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ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Crystallisation progress in Si-rich ultra-soft nanocomposite alloy fabricated by melt spinning Duc-The Ngo a,c,Ã, Mohamed Sultan Mahmud b, Hoang-Hai Nguyen c, Hong-Gam Duong c, Quang-Hoa Nguyen c, Stephen McVitie a, Chau Nguyen c a Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK Department of Physics, University of Asia Pacific, Dhaka-1209, Bangladesh c Center for Materials Science, College of Science, Vietnam National University Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam b a r t i c l e in f o a b s t r a c t Article history: Received 16 July 2009 Received in revised form September 2009 Available online 20 September 2009 The crystallisation process and the ultras-soft magnetic properties of amorphous/nanocomposite alloy Fe73:5 Si17:5 B5 Nb3 Cu1 fabricated by conventional melt-spinning technique are systematically investigated in terms of thermal analysis and in-situ measurement of magnetisation dynamics The thermal analysis using differential scanning calorimetry showed that crystallisation from Fe-based amorphous state to aFe(Si) started at 535 C Further heating the sample leads to a transformation from the a-Fe(Si) to Fe-B phases at 670 C Crystallisation activation energies were determined using two models: Kissinger and John–Mehl–Avrami (JMA), which were consistent to each other with a value of 2:81 0:03 eV High resolution transmission electron microscopy investigation revealed an ultrafine structure of a-Fe(Si) nanocrystallite with mean size of 12.5 nm embedded in an amorphous matrix At a volume fraction of 86% of a-Fe(Si) phase, optimum soft magnetic properties were obtained with very high permeability of 110,000 and a very low coercivity of 0.015 Oe by annealing the amorphous alloy at 530 C in 40 Crown Copyright & 2009 Published by Elsevier B.V All rights reserved PACS: 75.50.Kj 75.50.Tt 75.50.Bb 70.40.Gb Keywords: Amorphous alloys Nanocrystalline materials Permeability Finemet Introduction The Fe73.5Si13.5B9Nb3Cu1 amorphous/nanocomposite alloy (FINEMET) has been extensively studied by many world-wide researchers [1–4] due to its excellent soft magnetic properties Commonly, amorphous alloy is firstly produced as a raw material, and the nanocomposite alloy will be subsequently obtained by appropriate heat treatment for crystallising the nanocrystallite aFe(Si) with suitable volume fraction The microstructure and soft magnetic properties of Fe-based nanocomposite alloys are thus strongly influenced by heat treatment [1,3,5] The optimum structure of the FINEMET consists of an ultrafine structure of a-Fe(Si) nanocrystallites embedded in remaining amorphous matrix [2] A suitable volume fraction of a-Fe(Si) nanocrystallites in the materials leads to compensating the net magnetostriction of two phases (positive magnetostriction of amorphous and negative magnetostriction of nanocrystalline à Corresponding author at: Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK Tel.: + 44 141 339 8855x0895; fax: + 44 1413304464 E-mail address: d.ngo@physics.gla.ac.uk (D.-T Ngo) a-Fe(Si) grains) Furthermore, since the size of nanocrystallites is smaller than the ferromagnetic exchange interaction length (about 35 nm for FeSiBNbCu amorphous/nanocomposite system), ferromagnetically exchange coupling between the nanocrystalline a-Fe(Si) grains through amorphous matrix results in an averaging out of the magnetocrystalline anisotropy [5] As a result, ultrasoft magnetic properties with high saturation induction, very high permeability, low frequency losses and low coercivity are obtained [2,3] Understanding the crystallisation kinetics in these materials is of scientific interest for two crucial reasons Firstly, for the case of alloys that exhibit excellent magnetic properties in the amorphous phase, the crystallisation kinetics represents a limit at which these properties begin to deteriorate Therefore, thermal stability determines the magnetic stability of the materials in the amorphous phase Secondly, for the case of alloys that exhibit excellent magnetic properties in the nanocrystalline/amorphous matrix structure, controlling the crystallisation kinetics provides an ability to tailor the microstructure The amount of the nanocrystalline phase formed within the matrix can be controlled to achieve the desired magnetic performance Additionally, models of the crystallisation from the metastable amorphous state to the stable crystalline state depend on various parameters 0304-8853/$ - see front matter Crown Copyright & 2009 Published by Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2009.09.054 ARTICLE IN PRESS D.