RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES4003, doi:10.2205/2005ES000177, 2005
Specific magnetic structure forming in polymer nanocomposites containing magnetite nanoparticlesT. S. Gendler1, A. A. Novakova2, and E. V. Smirnov3
1Institute of Physics of the Earth, Russian Academy of Science, Moscow, Russia Contents
Abstract[1] The specific distribution of Fe 3 O 4 nanoparticles synthesized in situ in a polymeric PVA matrix was studied using the methods of magnetic measurements, transmission and depth-selective Mössbauer spectroscopy, and tunnel microscopy. The magnetic nanoparticles volumetric concentration Cv varied in the study samples from 0.6 vol. % to 43 vol. %. The size of the nanoparticles, measured using the X-ray diffraction, was found to be 10-20 nm. In the case of low Cv values the nanoparticles showed the composition of maghemite. At growing Cv concentrations, the product of the synthesis was partially oxidized magnetite. The contribution of the particles participating in the magnetic interaction at room temperature was estimated from the hyperfine magnetic splitting of the Mössbauer spectra. The blocking temperatures of the films of all compositions were found to be in the region of 300o C. This study revealed the high planar and linear magnetic anisotropy of the remanent saturation magnetization and of the remanent laboratory synthesis magnetization (LSM). The Mössbauer and microscopic studies revealed that during the synthesis the particles are distributed in the nanocomposite irregularly over the matrix, remaining almost isolated at the surface of the films and producing, in the lower part of the film, the chains of interacting nanoparticles, extending parallel to the film plain along the resulting trend. This chain structure is treated here as an artificial analog of fossil bacterial structures and biofilms, contributing to the magnetization of sedimentary rocks. Introduction[2] The recent years witnessed that the researchers, dealing with the magnetism of the environment, with the paleomagnetism of sedimentary rocks and biomagnetism, grew interested in the magnetic behavior of nanostructures, that is, in the structures of natural and artificial origin, consisting of the ensembles of ultrafine ferrimagnetic particles with sizes corresponding to the nanoscale. In spite of their insignificant amount in the total mass of the material, the magnetic nanoparticles of iron oxides and hydroxides play a significant role, for example, in surface sediments, in the enhancement of the magnetic signal in soil, providing information for climatic variations [Evans and Heller, 2003; Fassbinder et al., 1990]. Ultradispersed iron oxides and sulfides provide a basis for the vital activity of magnetotactic bacteria and are the products of the activity of iron-reducing and iron-oxidizing bacteria. Changes in the contents of ultradispersed biogenic magnetite, both of intracellular and extracellular origin, in the sediments are the excellent indicators of changes in the oxidation-reduction conditions. This was traced, for example, in the bottom sediments of the Baikal Lake [Peck and King, 1996], in the western equatorial Pacific Ocean (ODP, Site 805) [Tarduno et al., 1998], and in Lake Geneva [Gibbs-Eggar et al., 1999] using the disappearance of bacterial magnetite at the iron-redox boundary. Along with the chemical processes operating in the near-surface environment, biogeocenosis results in the formation of films, both of organic and inorganic origin, inside and at the surface of sediments, soils, and rocks [Hancock, 2001]. The formation of biofilms contributes both to the generation of new minerals and to the destruction of the minerals contained in the rocks [De Long et al., 1993; Krumbein et al., 2003]. Changes can be introduced into paleomagnetic records also as a result of the formation of nanoparticle ensembles. It is not accidental that the problems of the role of nanoparticles in the environmental magnetism and the necessity of combining the methods of physics, chemistry, biology, and materials technology in the study of nanoparticle ensembles were included into the program of the Conventional Conference on Rock Magnetism held in Santa Fe in 2004 [see the paper by Jackson and Banerjee, 2004].[3] The properties of the materials composed of nanoparticles, in particular their magnetic properties, differ fundamentally from those of their macroscopic analogs [Sohn et al., 1998]. For instance, ultrafine particles show significantly lower values of specific saturation magnetization Js, compared to their bulk analogs. In the case of magnetite nanoparticles this value is not higher than 30-60 Am2 kg-1, compared to the bulk value of 92 Am2 kg-1 at room temperature [Sato et al., 1987]. The Js/Jsbulk value declines nonlinearly with a drastic decrease in the particle size region from 15-5 nm. This decline of saturation magnetization is associated by many researchers with the formation of so-called dead layers at the surface of the particles. For example, in the case of acicular maghemite particles the nonmagnetic boundary is inferred to be about 6 nm [Berkowitz et al., 1968]; in the case of MnFe2O4 particles 5.6 nm in size it was found to be 0.6 nm [Zheng et al., 1998]. Moreover, the particles as small as that show hysteresis properties at room