Mechanochemical processing of Zn-ferrite powders and their magnetic characterization

Article abstract of DOI:10.1016/j.jallcom.2003.09.075

Powder mixture of magnetite (Fe₃O₄) and elemental zinc (Zn) was high energy ball milled looking forward to synthesize a composite material able to enhance magnetic coercivity (H_c). Results revealed up to 449.1Oe for powder mixture treated for 540min milling. Specific saturation magnetization (σ_s) of 50.35 and 43.7emu/g was respectively reached in samples ball milled for 300 and 540min. Moreover, specific remanent magnetization (σ_r) of 3.976 and 9.507emu/g was respectively obtained for samples ball milled for 60 and 540min. It is speculated that the degree of magnetization developed on studied samples might be influenced from the metallic-contamination derived from milling wear media such as Fe, Cr and Ni (stainless steel). The Zn-ferrite powder mixture processing and its preliminary magnetization results are discussed.

Full text of DOI:10.1016/j.jallcom.2003.09.075

Journal of Alloys and Compounds 369 (2004) 144–147
Mechanochemical processing of Zn–ferrite powders
and their magnetic characterization
a,b,∗
a
a
J.P. Muñoz Mendoza
, O.E. Ayala Valenzuela , V. Corral Flores ,
a
a
J. Matutes Aquino , S.D. De la Torre
a
Centro de Inv. Materiales Avanzados, CIMAV, M. de Cervantes 120, CP 31109 Chih, Mexico
b
Instituto Tecnológico de Chihuahua, ITCH, Av. Tecnológico, Chih, Mexico
Abstract
Powder mixture of magnetite (Fe
material able to enhance magnetic coercivity (H
saturation magnetization (σ ) of 50.35 and 43.7 emu/g was respectively reached in samples ball milled for 300 and 540 min. Moreover, specific
remanent magnetization (σ ) of 3.976 and 9.507 emu/g was respectively obtained for samples ball milled for 60 and 540 min. It is speculated
3
O
4
) and elemental zinc (Zn) was high energy ball milled looking forward to synthesize a composite
c
). Results revealed up to 449.1 Oe for powder mixture treated for 540 min milling. Specific
s
r
that the degree of magnetization developed on studied samples might be influenced from the metallic-contamination derived from milling
wear media such as Fe, Cr and Ni (stainless steel). The Zn–ferrite powder mixture processing and its preliminary magnetization results are
discussed.
©
2003 Elsevier B.V. All rights reserved.
Keywords: Magnetite; Mechanochemistry; Zinc–ferrites; MA
1
. Introduction
which are difficult to synthesize by using conventional tech-
niques.
The mechanical alloying (MA) technique might be under-
The reducing effect of zinc with magnetite, processed
using the MA technique has been previously studied by other
researchers [1,2]. Apart from understanding the particle size
refinement effect of the studied material, in this work it
has been verified the reducing effect of Zn on magnetite as
a function of the magnetic properties. Thus, the following
chemical reaction:
stood as a solid state reaction process, by which a powder
mixture is alloyed throughout a series of high energy col-
lisions occurring between milling media (balls) and pow-
der particles, under a given atmosphere (usually inert) [9].
During the MA process powder particles are repeatedly de-
formed, fractured and cold-welded. The sort of milling de-
vice used typically depends on the final product features
needed. Thus, either high or low energy mills can be used for
a variety of precursor and product powders. Parallel to the
particle size reduction and therefore creation of new surface
area, both the open surface and bulk of the powder particles
are mechanically energized up to the point of conferring the
powder mixture with a high chemical reactivity apart from
an atomic diffusion enhancement. Such phenomena might
result in generation of materials, which thermodynamically
are found out of equilibrium, such as solid solutions, in-
termetallics, amorphous, and/or unique composite materials
Fe3O4 + 4Zn → 3Fe + 4ZnO
proceeds in such a way that the appearance of francolinite
(Fe3O4–Zn) gradually occurs along the MA action, contin-
uing up to the point that iron oxide is finally metal-reduced
whereas zinc oxidized. Free energy versus temperature
(Ellingham) diagrams [4] indicate that elemental zinc may
reduce the iron present in magnetite to fulfill the above reac-
tion from left to right, but not the opposite. The conventional
casting routes for inducing the above reaction, however,
might lead to a microstructure fairly different from that at-
tainable by mechano-chemical routes. The finally developed
microstructure in the material influences its ultimate proper-
ties. The aim of this work is to analyze the magnetic behavior
of the mechano-chemically processed Zn–ferrite powders.
∗
Corresponding author.
E-mail address: sebastian.diaz@cimav.edu.mx (J.P. Muñoz Mendoza).
0
925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.09.075

