The structural and magnetic properties of Fe/Fe3O4 nanocomposites, synthesized by combined high
energy ball milling and controlled oxidation, have been studied. An X-ray diffraction analysis of the
crystal structure of the nanocomposites confirmed the coexistence of Fe and Fe3O4 phases. An increase of
the oxygen concentration during oxidation process led to the formation of a higher fraction of the Fe3O4
phase with good crystallinity and stoichiometry. The morphology of the nanocomposites revealed a
lamella-like structure with a thickness of about 30 nm. The saturation magnetization decreased when
the phase fraction of Fe3O4 increased. The coercivity was enhanced at low temperatures (100 K) but
decreased at high temperatures, due to thermal fluctuation effects on the anisotropy in the Fe3O4 phase.
Interestingly, the lamellae exhibited a sharp Verwey transition near 120 K, which is often suppressed or
absent in nanostructured Fe3O4 due to the poorly crystalline, off-stoichiometric characteristic. The
temperature dependence of high-field magnetization of the lamellae is analyzed by the modified Bloch
law. Our study demonstrates the possibility of tuning the magnetism in iron/iron oxide nanosystems
through controlled oxidation
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gu
g P
rials, Adv
d Faculty of Applied Sciences, Ton Duc Thang University
e Department of Physics, University of South Florida, Ta
f Institute of Experimental Physics, Slovak Academy of S
al University, Hanoi.
rg/licenses/by/4.0/).
den crystal lattice
bic inverse spinel
n accompanied by
erties. It has been
ear 125 K occurs
stoichiometry [9].
ons from the stoi-
It is not observ-
To the best of our knowledge, only Yamamuro et al. reported on
the structural and magnetic properties for Fe/Fe3O4 lamellae-like
structures, but not on their Verwey transition [4]. Furthermore,
the temperature dependence of magnetization in these materials
was not fully understood. Authors [13e15] reported that the Bloch
law (Ms(T)z T3/2) could be applied well for magnetic nanoparticles
* Corresponding author. Institute of Materials Science, Vietnam Academy of
Science and Technology, 18 Hoang Quoc Viet, Ha noi, Viet Nam.
** Corresponding author.
E-mail addresses: manhdh.ims@gmail.com (H.M. Do), skorvi@saske.sk
(I. Skorvanek).
Contents lists available at ScienceDirect
Journal of Science: Advance
journal homepage: www.el
Journal of Science: Advanced Materials and Devices 5 (2020) 263e269Peer review under responsibility of Vietnam National University, Hanoi.compounds containing Fe3O4 is the presence of Verwey phase able in Fe3O4 nanoparticles with average size below 6 nm [10e12].through controlled oxidation.
© 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam Nation
This is an open access article under the CC BY license (
1. Introduction
Heterogeneous magnetic systems composed of ferromagnetic
and ferrimagnetic phases, such as Fe/Fe3O4, have shown interesting
magnetic properties (exchange coupling, exchange bias, etc.) [1e6]
as well as their potential applications (enhanced specific absorp-
tion, microwave absorption, efficient removal of dyes from waste-
water, etc.) [3,7,8]. One of the interesting phenomena related to
transition (TV), which is widely known as a sud
change from a monoclinic structure to the cu
structure [9e11]. This structural change is ofte
other anomalies in electrical and magnetic prop
reported that the first-order phase transition n
only in bulk and single-crystal Fe3O4 of perfect
The TV tends to be suppressed because deviati
chiometry due to lattice defects, surface effect Verwey phase transition
Modified Bloch law
temperature dependence of high-field magnetization of the lamellae is analyzed by the modified Bloch
law. Our study demonstrates the possibility of tuning the magnetism in iron/iron oxide nanosystemsa r t i c l e i n f o
Article history:
Received 21 February 2020
Received in revised form
24 March 2020
Accepted 3 April 2020
Available online 10 April 2020
Keywords:
Lamellar structure
Fe/Fe3O4 nanocomposites
Oxidation-controlled magnetismhttps://doi.org/10.1016/j.jsamd.2020.04.001
2468-2179/© 2020 The Authors. Publishing services b
( Ho Chi Minh City, Viet Nam
mpa, FL 33620, USA
ciences, Watsonova 47, 040 01 Kosice, Slovakia
a b s t r a c t
The structural and magnetic properties of Fe/Fe3O4 nanocomposites, synthesized by combined high
energy ball milling and controlled oxidation, have been studied. An X-ray diffraction analysis of the
crystal structure of the nanocomposites confirmed the coexistence of Fe and Fe3O4 phases. An increase of
the oxygen concentration during oxidation process led to the formation of a higher fraction of the Fe3O4
phase with good crystallinity and stoichiometry. The morphology of the nanocomposites revealed a
lamella-like structure with a thickness of about 30 nm. The saturation magnetization decreased when
the phase fraction of Fe3O4 increased. The coercivity was enhanced at low temperatures (100 K) but
decreased at high temperatures, due to thermal fluctuation effects on the anisotropy in the Fe3O4 phase.