temperature and the paradoxical growth of the Curie temperature by 160 K compared to the bulk sample. This growth of the magnetic ordering temperature is believed to have been associated with the redistribution of the Mn and Fe cations in the tetra- and octahedral sublattices of the spinel structure in the thin boundary layer. The example of another specific feature of the magnetic properties of thin magnetite films, 50 nm thick, at the MgO surface, is the absence of saturation in the fields as high as 7T, which is believed to have been associated with the high density of the resulting antiphase domain boundaries [Rudee et al., 1997]. The exchange interaction at the boundaries of this type changes compared to the bulk Fe3O4 with the origin of a new 180o Fe-O-Fe interaction which leads to antiferromagnetic coupling at the boundaries of the domains [Voogt et al., 1998]. The growing interest in studying the peculiar properties of magnetic nanoparticles is proved by the great number of papers presented at several sections of the International Conferences on Magnetism (ICM-2003, ICM-2004). The analysis of these papers shows that in terms of the nanosizes the differences between amorphous, disordered, solid and even between biological structures become insignificant. Very productive in this connection is the use of modern supersensitive methods of physics, chemistry, biology, and materials technology for studying the products of biogeochemical processes and their artificial analogs. A significant role in this case belongs to laboratory modeling in the field of inorganic surface chemistry and, in particular, to the synthesis of polymeric nanocomposites. As proved by some researchers [Sohn et al., 1998], these materials show new magnetic properties, produced both by the size effects and by the significant role of the polymeric matrix. The study of the regular distribution and behavior of ferrimagnetic particles in the matrix of this kind, which is taken here as an analog of the intracellular or extracellular organic matter of the envelopes of biomineralized particles in nature, may throw light on the mechanism of the formation and magnetic properties of bacterial magnetosomes and biofilms. [4] Reported in this paper are some of the results obtained during the study of the magnetic properties of nanocomposite materials based on polyvinyl alcohol (PVA) and magnetite nanoparticles [Gendler et al., 2004; Novakova et al., 2002, 2003, 2005a, 2005b]. Preparation of Samples and the Methods of Their Study[5] The polymer nanocomposites examined in this study had been synthesized at the Chemical Faculty of the Moscow State University in Petri dishes in the ambient laboratory magnetic field, disregarding its direction. The synthesis was made using a 4% PVA water solution with the addition of a mixture of FeCl2![]() ![]() [6] The macroscopic magnetic characteristics of the films, such as saturation magnetization ( Js ), remanent saturation magnetization ( Jrs ), natural remanent magnetization ( Jn ), as well as their temperature dependence Js(T) and Jrs(T), were carried out in the Geomagnetic Laboratory of the Institute of Physics of the Earth, Russian Academy of Science, using a JR-4 magnetometer (AGIKO), a vibromagnetometer (VSM), and a two-component laboratory thermomagnetometer (Orion Company, Borok). The magnetic field used to produce Js and Jrs was equal to 450 mT. The coercive force ( Hc ) and the remanent coercive force ( Hcr ) were measured using a coercimeter in the maximum magnetic field of 1.7T (Orion Company, Borok). In this case Jn denotes the magnetization acquired by the films during the laboratory synthesis in the ambient geomagnetic field. Conventionally, we will refer to this magnetization as laboratory synthesis magnetization (LSM). In order to obtain the LSM vector and Jrs characteristics and study the anisotropy of these types of magnetization we measured the films in a cubic nonmagnetic organic glass container in three orthogonal directions X, Y, and Z. The Z axis was taken to be perpendicular to the film, the X and Y axes residing in the film plane. Since this synthesis was carried out without fixing the direction of the Earth magnetic field, the direction of the maximum values of the LSM and J rs vectors in the film plane was chosen after the measurements to be the X-axis. [7] The Mössbauer transmission spectra were obtained in the laboratory of the Solid State Physics Department, Moscow State University, using a spectrometer of constant acceleration. The radioactive source was Co57(Rh), the velocity scale of the spectrometer was calibrated using a standard a -Fe absorber. All experimental spectra were subject to compute fitting using a special program based on the Lorentz form of spectra lines. To check the homogeneity of the particle distributions of over the thickness of the film, Mössbauer spectra were obtained for different thicknesses below the surfaces of the study samples. Depth selective Mössbauer spectroscopy was performed in the geometry of back scattering with registration of two types of secondary radiation: conversion electrons (information from the subsurface layer ~0.3 m m) and conversion X-ray (information from the subsurface lay ~20 m m) [Kuprin and Novakova, 1992]. 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].