-T Ngo et al / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 such as the composition, the concentration of nucleation sites, the diffusion coefficients, the activation energy for diffusion, etc Investigation of crystallisation kinetics will be fruitful for fundamental understanding of thermal behaviours and for optimising producing process in industrial production In this article, the crystallisation evolution as well as the influence of heat treatment on the magnetic properties of Si-rich Fe73.5Si17.5B5Nb3Cu1 alloy fabricated by rapid quenching technique is investigated Experiments Ingot alloy with nominal composition of Fe73.5Si17.5B5Nb3Cu1 was initially fabricated by induction melting in high vacuum The sample is slightly modified from pure-Finemet as enriching Si content (from 13.5 to 17.5 at%) and lowering B content (from to at%) The amorphous ribbons were subsequently produced by rapidly quenching molten alloy on surface of a copper wheel rotating with longitudinal speed of 30 m sÀ1 The estimated cooling rate is about 106 K sÀ1 , which is high enough to ensure amorphous state in the ribbons The ribbons are in cm width and 16 mm thick The ribbons were isothermally annealed in Ar atmosphere at various temperatures ranging from 530 to 580 C for nanocrystallisation Thermal analysis was performed using a SDT 2960 TA Instruments differential scanning calorimeter (DSC) Microstructure of the sample was examined using a FEI Tecnai TF20 field emission gun transmission electron microscope (TEM) with an accelerated voltage of 200 kV Crystal structure of the sample was determined by X-ray diffraction (using a Bruker D5005 diffractometer) and electron diffraction on the TEM Magnetic properties were characterised using a DMS 880 vibrating sample magnetometer (VSM) and an AMH-401A Hysteresisgrapher Results and discussion X-ray diffraction and electron diffraction in TEM confirm the fully amorphous state in as-quenched ribbons Thermal analysis results performed on the DSC are shown in Fig The DSC curves exhibit two clearly exothermal peaks, corresponding to the Fig Differential scanning calorimetric curves of the as-quenched ribbon performed at various heating rates from 10 to 50 K minÀ1 343 crystallisation stages during heating progress It is well-known that the first peak, occurring around 530 C (so-called onset crystallisation temperature) corresponds to the crystallisation of crystalline a-Fe(Si) phase from amorphous phase [2,3,5] It is important to note that the onset temperature in the first peak is a little lower than that of pure Finemet with lower content of Si [1–3] Previous studies [2,3] suggested that the second peak (680 C) related to the formation of a boride phase (Fe–B) A small amount of boride phase Fe3B was found in the specimen heated to 680 C, confirming the transformation from a-Fe(Si) to Fe3B As seen from Fig the exothermal peaks on DSC curves slightly shifts to higher temperature with respect to heating rate According to Kissinger model [6], the temperature at exothermal peak, Tp , is dependent on the heating rate, b, as follows: ln b Tp2 ¼ À Ea þ const: kB Tp ð1Þ where b; Tp ; Ea are heating rate, exothermal temperature, and crystallisation activation energy, respectively; and kB is Boltzmann constant Fig shows the dependence of lnðb=Tp2 Þ on the inversed exothermal temperature, 1=Tp (Kissinger plot) The linear dependence of the lnðb=Tp2 Þ on the 1=Tp confirms that the Kissinger model is reasonable for describing the crystallisation in the studied sample As a result, the activation energy in this case can be easily determined by linear fitting the dependence in Fig 2, and shown to be Eka ¼ 2:80 70:4 eV This value is calculated for the crystallisation at the first peak—for a-Fe(Si) phase Crystallisation of the a-Fe(Si) phase at the first peak is also confirmed by using thermomagnetic measurement (Fig 3) on the VSM Firstly, the temperature dependence of the magnetisation of an amorphous specimen was measured by heating the specimen from room temperature to 780 C using flowing Ar gas on the VSM During the progress, a small magnetic field of 20 Oe is applied to the sample to detect the magnetisation of the specimen This progress can be described as follows: Firstly, the magnetisation rapidly decreases at the Curie temperature of the amorphous phase, TC ¼ 327 C and then remains at a small value because the sample is in a paramagnetic state Further increasing temperature to onset crystallisation temperature, the magnetisation drastically increases because of Fig Kissinger plot for determining crystallisation activation energy for amorphous specimens ARTICLE IN PRESS 344 D.