Interestingly, the lamellae exhibited a sharp Verwey transition near 120 K, which is often suppressed or
absent in nanostructured Fe3O4 due to the poorly crystalline, off-stoichiometric characteristic. Theb Graduate University of Science and Technolog
c Laboratory of Magnetism and Magnetic Mate anced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nama Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Ha Noi, Viet Nam
y, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Viet NamOriginal Article
Oxidation-controlled magnetism and Ve
lamellae
Hung Manh Do a, b, *, Thi Hong Le a, b, Xuan Phuc N
Thi Hong Ngo a, Trung Hieu Nguyen a, Thanh Phon
Jozef Kovac f, Ivan Skorvanek f, **y Elsevier B.V. on behalf of Vietnamey transition in Fe/Fe3O4
yen a, Hong Nam Pham a,
ham c, d, Manh Huong Phan e,
d Materials and Devices
sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license
(aexp)) [21,22]. Extra peaks of g-Fe2O3 were not seen in our XRD
patterns. Besides, the aexp value was estimated to be ~8.396 Å for
both M10 and M30 (see Table 1), which is equivalent to that of bulk
magnetite (8.396 Å) [21]. Moreover, the positions and the relative
intensity ratios of the diffraction peaks of the present samples
match well those of the published results for magnetite [25].
Fig. 1. XRD patterns of the Fe/Fe3O4 samples. The experimental and calculated patterns
are dots and full line, respectively.
Table 1
Average crystalline size (D), experimental lattice constant (aexp) and weight fraction
of Fe and Fe3O4 phases.
Sample D (nm) aexp (Å) % weight
Fe Fe3O4 Fe Fe3O4 Fe Fe3O4
M0 25.5 e 2.863 100 e
M10 62.4 67.5 2.866 8.396 62 38
ced Materials and Devices 5 (2020) 263e269with high TC up to room temperature, whereas authors [16e18]
noticed some deviation from Bloch law in such nanoparticles.
In this paper, we present the results of a detailed study of the
structural and magnetic properties of Fe/Fe3O4 lamellae prepared
by high-energy ball milling and subsequently controlled oxidation.
Following the structural and morphological characterizations, we
have investigated effects of varying Fe3O4 phase fraction on the
magnetic properties of the Fe/Fe3O4 lamellae. The occurrence of a
sharp Verwey transition at ~120 K as well as the temperature
dependence of high-field magnetization are analysed and
discussed.
2. Experimental
2.1. Synthesis of Fe3O4 lamellae
Commercial Fe powder with particle size less than 100 mm
(Merck) was milled in a planetary ball mill (Fritsch P 6) in air using
hardened steel containers and balls. The milling parameters were
used as follows: A ball-to-powder weight ratio of 15:1, a milling
speed of 450 rpm, and a milling time of 2 h (see Ref. [19] for more
details). The as-milled powder was used for controlled oxidation
under (oxygen and nitrogen) gas flow with oxygen of 10 and 30%
volume at 773 K (500 C) for 1 h and then cooled down to room
temperature. The gas sources of oxygen and nitrogenwere obtained
from Singapore Oxygen Air Liquid Pte Ltd and the desired con-
centrations were controlled by the flow-through mixing through
mass-flow-controllers (Aalborg, GFC17) [20]. We labeled M0 for Fe
milled for 2 h, and M10 and M30 for the subsequently oxidized
samples corresponding to the used oxygen concentrations.
