RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES4003, doi:10.2205/2005ES000177, 2005
Discussion of the Results
[8] The diffractograms obtained for the samples of all compositions showed a system of lines of a
cubic spinel structure with broad reflections, typical of magnetite or maghemite. The analysis of the
resulting X-ray diffraction line widenings, performed using the Scherrer formula
[Araki, 1989]
showed the average size of the particles in the regions of their low concentrations to be 10 nm. In the regions of
their higher concentrations the average size of the particles was as high as 25 nm, obviously because of
their growing coagulation. This was accompanied by the order-of-magnitude growth of microtension
[Novakova et al., 2002, 2003].
The significant broadening of the lines complicated the reliable distinction between magnetite and maghemite
with disordered vacancies. No superstructural reflections, typical of ordered maghemite, were found, this
being another typical feature of ultradispersed particles
[Haneda and Morrish, 1977].
|
Figure 1
|
[9] The Mössbauer transmission spectra, obtained at room temperature ( Tr ), showed substantial gradual
changes with the growth of magnetite nanoparticles concentration (Figure 1). In the case of low
Cv values the
significant part of the spectra were occupied by doublets corresponding to noninteracting
superparamagnetic particles of magnetite against the background of unresolved, low-intensity hyperfine
magnetic splitting. With the
Cv growth the intensity of the magnetically split portion of the spectrum
( Smagn ) grows higher, indicating the growing contribution of the particles involved in magnetic interaction.
The
Smagn
|
Figure 2
|
growth calculated after the computer processing of the results was found to be monotonic (Figure 2a).
[10] In contrast to the X-ray diffraction, the Mössbauer spectroscopy showed the high oxidation of the magnetite
particles in polymer film, especially in the case of low
Cv concentrations. The magnetically splited
spectra obtained for low
Cv concentrations (3-11%) are symmetrical and agree in terms of their parameters
only with the content of Fe3+ ions in the study maghemite. In the case of higher
Cv concentrations
the increased resolving of the first two lines of the spectrum was observed, which is characteristic of magnetite.
The isomeric shifts, obtained for various sextets after the computer fitting of the spectra also identify
the material to be magnetite. However, even at the highest
Cv values, such as 33% and 43%, the ratio between
the spectral intensities for tetra- and octahedral sublattices ( SA/SB )
are larger than 1, which does not
correspond to stoichiometric magnetite ( SA/SB = 0.5).
Therefore the particles produced by synthesis
and subsequent drying are composed of maghemite in the case of low
Cv values and of oxidized magnetite in
the case of high
Cv values.
[11] Proceeding from the visual observations of changes in the color of the films in
the course of their preparation and from the results of the Mössbauer spectroscopy, it can be inferred that
in the case of the low concentration of iron salts in the solution, the partial oxidation of magnetite
nanoparticles occurred as early as at their transformation to gel and ended during drying, because scattered
in the isolation manner in the cooling polymer material, the nanoparticles have high specific surface.
With the growth of the salt concentration in the solution up to 20% and higher, the formation rate of
ferrimagnetic particles increases resulting in their ability to produce agglomerates with a lower specific
surface. As a result, during the gel formation, the particles remained to be composed of magnetite, yet, the
drying of the composite material showed partial single-phase oxidation or the formation of maghemite
films at the surface of the magnetite nanoparticles. In the latter case, the particles showed a two-phase,
coherently conjugated magnetite-maghemite system. The similar synthesis of magnetite nanoparticles,
about ~10-15 nm in size, without the participation of any polymer, resulted in the complete oxidation
of the primary magnetite with the spectra showing no indications of magnetite components
[Novakova et al., 1992].
The spectra of nonstoichiometric magnetite, similar to those obtained for the samples with
Cv = 33-43%,
were observed by
Jolivet et al. [1992]
during the synthesis of magnetite from a FeCl2 and FeCl3 salt solution with
Fe2+/Fe3+ = 0.30-0.50
without polymer participation, yet in the
Ar atmosphere, which precluded potential oxidation. The size of the
synthesized particles varied from 4 nm to 20 nm, or from ~8.5 nm to 20 nm as indicated by electron microscopy.