-T Ngo et al / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 Fig Temperature dependence of magnetisation of the as-quenched specimen measured during heating and cooling cycles the formation of the ferromagnetic a-Fe(Si) phase with increasing volume fraction during the heating process When temperature is continuously increased near the Curie temperature of the a-Fe(Si) phase, a reduction of the magnetisation is observed In the cooling cycle, a single-phase temperature dependence of magnetisation is visible because of the unique exchangecoupled behaviour in multiphase-structure a-Fe(Si)/amorphous system Additionally, the Johnson–Mehl–Avrani (JMA) model [7,8] was also applied to clarify the crystallisation kinetics during heat treatment process The JMA model can be expressed that: Nucleation and growth occur at a constant temperature, e.g at the isothermal crystallisation temperature Nucleation is random throughout the bulk of the sample, which is assumed to be infinite Growth is isotropic until crystals impinge upon one another As a result, the volume fraction of the a-Fe(Si) phase, Xf ðtÞ depends on the annealing time, t, by the expression: n Xf tị ẳ ektị 2ị where k is the rate coefficient and n is the morphology index The rate coefficient, which is a temperature-dependent factor, is given by kTị ẳ k0 eEa =RT 3ị Here, Ea ; R; T are, respectively, crystallisation activation energy, ideal gas constant and the temperature The volume fraction of the a-Fe(Si) phase was determined using Leu and Chin method [2,3,9] by comparing DSC curves of asquenched and annealed samples at a sample heating rate Fig illustrates the volume fraction of the a-Fe(Si) phase as a function of annealing time as isothermally annealed at 530 C The data shows that the dependence of the volume fraction on the time is well consistent with JMA model described in Eq (2) Hence, values of k ¼ 1:04  10À3 sÀ1 and n ¼ 1:12 are calculated for rate coefficient and morphology index, respectively Repeating this process for specimens annealed at temperatures of 540, 550, 560, 570, and 580 C, the crystallisation activation Fig Volume fraction of the a-Fe(Si) phase as a function of annealing time (at 530 C annealing temperature) The inset shows an in situ measurement of magnetisation dependent on the time at 530 C in 10 kOe applied field energy is again estimated The activation energy in the JMA model is 2:83 70:03 eV, which is closed to that obtained by the Kissinger model mentioned above Furthermore, it is obviously seen that a higher Si content results in a lower crystallisation activation energy in comparing with original Finemet (3.2–3.4 eV [3,10,11]) This is crucial for tailoring the magnetic properties of the materials The lower activation energy, the lower annealing temperature is required, and as a result the better properties of the materials (soft magnetic, mechanical, etc properties) should be remained For example, using lower annealing temperature, it is indeed that crystallisation rate would be slow down, and better grain structure (finer structure, more homogeneous structure, etc.) could be controlled On the other hand, a lower annealing temperature could prohibit the hardness of the alloys, and keep the flexibility of the annealed alloy Furthermore, an enhancement of Si and a reduction of B content will prohibit the hardness of annealed alloy [11], an important feature for using in various purposes By the same ways, activation energy of phase transformation at the second peak on the DSC curve is also determined by the same methods and shown to be 4:7 0:07 eV The magnetisation evolution at onset crystallisation temperature, 530 C, is depicted in the inset of Fig In this measurement, an amorphous specimen was heated to 530 C and kept at that temperature Time dependence of the magnetisation was measured in an applied field of 10 kOe, which is high enough to saturate the sample The result in the inset of Fig shows that the time dependence of the isothermal magnetisation is again in agreement with the JMA model given by Eq (2) It is well-known that the saturation magnetisation in the specimen is the total magnetisation of two phases: the crystalline a-Fe(Si) phase and amorphous phase, which is given by [5] M ẳ Xf MFeSiị ỵ Xf ịMamor 4ị At high temperature, the Mamor % 0, therefore the total magnetisation is approximately proportional to the volume fraction of the a-Fe(Si) phase: n M % Xf MFeSiị ẳ MFeSiị ẵ1 eÀðktÞ ð5Þ Eq (5) expressed the time dependence of the magnetisation as shown in the inset of Fig Grain structure of the materials was determined by a transmission electron microscope using dark- ARTICLE IN PRESS D.