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Journal of Scientific and Engineering Research
15
Journal of Scientific and Engineering Research, 2017, 4(6):15-26
Research Article
ISSN: 2394-2630
CODEN(USA): JSERBR
Study of Fe60Cu40 Solid Solution Obtained By Attritor Ball Milling
Abdel Monem M. Shamah1, Mohamed M. El-Desoky2, Farid F. Hanna1, Peter N.G.
Ibrahim1*, Noor A. Ibrahim1
1Math and Sciences department, Faculty of Petroleum and Mining Engineering, Suez University,
Suez, Egypt
2Physics department, Faculty of Science, Suez University, Suez, Egypt
Abstract Single phase f.c.c. Fe60Cu40 solid solution was successfully obtained by attritor ball milling after 10
hours of milling. The formation of this solid solution after this short time of milling spots the light on the
advantage of attritor as a powerful tool of mechanical alloying, capable of providing sufficiently high energy
during the milling process. The formed f.c.c. Fe60Cu40 solid solution has a grain size in the nano-scale (about 10
nm), and a lattice parameters expanded with respect to that of pure f.c.c. Cu. The high temperature annealing of
the formed f.c.c. Fe60Cu40 solid solution results in the decomposition of the solid solution and in the separation
of phases. The formed f.c.c. Fe60Cu40 solid solutions were found to be ferromagnetic, which confirms the
existence of f.c.c. Fe atoms in these solid solutions in magnetic state with non-zero value of magnetic moment.
Our calculations of magnetic moment per Fe atom gave a value of 1.465 μB for the Fe60Cu40 sample after 10
hours of milling, and 1.373 μB after 20 hours of milling. These values are discussed in light of the possibility of
coexistence of -Fe in two magnetic states (HS and LS) in these solid solutions.
Keywords Fe-Cu, Mechanical alloying, Attritor, Magnetic properties
Introduction
According to the equilibrium phase diagram of the Fe-Cu system [1], It is known that, copper has a f.c.c.
crystalline structure, iron at room temperature has a b.c.c. crystalline structure (known as -Fe phase), which is
transformed when heated to temperature 1183K to a f.c.c. crystal structure (known as -Fe). The -Fe is a high
temperature phase and is not stable at room temperature. Both Fe and Cu have negligible mutual solid solubility
in equilibrium at temperatures below 700 oC. The Fe-Cu system has a large positive energy of mixing (Hf ~ +
13 kJ/mol) [2].
Recently, the mechanical alloying (MA) appeared as a powerful tool of preparation of supersaturated solid
solutions between elements immiscible, or with limited solubility, in the equilibrium conditions, such as the Fe-
Cu system, in which the formation of supersaturated solid solutions was reported in the whole range of iron and
copper compositions [2-8].
In the process of mechanical alloying (also known as ball milling), repeated mechanical deformations, cold
welding and fracture of the alloyed powder particles take place, due to their repeated collisions with steel balls
and vial. The process of alloying takes place due to the energy provided by these collisions [9]. Different types
of mills are used in the process of mechanical alloying [9-10], for example, the vibratory ball mill, the planetary
ball mill and the attritor (which have been used in the present work).
The formed FexCu100-x solid solutions can have a b.c.c. or f.c.c. crystalline structure, or even a mixture of both of
them, in dependence on the iron and copper concentrations. In the published data, there is not complete
agreement about the boundaries between the regions of existence of these different types of crystalline
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
Journal of Scientific and Engineering Research
16
structures, which seems to be dependent on the method of preparation of these solid solutions, and even on the
parameters of preparation [8, 11]. According to many of the published articles, concerning the formation of
FexCu100-x solid solutions by mechanical alloying, the f.c.c. phase is formed for iron concentration x ≤ 60, the
b.c.c. phase is formed for x ≥ 80, while a mixture of both phases is obtained for 60 < x <80 [8].
It can be noted, that the formation of f.c.c. Cu(Fe) solid solution by mechanical alloying is considered as an
interesting way to stabilize the f.c.c. -Fe at room temperature, by substituting it in the Cu-lattice, which has the
same f.c.c. crystalline structure, and which has lattice parameter close to that of -Fe [12-14]. Since Cu atoms
are non-magnetic, the f.c.c. Cu(Fe) solid solutions, formed by mechanical alloying, can be used to test the
magnetic properties of f.c.c. -Fe at room temperature. It is known that, the -Fe, in its equilibrium stability
(above 1183K) is also non-magnetic.