[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 /S B ) 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 Am2 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 O2- 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].
[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
20
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[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 [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.
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[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
[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.
Conclusion[33] The results of this study show that during the synthesis of polymer films the nanoparticles of magnetite are distributed over the matrix in the heterogeneously manner. They remain almost noninteracting and supermagnetic in the vicinity of the surface of the film, yet in the film itself they produce layers with chain structures, parallel to the film plane, similar to those developed in magnetotactic bacteria. The stability of these chain structures is controlled by the dipole-dipole interaction among the nanoparticles, performed via the polymer matrix at the expense of the elastic energy of the envelopes of the organic matter separating the particles. The result of this mechanism is the possibility of the formation of a specific domain structure and the fixing of the natural remanent magnetization in the ensemble of the superparamagnetic nanoparticles. As follows from the paper by Jianbao et al. [2000], along with the growth of the pressure applied to the maghemite particles covered by surfactant envelopes, the blocking temperature increased significantly in association with the growing magnetic interaction among the particles. In a composite material, its matrix ensured not only the solidity of the material but was also responsible for stress distribution at the expense of the interaction between the matrix and the filling material at the phase contacts. As to the nanocomposite material discussed, where nanoparticles were synthesized in situ in a nonmagnetic polymer matrix, the polymer elastic forces produce stress at the particle surface, similar to the external pressure. This stress arising in the space between the particles, filled with polymer molecules, leads to the growth of microstress in the particles themselves, recorded by X-ray diffraction. The further convergence of the particles and the elastic energy of the matrix create conditions favorable for high magnetic interaction among the superparamagnetic particles, which fixes magnetization with blocking temperature of about 300o C.[34] Shcherbakov et al. [1997] calculated the contribution of the elastic energy of envelopes from two-layer lipid membranes, 6 nm thick, to the stability of the chain structure of single-domain magnetite particles in magnetotactic bacteria. This problem can be transformed to a chain of superparamagnetic particles, separated by a PVA matrix, and this artificial system can be ranked as an analog of the biofilm produced by a colony of Fe-bacteria at the surface of minerals, where the bacteria become static. This result is important also for a new view for the formation of chemical remanent magnetization in nature, controlled by solutions. For instance, the laboratory experiments with amorphous ferric hydroxide, placed in the environment consisting of underground water, the samples of which had been collected from the overlying Cubero Sandstone, and the bacteria obtained from subsurface core samples (250 m below the ground surface) from the Morrison Formation, showed the substantial microbe recovery of the Fe(III) ions to their Fe(II) form [Fredrikson et al., 1998]. The primary reduction products of amorphous iron hydroxide (30% to 84%), produced during the incubation of 3 to 25 days, were siderite grains, 1 m m to 3 m m in size, vivianite crystals, 5-10 m m long and 0.5-1 m m wide, and the aggregates of magnetite grains of a few nanometers. The substantial magnetic interaction among magnetite particles in the aggregates of this kind might have been caused or intensified by the organic material of the cells. 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Keywords: magnetic nanoparticles, thin films, magnetic properties, magnetic anisotropy. Index Terms: 1500 Geomagnetism and Paleomagnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1518 Geomagnetism and Paleomagnetism: Magnetic fabrics and anisotropy. ![]() Citation: 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 SciencesPowered by TeXWeb (Win32, v.2.0). |