-T Ngo et al / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 345 Fig Bright-field TEM micrographs of the specimens annealed at 530 C (a), 560 C (b), 580 C (c) (in 40 keeping time) The frame (d) shows the temperature dependence of grain size The inset shows selected area electron diffraction pattern, and dark shading on the images is due to the thickness variation of the TEM specimens field and bright field imaging Fig shows a series of bright field TEM micrographs of the samples annealed at various temperatures No grain structure was observed in as-quenched specimen due to the amorphous structure By annealing, the grain structure of the a-Fe(Si) phase was formed Increasing annealing temperature results in an increase of grain size Namely, at 530 C, mean grain size is 12.5 nm, and rising to 14.0 nm when annealing temperature is 560 C And at 580 C annealing temperature, average grain size becomes 37.2 nm (see detail in Fig 5d) Diffraction analysis in TEM confirms a polycrystalline structure of bcc-Fe(Si) phase in the materials As increasing annealing temperature from 530 to 580 C, the lattice constant slightly decreases from 0.299 to 0.285 nm This indicates that increasing annealing temperature enhances the concentration of Si in the bcc-Fe(Si) phase Namely, at 530 C, lattice constant of a ¼ 0:299 nm of bcc-Fe(Si) phase corresponds to the bccFe87.6Si12.4 phase [12] Lattice constant decreases to 0.285 nm at 580 C confirming the existence of the bcc-Fe85.5Si14.5 phase [12] High resolution imaging of transmission electron microscopy (HRTEM) confirms the nanostructure of the nanosized bcc-Fe(Si) single crystals embedded in remaining amorphous matrix (Fig 6) At high annealing temperature, some lattice dislocations were observed in the bcc-Fe(Si) crystallites This suggests that high annealing temperature is not suitable for creating a perfect microstructure of the materials, which directly influences on the magnetic properties of the samples Fig displays hysteresis loops of the studied sample before and after heat treatment Amorphous as-quenched specimen exhibits a highly rectangular-shape hysteresis loop indicating that the magnetisation reversal is governed by pinning of domain wall displacement because of high mechanical stress in amorphous structure A quite large coercivity, Hc ¼ 0:20 Oe and low maximum permeability, mmax ¼ 6500 are measured for as-quenched specimen In the annealed specimens, the mechanical stress is reasonably reduced because of the formation of nanostructure of bcc-Fe(Si) crystallites, resulting in a narrow S-shape hysteresis loop of domain wall movement of magnetisation reversal The formation of the bcc-Fe(Si) phase with appropriate volume fraction leads to satisfying the criteria for the formation of ultrasoft magnetic properties Excellent soft magnetic properties are indeed obtained as shown in Fig with a very small coercivity of 0.013 Oe, a very high maximum permeability up to 110,000 at optimum annealing condition (530 C for 40 min) These are better than other ultrasoft Finemet-based alloys studied previously [1–4] As seen from Fig 8, as increasing the time from to 40 min, the volume fraction of the a-Fe(Si) phase raises from 18% to 86% leading to rapid reduction of net saturation magnetostriction of nanocomposite system due to the combination of two phases, whereas the particle size increases very slowly This effect leads to decreasing the coercivity from 0.20 Oe (for as-cast specimen) to 0.015 Oe (annealing in 40 min) In longer annealing time (50 and 60 min), the higher volume fraction and large grain size of the a-Fe(Si) phase make an increase of coercivity Contrary to the variation of the coercivity, as increasing annealing temperature, the permeability increases and goes through a maximum peak and finally decreases when the keeping time is too long (see Fig 8) It is noticed that very high permeability (a detail of magnetic characteristics is also shown in Table 1) was ARTICLE IN PRESS 346 D.