Abstract:
Amorphous ribbon of composition (Fe0.8Co0.2)73.5Cu1Nb3Si13.5B9 was prepared by rapid
quenching method at wheel speed of 25 m/s in an Ar atmosphere. The alloy has been
annealed in a controlled way in the temperature range of 400-700°C for 30 minutes.
Crystallization onset temperature for FeCo(Si) phase was found 525ºC from Differential
Thermal Analysis Experiment and from the obtained data, the activation energy was
calculated to be 3.11eV. Amorphosity of the ribbon and nanocrystalline state was evaluated
by X-ray diffraction. In the optimized annealing condition the grain size has been obtained in
the range of 2-26 nm. Temperature and frequency dependence of permeability of amorphous
and devitrified toroid shaped samples have been measured. The low frequency value of m¢
increases with the increase of annealing temperature up to 550°C. Very low value of relative
loss factor of the order of about 10-2 has been observed at this annealing temperature. It is
notable that the addition of Co has significant effects on enhancement of Curie temperature.
In the as-prepared condition the Curie temperature has been found to be 421°C, which is
quite high compared to the Curie temperature of conventional FINEMET (i.e. sample without
Co), which is about 354°C. Enhancement of Curie temperature occurs with increasing
annealing temperature up to 475°C. As the nanocrystalline phase appeared, it was found that
at and above the crystallization temperature, the Curie temperature of the residual amorphous
phase gradually decreases with the increase of annealing temperature. This is because the
amorphous matrix is depleted with Fe and relative amount of Nb in the amourphous matrix
increases, which weakens the exchange interaction resulting in reduction of Curie
temperature.
