- Email: [email protected]
Magnetic polymers of maghemite (γ-Fe2O3) and polyvinyl alcohol
ARTICLE IN PRESS
Physica B 354 (2004) 149–153 www.elsevier.com/locate/physb
Magnetic polymers of maghemite (g-Fe2O3) and polyvinyl alcohol Cecilia Albornoza, Elsa E. Sileob, Silvia E. Jacoboa, a
LAFMACEL, Facultad de Ingenierı´a, UBA, Paseo Colo´n 850 (1063) Capital Federal, Argentina b INQUIMAE, Facultad de Cs Exactas y Naturales, UBA, Capital Federal, Argentina
Abstract Oxide particles synthesized and dispersed in aqueous medium, containing different polymeric forms, can be trapped in solid matrices keeping a high dispersion degree. The oxide/polymer mass ratio determines the average distance between the particles existing in the sol. In this work, ultrafine maghemite particles were prepared in aqueous solution containing different concentrations of polyvinyl alcohol (PVA). The average diameter of the obtained particles was below 20 nm. Adsorption of PVA on the surface of maghemite was examined by infrared spectra and thermogravimetric analysis. The dispersion and agglomeration of the particles were different in an aqueous solution at 4 wt% because the magnetite particles were of a small diameter. Magnetic properties of these composites are preserved. r 2004 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Composites; Superparamagnetic particles; PVA; g-Fe2O3
1. Introduction An extensive literature on iron-oxide and ferrite nanostructures and their composites [1–5] attests to the vast technological importance of these materials for broad application in the nanotechnologies of information storage, bioprocess and ferrofluids [6]. Thus, the design and synthesis of magnetic nanostructures with controlled size
Corresponding author.
E-mail address: sjacobo@fi.uba.ar (S.E. Jacobo).
and uniform dispersion is an important subject of current materials research [7]. The particles synthesized and dispersed in aqueous medium can be trapped in different polymeric solid matrices keeping a high dispersion state. The oxide/polymer mass ratio determines the average distance between the nuclei existing in the sol, isolated particles or aggregates. Various surfactants, e.g., sodium oleic acid or dodecylamine, are usually added to the solution containing metal oxide particles in order to enhance the dispersibility in an aqueous medium [8,9]. In the case of maghemite polyvinyl alcohol (PVA) may
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.09.038
ARTICLE IN PRESS 150
C. Albornoz et al. / Physica B 354 (2004) 149–153
be added after the precipitation process as a protective agent to stabilize the colloidal dispersions [10]. In this work, we report the synthesis of maghemite particles in the presence of PVA, in an attempt to modify the surface of freshly prepared particles with the coexisting hydrophilic polymer. We examined the effects of the PVA concentration on particle size, crystalinity and magnetic properties of the composites.
2. Materials and methods 2.1. Preparation of maghemite particles and its slurries Several solutions (100 ml, 0.064 mol/l) containing Fe(III) and Fe(II) in the molar ratio 1:9, were prepared at room temperature, from FeCl2 6H2O, FeCl3 3H2O, and water containing different amounts of PVA. The solutions were oxidized in a closed vessel at a constant pH of 7 using an air flow rate of 10 ml/min. The PVA used presented a saponification value of 87% and an average molecular weight of 22,000. Precipitates and supernatant were separated by filtration. Precipitates were dried, after washing three times with deionized water, under reduced pressure at 40 1C for 48 h. Concentration of PVA in the prepared solutions were 0, 1, 2, 3 and 4 wt%.
measured isomer shifts (IS) are referred to a-iron. Magnetic measures were performed at room temperature using a vibrating sample magnetometer (VSM).
3. Results and discussion The XRD peaks of all samples corresponds to a cubic cell identified as g-Fe2O3 (maghemite). The peaks did not shift but became broader by adding PVA. The broadening is mainly attributed to the decrease in the crystalline size [11]. The dispersibility of particles also increased significantly in the PVA aqueous solution. Calculated crystal size for the as-prepared maghemite, without PVA, was estimated as 22 nm. All samples prepared in the presence of PVA showed a significant reduction in crystalline size (15–10 nm). Fig. 1 shows the total weight loss at 720 1C for the PVA-coated g-Fe2O3 particles. The loss increases with increasing PVA concentration in the original solutions. The weight loss after heating is attributed predominantly in samples up to 2% to the amount of PVA adsorbed on the maghemite. Samples prepared in 3% and 4% PVA do not achieve weight constant at 720 1C, so the amount of PVA shown in Fig. 1 does not probably correspond to the total amount adsorbed on the maghemite.