-T Ngo et al / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 Fig HRTEM micrographs of the specimens annealed at 530 C (a) and 580 C (b) (in 40 keeping time) Table Volume fraction, coercivity (Hc ), initial permeability (mi ), maximum permeability (mmax ) and saturation induction ðBs Þ as a function of annealing temperature (annealed in 40 min) Asquenched vol% Fe(Si) 0.200 Hc (Oe) mi 1300 mmax 43,000 Bs (kG) 3.0 Fig Hysteresis loops of studied sample: as-quenched specimen and optimally annealed specimen 530 C 540 C 550 C 560 C 570 C 580 C 86% 0.015 22,000 110,000 12.5 87% 0.018 22,500 100,000 12.6 89% 0.022 19,000 90,000 12.3 93% 0.034 13,200 62,800 12.2 95% 0.082 10,000 50,800 12.4 98% 0.120 8500 48,000 12.0 obtained in very low applied field (below Oe) It allows suggesting that the studied material is a suitable candidate for applications of sensitive response e.g sensitive magnetic sensor, small transformation Moreover, at optimally annealed condition, a high saturation induction up to 12.5 kG is measured, slightly higher than that of pure Finemet [1] It is noted that lower annealing temperature (e.g 520 C) had been tested At temperatures lower than 530 C, the crystallisation process became slowly, resulting in a unexpected mechanical hardness of the alloy These temperatures are lower than onset crystallisation temperature of the amorphous phase (530 C, hence, the heat treatment required long keeping times to obtain a proper volume fraction of the a-Fe(Si) phase (e.g 120 to obtain 80% vol of the a-Fe(Si) at 530 C annealing temperature) This causes the alloy to be brittle-fracture and start oxidising slightly Therefore, lower annealing temperature ð o530 CÞ is not interesting so far for tailoring the magnetic properties of the alloy, and annealing at 530 C in 40 (either at vicinity of onset crystallisation temperature) should be considered as optimal condition to obtain the best soft magnetic properties Conclusions Fig Magnetic characteristics coercivity and maximum permeability as a function of annealing time as annealed at 530 C Crystallisation evolution, microstructure and magnetic properties of Finemet-like Fe73.5Si17.5B5Nb3Cu1 nanocomposite material have been systematically investigated Differential scanning calorimetry and thermomagnetic measurements revealed the crystallisation kinetics in the amorphous specimens relating to the crystallisation of the a-Fe(Si) phase with the crystallisation activation energy of 2:81 0:03 eV, which is lower than pure FINEMET containing a lower Si content The crystallisation evolution can be reasonably described by Kissinger’s and John– Mehl–Avrami (JMA) models Diffraction analysis on TEM indicated ARTICLE IN PRESS D.-T Ngo et al / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347 that the lattice parameter of the a-Fe(Si) in the annealed specimens increases by increasing annealing temperature due to the enhancement of diffusion of Si atoms into the crystal lattice of the bcc-Fe By optimally annealing at 530 C in 40 min, excellent soft magnetic properties are obtained with very high permeability of 110,000 and a very low coercivity of 0.015 Oe Ultrasoft magnetic softness made the studied material is suitable for highly sensitive applications such as sensitive magnetic sensor, lowpower transformer Acknowledgements This work is completed by the financial support from the Vietnam Fundamental Research Program for Natural Sciences through the Grant 406506 Authors from Glasgow would like to thank Engineering and Physical Sciences Research Council and Overseas Research Students Awards Scheme (ORSAS) for supporting for the work We would like to express our sincere thanks to 347 Dr S McFadzean and Mr B Miller (Kelvin Nanocharacterisation Centre, University of Glasgow) for their technical support of TEM measurements References [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] N Chau, N.X Chien, N.Q Hoa, P.Q Niem, N.H Luong, N.D Tho, V.V Hiep, J Magn Magn Mater 282 (2004) 174 [3] N Chau, N.Q Hoa, N.D The, L.V Vu, J Magn Magn Mater 303 (2006) e415 [4] E.Y Kang, Y.B Kim, K.Y Kim, Y.H Chung, J Appl Phys 99 (2006) 08F111 [5] G Herzer, IEEE Trans Magn 25 (1989) 3327 [6] H Kissinger, Anal Chem 29 (1957) 1702 [7] M Al-Haj, J Barry, J Mater Sci Lett 16 (1997) 1640 [8] J Bigot, N Lecaude, J.C Perron, C Milan, C Ramiarinjaona, J.F Rialland, J Magn Magn Mater 133 (1994) 299 [9] M.S Leu, T.S Chin, MRS Symp Proc 577 (1999) 557 [10] A.C Hsiao, M.E McHenry, D.E Laughlin, M.R Tamoria, V.G Harris, IEEE Trans Magn 37 (2001) 2236 [11] C.-Y Um, F Johnson, M Simone, J Barrow, M.E McHenry, J Appl Phys 97 (2005) 10F504 [12] F Richter, W Pepperhoff, Arch Eisenhuettenwes 45 (1974) 107 ... is in a paramagnetic state Further increasing temperature to onset crystallisation temperature, the magnetisation drastically increases because of Fig Kissinger plot for determining crystallisation. .. Experiments Ingot alloy with nominal composition of Fe73.5Si17.5B5Nb3Cu1 was initially fabricated by induction melting in high vacuum The sample is slightly modified from pure-Finemet as enriching Si... Differential scanning calorimetric curves of the as-quenched ribbon performed at various heating rates from 10 to 50 K minÀ1 343 crystallisation stages during heating progress It is well-known that the
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