1100 K did not exhibit the same. The variations in the saturation magnetization with annealing temperatures were systematically correlated. The saturation magnetization and the martensitic transformation temperature show a correlation to the increase in the degree of Mn ordering.