To the best of our knowledge, the majority of researches, concerning the formation of FexCu100-x solid solutions
by mechanical alloying, used the vibratory ball mill or the planetary ball mill to prepare these solid solutions,
while the published data about the FexCu100-x solid solutions, formed by attritor ball mill, are rare. Consequently,
this present work is aimed to study the formation of Fe60Cu40 solid solution by the attritor ball milling, and to
study the microstructure of the formed phase(s). The composition is chosen at the boundary of formation of the
f.c.c. phase, according to the published literature, to provide new data, based on the attritor ball mill, that
confirm (or disagree with) the previously published data, about this boundary of formation of the f.c.c. phase.
The study of magnetic properties of the Fe60Cu40 solid solution, prepared by the attritor ball milling, is another
objective of the present work. It was reported [5,6,15,16] that, most of the obtained FexCu100-x solid solutions are
ferromagnetic, in exception of those solid solutions very rich in copper, which are paramagnetic. The
appearance of ferromagnetism in the f.c.c. FexCu100-x solid solutions is very interesting, since both Cu and f.c.c-
Fe are non-magnetic [14]. The ferromagnetism of these f.c.c. FexCu100-x solid solutions was discussed [12-14], in
the light of the model of Weiss [17], according to which, the -Fe can exist in two different spin-electronic
states; known as “low spin-low volume” state (LS) with lattice parameters around 3.5 Å, and “high spin-high
volume” state (HS) with lattice parameters above 3.6 Å. The value of magnetic moment per iron atom in the LS
state is estimated to be below 1 μB, while for HS state can reach values over 2.5 μB. Therefore, it was pointed,
that the magnetic state of -Fe depends on the lattice parameter of the crystalline structure, in which -Fe atoms
are stabilized (and hence depends on the change of the crystalline field inside the formed solid solutions).
Experimental Procedure
Iron powder (of purity 99%) and copper powder (of purity 99.9%) have been weighted to obtain the desired
composition Fe60Cu40, and placed in high energy attritor ball mill. Several samples Fe60Cu40 have been prepared,
after different milling times. The powders were treated with hard steel balls at different milling times with the
balls to powder ratio equals to 20:1, and rotation speed 500 rpm. The milling process was performed in
continuous argon atmosphere to prevent the oxidation of the samples.
XRD was performed using D5000 powder diffractometer using Cu Kα radiation (wavelength λ = 0.15406 nm)
with a nickel filter at 40 kV and 30 mA. The diffractometer was operated within range of 10°< 2θ<100° with
one second step-time and 0.05 degree step-size. Diffraction signal intensity throughout the scan was monitored
and processed with DIFFRAC plus software. Scanning electron microscope images of the Fe60Cu40 sample, after
10 hours and 20 hours of milling were obtained by quanta fei 250 Scanning Electron Microscope.
To investigate the thermal stability of the obtained solid solution, sample Fe60Cu40, after 20 hours of milling,
was compressed in tablets, annealed at temperatures 200C, 300C and 400C for one hour, followed by cooling in
furnace. The magnetic hysteresis loops of the obtained Fe60Cu40 samples, after 10 hours and 20 hours of
milling, were measured by a Vibrating Sample Magnetometer Lake Shore model 7410 (USA) at different
temperatures (100K, 150K, 300K and 450K).
Results and Discussion
X-ray diffraction patterns
X-rays diffraction patterns of the Fe60Cu40 sample after different milling times are represented in fig 1 and fig 2.
The mechanical alloying process of the Fe60Cu40 powder resulted in the reduction of the intensities of x-ray
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
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17
diffraction peaks and their broadening, as seen in fig 1 and 2, which is attributed to the reduction of the grain
size, and also to the introduction of internal strain during the milling process. According to the x-ray diffraction
pattern, the formation of new phases were not detected up to 30 hour milling, but it was observed, that the peaks
of b.c.c. iron disappeared after 10 hours of milling. Also a shift of the Cu peaks toward the smaller 2 angles
was observed (see fig 2). The disappearance of the Fe peaks and the shift of the Cu peaks gave an indication of
the formation of solid solution of Fe in Cu matrix (Cu(Fe) solid solution), which took place, according to our
results, after 10 hours of milling.
