Role of Si on structure and soft magnetic properties of Fe87−xSixB9Nb3Cu1 ribbons

Role of Si content on the structural, thermal and soft magnetic properties of melt spun and annealed Fe87−xSixB9Nb3Cu1 (x = 11, 13, 18, 20, 25 and 30 at.%) alloys has been investigated. Processing of ribbons at a higher wheel speed (47 m/s) during melt spinning results in the formation of amorphous phase in all the alloys expect for 30 at.% Si whereas at lower wheel speed (34 m/s), amorphous phase has been observed only in 18 at.% Si alloy ribbon. Detailed thermal analyses revealed that higher glass forming ability of 18 at.% Si alloy is due to the near eutectic point in these alloy series and higher ratio between the crystallization and liquidus temperature (Tx/TL). On annealing, nanocrystalline Fe3Si phase was found to be precipitated out in amorphous matrix in alloys up to 20 at.% Si ribbons whereas paramagnetic Fe4.9Si2B phase along with Fe3Si phase was observed in 25% Si ribbon. Detailed Mössbauer spectroscopy of the annealed ribbons revealed that the composition of the nanocrystalline Fe3Si phase increases from Fe-21 at.% Si towards the stoichiometric composition (Fe-25 at.% Si) with Si content. The results are also corroborated with precision lattice parameter measurements. The coercivity of the annealed ribbons initially decreases, attains a minimum at 18 at.% Si and then increases abruptly with Si content.