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.