Study on the thermomagnetic properties of alnico v magnetic ribbon

Abstract

The structural and thermomagnetic properties of melt-spun Alnico V magnetic ribbons have been studied as function of temperature and magnetic field. Ribbons fabricated at different wheel speeds ( 25 m/s – 50 m/s) have been studied for their thermal , structural and magnetic properties. The effect of wheel speed on the magnetic properties have been investigated. It is found that change in wheel speed has an effect on the ductility of the ribbons. All the as made and annealed samples have shown pure crystalline structure as revealed by the X-ray Diffraction analysis unlike most of the iron, cobalt and nickel based ribbons which show amorphous or glassy structure after melt-spinning. A significant enhancement in coercive field has been observed after subjecting the Alnico V ribbons through regular heat treatment followed by the aging process. It is observed through this studies that aging the ribbons at high temperature has a positive effect on the coercivity enhancement of Alnico V alloy. The as made ribbons have a relatively low coercivity and only a uniaxial anisotropy is observed which is induced due to mechanical stress developed during meltspinning. However annealing at high temperature followed by aging has yielded a significant increase in coercivity. Elemental addition of Boron has shown to have no effect on the microstructure of the ribbon except for improving the elastic properties of the ribbon. The magneto resistance behavior is found to be typical in nature and has shown a minimum around 1 kOe which is also near the coercieve field obtained after Alnico regular heat treatment followed by aging. No significant change is observed in the differential thermal analysis meaning that no structural phase transformation has taken place. However a selective annealing under an external applied magnetic field has caused the nucleation of a crystalline and amorphous mixed phase still dominated by the crystalline phase. It is proposed that this behavior of the alloy at elevated temperature is related to the onset of spinodal decomposition of the alloy where the strong magnetic phase is split into two different magnetic phases with one phase magnetically stronger than the other and the relatively weaker phase grows at the expense of stronger phase causing the deterioration of the overall magnetic properties of the ribbon shaped alloy.