2.2. Characterization The solids were characterized by X-ray diffractometry (XRD). Relative crystalline size was determined from the full-width at half-medium of the 311 X-ray diffraction peak. Adsorption of PVA on the surface of maghemite powders was examined by infrared spectroscopy by a ThermoNicolet FT-IR Avatar 320. The amounts of PVA attached to the maghemite particles were calculated from thermogravimetric measures (TG) under a linear heating of 10 K/min, in a nitrogen atmosphere. The final temperature was 720 1C. A 57 Co source (Rh matrix) was used to record the Mo¨ssbauer spectra at room temperature of the samples in constant acceleration mode. The
Fig. 1. The amount of total weight loss as a function of initial PVA concentration after heating PVA coated g-Fe2O3 up to 720 1C.
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
Figs. 2 and 3 show different regions of the IR spectra of PVA and of precipitated g-Fe2O3 particles in solutions containing different wt% PVA. As shown in Fig. 2 three absorption bands
Fig. 2. FTIR spectra of PVA and of composites synthesized at different concentrations of PVA.
Fig. 3. FTIR spectra of g-Fe2O3 and of composites synthesized at different concentrations of PVA.
151
are observed at 3420, 2930, and 2347 cm1 being assigned to O–H and C–H stretching of PVA, while in Fig. 3, absorption bands assigned to the C–H deformation vibration of PVA and to the C–O stretching vibration of PVA are shown. Table 1, related to Fig. 3, shows some characteristic absorption bands of g-Fe2O3 and composites. These results indicate that the interaction between g-Fe2O3 particles and PVA was not significant to the absorption in the IR band owing to g-Fe2O3, since little chemical shift of IR band due to PVA was observed (Table 1). In all samples, we can appreciate, between 1500 and 450 cm1, three absorption bands assigned to iron oxides. The intensity of the band from g-Fe2O3 at (500 cm1) decreases relative to the bands of PVA with increasing PVA, particularly for 2 wt% PVA concentration where this band disappears. These results indicate that PVA is irreversibly adsorbed on the surface of maghemite particles even after careful washing, and that at 2 wt% PVA the surface of the particle is significantly covered by PVA. The observation of the absorption in the 3% PVA sample implies that probably some clusters of maghemite have been formed. The room temperature Mo¨ssbauer spectra of g-Fe2O3/PVA composites for different percentages of PVA are shown in Fig. 4. The spectra show a gradual transition from a six-line (magnetically split) spectrum to a broad singlet and a quadrupole doublet as the wt% PVA increases from 0% to 2%. Evaluation of Mo¨ssbauer spectra with two sextets and a central doublet allowed the area ratio of the paramagnetic and magnetic parts to be determined. While in the bulk g-Fe2O3 (0 wt% PVA) the highly symmetric octahedral and tetrahedral iron sites of the spinel structure produce vanishing quadrupole splitting values, distorted coordination symmetry at the surface of the
Table 1 Characteristic absorption bands (cm1) of g-Fe2O3 and of composites synthesized at different concentrations of PVA
g-Fe2O3 (0% PVA) g-Fe2O3(0.5% PVA) g-Fe2O3 (2% PVA) g-Fe2O3 (3% PVA)
C–H (cm1)
C–O (cm1)
g-Fe2O3 (cm1)
— 1621–1410 1627–1416 1644–1416
— 1088 1088 1094
890–790–597 883–790 848 842–573
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
152
Fig. 4. Mo¨ssbauer spectra of g-Fe2O3 and of composites synthesized at different concentrations of PVA. Table 2 Magnetic properties of g-Fe2O3 and of composites synthesized at different concentrations of PVA wt% PVA
Ms (emu/g)
Hc (Oe)
0 1 2 3 4
75 31.7 7.7 8.2 12.9
15 10 7 6.7 5
the samples with crystalline size greater than 20 nm showed a saturation magnetization of 74 emu/g (theoretical: 76 emu/g) but it decreased to 34 emu/g for particles in which the crystalline size was about 50 A˚. Berkowitz and Lahut [15] found that the presence of a surfactant layer on particles can reduce Ms by more than 50%. They explained the effect in terms of a chemical bonding of the organic molecules to the ferrite which decreases the magnetization on the surface layers of the particles. Sohon et al. [16] found that the origin of this reduction is a manifestation of quantum-size effects in nanometer-size particles, which result in noncolinear spin arrangement within size-confined structures. The coercive force of ferromagnetic particles decreases rapidly when the particle size decreases and so does the saturation magnetization. The anomalous behavior of samples prepared in 3% and 4% PVA can be explained as follows: in the more concentrated samples, the particles agglomerate in clusters so interparticle interactions increases. Magnetic and Mo¨ssbauer results agree with this suggestion.
