Magnetization anomalies in melt-spun Ni-Mn-Ga ribbons

Temperature and field dependencies of the magnetization of melt-spun ribbons of Ni45.7Mn37.2Ga17.1 (T-C < T-A) and Ni54.3Mn20.5Ga25.2 (T-C > T-A), where T-A and T-C are the reverse martensitic transformation and Curie temperatures, respectively, have been studied in broad ranges of magnetic fields (+/-30 kOe) and temperatures (10-350 K). It was found that the magnetic and thermomagnetic properties have a number of peculiarities and anomalies. Particularly, the magnetization values measured at both low and high magnetic fields increase significantly after annealing. Low-temperature anomalies of magnetization suggest the presence of the superparamagnetic behavior and/or phenomena related to the concurrent exchange interactions in the as-spun state. The obtained results can be explained by the influence of concentration inhomogeneities and atomic disordering in the rapidly quenched ribbons, which can be reduced by annealing. (C) 2007 Published by Elsevier B.V.