2.2. Characterization of the samples
The structural characterization was performed by SIEMENS
D5005 diffractometer using Cu-Ka radiation at l ¼ 0.154 nm. The
diffraction patterns were collected with 2q in the range of 20oe70.
A careful refinement for X ray diffraction (XRD) patterns for sam-
ples have done before evaluating the average crystal size, lattice
constant, and the proportion of the Fe and the Fe3O4 phases by
using a commercial X'Pert HighScore Plus software [15]. The
morphology was studied using scanning electron microscopy
(SEM) and bright field transmission electron microscopy (BFTEM)
techniques with a Hitachi S-4800 equipment. The temperature and
field dependence of magnetization was measured using the
superconducting quantum interference device (SQUID) magne-
tometer MPMS-XL-5 (Quantum Design) in the temperature range
of 5e300 K and in magnetic fields of up to 50 kOe.
3. Results and discussion
3.1. Structural and morphological analysis
The XRD patterns of the samples are shown in Fig. 1. Two peaks
at the 2q values of 44.7 and 65.0 were indexed to body-centered
cubic (bcc) Fe. We did not see any signature of secondary phases
indicating that sample M0 was single phase. All new peaks of
samples M10 and M30 were indexed to magnetite (Fe3O4) and
maghemite (g-Fe2O3) patterns. They were strong and sharp, indi-
cating that the both samples are well crystallized. It is generally
known that the basic structure of geFe2O3 is close to that of Fe3O4,
resulting in appearance of the XRD peaks at the similar positions
[21,22]. There are several approaches that have been used to
distinguish these phases such as neutron diffraction [23] and XRD
methods (extra peaks due to (321) and (221) lattice planes corre-
H.M. Do et al. / Journal of Science: Advan264spond to the g-Fe2O3 [24], or the experimental lattice constants
M30 45.5 52.4 2.868 8.396 57.8 42.2
Therefore, we can assume that the new peaks belong to the major
crystalline phase of Fe3O4 with good crystallinity and stoichiom-
etry. This hypothesis has been further supported by the M(T) data,
as shown in Fig. 5, in which a sharp change in the magnetization
near the TV ~ 120 K appears to observe.
Values of average crystallite size D were evaluated from all
peaks of Fe3O4 and Fe and are summarized in Table 1. It can be seen
in Table 1 that D of the Fe phase in M10 and M30 increased about
two times as compared to M0 due to the annealing at 773 K for 1 h.
The estimated fractions of Fe and Fe3O4 are ~62% and ~38% for M10,
and ~57.8% and ~42.2% for M30, respectively. This finding also
pointed out that the presence of the higher oxygen content helped
accelerate oxidation of Fe, leading to an increasing growth rate of
the Fe3O4 phase. This observation is consistent with reports of Chin
et al., in which the growth rate of Fe3O4 nanosheets was enhanced
by flowing O2 gas into the solution during oxidation [26]. The effect
of the Fe3O4 fraction on the magnetic properties of the Fe/Fe3O4
nanocomposites will be discussed below.
Fig. 2 shows the representative SEM and BFTEM images of M10
andM30. These images revealed a high density of lamella structure.
The edges of the lamellae are irregular and the lengths of the
lamellae are in the range of a micrometer, with thicknesses of about
30 nm. Since the lamellae are relatively thin (in the nanometer
range), they are almost transparent under high-energy electron
beam (see Fig. 2b and d). It was also found that increasing the
amount of oxygen in the oxidation process did not change the
morphology of the samples. It can be noted that completely
different morphologies of the samples indicated in our previous
work [19] and that of this work are due to the different fabrication
3.2. Magnetic properties
Magnetization versus field (MH) measurements were per-
formed on all samples at various temperatures from 5 to 300 K and
shown in Fig. 3. The M(H) curves shown in Fig. 3 are smooth and
shoulderless, irrespective of the presence of two magnetic phases
(Fe and Fe3O4). The existence of shoulders in the M(H) curve in a
dual-phase magnetic system is often derived from the decoupling
of cooperative magnetic moments between two components’
magnetic phases [4,27]. The presently observed smooth and
shoulderless M(H) curves would thus be a signature of relatively
good exchange-coupling between two magnetic phases (Fe and
Fe3O4) in our samples.
