The Effect of Grain Size and Phase constitution on the Magnetic Properties of Higher Content Cr substituted Fe73.S.XCrXNb3CUISiI3 5B9 Nanocrystalline Alloys

Abstract

This thesis is based on the experimental investigation of the effect of grain size and phase constitution on the magnetic behavior of Fe73.5-xCrxNb3Cu1Si13.5 B9 [x = 7, 9, 10 & 12.5] alloys in the amorphous and annealed states. The samples are initially prepared in the amorphous state in the form of thin ribbons by rapid quenching technique at wheel speed of 25m/s in an Ar atmosphere. The alloy has been annealed in a controlled way in the temperature range of 450-800°C for 30 minutes. The kinetics of glass formation and crystallization in composition is studied as affected differential thermal analysis (DTA). Higher content Cr substituted Fe73.5-x CrxNb3Cu1S13.5B9 amorphous alloys upon annealed undergoes phase transformation due to primary crystallization of α -Fe(Si) and secondary crystallization of Fe2B phase re4spectively revealed by DTA and confirmed by XRD. Thermal analysis experiment and from the obtained data activation energy of primary crystallization products α -Fe(Si) phase are varies 2.35 to 4.88 eV and secondary crystallization products Fe2B3.28 to 3.76 eV. Amorphosity of the ribbon and nanocryatalline state was evaluated by XRD. In the optimized annealing condition the grain size has been obtained in the range of 11 - 30 nm. The primary crystallization phase shifts to higher annealed temperature with Cr content implying the enhancement of thermal stability of the amorphous alloys against crystallization due to increasing amount of Cr. The average grain size of the a-Fe(Si) phase, almost same under the identical annealing condition as the higher content Cr is increased. The peak shifts indicate the change of the values of Si-content of nanograins and therefore, the change of the values of lattice parameter of nanograins. The saturation magnetization (Ms) and Curie temperature (Tc) of these alloys decrease linearly with the increase of Cr content for the entire composition range due to dilution of Fe magnetic moment and weakening of exchange interaction between of magnetic atoms. The critical composition for disappearance of ferromagnetism fall of curve Ms with the replacement Fe by Cr, where the nearest neighbor coupling is longer dominant and intermediate range occur, giving rise to a significant portion of antiferromagnetic interaction. This causes a weaker exchange interaction among the Fe magnetic moment resulting in a decrease of Tc. This increase of magnetization (M) up to annealed temperature 600°C is connected to the structural relaxation and varying degrees of chemical disorder with reference to enhancement of M of annealed samples. Crystallization seems to take place and accordingly M Vs H curve sharply rises and look ferromagnetic for x = 12.5 which is paramagnetic in the amorphous condition with Tc = 246K. This increase of M for the four samples are due to the evolution of ferromagnetic α-Fe(Si) nanograin crystallites.