4. Conclusion particles can produce sizable quadrupole splittings [12]. For sample with 2 wt% PVA, the spectrum primarily consists of a single central doublet with isomer shift d ¼ 0:35 mm=s and quadrupole splitting DE Q ¼ 0:65 mm=s; characteristic of nanometer-size superparamagnetic iron oxide particles above their blocking temperature [13]. Table 2 shows magnetic values of samples prepared in different solutions of PVA. The saturation magnetization (Ms) at room temperature was measured from a hysteresis loop at a field of 10,000 Oe. For the maghemite sample with 0% PVA (about 75 emu/g, crystalline size 22 nm) was very close to the theoretical value. At room temperature, a coercive force of a few oersted was observed, and no remanent magnetization (Mr) was observed in samples coated with PVA. The extrinsic magnetic properties of particles depend strongly upon their shape and size. Berkowitz and Schuele [14] found a marked dependence on crystalline size for saturation magnetization (Ms) of g-Fe2O3. All
In this work, ultrafine maghemite particles with an average size of about 10–15 nm were prepared in an aqueous solution containing different concentrations of PVA. Experimental results show that g-Fe2O3 particles prepared in a solution with 2 wt% PVA are completely covered by the polymer as IR results and Mo¨ssbauer analysis indicated. The saturated magnetization and coercive force show low magnetic values owing to the presence of a nonmagnetic layer which reduces interparticle interaction, although some soft agglomerates cannot be discarded. Samples prepared with higher wt% PVA seem to form a large number of clusters and agglomerated structures. Further investigation is now in progress.
Acknowledgements We thank F. Vignolo for diffraction measures and B. Arcondo for Mo¨ssbauer patterns (FIUBA),
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
C. Chiliotte for magnetic measures (FCEyN, UBA) and UBACyT (I-015 ) for financial support.
References [1] D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770. [2] C.L. Chien, Annu. Rev. Mater. Sci. 25 (1995) 129. [3] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [4] R.D. Shull, H.M. Kerch, J.J. Ritter, J. Appl. Phys. 75 (1994) 6840. [5] F. Bodker, S. Mørup, S. Linteroth, Phys. Rev. Lett. 72 (1994) 282. [6] A.E. Berkowitz, F.T. Parker, F.E. Spada, D. Margulies, Magnetic Properties of Fine Particles, Elsevier, Amsterdan, 1991, p. 309.
153
[7] D. de Caro, T.O. Ely, A. Mari, B. Chaudret, E. Snoeck, M. Respaud, J.M. Broto, A. Fert, Chem. Mater. 8 (1996) 1987. [8] A. Wooding, M. Kilner, D.B. Lambrick, J. Colloid Interface Sci. 144 (1991) 236. [9] V. Pillai, P. Kurman, D.O. Shah, J. Magn. Magn.Mater. 116 (1992) L299. [10] R.K. Iler, J. Colloid Interface Sci. 51 (1975) 388. [11] J.W. Lee, T. Isobe, M. Senna, J. Colloid Interface Sci. 177 (ISS2) (1997) 490. [12] L. Zang, G.C. Papaefthymiou, J.Y. Ying, J. Appl. Phys. 81 (1997) 6892. [13] W. Kundig, H. Bommel, G. Konstabaris, R. Lundquist, Phys. Rev. 142 (1966) 327. [14] A.E. Berkowitz, W. Schuele, J. Appli. Phys. 30 (1959) 154S. [15] A.E. Berkowitz, J.A. Lahut, AIP Conf. Proc. 10 (Part 2) (1973) 966. [16] B.H. Sohon, R.E. Cohen, G.C. Papaefthymiou, J. Magn. Magn. Mater. 182 (1998) 216.