As seen in fig 1 and fig 2, the diffraction peaks of f.c.c. Cu are shifted toward the smaller 2 angles with
increasing the milling time, which corresponds to the expansion of f.c.c. lattice parameters during the formation
of Cu(Fe) solid solution, due to the substitution of Cu atoms by Fe atoms. It should be mentioned here, that the
atomic radius of copper (rCu = 1.28 Å) is greater than that of iron (rFe = 1.26 Å) [18], consequently, the
expansion of lattice during the formation of f.c.c. Cu(Fe) solid solution can’t be explained by the difference in
atomic radii of Fe and Cu atoms, but it was suggested that, this increase of lattice parameters of the f.c.c. phase
is most likely attributed to magnetovolume effects (The f.c.c. phase becomes ferromagnetic, which expands the
unit-cell volume, as compared to the corresponding paramagnetic material, because of the magnetovolume
effect) [19-21].
Figure 1: X-rays diffraction patterns of the Fe60Cu40 sample after different milling times
Figure 2: a detailed view of the main peak of b.c.c. Fe and f.c.c. Cu in the x-ray diffraction patterns of the
Fe60Cu40 powder after different milling times.
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18
The variation of the lattice parameter (a) of the f.c.c. phase with increasing the milling time of the Fe60Cu40
sample, calculated from the peak position of the main Cu peak in the x-ray diffraction patterns, is represented in
fig 3.
It can be suggested that, the dependence of lattice parameter (a) on milling time, as shown in fig 3, seems to
pass by two different stages during the milling of the Fe60Cu40 sample. The first stage, up to 10 hours of milling,
is characterized by an increase of the lattice parameters. By the end of this stage, all the iron atoms are
introduced inside the copper f.c.c. lattice, and a solid solution of Fe in Cu lattice is formed. The second stage,
observed for further milling time, can be characterized as a steady state stage, in which the change in lattice
parameters is very small, as compared with the first stage. In this second stage, no more Fe atoms are introduced
inside the Cu lattice, therefore, the lattice parameters do not continue to increase, but it can be suggested that, in
this stage the Fe atoms, which already exists inside the Cu lattice, continue to diffuse inside the f.c.c. lattice to
occupy energetically preferred positions, and to complete the alloying process to atomic level.
Figure 3: The variation of the lattice parameter (a) of the f.c.c. phase with increasing milling time of the
Fe60Cu40 powder.
It can be noted that, this trend (dependence) of lattice parameter of the formed Cu(Fe) solid solution on milling
time is in good agreement with previously published data [8, 21-23], but it should be mentioned also, that, there
is some kind of disagreement in these publications about the rate of expansion of lattice parameters and the
milling time required to reach the steady state stage, which can be explained by the dependence of these
parameters on the milling parameters and the amount of energy transferred to the alloyed powder.
It should be also mentioned, that according to our results, obtained on attritor ball mill, the formation of f.c.c.
Cu(Fe) solid solution was achieved after 10 hour of milling, which is considered as one of the shortest milling
time reported for the formation of solid solution in this system, when comparing with previously published data.
Consequently, our results spot the light on the advantage of the attritor ball mill, with the milling parameters
indicated in the present work, as a tool of preparation of solid solution on the Fe-Cu system, due to its capability
to provide high rate of energy transfer to the alloyed powder, necessary to achieve the formation of solid
solutions in minimum interval of milling time.
The grain size of the formed f.c.c. Cu(Fe) solid solution was estimated from the x-ray diffraction pattern by
using Scherrer’s formula [24], D = 0.9 / B(2) cos(), where  is the wavelength of the x-ray beam ( in the
present study  = 0.15406 for Cu Kα radiation), and B(2) is the FWHM of the diffraction peak located at
angular position 2 [corrected for instrumental broadening, by using the formula B2(2) = B2
measured (2) –
B2
instrumental (2)]. Scherrer’s formula was applied to the main peak of the f.c.c. phase. It should be noted here,
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
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19
that the complete Williamson Hall formula, which is used to evaluate both the grain size and microstrain, was
not applicable in our x-ray patterns, due to the considerable broadening of the diffraction peaks and their
reduction in intensity, which make it so difficult, except for the main peak, to separate accurately the peak from
the background for accurate determination of FWHM. Therefore, the values of grain size, calculated by
Scherrer’s formula, can be taken only as a lower limit of the grain size, since the broadening due to the internal
strain is not included in calculations.