The magnetic parameters of the samples at 300 K were deduced
from this figure and summarized in Table 2. As can be seen from
this table, MS decreased from 220 emu/g for M0 to 177 emu/g and
172 emu/g for M10 and M30. The results are fully consistent with
increasing Fe3O4 content due to the controlled oxidation. It should
be noted that theMS of bulk Fe3O4 at room temperature (92 emu/g)
[28] is lower than that of Fe (220 emu/g) (see Table 2). Therefore,
the larger values ofMS obtained for M10 (177 emu/g) and M30 (172
emu/g) relative to bulk Fe3O4 also affirm the presence of both Fe
and Fe3O4 phases in these samples, which is consistent with the
XRD analysis (see Fig. 1 and Table 1).
Fig. 4 shows a comparison of the coercivity as a function of
temperature for the three samples. The coercivity at low temper-
atures (100 K) of M10 and M30 has increased three times as
compared to M0. For these samples, we assume that a thin layer of
Fe3O4 could be formed on to surface of Fe due to oxidation (a
H.M. Do et al. / Journal of Science: Advanced Materials and Devices 5 (2020) 263e269 265conditions. The first is the morphology of the 10 h as-milled Fe,
while the second is that of the Fe/Fe3O4 lamellae (Fe powder was
only milled for 2 h and then annealed at 500 C for 1 h). Besides, the
size of lamellae is larger than their crystal size because a lamellar
contains many different crystals.Fig. 2. SEM and BFTEM morphologies ofthickness of ~30 nm for the Fe/Fe3O4 lamellae as shown in Fig. 2).
The interfacial coupling between Fe and Fe3O4 could be a main
driving force for the enhanced coercivity below 100 K. The authors
[6] stated that Fe3O4 shell shows typically a soft magnetic behavior.
This is different from the g-Fe2O3 shell which exhibits strong spinsamples M10 (a, c) and M30 (b, d).
ced Materials and Devices 5 (2020) 263e269H.M. Do et al. / Journal of Science: Advan266disorder and hence an enhanced magnetic anisotropy [29]. How-
ever, at room temperature, the coercivity decreased and began to
converge (13 Oe, 20 Oe and 21 Oe for M0, M10 and M30, respec-
tively) due to the thermal fluctuations in the shell overcoming the
anisotropy of Fe3O4 [27].
The phase fraction of Fe and Fe3O4 in M10 and M30 could be
determined by comparing the room temperature MS values,
normalized to the total mass of the sample, using the following
equation:
Msðaþ bÞ¼ aMFes þ bMFe3O4s (1)
Fig. 3. M(H) hysteresis loops of M0, M10 and M30 samples taken at indicated tem-
peratures. The insets show a magnified low-field region of the M(H) loops.where a and b are % wt. of Fe and Fe3O4 phases and (a þ b) ¼ 100%,
MFes andM
Fe3O4
s are the saturation magnetization of Fe and Fe3O4.
From the values of MS at 300 K, the weight fractions of the Fe3O4
phase was determined to be 32.3% wt. and 36.9% wt. (see Table 2).
These results are relatively consistent with that derived from the
XRD data.
Fig. 4. Temperature dependent coercivity for samples M10 and M30.
along their respective easy axes, when the magnetocrystalline
anisotropy of Fe3O4 becomes dominant at low field, derived from
the ferrimagnetic shell region.
The temperature dependence of magnetization in heteroge-
neous magnetic systems, including core/shell nanoparticles of
Fe3O4/g-Fe2O3 and Fe3O4/a-Fe2O3 [31,32], has been reported,
indicating the important roles of surface and interface spins. As
discussed in the previous section, the magnetic phases in M10
and M30 are expected to be exchange coupled. Therefore, the
change of high field magnetization in the temperature range of
5e300 K for these samples was also investigated. The tempera-
ture dependence of magnetization well below the Curie tem-
perature for most ferromagnetic materials is well described by
Bloch law [14]:
MSðTÞ¼MSð0Þ
h
1BT3=2
i
ðfor T << TCÞ (2)
ced Materials and Devices 5 (2020) 263e269 267H.M. Do et al. / Journal of Science: AdvanTemperature dependence of field-cooled magnetization, MFC(T)
in an applied field of 100 Oe is shown in Fig. 5 for M10 andM30. The
MFC(T) curves show a sharp change of magnetization at ~120 K that
coincides well with TV of Fe3O4 [9,30]. So far, it is well known that
TV disappears due to loss of crystal quality (imperfect stoichiometry
or/and poor crystallinity) in bulk, single crystals of Fe3O4 [9e11].
