Journal of Magnetism and Magnetic Materials 226}230 (2001) 1498}1500 Enhancement of the coercive force with addition of Nb in -FeSi as-quenched ribbons R.Sato Turtelli *, M SchoK nhart , H Sassik , R GroK ssinger , C Kolbeck , Vo Hong Duong , E Ferrara Institut fu( r Experimentalphysik, Technische Universita( t Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria Institut fu( r Technische Elektrochemie und Festko( rperchemie, Getreidemarkt, Wien, Austria Department of Solid State Physics, Vietnam National University, Hanoi, Viet Nam Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy Abstract The e!ect of the addition of Nb in Fe}Si alloys was investigated by measuring the magnetic properties and investigating the microstructure Nb enhances (about 10 times) the coercivity and changes the microstructure as well as the temperature dependence of the coercivity There exists indication that Nb enters in the lattice 2001 Elsevier Science B.V All rights reserved Keywords: Soft magnetic materials; Coercivity; Structure FINEMET is the best-known commercial nanocrystalline material which makes use of the primary crystallisation process According to the authors of Ref [1], in the early primary crystallisation stage, the Nb and B concentrations are higher in the remaining amorphous phase, while those in the -Fe is lower Additionally they explain that once -Fe forms, Nb and B are excluded from the -Fe lattice, enriching the remaining amorphous phase These rejected atoms are found to generate di!usion layers with sharp concentration gradients at the -Fe crystallisation interface [2] The optimum magnetic properties correspond to an iron rich phase with about 20}25 at% Si The discovery of the excellent soft magnetic properties for temperatures lower than the Curie temperature, ¹ , of the amorphous phase and di!erent ! behaviours of the temperature dependence of H , H (¹), with di!erent volume fraction of Fe}Si crystals in FINEMET induced considerable interest in Fe}Si compounds having D0 and B2 structures [3,4] * Corresponding author Tel.: #43-1-58801-13151; fax: #43-1-58801-13199 E-mail address: sato@xphys.tuwien.ac.at (R.S Turtelli) The theoretical description of H (¹), is derived from the degree of magnetic coupling through an e!ective inter-granular exchange constant [5] However, the reason of the large H appearing in samples with low volume fraction of crystallites embedded in the amorphous matrix above ¹ of amorphous phase and the broad and ! low-intensity maximum of H found in those samples with high volume fraction is not completely clear The aim of this work is to investigate the e!ect of the addition of Nb in -Fe}Si alloys on H (¹) because in nanocrystalline materials, the participation of Nb in nanocrystals cannot be totally excluded Ribbons of Fe Si (thickness 30 m, width mm) \V V and of Fe Si Nb (thickness 30 m, width \ V>W V W 1.5 mm) were prepared by rapidly solidi"cation method Structural investigations were made by means of X-ray di!raction using CoK and CoK radiation and scan ning electron microscopy Hysteresis loops were measured with a frequency of 0.7 Hz and a maximum intensity of 180 kA/m ¹ was determined by means of ! temperature dependence of the AC-susceptibility (4 A/m and kHz) Fig shows H as function of the Si content obtained for both groups of as-cast ribbons, at room temperature Although both curves show the same behaviour with 0304-8853/01/$ - see front matter 2001 Elsevier Science B.V All rights reserved PII: S - 8 ( 0 ) 0 - R.S Turtelli et al / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1498}1500 Fig Coercivity as function of Si content measured on Fe Si and Fe Si Nb as-cast ribbons \V V
\V V
1499 Fig Temperature dependence of the coercivity of as-cast samples of Fe Si and Fe Si Nb alloys a minimum at x"12, a large increase of H (about 10 times) is found when Nb is added in Fe}Si alloys A decrease of the grain size was not detected by studying the line broadening of X-ray di!raction and also by microstructural study (see Fig 2) However a signi"cant change in the shape of the grains and the presence of large cavities at the grain boundaries, which look like free volumes (see arrows in Fig 2, below) in Fe Si Nb alloys were observed These results \ V>W V W suggest that: (a) atoms of Nb exert a large stress around the grain boundaries causing cavities between grains and, (b) change shapes of the grains (c) eventually some atoms can enter in the lattice The "rst suggestion can be proved by annealing the samples, such experiments are in progress In relation to the later, X-ray measurements show a displacement of lines for lower values of with Nb addition of Fe Si and Fe Si Nb are, respec tively, 52.99 and 52.883 A linear decrease of H with temperature is found in Fe Si ribbon However in Fe Si Nb , the coercive force up to around 673 K is nearly temperature independent Above this temperature it decreases monotonically (see Fig 3) This result leads to suggest that the large H appearing in FINEMET with low volume fraction of crystallites above ¹ of amorphous phase can have the contribution ! of the presence of Nb in Fe}Si nanocrystals Acknowledgements Fig Microstructure of Fe Si (above) and Fe Si Nb (below) alloys obtained at room temperature This work was partly supported by KELAG 1500 R.S Turtelli et al / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1498}1500 References [1] K Hono, D.H Ping, Mater Sci Forun 307 (1999) 69 [2] R.Yavari, in: M VaH zquez, A Hernando (Eds.), Nanostructured and Non-Crystalline Materials, 1994, p 35 [3] R Sato Turtelli, E de Morais, G Wiesinger, Ch Reichl, Vo Hong Duong, R GroK ssinger, J Magn Magn Mater 205 (1999) 290 [4] R Sato Turtelli, V.H Duong, R GroK ssinger, M Schwetz, E Ferrara, N Pillmayr, IEEE Trans Magn 36 (2000) 508 [5] A Hernando, P MarmH n, M VaH zquez, J.M BarandiaraH n, G Herzer, Phys Rev B 58 (1998) 366 ... that: (a) atoms of Nb exert a large stress around the grain boundaries causing cavities between grains and, (b) change shapes of the grains (c) eventually some atoms can enter in the lattice The "rst... annealing the samples, such experiments are in progress In relation to the later, X-ray measurements show a displacement of lines for lower values of with Nb addition of Fe Si and Fe Si Nb are,... leads to suggest that the large H appearing in FINEMET with low volume fraction of crystallites above ¹ of amorphous phase can have the contribution ! of the presence of Nb in Fe}Si nanocrystals