The evolution of the grain size of the f.c.c. Cu(Fe) solid solution with the increase of milling time is shown in
fig 4. As seen in fig 4, increasing the milling time leads to the decrease of the grain size of the f.c.c. Cu(Fe) solid
solution to about 10 nm after 10 hours of milling. For further milling the grain size is weakly changes, and have
nearly steady-state value .The obtained value of grain size confirms the ability of the mechanical alloying
(attrition ball mill) to produce nanocrystalline f.c.c. Cu(Fe) solid solutions (with grain size in the nanoscale).
These obtained results are in good agreement with the previously published data about the mechanically alloyed
Fe-Cu system [3,8,11,25,26].
Figure 4: The dependence of the grain size (D) of the f.c.c. phase with on the milling time of the Fe60Cu40
powder
Scanning Electron Microscope Images
The scanning electron microscope SEM images of the mechanically alloyed Fe60Cu40 sample after 10 hours and
20 hours of milling are presented in fig 5. For comparison, the SEM image of the same sample without milling
(zero time) is also presented in fig 5. The sample after 10 and 20 hours of milling has a plate-like morphology,
as seen from fig 5, in contrast with the initial sample (before milling) which has a spherical morphology
(characteristic for the initial non-milled Fe and Cu particles). The appearance of plate-like morphology in the
Fe60Cu40 sample after 10 hours and 20 hours of milling is a consequence of the mechanical alloying process, in
which successive cold welding, fracture and plastic deformation of the milled powder take place, which in turn
causes the flattening of the milled particles, changing their morphology to the plate-like shape.
The size of the mechanically alloyed Fe60Cu40 particles, as seen in SEM images, after 10 and 20 hours is
estimated to be in the range of some hundreds nanometers. It should be noted here, that this estimated particle
size is completely different from the grain (crystallite) size calculated by Scherrer’s formula. The mechanically
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
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20
alloyed particles, as seen in SEM image are the result of agglomeration of a great number of crystallites, which
in turn make it not possible to view separate grains by SEM.
Figure 5: SEM of Fe60Cu40 alloy. (A) Without milling. (B) After 10 hours. (C) After 20 hours
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
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21
Thermal stability of the Fe60Cu40 solid solution obtained by mechanical alloying
To investigate the thermal stability of the Fe60Cu40 solid solution obtained by mechanical alloying, the sample
after 20 hours of milling were compressed and annealed at temperatures 200C, 300C and 400C for 1 hour at
each temperature. The x-ray diffraction patterns, at the 2-range around the main peak of the f.c.c. phase, after
these heat treatments are shown in fig 6.
As shown in fig 6, with the increase of annealing temperature, the b.c.c. Fe-peak reappeared, and in clearly seen
after annealing at temperature 400C. These results show that, the f.c.c. Fe60Cu40 solid solution formed by
mechanical alloying is not thermally stable, and undergoes a phase separation into b.c.c. Fe and f.c.c. Cu phases.
The degree of separation of these phases increases with increasing the temperature of annealing, and become
obvious after annealing at temperature 400C.
It can be also noticed, that a shift of the main f.c.c. diffraction peak toward the higher 2 is observed with the
increase of temperature of annealing, as shown in fig 6, which corresponds to the contraction of the f.c.c. lattice.
This is easly explained in the light of the separation of the b.c.c. Fe from the f.c.c. lattice, and the return of the
lattice parameters of the f.c.c. phase to that of pure f.c.c. Cu.
Figure 6: X ray diffraction patterns of the Fe60Cu40 solid solution obtained by mechanical alloying for 20 hours,
followed by annealing at the indicated temperatures for 1 hour
Magnetic Measurements
The curves of the field dependence of magnetization (hysteresis loops) of the Fe60Cu40 sample after 10 hours
and 20 hours of milling are shown in fig 7 and fig 8, respectively. The shape of the measured hysteresis loops
are characteristic to the existence of ferromagnetic ordering in these Fe60Cu40 solid solutions, formed by
mechanical alloying.
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Figure 7: The dependence of magnetization (M) on the intensity of magnetic field (H) for the Fe60Cu40 sample
after 10 hours of milling
Figure 8: The dependence of magnetization (M) on the intensity of magnetic field (H) for the Fe60Cu40 sample
after 20 hours of milling
It is interesting, why the mechanically alloyed Fe60Cu40 solid solutions show ferromagnetic behavior, while both
f.c.c. Cu and f.c.c. Fe (-Fe) are non-magnetic. The appearance of ferromagnetism in the f.c.c. Cu(Fe) solid
solutions can be attributed, as was discussed in the work of Gorria [12-14], to the existence of -Fe in high spin
(HS) state in the f.c.c. environment of Cu lattice. Since the lattice parameter of the pure f.c.c. Cu is about 3.615
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
Journal of Scientific and Engineering Research
23
Å (see fig 3), which is close to that of the -Fe in the HS state, therefore it is more favorable for the -Fe in the
f.c.c. environment of Cu lattice to exist in the HS state, leading to the ferromagnetism of these f.c.c. Cu(Fe)
solid solutions, as confirmed our results for the mechanically alloyed Fe60Cu40 solid solutions.
From another point of view, our results (in agreement with previously published data) showed, that the
substitution of f.c.c. Cu atoms by the f.c.c. Fe atoms during the formation of f.c.c. Cu(Fe) solid solutions
resulted in the expansion of the lattice and the increase of lattice parameters (see fig 3). This can be explained by
the existence of -Fe in its HS magnetic state in these solid solutions, and taking into consideration the
magnetostriction effect caused by the change of the magnetic state of -Fe, located inside the Cu lattice.
From the hysteresis loops (fig. 7 and 8), it can be seen that, the values of coercivity Hc, remanence Mr and
saturation magnetization Ms decrease, as expected, with increasing the temperature, due to the effect of thermal
agitation, which opposes the ordering of magnetic moments (tends to their disordering).
To determine the saturation magnetization Ms(0) at nearly 0K, a plot of the Ms(T) as a function of T3/2 were
performed, in accordance of Bloch T3/2 law [M(T)=M(0)(1−BT3/2)] based on the spin-wave excitation in
ferromagnetic materials [15]. The saturation magnetization Ms(0) at nearly 0 K is obtained from the
extrapolation of the linear part of the Bloch T3/2 plot to T = 0 K, as shown in fig 9. It should be noted, that our
experimental data are well fitted by Bloch T3/2 law in temperature range up to 300 K, which confirms the
existence of ferromagnetic ordering in these samples. The obtained values of Ms(0) are 83.3 emu/g for the
Fe60Cu40 sample after 10 hours of milling, and 78.1 after 20 hours of milling.
Figure 9: the saturation magnetization as a function of T3/2, based on Bloch T3/2 law, for the Fe60Cu40 sample
after 10 hours and 20 hours of milling
From these calculated values of saturation magnetization at zero Kelvin Ms(0) the magnetic moment μ per unit
formula (Fe60Cu40) can be estimated, by using the relation μ = (Ms(0) × M.W) / (NA × μB) = (Ms(0) × M.W) /
5585, where M.W is the molecular weight of the unit formula, NA is Avogadro’s number (6.02252 × 1023 mole-
1) and μB is Bohr magneton (9.2732× 10-21erg/G). After that, the magnetic moment per iron atom μFe can be
estimated, assuming a ferromagnetic ordering of magnetic moments and assuming that, Cu atoms have no
magnetic moment. In this way, our calculations leaded to a value of magnetic moment per iron atom μFe of
1.465 μB for the Fe60Cu40 after 10 hours of milling, and 1.373 μB after 20 hours of milling. The existence of
none-zero value of magnetic moment per iron atom accounts for the appearance of ferromagnetism in the
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
Journal of Scientific and Engineering Research
24
mechanically alloyed Fe60Cu40 samples. The values of magnetic moment per Fe atom, as calculated from our
magnetic measurements, are in acceptable agreement with some previously published data [3], while other
published data gave higher values of magnetic moment per Fe atom (around 2μB) [2, 27-28]. Our calculated
value of magnetic moment per Fe atom is also lower than that, suggested by Weiss model for the -Fe in the HS
state. It seems that, not all the Fe atoms existing in the f.c.c. Cu-lattice are in high spin state, but some Fe atoms
are in low spin state or non-magnetic state, in dependence on the local environment surrounding each atom. The
existence of fraction of Fe atoms in non-magnetic state in the mechanically alloyed Fe-Cu system was also
suggested by the results of Mossbauer spectroscopy [15], and the coexistence of Fe atoms in two different
magnetic state (HS favoring the ferromagnetic coupling and LS favoring the antiferromagnetic coupling] in the
mechanically alloyed Fe-Cu system, was confirmed by the results of XMCD [29].