This is the first time that the Verwey transition has been observed
in a Fe/Fe3O4 nanocomposite system with a lamellar structure. It
also implies that the Fe3O4 phase has a good crystallinity and
stoichiometry, which is consistent with both the XRD and SEM
analyses.
M30 sample with the higher phase fraction of Fe3O4 exhibits a
sharper change in magnetization at ~120 K than that of M10 (see
Fig. 5). We can define “jump” at low field DM ¼ Mtop e Mbottom at
120 K from the M(T) curve. The DM values are 0.8 emu/g and 1.2
emu/g coresponding to 14% and 23% of the overall magnetization
for FC conditions of M10 and M30. Ong et al. [6] believed that the
low-field jumps are due to a sudden reorientation of moments
where B is a constant, which is related to the exchange integral Je
(B ~ 1/Je 3/2) and MS(0) is the saturation magnetization at zero
temperature. However, theoretical calculations and experimental
results [16e18] have reported that the temperature dependence of
magnetization for small ferromagnetic clusters and nanoparticles is
given by a modified Bloch law:
MSðTÞ¼MSð0Þ½1BTa (3)
Fig. 5. Magnetization changes as a function of temperature under field-cooled con-
dition for samples M10 and M30. Field applied during the measurement was 100 Oe.
Table 2
The saturation magnetization (MS) and coercivity (HC) of the samples at 300 K. The
fraction of Fe and Fe3O4 phases calculated from Eq. (1) in the text.
Sample MS (emu/g) HC (Oe) % wt.
Fe Fe3O4
M0 220 13 100 e
M10 177 20 67.7 32.3
M30 172 21 63.1 36.9Fig. 6. Temperature dependence of saturation magnetization for M10 and M30. Solid
curves are the best-fit curves to the experiment data with error bars of near 1% using
Eq. (3) described in the text.
ced MSince the magnetization tends to saturate at ~50 kOe, we can
consider that MS(T) ¼ M(T, H ¼ 50 kOe). Then, the temperature
dependence of MS data was fitted by using Eq. (3). The best-fit
curves were obtained, as presented in Fig. 6 (solid curve). It
points out that MS(T) is indeed well described by a Ta law in the
whole experimental temperature range (5 K < T < 300 K). The a
values were 1.74 and 1.72 for M10 and M30, respectively. These
results are different from those for single-phase ferromagnetic
nanoparticle systems with high TC [14,15] in which the Bloch law
could be applied well up to room temperature. However, authors
[31] also reported some deviation from the original Bloch's law for
Fe3O4/g-Fe2O3 ferrimagnetic/ferrimagnetic core/shell nano-
particles, where the surface and interface have contributed to the
magnetization effectively.
The deviation from the Bloch T3/2 law of MS(T) lamellae is
smaller for the Fe/Fe3O4 ferromagnetic/ferrimagnetic lamellae than
for the Fe3O4/g-Fe2O3 core/shell nanoparticles. This result is
reasonable because the lamellae have a magnetic volume larger
than that of Fe3O4/g-Fe2O3 core/shell nanoparticles leading to the
smaller contributions of surface and interface to MS and its tem-
perature dependence.
4. Conclusion
In summary, the Fe/Fe3O4 lamellae with a thickness of about
30 nm have been successfully synthesized by high energy ball
milling and subsequently oxidized via controlling oxygen con-
centration at 500 C for 1 h. The Fe3O4 phase in this structure
has good crystallinity and stoichiometry and its content in-
creases with oxygen concentration during oxidation. The
magnetic properties of the Fe/Fe3O4 lamellae were systemati-
cally studied. The Verwey transition in these Fe/Fe3O4 lamellae
was observed for the first time. The saturation magnetization
decreased as the phase fraction of Fe3O4 increased, whereas the
Verwey transition became sharper. The temperature depen-
dence of the saturation magnetization in the Fe/Fe3O4 lamellae
could be predicted by the modified Bloch law i