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].

2005ES000177-fig01
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
2005ES000177-fig02
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].

2005ES000177-fig03
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''.

2005ES000177-fig04
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 Cvge 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.

2005ES000177-fig05
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/Jsf0sim0.15-0.5 ), 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.

2005ES000177-fig06
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 Tsim 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 Tsim 300o C as a true blocking temperature.

2005ES000177-fig07
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 times 40 times 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.

2005ES000177-fig08
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 cdot E-2 to 18 cdot 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.

2005ES000177-fig09
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.

2005ES000177-fig10
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.


RJES

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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