DSpace at VNU: Crystallisation progress in Si-rich ultra-soft nanocomposite alloy fabricated by melt spinning

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Journal of Magnetism and Magnetic Materials 322 (2010) 342–347

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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
7 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 3 C. Further heating the sample leads to a transformation from the a-Fe(Si) to Fe-B
phases at 670 3 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 7 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 3 C in 40 min.
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

1. 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.-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


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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.

2. 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 9 to
5 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 1 cm width and
16 mm thick. The ribbons were isothermally annealed in Ar
atmosphere at various temperatures ranging from 530 to 580 3 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.

3. 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. 1. The DSC curves
exhibit two clearly exothermal peaks, corresponding to the

Fig. 1. Differential scanning calorimetric curves of the as-quenched ribbon
performed at various heating rates from 10 to 50 K minÀ1 .

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crystallisation stages during heating progress. It is well-known
that the first peak, occurring around 530 3 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 3 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 3 C, confirming the transformation from a-Fe(Si) to Fe3B.
As seen from Fig. 1 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. 2 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 3 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 3 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. 2. Kissinger plot for determining crystallisation activation energy for
amorphous specimens.


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D.-T. Ngo et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347

Fig. 3. 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 ðtÞ ¼ 1 À eÀðktÞ

ð2Þ

where k is the rate coefficient and n is the morphology index. The
rate coefficient, which is a temperature-dependent factor, is given
by
kðTÞ ¼ k0 eÀEa =RT

ð3Þ

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. 4 illustrates the volume fraction of the a-Fe(Si) phase as a
function of annealing time as isothermally annealed at 530 3 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 Â 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 3 C, the crystallisation activation

Fig. 4. Volume fraction of the a-Fe(Si) phase as a function of annealing time (at
530 3 C annealing temperature). The inset shows an in situ measurement of
magnetisation dependent on the time at 530 3 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 7 0:07 eV.
The magnetisation evolution at onset crystallisation temperature, 530 3 C, is depicted in the inset of Fig. 4. In this measurement,
an amorphous specimen was heated to 530 3 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. 4 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 ¼ Xf MFeðSiÞ þ ð1 À Xf ÞMamor

ð4Þ

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 MFeðSiÞ ¼ MFeðSiÞ ½1 À eÀðktÞ Š

ð5Þ

Eq. (5) expressed the time dependence of the magnetisation as
shown in the inset of Fig. 4. Grain structure of the materials was
determined by a transmission electron microscope using dark-


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Fig. 5. Bright-field TEM micrographs of the specimens annealed at 530 3 C (a), 560 3 C (b), 580 3 C (c) (in 40 min 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. 5 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 3 C,
mean grain size is 12.5 nm, and rising to 14.0 nm when annealing
temperature is 560 3 C. And at 580 3 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 3 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 3 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 3 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. 7 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. 8 with a very small coercivity
of 0.013 Oe, a very high maximum permeability up to 110,000 at
optimum annealing condition (530 3 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 5 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


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D.-T. Ngo et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 342–347

Fig. 6. HRTEM micrographs of the specimens annealed at 530 3 C (a) and 580 3 C (b) (in 40 min keeping time).

Table 1
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
0.200

Hc (Oe)
mi
1300
mmax
43,000
Bs (kG)
3.0

Fig. 7. Hysteresis loops of studied sample: as-quenched specimen and optimally
annealed specimen.

530 3 C

540 3 C

550 3 C

560 3 C

570 3 C

580 3 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 1 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 3 C) had been tested. At
temperatures lower than 530 3 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 3 C,
hence, the heat treatment required long keeping times to obtain
a proper volume fraction of the a-Fe(Si) phase (e.g. 120 min to
obtain 80% vol. of the a-Fe(Si) at 530 3 C annealing temperature).
This causes the alloy to be brittle-fracture and start oxidising
slightly. Therefore, lower annealing temperature ð o530 3 CÞ is not
interesting so far for tailoring the magnetic properties of the alloy,
and annealing at 530 3 C in 40 min (either at vicinity of onset
crystallisation temperature) should be considered as optimal
condition to obtain the best soft magnetic properties.

4. Conclusions

Fig. 8. Magnetic characteristics coercivity and maximum permeability as a
function of annealing time as annealed at 530 3 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 7 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


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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 3 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.

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