Role of Si on structure and soft magnetic properties of Fe 87x Si x B 9 Nb 3 Cu 1 ribbons M. Srinivas 1 , B. Majumdar ⇑ , S. Bysakh 2 , M. Manivel Raja, D. Akhtar 3 Defence Metallurgical Research Laboratory, Hyderabad 500058, India article info Article history: Received 2 May 2013 Received in revised form 22 July 2013 Accepted 6 August 2013 Available online 25 August 2013 Keywords: Rapid solidification Melt spinning Amorphous & nanocrystalline Finemet abstract Role of Si content on the structural, thermal and soft magnetic properties of melt spun and annealed Fe 87x Si x B 9 Nb 3 Cu 1 (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 (T x /T L ). On annealing, nanocrystalline Fe 3 Si phase was found to be precipitated out in amorphous matrix in alloys up to 20 at.% Si ribbons whereas paramagnetic Fe 4.9- Si 2 B phase along with Fe 3 Si phase was observed in 25% Si ribbon. Detailed Mössbauer spectroscopy of the annealed ribbons revealed that the composition of the nanocrystalline Fe 3 Si phase increases from Fe- 21 at.% Si towards the stoichiometric composition (Fe-25 at.% Si) with Si content. The results are also cor- roborated 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. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline Fe–Si–B–Nb–Cu alloy system (FINEMET), pro- duced by controlled annealing of amorphous melt spun ribbon has attracted considerable interest because of the unique combina- tion of soft magnetic properties such as high saturation magnetiza- tion, permeability, resistivity, Curie temperature and very low coercivity [1]. The soft magnetic properties in these alloys arise due to the dual phase microstructure consisting of nanocrystalline Fe–Si phase in amorphous matrix which leads to the nullification of magnetostriction and magnetocrystalline anisotropy [2,3]. The effective magnetostriction (k eff ) is nullified by the positive magne- tostriction of the residual amorphous matrix (k amor = 20  10 6 ) and negative contribution of a-Fe(Si) nanocrystalline phase (k nc = 6  10 6 ) in such a way that k eff = (1  v)k amor + vknc. Again, based on the Random Anisotropy Model [4], the magnetocrystal- line anisotropy value gets averaged out when the crystalline size is less than the exchanged interaction length which is of the order of 35 nm in case of Fe–Si phase. Since the magnetic properties are dependent on the microstructural parameters such as volume frac- tion, size and distribution of the nanocrystalline phase and compo- sition of nanocrystalline and amorphous phases, it is essential to optimize the initial composition and process parameters for obtaining best combination of soft magnetic properties. The most widely studied composition investigated in this alloy system is Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 ribbons because of their superior soft magnetic properties [1]. The effect of Si and B ratio on micro- structure and soft magnetic properties in Fe 73.5 Si x B 22.5x Nb 3 Cu 1 (x = 7–17.5 at.%) alloys have been investigated in details [5,6]. It has been reported that the magnetostriction value of the annealed ribbons is zero for compositions above 16 at.% Si [5]. Conde et al. [6] reported that the grain size and volume fraction of nanocrystal- line phase increase with the Si content. Several techniques such as X-ray diffraction (XRD) [7], energy dispersive spectroscope (EDAX) [8], Curie temperature measurements [9], and atom probe field ion microscopy [10]. have been used to estimate the composition (Si content) of the nanocrystalline phases. Recently Mössbauer tech- nique has proved to be a useful tool for determining the exact Si content in the nanocrystalline phase [10–13]. However, the varia- tion of the structure and soft magnetic properties of Si rich Fe–Si– B–Nb–Cu alloys has not been studied extensively. It has been observed that the addition of Si up to 18 at.% is ben- eficial as it lowers the coercivity [14]. Recently, a detailed investi- gation was carried out by the same group [15] in Fe 68.5 Si 18.5 B 9 Nb 3 Cu 1 alloy produced by planar flow melt spinning 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.047 ⇑ Corresponding author. Tel.: +91 40 24586743; fax: +91 40 24340884. E-mail address: bhaskar@dmrl.drdo.in (B. Majumdar). 1 Present address: Naval Science and Technological Laboratory, Visakhapatnam 530027, India. 2 Present address: Central Glass and Ceramic Research Institute, Kolkata 700032, India. 3 Present address: ER&IPR, DRDO Head Quarters, New Delhi 110011, India. Journal of Alloys and Compounds 583 (2014) 427–433 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom