DSpace at VNU: Effect of annealing on the microstructure and magnetic properties of Fe-based nanocomposite materials

Composites: Part A 37 (2006) 191–196
www.elsevier.com/locate/compositesa

Effect of annealing on the microstructure and magnetic properties
of Fe-based nanocomposite materials
Manh-Huong Phana,*, Hua-Xin Penga, Michael R. Wisnoma, Seong-Cho Yub, Nguyen Chauc
a

Department of Aerospace Engineering, Bristol University, Queen’s Building, University Walk, Bristol BS8 1TR, UK
b
Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea
c
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
Received 4 October 2004; revised 8 January 2005; accepted 11 January 2005

Abstract
The influence of annealing on microstructure, magnetic properties including the giant magnetoimpedance (GMI) effect of a Fe-based
nanocomposite has been investigated. The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix
was attained by annealing the Fe-based amorphous alloy prepared by rapid quenching method. The GMI profiles were measured for samples
annealed at different temperatures ranging from 350 to 650 8C in vacuum and for 30 min. It is found that the mean grain size of the a-Fe(Si)
crystallites in the order of 12 nm remains almost unchanged until the annealing temperature reached 540 8C. A decrease of anisotropy field
and an increase of GMI with increasing annealing temperature up to 540 8C were observed and ascribed to the increase of the magnetic
permeability and the decrease of the coercivity, whereas the opposite tendency was found for the sample annealed above 600 8C which is
likely due to the microstructural change caused by high-temperature annealing. This indicates that variation in the magnetic characteristic of
the amorphous phase upon annealing changed the intergrain exchange coupling. This altered both the magnetic softness and the effective
anisotropy and consequently modified the GMI features. The study of the temperature dependence of the GMI effect provides further
understanding of the magnetic exchange between these crystallized grains through the amorphous boundaries in Fe-based nanocrystalline
materials.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: B. Anisotropy; Magnetoimpedance

1. Introduction
Recent advances in magnetic sensing applications,
especially in the high-density magnetic recording technology, has benefited from the discovery of new magnetic
materials with amorphous structure [1–3]. In contrast to
crystalline magnetic materials where the periodicity of
constituent atoms plays an essential part, in an amorphous
substance, atoms are distributed randomly, taking a
topologically disordered structure. The absence of crystal
structure (i.e. the presence of a short range order and the
absence of a long range order) leads to superior properties

* Corresponding author. Tel.: C44 783 823 2277; fax: C44 117 927
2771.
E-mail address: (M.-H. Phan).

1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2005.01.033

(e.g. mechanical, chemical, electrical, and magnetic properties) observed in these materials.
It is known that the absence of magnetocrystalline
anisotropy and grain boundaries in an amorphous magnetic
material results in excellent soft magnetic properties (e.g.
high magnetic permeability and saturation induction), high
electrical resistivity leading to small eddy current losses,
high hardness and stiffness etc. Importantly, a variety of
properties can be achieved by the applications of external
parameters (e.g. magnetic field, pressure, temperature, etc.)
and/or by controlling the fabrication processes [1]. The
combined magnetic, electrical, mechanical and chemical
properties are making an amorphous magnetic material the

most promising candidate material for many engineering
applications [3].
In view of the existing materials, the discovery of
Finemet-type nanocomposite magnetic materials with a
composition of Fe 73.5Si13.5B9Cu1Nb3 provided some
insights into the science and technology of soft magnetic


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materials [4–8]. This kind of materials, routinely obtained
by an appropriate heat treatment of an amorphous precursor,
exhibits excellent magnetic properties due to its unique
microstructure, namely, ultrafine nanocrystalline a-Fe(Si)
grains embedded in an amorphous matrix [6]. This is
directly depending on the magnetic exchange coupling
between the grains through the amorphous boundaries
[9,10]. However, the underlying physical mechanism of the
magnetic exchange coupling in a nanocrystalline magnetic
material is not well understood.
Fortunately, a number of recent studies on the giant
magnetoimpedance (GMI) effect in Fe-based amorphous
soft magnetic alloys subjected to heat treatment showed
some insights into the nature of the magnetic exchange
coupling between these grains through the amorphous
boundaries in Fe-based nanocrystalline materials [7,8,11–
13]. Because of the fact that the two-phase Fe-based
nanocomposite material has two distinct Curie temperatures, one for the nanocrystalline grains and the other

belonging to the amorphous phase, the roles of the two
magnetic phases in the intergrain magnetic coupling can be
taken apart in a sufficiently high temperature region.
In this context, the study of the temperature dependence
of the magnetic properties and the GMI effect in such a Febased nanocomposite material composed of a nanocrystalline phase in an amorphous matrix can be of significant
importance in gaining more rudimentary insights into the
nature of the magnetic coupling in the material. This paper
reports the effect of annealing on the structural and magnetic
properties and the GMI effect in a Fe73.5Si15.5Nb3Cu1B7
amorphous alloy.

2. Experiment
Fe73.5Si15.5Nb3Cu1B7 ribbons with a width of 4 mm and
a thickness of 20 mm were prepared by rapid quenching
method. The nanocomposite materials composed of a
nanocrystalline phase in an amorphous matrix were
obtained by annealing these as-quenched amorphous
ribbons at different temperatures ranging between 350 and
650 8C for 30 min in vacuum. The structures of the asquenched amorphous ribbons and the annealed ones were
examined by X-ray diffraction (XRD). Differential scanning
calorimeter (DSC) measurements on as-cast and annealed
ribbons were conducted with increasing temperature at a
rate of 20 8C/min in Ar atmosphere. Accordingly, the
crystallization processes can be monitored by DSC.
Transmission electron microscopy (TEM) images of the
nanocrystallized ribbons have been obtained for samples
thinned by using a Philips C30 ion etching device. The M–H
hysteresis loops were measured using a vibrating sample
magnetometer (VSM). Magneto-impedance (MI) measurements were carried out along the ribbon axis with the
longitudinal applied magnetic field. The samples with a

length of about 15 mm were used for all MI measurements.

Details on a MI measurements system can be found
elsewhere [14].

3. Results and discussion
3.1. Microstructural analyses
Fig. 1 shows the XRD pattern of the Fe-based asquenched amorphous ribbon. It is clear that the pattern
exhibited only one broad peak around 2qZ458, which is
often known as a diffuse halo, indicating that the sample
prepared is amorphous. No indication of presence of
crystallites was observed by TEM. This reflects the absence
of crystal structure, i.e. the absence of a long range order.
To find out a proper annealing regime for as-quenched
amorphous ribbon samples, we carried out DSC measurements. Typical DSC curves for the as-cast and 540 8Cannealed ribbons are shown in Fig. 2. It is easy to see clearly
from Fig. 2 that for the as-cast sample the curve has a typical
behavior with the two mainly exothermic peaks; the first
exothermic peak (first peak at w550 8C) is attributed to the
primary crystallization of the nanocrystalline phase (e.g. the
a-Fe(Si) soft magnetic phase) while the second one is
attributed either to the further crystallization of the
remaining amorphous phase, or to phase transformation of
existing metastable phases, such as Fe3B, following the
primary crystallization. In the case of the annealed sample,
the crystallization peaks shifted to a higher temperature due
to a significant contribution of nucleation. It was also found
that the first peak disappeared for the sample annealed at
650 8C for 30 min, indicating a full crystallization state.
Based on the DSC results, as-cast amorphous alloys were
annealed at different temperatures ranging between 350 and

650 8C for 30 min in vacuum to achieve the nanocrystalline
materials with a-Fe(Si) phase.
Furthermore, it is known that the crystallization fraction
determines magnetostriction of the ribbon while the grain

Fig. 1. The X-ray diffraction pattern of the Fe73.5Si15.5Nb3Cu1B7 as-cast
amorphous alloy.


M.-H. Phan et al. / Composites: Part A 37 (2006) 191–196

193

Fig. 2. DSC curves for Fe73.5Si15.5Nb3Cu1B7 ribbons (as-cast and annealed
at 540 8C for 30 min).

size determines the magnitude of the effective magnetic
anisotropy. Both magnetostriction and effective magnetic
anisotropy play a decisive role in the soft magnetic
properties of nanocrystalline magnetic materials. It is
therefore necessary to evaluate the crystallization fraction
of the sample after annealing. Recently, Leu and Chin [15]
have first proposed the method that allows one to evaluate
the crystallization fraction (cf) from the DSC diagram,
which is expressed by:
cf Z

DHa KDHt
;
DHa

(1)

where DHa and DHt are the crystallization enthalpy of the
as-cast amorphous ribbon and the ribbon annealed for a time
t, respectively. An example is also shown in Fig. 2, where
the crystallization fraction of the sample annealed at 540 8C
for 30 min, corresponding to the a-Fe(Si) phase at the first
peak, reaches a value of 82%. We have found that the
amorphous sample became fully crystallized (cfZ100%)
when annealed above 650 8C. It should, however, be noted
that the soft magnetic property may be degraded by
excessive crystallization. Because, for annealing over
650 8C, the BCC crystallites will grow, and large crystallites
lead to the decoupling of magnetic exchange, and
consequently the good soft magnetic properties are lost.
To further scrutinize this feature, the structure of the
amorphous samples after annealing was examined by XRD
and TEM [see Fig. 3, for example]. After the thermal
treatments the XRD peaks of a-Fe(Si) are seen to emerge
from the amorphous halos. The relative intensity of the
various peaks indicates that there is no preferred orientation
in the crystallized phase. Furthermore, the mean grain size
(t) of a-Fe(Si) was determined according to the Scherrer
expression [16]:
0:96l
tZ
;
B cos q

(2)

Fig. 3. X-ray diffraction pattern (in the upper panel) and TEM image (in the
lower panel) for the Fe73.5Si15.5Nb3Cu1B7 amorphous alloy annealed at
540 8C for 30 min.

˚ ), q is the
where l is the X-ray wavelength (lZ1.54056 A
diffraction angle, and B is the full width at half maximum
(FWHM).
In Fig. 4 we display the annealing-temperature dependence of the mean grain size of the bcc phase estimated from
broadening its relation to the peak in the X-ray diffraction
patterns using Eq. (2). It should be noted that the growth of
a-Fe(Si) is controlled by the slow diffusion of Nb and Cu
which leads to a nanocrystalline structure. As can be seen

Fig. 4. The annealing temperature dependence of the mean grain size of the
bcc phase estimated from the broadening of the relations in the X-ray
diffraction patterns using Eq. (2).


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clearly from Fig. 4, the mean grain size is about 12 nm and it
remains almost constant until the annealing temperature
reaches 540 8C. This indicates that the primary crystallization (the first peak of the DSC curve in Fig. 2) is actually
the formation of nanocrystalline structure, where a-Fe(Si)
grains are embedded in an amorphous matrix [see the TEM

image in Fig. 3(b)]. Annealing at higher temperatures not
only leads to grain growth of a-Fe(Si), but also additional
phases are formed from the amorphous matrix phase at Ta Z
650 8C. This second stage of crystallization corresponds to
the second peak in the DSC curve (Fig. 2) and boride phases
(e.g. Fe3B and Fe2B) are found.
3.2. Magnetic characteristics
The crystallization kinetics of the ribbons can be
observed by measurements of thermomagnetic curve, as
shown in Fig. 5. It is clear to see from this figure that the
ribbon is amorphous at room temperature. As the
temperature increases, the magnetization is abruptly
reduced marking the Curie temperature (TC) of the
amorphous phase. With further increasing temperature, the
magnetization is small and constant over a large temperature interval up to a region where crystallization of a-Fe(Si)
leads to an increase of the magnetization. The increase of
the magnetization at the crystallization onset (w500 8C
seen in the DSC curve of Fig. 2) indicates the formation of
some crystalline magnetic phase(s). On returning from high
temperature, a large amount of a-Fe(Si) grains are crystallized in the sample and this leads to a strong increase of the
magnetization below the TC of a-Fe(Si) [see the curve 2 in
Fig. 5]. This reflects that any variation in the magnetic
nature of the amorphous phase could change the intergrain
exchange coupling and consequently the magnetic softness
of the nanocomposite material.
In order to further evaluate influences of annealing on the
magnetic properties, we measured hysteresis loops and the
annealing-temperature dependence of the coercivity (Hc) is
displayed in Fig. 6. It is clear that the coercivity decreased

Fig. 6. The coercive force (Hc) as a function of annealing temperature for
Fe73.5Si15.5Nb3Cu1B7 alloys annealed for 30 min.

with increasing annealing temperature (Ta) up to 540 8C
and then increased at higher temperatures. This can be
interpreted as following: the gradual decrease of Hc at Ta
well below the onset crystallization temperature (i.e.
w500 8C, see Fig. 2) is a result of structural relaxation,
while the drop of Hc in the temperature range of the first
crystallization stage (w540 8C) is likely due to the
appearance of nanosized a-Fe(Si) grains where magnetocrystalline anisotropies are averaged out. Annealing over
540 8C caused a rapid increase of Hc, indicating a large
degradation of the soft magnetic properties. This coincides
well with microstructural change (i.e. the abrupt increase of
the mean grain size for annealing above 540 8C as seen in
Fig. 4). In this case, the increase of nanoparticles size can
considerably reduce the magnetic exchange coupling in the
nanocrystalline material [9]. Furthermore, it is found that
the change of Hc with annealing temperature is correlated
well to the temperature dependence of the permeability,
where the permeability resulting from the rotational
magnetization increased with increasing annealing temperature up to 540 8C and then decreased at higher
temperatures [13]. It is known that the permeability is
inversely proportional to the coercivity in the temperature
range investigated.
3.3. Magnetoimpedance analyses

Fig. 5. Thermomagnetic curves of the Fe73.5Si15.5Nb3Cu1B7 amorphous
alloy: (1) heating cycle and (2) cooling cycle.

The magnetoimpedance ratio DZ/Z can be defined as
DZ=Zð%ÞZ ZðHÞ=ZðHmax ÞK1 where Hmax is the external
magnetic field sufficient to saturate the impedance and
equals to 150 Oe in the present study. The GMI profiles of
the amorphous samples annealed at different temperatures
ranging between 350 and 650 8C were measured and used
to assess the anisotropy field.
As reported in Ref. [17], the contribution of the
transverse permeability to GMI from magnetization rotation
becomes dominant in the high frequency range (w10 MHz)
and a simple single-domain model was proposed. According
to this model, the width of measured GMI peak could reflect


M.-H. Phan et al. / Composites: Part A 37 (2006) 191–196

Fig. 7. GMI profile at fZ10 MHz in Fe73.5Si15.5Nb3Cu1B7 as-cast and
annealed alloys.

the distribution of anisotropy field. As shown in Fig. 7, at a
fixed frequency of 10 MHz, the anisotropy field and GMI
change sensitively with annealing temperature. This also
implies that the permeability in the transverse direction
changes sensitively with annealing temperature [13]. The
changes in anisotropy field (Hk, as depicted in Fig. 7) and
the magnitude of GMI [DZ/Z(%)] are plotted as a function
of the annealing temperature in Fig. 8. It is clear that, with
increasing temperature up to 540 8C, a decrease of the
anisotropy field and an increase of GMI were observed, but
an opposite tendency was found when the annealing

temperature exceeded 600 8C. This is respectively related
to the increase of magnetic softness and the microstructural
change of the sample as the annealing temperature is
increased, as discussed in Sections 3.1 and 3.2. These results
also coincided with the annealing-temperature dependence
of the magnetostriction saturation and the effective
anisotropy constant evaluated by separate magnetization
measurements [4–8].
Now let us discuss the interaction between the magnetic
properties and the GMI effect in the Fe-based amorphous
alloy upon annealing by considering a two-phase random
anisotropy model [9]. Within the framework of this model,
the nanocrystalline grains in nanocrystalline alloys are

Fig. 8. Variation of the anisotropy field and magnitude of GMI with
annealing temperature.

195

strongly coupled through magnetic exchange interactions,
and the local magnetocrystalline anisotropies of grains are
averaged out. Meanwhile, the intergranular amorphous
phase plays an indispensable role, because, only through
it, can the exchange coupling be conveyed. Thus, any
variation in the magnetic nature of the amorphous phase will
consequently change the intergrain exchange coupling, then
alter the magnetic softness and the effective anisotropy, and
finally modify the GMI features. Here, we assume that, for
an amorphous alloy (i.e. as-cast state), the amorphous phase
is ferromagnetic and maintaining the exchange coupling. As

the amorphous sample was annealed at a temperature close
to the crystallization temperature of the soft magnetic phase
of a-Fe(Si), a combination of stress release and the
magnetocrystalline anisotropy decrease in the amorphous
phase further softens the ribbon magnetically, thus enhance
the GMI effect. In the present work, the optimal GMI profile
was observed for the alloy annealed at 540 8C, as a result of
the largest increase in magnetic softness (i.e. the largest
permeability and the smallest coercivty as seen in Fig. 6).
The annealing of amorphous ribbons drastically reduced the
coercive force and increased the effective magnetic
permeability, thus resulted in an increase in GMI effect.
When annealing temperatures were relatively high, e.g. over
600 8C, and close to the crystallization temperature of the
hard magnetic Fe-B phase (725 8C, see Fig. 2), annealing
may damage the soft magnetic phase of a-Fe(Si) and cause a
ferromagnetic to paramagnetic transition, the material
becomes incapacitated in conveying the intergrain magnetic
exchange coupling, an overwhelming decrease in GMI was
observed.

4. Conclusions
A thorough study of the effect of annealing on structural
and magnetic properties and the giant magnetoimpedance
effect in the Fe73.5Si15.5Nb3Cu1B7 amorphous alloy has
been made. It is found that the mean grain size of the aFe(Si) crystallites in the order of 12 nm remains almost
unchanged until the annealing temperature reached 540 8C.
The decrease of anisotropy field and the increase of GMI
with increasing annealing temperature up to 540 8C were
observed and ascribed to the increase of the magnetic

permeability and the decrease of the coercivity, whereas the
opposite tendency was found for the sample annealed above
600 8C which is likely due to the microstructural change
caused by high-temperature annealing. This indicates that
variation in the magnetic characteristic of the amorphous
phase upon annealing changed the intergrain exchange
coupling. This altered both the magnetic softness and the
effective anisotropy and consequently modified the GMI
features. It is proposed that the temperature-dependent GMI
profile is useful to further understand the magnetic exchange
coupling between these grains through the amorphous
boundaries in Fe-based nanocrystalline materials.


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M.-H. Phan et al. / Composites: Part A 37 (2006) 191–196

Acknowledgements
The authors wish to acknowledge the Scientific
cooperation between UK, Korea and Vietnam. Research at
Chungbuk National University supported by the Korean
Science and Engineering Foundation through the Research
Center for Advance Magnetic Materials at Chungnam
National University. Research at Center for Materials
Science was supported by the Vietnam National Program
for Fundamental Research Grant No. 420110.

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