Physica B 354 (2004) 149–153 www.elsevier.com/locate/physb
Magnetic polymers of maghemite (g-Fe2O3) and polyvinyl alcohol Cecilia Albornoza, Elsa E. Sileob, Silvia E. Jacoboa, a
LAFMACEL, Facultad de Ingenierı´a, UBA, Paseo Colo´n 850 (1063) Capital Federal, Argentina b INQUIMAE, Facultad de Cs Exactas y Naturales, UBA, Capital Federal, Argentina
Abstract Oxide particles synthesized and dispersed in aqueous medium, containing different polymeric forms, can be trapped in solid matrices keeping a high dispersion degree. The oxide/polymer mass ratio determines the average distance between the particles existing in the sol. In this work, ultrafine maghemite particles were prepared in aqueous solution containing different concentrations of polyvinyl alcohol (PVA). The average diameter of the obtained particles was below 20 nm. Adsorption of PVA on the surface of maghemite was examined by infrared spectra and thermogravimetric analysis. The dispersion and agglomeration of the particles were different in an aqueous solution at 4 wt% because the magnetite particles were of a small diameter. Magnetic properties of these composites are preserved. r 2004 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Composites; Superparamagnetic particles; PVA; g-Fe2O3
1. Introduction An extensive literature on iron-oxide and ferrite nanostructures and their composites [1–5] attests to the vast technological importance of these materials for broad application in the nanotechnologies of information storage, bioprocess and ferrofluids [6]. Thus, the design and synthesis of magnetic nanostructures with controlled size
Corresponding author.
E-mail address: sjacobo@fi.uba.ar (S.E. Jacobo).
and uniform dispersion is an important subject of current materials research [7]. The particles synthesized and dispersed in aqueous medium can be trapped in different polymeric solid matrices keeping a high dispersion state. The oxide/polymer mass ratio determines the average distance between the nuclei existing in the sol, isolated particles or aggregates. Various surfactants, e.g., sodium oleic acid or dodecylamine, are usually added to the solution containing metal oxide particles in order to enhance the dispersibility in an aqueous medium [8,9]. In the case of maghemite polyvinyl alcohol (PVA) may
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.09.038
ARTICLE IN PRESS 150
C. Albornoz et al. / Physica B 354 (2004) 149–153
be added after the precipitation process as a protective agent to stabilize the colloidal dispersions [10]. In this work, we report the synthesis of maghemite particles in the presence of PVA, in an attempt to modify the surface of freshly prepared particles with the coexisting hydrophilic polymer. We examined the effects of the PVA concentration on particle size, crystalinity and magnetic properties of the composites.
2. Materials and methods 2.1. Preparation of maghemite particles and its slurries Several solutions (100 ml, 0.064 mol/l) containing Fe(III) and Fe(II) in the molar ratio 1:9, were prepared at room temperature, from FeCl2 6H2O, FeCl3 3H2O, and water containing different amounts of PVA. The solutions were oxidized in a closed vessel at a constant pH of 7 using an air flow rate of 10 ml/min. The PVA used presented a saponification value of 87% and an average molecular weight of 22,000. Precipitates and supernatant were separated by filtration. Precipitates were dried, after washing three times with deionized water, under reduced pressure at 40 1C for 48 h. Concentration of PVA in the prepared solutions were 0, 1, 2, 3 and 4 wt%.
measured isomer shifts (IS) are referred to a-iron. Magnetic measures were performed at room temperature using a vibrating sample magnetometer (VSM).
3. Results and discussion The XRD peaks of all samples corresponds to a cubic cell identified as g-Fe2O3 (maghemite). The peaks did not shift but became broader by adding PVA. The broadening is mainly attributed to the decrease in the crystalline size [11]. The dispersibility of particles also increased significantly in the PVA aqueous solution. Calculated crystal size for the as-prepared maghemite, without PVA, was estimated as 22 nm. All samples prepared in the presence of PVA showed a significant reduction in crystalline size (15–10 nm). Fig. 1 shows the total weight loss at 720 1C for the PVA-coated g-Fe2O3 particles. The loss increases with increasing PVA concentration in the original solutions. The weight loss after heating is attributed predominantly in samples up to 2% to the amount of PVA adsorbed on the maghemite. Samples prepared in 3% and 4% PVA do not achieve weight constant at 720 1C, so the amount of PVA shown in Fig. 1 does not probably correspond to the total amount adsorbed on the maghemite.
