Effect of Grain Size and Phase Constitution on the Magnetic Properties of Fe-X-Nb-Si-B [X = Cu, Au & Ag] Alloys

Abstract

Amorphous FINEMET type alloy nominal composition of Fe73.5X1Nb3Si13.5B9 [ X = Cu, Au and Ag] has prepared by melt spinning technique. 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 ribbon has been annealed in a controlled way in temperature range 450o – 800oC for 30 minutes. The kinetics of crystallization amorphous FINEMET type alloys were investigated by the use of differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. DTA runs for these three samples show the existence of two exothermic peaks one for α-Fe(Si) and other for Fe2B phase. The crystallization of each phase has occurred over a wide range of peak temperatures and the primary crystallization like α-Fe(Si) phase temperature shift higher temperature as the replace Ag>Au>Cu concentration. DTA experiment and from the obtain data activation energy of primary crystallization α-Fe(Si) phase are varies 2.18 - 4.37eV and secondary crystallization products Fe2B phase is 3.62 - 4.45 eV. Amorphosity of the ribbon and nanocrystalline state has been evaluated by XRD. The primary crystallization phase shifts to higher annealed temperature with Cu replacement Au or Ag implying the enhancement of thermal stability of amorphous alloys against crystallization. All the three samples has been annealed in the temperature range 6000C -8000C for 30 minutes and taken under XRD experiment in order to study the effect of structural parameters such as lattice parameter, grain size and silicon content of the nanocrystalline α-Fe(Si) grain. In optimizing annealing condition the grain size has been obtained in the range of 14 - 20nm for Cu, 13 - 25nm for Au and 09 - 30nm for Ag. Saturation magnetization Ms increase in order with condition Ag > Cu > Au in these experimental samples. The reduction curie temperature (Tc) with substitution of Cu by Au or Ag in order Ag < Au < Cu may attributed to this simultaneous weakening of strength of exchange interaction between the magnetic moments. This is because the amorphous matrix is depleted Fe and relative amount of Nb in the amorphous matrix increases Cu replace Au or Ag which weakness the exchange interaction resulting in reduction of Tc.