J.P. Muñoz Mendoza et al. / Journal of Alloys and Compounds 369 (2004) 144–147
145
2
. Experimental
Magnetite and zinc powders were processed under inert
gas atmosphere (Ar), using a SPEX 8000 mill. Used pre-
cursor raw materials were commercial Fe3O4 (97% pure)
and Zn (99.9% pure) powders. The pot mill and the milling
balls were made of stainless steel. The material/balls ratio
was set to 59, 54/2, 634 and 22.6. This was decided consid-
ering that a smaller ratio would cause a less efficient par-
ticle diminution as well as an increase of the milling time.
Milling times were set to 60, 180, 300 and 540 min from
which four samples were eventually selected for analysis.
Obtained specimens were characterized using some experi-
mental techniques such as X-ray diffraction (XRD), atomic
absorption (AA) and magnetic hysteresis loops. Hysteresis
loops were measured using a LDJ vibration magnetometer
through a maximum applied field of 16,000 Oe. The real cal-
culus of the specific remanent magnetization was omitted
for all hysteresis loops shown in this work. This was due to
a mathematical correction done to fit our measuring system
involving the specimen holder.
Fig. 2. XRD patterns of powder mixture in Fig. 1 after being milled at
indicated times (min).
ated to another unidentified crystalline phase. For this later
◦
◦
specimen, two new peaks diffracting at 44 and 61 were
visible indicating the presence of elemental Fe, as well as
◦
◦
Fe3O4 [1]. Similarly, two other peaks found at 73 and 76
3
. Results and discussion
indicate that Fe3O4 is left in the powder product. Elemental
◦
Zn is revealed from reduction of the peaks intensity at 31.5
3
.1. XRD
◦
and 56 ZnO is also evident from its main diffraction peak
◦
occurred at around 36.5 .
Figs. 1 and 2 correspond to the XRD patterns of magnetite
and zinc precursor powders and its stoichiometric mixture
after being treated for several milling times, respectively.
These figures reveal that main diffraction peaks of precur-
sors tend to shorten and meld as the milling time rose. Al-
though with some 2θ angle deviation, nearly similar shape
of patterns was obtained for the 60–300 min milled speci-
mens. This suggests the gradual lattice network-combination
of reactants to form francolinite. Exception is powder sam-
ple obtained after 540 min where the XRD pattern is associ-
3
.2. Metallic contamination of studied powder
mixture
The milling action of the precursor powders, carried
out using stainless steel media ended up with a sort of
elemental-metallic contamination, which was monitored
using atomic absorption analysis is reported in Fig. 3.
Not only the concentration of iron but also that of
chromium was found to rise as the milling time increased.
For magnetic effect purposes, however, Fe plays a stronger
effect. The reduction of zinc, on the other hand, is due to
the fact that it oxidizes as ZnO.
3.3. Magnetic analysis
Hysteresis loops shown Fig. 4 corresponds to the obtained
from powder samples milled for 540, 300, 180 and 60 min
and the original mixture of zinc and magnetite before the
milling operations. In this figure, both the coercitivity rise
and hysteresis loop area are seen to enlarge as the milling
time rose. The appearance of hysteresis loops lightly tilted
to the x-axis is due to the omission in calculating specific
remanent magnetization.
It is highly probable that the gradual incorporation of Fe
into the powder product due to erosion of the milling media
has enhanced Hc making the material harder magnetically
speaking.
Fig. 1. XRD patterns of original precursor powders.