2.2. Characterization The solids were characterized by X-ray diffractometry (XRD). Relative crystalline size was determined from the full-width at half-medium of the 311 X-ray diffraction peak. Adsorption of PVA on the surface of maghemite powders was examined by infrared spectroscopy by a ThermoNicolet FT-IR Avatar 320. The amounts of PVA attached to the maghemite particles were calculated from thermogravimetric measures (TG) under a linear heating of 10 K/min, in a nitrogen atmosphere. The final temperature was 720 1C. A 57 Co source (Rh matrix) was used to record the Mo¨ssbauer spectra at room temperature of the samples in constant acceleration mode. The
Fig. 1. The amount of total weight loss as a function of initial PVA concentration after heating PVA coated g-Fe2O3 up to 720 1C.
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
Figs. 2 and 3 show different regions of the IR spectra of PVA and of precipitated g-Fe2O3 particles in solutions containing different wt% PVA. As shown in Fig. 2 three absorption bands
Fig. 2. FTIR spectra of PVA and of composites synthesized at different concentrations of PVA.
Fig. 3. FTIR spectra of g-Fe2O3 and of composites synthesized at different concentrations of PVA.
151
are observed at 3420, 2930, and 2347 cm1 being assigned to O–H and C–H stretching of PVA, while in Fig. 3, absorption bands assigned to the C–H deformation vibration of PVA and to the C–O stretching vibration of PVA are shown. Table 1, related to Fig. 3, shows some characteristic absorption bands of g-Fe2O3 and composites. These results indicate that the interaction between g-Fe2O3 particles and PVA was not significant to the absorption in the IR band owing to g-Fe2O3, since little chemical shift of IR band due to PVA was observed (Table 1). In all samples, we can appreciate, between 1500 and 450 cm1, three absorption bands assigned to iron oxides. The intensity of the band from g-Fe2O3 at (500 cm1) decreases relative to the bands of PVA with increasing PVA, particularly for 2 wt% PVA concentration where this band disappears. These results indicate that PVA is irreversibly adsorbed on the surface of maghemite particles even after careful washing, and that at 2 wt% PVA the surface of the particle is significantly covered by PVA. The observation of the absorption in the 3% PVA sample implies that probably some clusters of maghemite have been formed. The room temperature Mo¨ssbauer spectra of g-Fe2O3/PVA composites for different percentages of PVA are shown in Fig. 4. The spectra show a gradual transition from a six-line (magnetically split) spectrum to a broad singlet and a quadrupole doublet as the wt% PVA increases from 0% to 2%. Evaluation of Mo¨ssbauer spectra with two sextets and a central doublet allowed the area ratio of the paramagnetic and magnetic parts to be determined. While in the bulk g-Fe2O3 (0 wt% PVA) the highly symmetric octahedral and tetrahedral iron sites of the spinel structure produce vanishing quadrupole splitting values, distorted coordination symmetry at the surface of the
Table 1 Characteristic absorption bands (cm1) of g-Fe2O3 and of composites synthesized at different concentrations of PVA
g-Fe2O3 (0% PVA) g-Fe2O3(0.5% PVA) g-Fe2O3 (2% PVA) g-Fe2O3 (3% PVA)
C–H (cm1)
C–O (cm1)
g-Fe2O3 (cm1)
— 1621–1410 1627–1416 1644–1416
— 1088 1088 1094
890–790–597 883–790 848 842–573
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
152
Fig. 4. Mo¨ssbauer spectra of g-Fe2O3 and of composites synthesized at different concentrations of PVA. Table 2 Magnetic properties of g-Fe2O3 and of composites synthesized at different concentrations of PVA wt% PVA
Ms (emu/g)
Hc (Oe)
0 1 2 3 4
75 31.7 7.7 8.2 12.9
15 10 7 6.7 5
the samples with crystalline size greater than 20 nm showed a saturation magnetization of 74 emu/g (theoretical: 76 emu/g) but it decreased to 34 emu/g for particles in which the crystalline size was about 50 A˚. Berkowitz and Lahut [15] found that the presence of a surfactant layer on particles can reduce Ms by more than 50%. They explained the effect in terms of a chemical bonding of the organic molecules to the ferrite which decreases the magnetization on the surface layers of the particles. Sohon et al. [16] found that the origin of this reduction is a manifestation of quantum-size effects in nanometer-size particles, which result in noncolinear spin arrangement within size-confined structures. The coercive force of ferromagnetic particles decreases rapidly when the particle size decreases and so does the saturation magnetization. The anomalous behavior of samples prepared in 3% and 4% PVA can be explained as follows: in the more concentrated samples, the particles agglomerate in clusters so interparticle interactions increases. Magnetic and Mo¨ssbauer results agree with this suggestion.
