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.
