Ultra-soft magnetic properties of two phase nanocrystalline (Fe0.8Co0.2)75.3Cu1Nb3Si13.5B9 alloy

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.