To investigate the effect of milling time on the magnetic characteristics Mr, Hc and Ms of the mechanically
alloyed Fe60Cu40 sample, the hysteresis loops M(H) at room temperature (T = 300K) of the Fe60Cu40 sample
after 10 and 20 hours of milling are plotted in fig 10, together with the hysteresis loop of the starting Fe60Cu40
sample without mechanical alloying. As seen from fig 10, and also from fig 8, the coercivity and the remanence
of the Fe60Cu40 sample increases considerably with the increase of milling time. The coercivity increased from
about 20 Oe before milling at RT to about 160 Oe after 20 hours of milling at RT, which reached about 305 Oe
at T=100K. The increase of coercivity can be attributed to the decrease of grain size and the introduction of
micro strain accompanying the mechanical alloying process.
Figure 10: The dependence of magnetization (M) on the intensity of magnetic field (H) for the Fe60Cu40 sample
without milling and after 10 hours and 20 hours of milling at T = 300 C
It can be also noticed, that the magnetization of the initial Fe60Cu40 sample (without mechanical alloying) does
not saturate up to an applied magnetic field of 18 KOe, while the mechanically alloyed samples saturate easily
in lower magnetic fields, as shown in fig 10. Generally, it is seen that, the magnetization at high magnetic fields
tend to decrease with increasing the milling time, which can be related to the change of iron from b.c.c. structure
to f.c.c. structure, and the change of magnetic moment of iron atoms, due to the change of their surrounding
crystalline field. It can be also noted that, there is a difference in the value of saturation magnetization in the
samples after 10 hours and 20 hours of milling, which seems to be related to the change of the value of magnetic
moment of Fe atom, and/or the change of fraction of f.c.c. Fe existing in HS state, due to the change of local
environment surrounding the Fe atoms.
Shamah AMM et al Journal of Scientific and Engineering Research, 2017, 4(6):15-26
Journal of Scientific and Engineering Research
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Conclusions
The main conclusions of the present work can be summarized in the following points:
Single phase nanocrystalline f.c.c. solid solution Fe60Cu40 was successfully prepared by attritor ball milling after
10 hours of milling, which confirms the ability of attritor to provide sufficiently high energy during the milling
process, resulting in the formation of single phase solid solution after considerably short time of milling,
comparable and even better than that reported in the published data by using other tools of mechanical alloying.
The formation of the solid solution Fe60Cu40 was accompanied by f.c.c. lattice expansion, compared with that of
pure f.c.c. Cu, and which was suggested to be attributed to magnetovolume effects.
The mechanically alloyed Fe60Cu40 solid solutions are thermally unstable, and their annealing at relatively high
temperature (300 °C, 400 °C) results in the decomposition of the solid solution and in the separation of b.c.c. Fe
and f.c.c. Cu phases.
The formed Fe60Cu40 solid solutions are found to be ferromagnetic, which confirm the existence of f.c.c. Fe
atoms in these solid solutions in magnetic state with non-zero value of magnetic moment. Our calculations of
magnetic moment per Fe atom gave a value of 1.465 μB for the Fe60Cu40 sample after 10 hours of milling, and
1.373 μB after 20 hours of milling. These values are discussed in light of the possibility of coexistence of -Fe in
two magnetic states (HS and LS) in these solid solutions.
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