The PVA solution was added to the resulting suspension to produce films for Mössbauer spectroscopy.
[12] The similarity of the spectra obtained in our study and those reported by
Jolivet et al. [1992]
suggests that in the case of high iron salt concentrations in the solution the PVA addition in the course of
the reaction precludes the complete oxidation of nanoparticles. However, this is not the only effect of the
polymer. Numerous Mössbauer studies of fine-dispersed maghemite particles varying little in size (6-10 nm),
demonstrated that the spectra obtained at room temperature showed either a paramagnetic doublet or
partial relaxation effects characteristic of systems with noninteracting supermagnetic particles
[Coey and Khalafalla, 1972;
Mørup, 1990;
Pankhurst, 1994].
In the cases where the particles synthesized from salt solution were coated with oleic acid, 1.0-1.5 nm thick,
the relative doublet content grew substantially compared to the pressed uncoated particles
[Mørup, 1990].
This proved the lower magnetic interaction among the coated particles. Our study showed a different situation: with
a significant separation of particles by a polymer at the synthesis stage (the
Cv values being lower than or equal
to 11%) the spectra demonstrated stable magnetic interaction between some of the iron oxide particles. The number
of these particles increased with the growth of their total volumetric concentration ( Cv ). The same type of
magnetic splitting, "prohibited'' for the isolated particles of this size, was observed in the case of
monocrystalline magnetite layers merely 5.3 nm thick at the MgO surface
[Voogt, 1998].
[13] The specific feature of the spectra of the polymer films examined, in spite of their external similarity with the spectra
obtained for a bulk magnetite, was the low values of the hyperfine magnetic fields ( Hhf ). This
lowering was especially significant where the contribution of the superparamagnetic doublet was low and
the relaxation was absent, and, hence, cannot be explained in terms of superparamagnetic relaxation. The
Hhf/Hhf(bulk) ratios for the Fe3+ in the films with the low
Cv concentrations (3-5.7%),
where the particles showed the best isolation, was found to be 0.96-0.94; it declined from 0.9 to 0.86 as the
Cv value grew from 11% to 43%. A similar trend of the
Hhf decline, with the paramagnetic doublet
declining to zero, was reported by
Jolivet et al. [1992]
and Voogt et al. [1998].
The
Hhf decline is a specific feature of the physics of nanoparticles and thin layers.
The magnetic interaction in the case of particles is more controlled by the extent of
their isolation than grain size
[Mørup, 1990].
In the present case the effects responsible for the lowering of hyperfine magnetic fields is the active role of a polymer
matrix which, on the one hand, separates the conglomerates of interacting nanoparticles and, on the other hand, have an
influence on the exchange interaction because of medium elastic forces.
[14] The specific saturation magnetization of films ( Jsf ), measured in the magnetic field of 450 mT (the
lower curve in Figure 2b), demonstrates also its monotonous growth with the growing
Cv value in a broad
range of the values varying from 0.47 Am2 kg-1
to 60 Am2 kg-1. The scatter of the
Js values
for different pieces of the film of the same composition amounts to 20-25%, this possibly being caused by the nonuniform
distribution of the nanoclasters inside the polymer material. Since the measured magnetization values are
controlled primarily by the concentration of ferrimagnetic particles, the monotonic
Jsf growth proves the
validity of the volumetric
Cv concentrations in the case of different batches. Despite the
insignificant polymer weight, the
Jsf value of the films is much lower than that of magnetite (92 Am
2 kg
-1 )
and than that of maghemite (74 Am2 kg-1 ).
[15] In computing the true specific value of the saturation magnetization of any ferromagnetic phase,
formed in the course of the reaction, the unknown value is the mass of nanoparticles contributing to the
measured
Js value. In the case discussed, to calculate the
Js values of nanoparticles ( Jsn ), the magnetization
values measured for the specimens, for which Mössbauer spectra had been obtained, were corrected in two
stages. The first correction stage was based on the formal view that the only mineral produced as the result
of the reaction had been magnetite the weight percentage of which ( Cp ) was calculated for each individual
synthesis, proceeding from the amounts of the respective components in the solution. The results of the
Jsn computation are shown in blue color in Figure 2b. One can see that the
Jsn values are higher than the
respective
Jsf values and show two intervals of the peak
Jsn values: 18-48 Am2 kg-1 for the
Cv values lower or equal to 3% and 55-65 Am2 kg-1 for the
Cv concentrations higher or equal to 5.7%.
These values are lower than the
Js values of magnetite or maghemite. In this connection, the samples with
purely paramagnetic spectra ( Cv = 0.6% to 1.2%) were discarded because the contribution of SP particles,
stabilizing in the magnetic field during
Js measurements, could not be estimated correctly.
[16] In the case of the other film compositions the mass was calculated including the percentage of the particles that participated
in magnetic interaction, this being determined from a ratio between the areas of the magnetic and superparamagnetic
components in the spectra (Figure 2a). In this case the weight percentages ( Cp) of the ferrimagnetic phases
of the samples with the
Cv values of 3% and 5.7% were recalculated for maghemite in accordance with the
spectroscopic data available. The values obtained after the second stage of corrections ( Js ) also showed two
groups (red dots in Figure 2b). One group, corresponding to the low concentrations, was located in the
vicinity of the
Js value obtained for maghemite.
[17] The other group of the corrected
Js values resided in the
region of 82-89 Am2 kg-1 which accounted for 0.90-0.96 of the
Js value obtained for stoichiometric
magnetite and can be easily explained for the case of nonstoichiometric magnetite or for the case of a two-phase
g Fe2O3 + Fe3O4 system.
A more exact calculation was found to be possible only after getting
a few versions of the computer processing of the spectra, which was beyond the scope of this paper. Our
correction procedure proved that the use of the results of merely macroscopic magnetic measurements may
lead to the underestimation of the specific magnetic saturation of ferrimagnetic nanoparticles.
[18] As mentioned in the introduction to this paper, the underestimated values of specific saturation
magnetization is a characteristic feature of the physics of nanoparticles of ferrimagnetic materials and thin
films, which is a subject of discussion in the modern literature. Various models, often precluding one
another, are offered to explain this effect. The examples are the formation of magnetically inactive or
magnetically dead layers at the surface of the particles; changes in the cation distribution; the
noncolinearity of spins in the A and B sublattices; variations of the
K1 crystallographic anisotropy constant;
the fragmentary distribution of cation sublattices with the preservation of a continuous O
2- sublattice; and the
antiphase boundaries precluding ferrimagnetic interactions among magnetic domains and producing their patchworks
[Coey and Khalafalla, 1972;
Goss, 1998;
Han et al., 1994;
Mørup, 1990;
Pankhurst, 1994;
Voogt, 1998;
Zheng et al., 1998].
In the case of polymer nanocomposites two coexisting versions can
be proposed, instead of complex models, to explain the underestimation of
Js nanoparticles.
One of them is the heterogeneous distribution of particles in terms of their sizes in the course of the synthesis,
where only a small number of the particles have a superparamagnetic size. The other version is the heterogeneous
distribution of nanoparticle clusters, both over the film thickness and inside the layers (isolated
noninteracting particles and the conglomerates of interacting ones). The former proposition is based on the
general considerations and on the results of the suspension fractionation study performed by
Jolivet et al. [1992]
after the similar synthesis of magnetite without any polymer participation. The latter was
confirmed by the study of film layers using depth-selective Mössbauer spectroscopy
[Novakova et al., 2003, 2005b].
|
Figure 3
|
[19] The conversion Mössbauer spectra were collected from both sides of the studied polymer films
at the surface and at a depth of a 20-micron thick layer. Figure 3 shows the results of the study of this kind
for a sample with the
Cv concentration of 33%. One can see from this figure that the spectra obtained for
different thicknesses of the film differ substantially. It is worth mentioning that the thin surface layers
on both sides of the film showed similar doublet spectra indicating the absence of magnetic interaction
among the superparamagnetic iron oxide particles. It appears that the particles in the surface layers are
scattered for higher distances from one another. The layers 20
m m thick showed different spectra. The upper
layer showed mainly a doublet component at the background of poor hyperfine magnetic splitting.
However, the spectrum of the lower layer of the same thickness showed a clearly expressed magnetic
structure typical of the magnetite spectrum. Its form coincides almost wholly with the spectrum obtained
for the total thickness of this sample, shown in the same figure. These experiments demonstrate clearly the
heterogeneous distribution of particles in the course of the synthesis in a polymeric material and, as a
consequence, a difference in their magnetic interactions.
[20] Although most of the particles are located in the lower 20-micron film layer, this
layering cannot be identified as exceptionally gravitational one, because the spectra of the subsurface layers
are identical. It can be supposed that the surface layers of the film grow faster than the internal ones. The
slower drying of the internal part of the film seems to produce conditions favorable for the agglomeration
of the particles separated by the polymeric material. In this case, in spite of the superparamagnetic size of
the particles, magnetic interaction exists inside the agglomerate. Similar aggregates of magnetite particles,
of a few nanometers, were observed in the case of the bacterial reduction of amorphous ferric hydroxide
[Fredrickson et al., 1998].
The theory of exchange interaction between the closely spaced
particles of superparamagnetic size was offered by
Mørup [1990],
and this interaction phenomenon itself was referred to superferromagnetism. It is obvious that in the case of
nanocomposite materials the decisive role in the magnetic interaction of nanoparticles is played by a polymer and
its elastic characteristics. As follows from Figure 3, the organizing effect of the polymer in this kind of
synthesis develops slowly in a
20180 micrometer layer, where both surfaces can be classified as "dead layers''.
|
Figure 4
|
[21] The shapes of the saturation magnetization vs. temperature curves and the measured
Tc values
provide additional information for the type of the magnetic interaction among fine-dispersed particles. In
the case of polymer films containing magnetite or maghemite particles some difficulties arise in association
with the fact that the temperature of the polymer destruction may be lower than that of the destruction of
the magnetic order in the ensemble of the particles. Nevertheless, in the study reported here the
thermomagnetic
Js(T) curves were measured in the temperature interval of 20-700o C in the cases of all film
compositions. The examples of the normalized curves obtained during the first and second heatings are shown
in Figure 4, the results of their processing being summarized in Table 1. The first feature that attracts attention
in the behavior of the first-heating curves is a break in their monotonic behavior in the T
1-T2 temperature
interval, associated with the destruction of the polymer. The T1 temperature varied from 211o C to 231o C
and T2, from 298o C to 366o C, for the films of different compositions.
The T2-T1 interval grew regularly
with the growth of the volumetric iron oxide concentration. Another peculiar feature of the curves behavior was the
difference in their steepness prior to and after the break, this proving the different type of the magnetic
interaction between the particles in the film prior to and after its decomposition. Since it was impossible to
connect the segments of the curves obtained for the situations prior to and after the break, the curves were
processed using polynomials in the segments of (20o C) - T1 and T2 - (600o C).
It was found that in the case
of the compositions with the
Cv values varying from 0.6% to 3% their curves were approximated in an excellent
way by a linear function in the former segment, and in the case of the compositions with
Cv
11%, by the
polynomials of grade 4. The second segment of the curves could be approximated adequately by grade-4
polynomials in the cases of
Cv values equal to or lower than 3% and by grade-8 polynomials in the case of
the
Cv equal to or higher than 11% (Table 1, figures in brackets). Variations in the shapes of the
Js(T) curves in the first
segment were caused by the substantial content of superparamagnetic particles in the case of the low
content of iron oxide in the film. In the case of its higher concentrations magnetic interaction remained to
be high enough in spite of the fine size of the particles. As an additional parameter ( a
), controlling
magnetic interaction between the nanoparticles in the film, we preferred to use the decline of the initial
saturation magnetization
Jsf0 as a result of heating to the temperature of 200o C:
a=Jsf0-Jsf (200)/Jsf0.
As shown in Table 1, the
a value diminishes with the growing volumetric concentration of particles in the film,
this indicating the growth of magnetic interaction. In the case of massive magnetite,
a = 0.09 in this
temperature interval.
|
Figure 5
|
[22] For the formal determination of the Curie temperatures of the films, the polynomials of the first
segment were extrapolated for a greater temperature range, and the point of the intersection with the
temperature axis was assumed to be
Tcf. An example of this extrapolation is shown in Figure 5 for the
composition with
Cv = 43%. The
Tc values determined after this procedure were in
the range of 415-457o C, without of any visible correlation with
Cv. The exclusion was a sample with
Cv = 20%, for which the computed
Tcf value was 352o C. The virtual point of the intersection
of the two
Js(T) curve branches could also be taken to be the Curie temperature. In this calculation
procedure the
Tcf values were in the vicinity of 300o C. It could be supposed that
the temperature of about 300o C was not the Curie temperature of the films, but the temperature of
maghemite to hematite transition. However, the saturation magnetization did not decline after the cooling
from 300o C, this allowing one to interpret the computed
Tcf values as Curie temperatures.
In this case the particular
Tcf values are not important. What is important is
the fact that in the cases of all nanoparticle concentrations in the films the Curie temperatures are higher
than the room temperature, yet, lower than that of bulk magnetite ( Tc = 585o C). It follows that
in the case of a high outer magnetic field the superexchange magnetic interaction of nonstoichiometric magnetite
particles, "frozen'' in the films, is fairly strong, yet weaker than in massive magnetite or maghemite. This
weakening of the exchange interaction explains the above-mentioned decline of the
Hhf values in the
Mössbauer spectra of the films.
[23] The Curie temperatures ( Tc ) of the free-particle conglomerates that remained after the destruction
of the polymer were determined using the first derivatives of the experimental curves. The
Tc values
were found to decline form 532o C to 517o C with the growth of the
Cv concentration. These values were also
lower than the
Tc values of the bulk stoichiometric and, moreover, nonstoichiometric magnetite or
maghemite. It could be supposed that the destruction of the film and the surface liberation of the particles
was followed by the further oxidation or their sintering of the fine-dispersed particles. However, as follows from
the
Tc
measurements, no substantial oxidation or welding of the particles were observed prior to the heating
to 600o C. It was only after the heating up to 700o C and the cooling down to room temperature, that the
JsfT values declined by factors of 2 to 7 ( JsfT/Jsf0
0.15-0.5 ), the Curie temperatures
of the resulting mineral phase being close to
600o C (see the right-hand column in Figure 4). In some cases the curves had tails up to 650o C. Although the
Jsf value declined significantly, the specific values of this parameter, calculated for the weight content of
magnetite or maghemite, were higher than 10 Am2 kg-1.
Hence, most of this magnetization belonged to the
particles of oxidized magnetite or maghemite, that is, the analyzed material was not fully transformed to
hematite during the heating of the films. The exception was a sample with
Cv = 11%, whose
Js value was
found to be 50 times lower, and whose second-heating
Js(T) curve was obviously of a two-phase type and showed
the high content of hematite (Figure 4).
[24] The significant role of polymer in the stable magnetic interaction of nanoparticles was manifested
even more obviously during the study of the remanent magnetic characteristics of films
[Novakova et al., 2003, 2005a].
In the case of magnetite nanoparticles, ~10 nm in size, derived from GS-15 bacteria, the
blocking temperature ( Tb ) was found to be lower that the room temperature
[Moscowitz et al., 1989].
The hyperfine magnetic splitting in the film spectra at room temperature ( Tr ) cannot, in principle, be interpreted
as contradictory, because the relaxation times
t for magnetic and Mössbauer measurements are not much
different (about
102 s and about
10-8 s, respectively). This difference diminishes more than four
times the
Tb values observed in magnetic measurements, compared to Mössbauer ones. For instance, in the case of
maghemite nanoclasters, 5 nm in size, in block copolymer, the
Tb values derived from magnetic and
Mössbauer measurements were found to be 16 K and 50 K, respectively. Hence, it was natural to infer the
absence of hysteretic properties in the study films. However, all of the samples showed the measurable
Jrs,
Hc, Hcr, and LSM values even in the cases of the lowest iron oxide concentrations. This is another proof of
the existence of magnetic interaction between the nanoparticles separated by a polymer, which results in
the formation of a specific domain structure in the films.
|
Figure 6
|
[25] The blocking temperatures of the films ( Tbf ) were derived from the
Jrs(T) curves, the examples of
which are shown in Figure 6. The concave shapes of the curves with
Tb values of about 300o C characterize
the fine dispersion of the particles even in the cases of their highest concentrations in the films. As mentioned
above, we must certainly bear in mind that
T
300o C is simultaneously the temperature of the polymer
destruction in the case of the heating as long as this one. However, the monotonous decline of the
Jrs value
to zero in the course of heating, as well as the stability of the
Jrs values after the cooling from 400o C to the
room temperature, allow one to rank
T
300o C as a true blocking temperature.
|
Figure 7
|
[26] In contrast to the
Jsf value, the specific
Jrsf values of the films showed a nonmonotonic variation
with the growth of the Fe
3 O
4 concentration. It attained its maximum value where
Cv was equal to 20% and
declined with its further growth (Figure 7a). The nonsporadic recording of the
Jrsf behavior at
Cv = 20% and
its decline at higher concentrations was confirmed by the measurements performed using samples with the
Cv values equal to 18% and 31% obtained during the repeated synthesis. This phenomenon remains to be
clarified. The
Jrs values obtained for different concentrations were compared at the time interval of 80 s
after the magnetization because significant magnetic viscosity was observed
[Novakova et al., 2003].
The nonmonotonous character of the
Jrsf variation showed good correlation with the variations of
Hc and
Hcr values
(Figures 7b and 7c), which also showed peaks at
Cv = 20%. The
Hc
and
Hcr values measured in this study for the film
with
Cv = 3% were found to be 3.6 mT and 17 mT, respectively. The low coercivity values seem to have been associated
with the significant content of superparamagnetic grains at the low maghemite concentrations observed in the
spectrum of the sample concerned (Figure 1). The maximum
Hc and
Hcr values were found to be 8 mT and 45.3 mT,
respectively, which are feasible for the magnetite-maghemite system, as follows from the highly
variable published experimental values and theoretical estimates
[Dunlop, 1981;
Gendler et al., 2005;
Goss, 1988;
Moscowitz et al., 1989;
Sato et al., 1987;
Sohn et al., 1998].
These variations are associated with
the fact that at constant temperature, coercivity is controlled by a large number of factors, such as, grains size
the patterns of their distribution, their morphology, and structural defects, the external tension,
and the magnetic anisotropy and interaction among the particles. All of these structural characteristics are
controlled, in turn, by the type of their formation. For instance, the values of the coercivity,
Hc, of
bulk maghemite reside in the range of 25
- 40 mT
[Sohn et al., 1998].
The theoretical
Hc values, available for isotropic, single-domain grains with the predominance of magneto-crystalline
anisotropy were found to be 15 mT for maghemite and 19 mT for magnetite. However, in the vicinity of
the single-domain/superparamagnetic boundary ( ds ), the calculations show the
Hc value to be 6.7 mT for the particles
with a size of 40 nm and
ds=30 nm
[Dunlop, 1981].
In the case of maghemite grains of 37 nm in size and of the growing relative content of superparamagnetic particles,
the coercivity calculated by
Goss [1988]
was found to be 4 mT. The experimental
Hc values obtained for the magnetite and maghemite
synthesized from the iron salt solution, without adding any PVA, varied from 0 mT to 15 mT for the cubic
particles ranging from 7.5 nm to 17 nm in size
[Coey et al., 1972;
Sato et al., 1987],
and from 23 mT to 36 mT for the needle-shape particles about 100 nm long
[Morrish and Yu, 1955].
In the case of the fine-dispersed maghemite, obtained from lepidocrocite as a result of its long low-temperature
annealing, the resulting
Hc and
Hcr values were found to be 3 mT and 6 mT, respectively
[Gendler et al., 2005].
The nanoclasters of maghemite particles with the size of 5 nm, contained in block copolymers, and the particles
with a size of 8.5 nm, contained in ion-exchange resins, showed a pure superparamagnetic behavior, with
the
Hc values equal to zero at room temperature
[Sohn et al., 1998].
The magnetite grains of 40
40
60 nm in size, from magnetotactic bacteria MV-1,
isolated from sulfide-rich sediments of an estuarian salt marsh, showed the
Hc and
Hcr values to be 28.5 mT
and 48.5 mT, respectively, and the behavior typical of that observed for the assemblages of noninteracting
single-domain grains. In turn, the magnetite grains produced by iron-reducing bacteria GS-15, showed
the
Hc and
Hcr values equal to 0.8 mT and 30 mT, respectively, and the behavior consistent with
the effects of magnetostatic interaction because of the particle agglomeration
[Moskowitz et al., 1989].
This significant difference in the coercitivity of the bacterial magnetite can be explained by a difference
in the spectra of the grain sizes: narrow in the former, and broad in the latter case with a great contribution
of SP grains. A similar situation seems to be observed in the study films in the case of the low concentrations
of nanoparticles and the significant content of SP-size grains. The factors responsible for the growth of remanent
magnetization and coercivity in the sample with
Cv = 20% remain to be found. The only one real experimental
fact is the appearance in this sample spectrum of the unresolved sextets conforming to iron ions with the valence of 2.5 and lower.
This suggests the presence of Fe
2+ ions in the octahedral sublattice of spinel, which is known to increase
the constant of magnetocrystalline anisotropy and, hence, the coercivity. However, the concentration of Fe
2+ ions grows with the further growing
Cv values and the amount of the superparamagnetic particles declines,
while the coercivity declining as well, similar to the remanent magnetization. It can be assumed that at high
Cv values the mechanism responsible for the coercivity decline is the factor of the growing magnetostatic
interaction between the ultradispersed particles in the films depth.
|
Figure 8
|
[27] The measurements of the remanent magnetization acquired in the magnetic field of two
orthogonal directions (parallel and perpendicular to the film plane) were used to study the
Jrs anisotropic
properties. It was found that, irrespective of the magnetizing field direction, the
Jrsmax vector does
not only resides in the film plane, but also has a preferable orientation in it. For instance, the results of measuring
the
Jrs vector component ratios, normalized for the J
rsz vector for the sample with
Cv = 43% Fe3O4,
were found to be as follows:
Jrsx:Jrsy:Jrsz=50:3.5:1 and 10.6: 2.4: 1 for the external field
parallel to and perpendicular to the surface of the film, respectivly. Figures 8a and 8b shows the values of
the coefficients of the planar ( kp ) and linear ( kl ) anisotropy for the films of different
compositions
(kp=(Jrsx)2+(Jrsy)2/Jrsz, k1=Jrsx/Jrsy).
One can see in Figure 8 that the planar anisotropy of all films compositions
is higher than 10 and amounts to 50-55 for the
Cv values higher or equal to 20% for the magnetization
parallel to the plane surface. The
kp values declined by a factor of five for a sample with
Cv = 43%,
in the case of the magnetization perpendicular to the film surface, yet remained to be significantly higher
than 1 ( kp=10.5 ). The linear anisotropy was usually weaker than the planar one ( kl ):
the
kl value varied averagely from 4 to 6 in the case of the magnetization parallel to the film surface
for the
Cv value lower than or equal to 33% and was found to be as high as 13.4 for a sample with
the
Cv concentration equal to 43% (see Figure 8b). Although the values of the linear anisotropy varied
from 20% to 25% for different segments of the film and for different syntheses, yet, always remained
to be higher than 1. Proceeding from the results of our measurements of
Jrs anisotropy, we proposed
the heterogeneous, lamellar-like or chain-shaped structure of the nanoparticles originating in
the film depth parallel to the film surface. In other words, the anisotropy
might have been caused by some magnetic texture produced by the active role of the organic nonmagnetic
matrix in which nanoparticles are synthesized.
[28] Since the synthesis of the polymer nanocomposite material was carried out in the laboratory
ambient magnetic field, it was of interest to check whether the magnetization acquired in the magnetic field
of the Earth remained to be fixed in spite of the long procedure of the long washing with water during the
synthesis. For this purpose we measured the natural remanent magnetization of the films
[Gendler et al., 2004];
this magnetization has been referred to above as laboratory synthesis magnetization (LSM).
As far as we know, those were the first LSM measurements in nanocomposite materials.
[29] In our case all samples were found to show LSM with its specific values varying from 0.08
E-2
to 18
E-2 Am2 kg-1
calculated for the real content of magnetite. This means that the magnetic
structure responsible for the interaction among the nanoparticles is forming as early as during the synthesis with the
indisputable role of the polymer.
[30] The scatter of the LSM values measured for the samples of the same composition, but taken
from different parts of the film, was found to be higher than the differences associated with changes in the
film composition. Therefore we did not find any LSM variations produced by changes in the film
composition, considering the limited amount of the material obtained in each synthesis, available for our
measurements.
|
Figure 9
|
[31] The laboratory synthesis magnetization (LSM) showed distinctly expressed anisotropy, similar to
the remanent magnetization acquired in the magnetic field of 450 mT. The parameter of planar anisotropy
kp for the
Cv concentrations ranging from 1.2% to 43% was substantially higher than 1, varying
nonmonotonically over the range of 2 to 11 (Figure 9). Therefore the total vector of natural remanent
magnetization acquired by polymer films in the course of the synthesis performed in the laboratory
conditions in the Earth magnetic field resides primarily in the film plane for all concentrations beginning
with
Cv=1.2. The exception was a sample with the lowest
Cv
concentration equal to 0.6%, which showed
kp=0.34, that is, the LSM vector was perpendicular to the film plane. The aim of the next step was to
calculate the coefficient of linear anisotropy,
k1. It was found that in the cases of all concentrations
k1 was higher than 1 and varied from 1.1 to 16.6 depending on the film composition (Figure 9).
The highest values of the anisotropy were found for the lowest concentrations of nanoparticles, this having
been potentially associated with large errors in measuring low LSM values, and calling for checking.
Therefore the total vector of the remanent magnetization acquired by polymer films of all concentrations,
beginning with
Cv=1.2, during the laboratory synthesis in the Earth magnetic field, lies not only in
the film plane, but also along its certain axis. This suggests the formation of chain-type structures
of nanoparticles in the Earth magnetic field at the expense of the PVA organizing role. It would be useful
to verify the presence or absence of this structure using a similar synthesis in a screened off space and
in the magnetic field of some fixed trend direction.
|
Figure 10
|
[32] The direct observation of the chain structures inferred from the magnetic measurements of chain
structures was performed using a Femto Scan probing microscope, operating as an atomic-force
microscope in the Center of Advanced Technologies, Moscow,
[Novakova et al., 2005a, 2005b].
Presented in Figure 10 is a photograph of the lower surface of the film ( Cv = 33%), borrowed from these papers.
In the case of large scales (~6
m m) one can see the rows of clustered particles arranged in one direction.
Judging by their contrasting behavior, these rows are located at different depths below the sample surface.
Moreover, they are scarce at the surface of the sample and grow in number with its depth, this being in agreement with the results
of Mössbauer spectroscopy. The distances between the nearest parallel rows of the particles, lying in
the same plane, vary within 200-400 nm, the thickness of the polymer envelopes between the nanoparticles
ranging from 8 nm to 10 nm.

Citation: Gendler, T. S., A. A. Novakova, and E. V. Smirnov (2005), Specific magnetic structure forming in polymer nanocomposites containing magnetite nanoparticles, Russ. J. Earth Sci., 7, ES4003, doi:10.2205/2005ES000177.
Copyright 2005 by the Russian Journal of Earth Sciences
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