4. Conclusion particles can produce sizable quadrupole splittings [12]. For sample with 2 wt% PVA, the spectrum primarily consists of a single central doublet with isomer shift d ¼ 0:35 mm=s and quadrupole splitting DE Q ¼ 0:65 mm=s; characteristic of nanometer-size superparamagnetic iron oxide particles above their blocking temperature [13]. Table 2 shows magnetic values of samples prepared in different solutions of PVA. The saturation magnetization (Ms) at room temperature was measured from a hysteresis loop at a field of 10,000 Oe. For the maghemite sample with 0% PVA (about 75 emu/g, crystalline size 22 nm) was very close to the theoretical value. At room temperature, a coercive force of a few oersted was observed, and no remanent magnetization (Mr) was observed in samples coated with PVA. The extrinsic magnetic properties of particles depend strongly upon their shape and size. Berkowitz and Schuele [14] found a marked dependence on crystalline size for saturation magnetization (Ms) of g-Fe2O3. All
In this work, ultrafine maghemite particles with an average size of about 10–15 nm were prepared in an aqueous solution containing different concentrations of PVA. Experimental results show that g-Fe2O3 particles prepared in a solution with 2 wt% PVA are completely covered by the polymer as IR results and Mo¨ssbauer analysis indicated. The saturated magnetization and coercive force show low magnetic values owing to the presence of a nonmagnetic layer which reduces interparticle interaction, although some soft agglomerates cannot be discarded. Samples prepared with higher wt% PVA seem to form a large number of clusters and agglomerated structures. Further investigation is now in progress.
Acknowledgements We thank F. Vignolo for diffraction measures and B. Arcondo for Mo¨ssbauer patterns (FIUBA),
ARTICLE IN PRESS C. Albornoz et al. / Physica B 354 (2004) 149–153
C. Chiliotte for magnetic measures (FCEyN, UBA) and UBACyT (I-015 ) for financial support.
References [1] D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770. [2] C.L. Chien, Annu. Rev. Mater. Sci. 25 (1995) 129. [3] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [4] R.D. Shull, H.M. Kerch, J.J. Ritter, J. Appl. Phys. 75 (1994) 6840. [5] F. Bodker, S. Mørup, S. Linteroth, Phys. Rev. Lett. 72 (1994) 282. [6] A.E. Berkowitz, F.T. Parker, F.E. Spada, D. Margulies, Magnetic Properties of Fine Particles, Elsevier, Amsterdan, 1991, p. 309.
153
[7] D. de Caro, T.O. Ely, A. Mari, B. Chaudret, E. Snoeck, M. Respaud, J.M. Broto, A. Fert, Chem. Mater. 8 (1996) 1987. [8] A. Wooding, M. Kilner, D.B. Lambrick, J. Colloid Interface Sci. 144 (1991) 236. [9] V. Pillai, P. Kurman, D.O. Shah, J. Magn. Magn.Mater. 116 (1992) L299. [10] R.K. Iler, J. Colloid Interface Sci. 51 (1975) 388. [11] J.W. Lee, T. Isobe, M. Senna, J. Colloid Interface Sci. 177 (ISS2) (1997) 490. [12] L. Zang, G.C. Papaefthymiou, J.Y. Ying, J. Appl. Phys. 81 (1997) 6892. [13] W. Kundig, H. Bommel, G. Konstabaris, R. Lundquist, Phys. Rev. 142 (1966) 327. [14] A.E. Berkowitz, W. Schuele, J. Appli. Phys. 30 (1959) 154S. [15] A.E. Berkowitz, J.A. Lahut, AIP Conf. Proc. 10 (Part 2) (1973) 966. [16] B.H. Sohon, R.E. Cohen, G.C. Papaefthymiou, J. Magn. Magn. Mater. 182 (1998) 216.