1
46
J.P. Muñoz Mendoza et al. / Journal of Alloys and Compounds 369 (2004) 144–147
Fig. 3. Atomic percentage of metallic traces monitored as a function of milling time. These metals were incorporated into the precursor powder mixture
of magnetite and elemental zinc.
However, it is evident from Table 1 that the specific mag-
netization saturation keeps on rising up to 300 min milling
before dropping at 540 min. This magnetization enhance-
ment is associated to the gradual increase of metallic Fe,
whereas the final drop concerns formation of non-magnetic
ZnO.
real σr data, however, an intrinsic de-magnetization factor
must be considered since the studied material is one of the
medium-soft types. At the moment of writing this report no
correction has been performed on the data listed in Table 1,
nor in the hysteresis loops of this work figures. Literature
suggests that this sort of ferrites might have σr values equiv-
alents to 80% of the specific saturation magnetization [3].
The atomic diffusion rate of metallic elements and ox-
ides in most solids is a function of several factors [5], in-
cluding working temperature and pressure, microstructural
and crystalline defects [6], reactivity (nature) of materials,
among other possible experimental parameters [7]. It has
long be known that activating metallic and/or oxide systems
through the usage of the mechanical alloying technique, also
called mechanochemistry (depending on the sort of solid
reactions occurring) might substantially enhance atomic
diffusion after a given time of mechanical energization [2].
Thus, due to larger surface area and promotion of more
grain boundaries thermodynamic barriers can be overcome
Coercitivity Hc reached its maximum value at 540 min
milling, whereas σs did at 300 min. In order to obtain the
Table 1
Magnetic parameters measured from Fe2O3 and Zn powder mixture milled
at indicated times
Sample (min)
Hc (Oe)
σr (emu/g)
σs (emu/g)
Fe3O4 + Zn
115.8
204.7
272.6
354
3.06
4.483
8.318
12.314
11.46
27.07
22.12
44.19
50.35
43.7
6
1
3
5
0
80
00
40
Fig. 4. Magnetization curves of powder mixture in Fig. 3 after being
milled at indicated times, and the curve of the original powder mixture
of zinc and magnetite.
449.1

J.P. Muñoz Mendoza et al. / Journal of Alloys and Compounds 369 (2004) 144–147
147
[
8]. The studied system is energetically favored so that it
civity (Hc) was enhanced. It is speculated that the degree of
magnetization developed on studied samples might be influ-
enced from the presence of metallic-traces such as Fe and
Cr.
is not surprising to form a magnetic-properties disclosing
zinc–ferrite. Once the proposed reaction starts taking place
upon milling and either elemental traces of Fe increase,
enhancing magnetization of the system and parallel making
Zn to oxidize reducing magnetization, the finally attained
Hc increases because the number of magnetic domains was
also increased. Although not shown here, it was found by
SEM microscopy that sub-micrometer sized clusters de-
velop as the milling time increases. Once oriented due to
the magnetic field applied these clusters, in turn, acting as
tiny magnetic domains tend to rise Hc. After the magnetic
field is removed such clusters might retain its orientation
such that coercitivity remained risen.
Acknowledgements
This work was partially supported by Conacyt through
the project no. 37507-U.
References
[
1] L. Takacs, Nanocomposite formation and combustion induced by re-
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Nanophase and Nanocomposite Materials, MRS Symposium Proceed-
ings, vol. 286, 1993, pp. 215–220.
Fe3O4 + 4Zn → 3Fe + 4ZnO containing francolinite.
Apparently, the above reaction might be fully completed
providing that the milling time was allowed to extend. This,
however, might not be the case since the formation of a
metastable phase called francolinite temporarily tricked the
system’s thermodynamic. Francolinite might be used in
applications such that magnetization is required to occur at
intermediate times demanded by soft and hard materials,
such as videotape recording ribbons.
[2] L. Takacs, On the mechanism of displacement reactions initiated by
ball milling, in: Proceedings of the 123rd TMS Annual Conference,
San Francisco CA, 1994, p. 26.
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3] G. Bertotti, Hysteresis in Magnetism, Academic Press, New York,
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2
[
[5] G.E. Mursh, Atomic Diffusion in Defective Solids, Trans Tech Pub-
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[6] Y. Ming Chiang et al., Physical Ceramics Principles for Ceramic
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[
[
7] S. Mitura, Nanomaterials, Elsevier, Amsterdam, 2002.
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4
. Conclusions
A powder mixture of magnetite (Fe3O4) and elemental
[
zinc (Zn) was high energy ball milled and its magnetic coer-