Study of Structural and Magnetic Properties of Nanocrystalline (Fe095Co0.05)73.5Cu1Nb3Si13.5B9 Alloy

Abstract

This thesis is based on the experimental investigation of nanocrystal formation from FINEMET type of amorphous ribbons subjected to controlled thermal treatment and evolution of unique soft magnetic properties that are microstructure dependent. Amorphous ribbon of composition (Fe095Co0.05)73.5Cu1Nb3Si13.5 B9 was prepared by rapid quenching method at wheel speed of 25m/s in an Ar atmosphere. The alloy has been annealed in a controlled way in the temperature range of 550oc to 750°C for 30 minutes. Crystallization onset temperature for FeCo(Si) phase was found around 550oC. Thermal analysis experiment and from the obtained data, the activation energy was calculated to be 2.40eV. Nanocrystalline state was evaluated by X-ray diffraction. In the range of optimizing annealing temperature 550°C to 750°C the grain size has been obtained in the range of 9 - 26nm. Temperature and frequency dependence of permeability of amorphous and devitrified toroid shaped samples have been measured. The initial permeability for the optimum annealed samples has been found to 5.8× 103 and the highest value of quality factor is found for the sample annealed at 550°C; which also indicates the best heat treatment temperature to obtain the highest value of quality factor. It is notable that the addition of Co has significant effects on enhancement of Curie temperature (Tc). In the as-prepared condition the Tc has been found to be 422°C, which is quite high compared to the Tc of conventional FINEMET (i.e. sample without Co), which is 354oC. As the nanocrystalline phase appeared, it was found that at and above the crystallization temperature. Tc 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 amorphous matrix increases, which weakens the exchange interaction resulting in reduction of Tc. The high permeability of the optimum nanocrystallized sample has been achieved due to drastic of effective anisotropy resulting from nanometric grain size effect and strong magnetic coupling. The results show that amounts of Cu and Nb are very important for the soft magnetic properties of FINEMET alloys.