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Keywords:

  • block copolymers;
  • electron microscopy;
  • ESR/EPR;
  • magnetic properties;
  • nanoparticles

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

The preparation and characterization of titanium-doped iron oxides–styrene isoprene styrene block copolymer composites is reported. The precursor, as-synthesized nanoparticles, contains akaganeite, maghemite, and titanium-doped maghemite. Hysteresis loops obtained by SQUID measurements revealed an exchange biased field assigned to the presence of akaganeite nanoparticles. It was found that the coercive field and the exchange bias field decreases as the temperature of the sample is increased. The temperature dependence of the magnetization revealed a blocking temperature of about 220 K. The ESR spectra in the temperature range 150 K to about 300 K are single broad resonance lines. It was observed that the resonance line position is strongly temperature dependent due to the combined action of the external and molecular magnetic fields. The decrease of the resonance line width as the temperature is increased is governed by the reorientation of the magnetization. Thermally activated movements of magnetic nanoparticles within the soft phase (polyisoprene) of the matrix may also contribute to resonance line narrowing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3432–3437, 2005


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

Polymers can be excellent matrix materials to control particle size and particle assembly in the production of nanocomposites with different functionalities. The dispersion of magnetic particles in polymeric matrices presents a particular interest for future ultrahigh-density recording media and electromagnetic filters,1 specific drug delivery through magnetic guidance, and contrast agents in magnetic resonance imaging. The porous structure of polymers and the relatively large free-volume above the glass-transition temperature provides the space necessary for the free rotation of magnetic nanoparticles. Hence, the magnetic composites based on polymeric matrices are expected to present new properties.1, 2 This study focused on the physical properties of magnetic nanoparticles dispersed in a block copolymer.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

Titanium-doped γ-iron oxides were prepared by laser-induced pyrolysis of a gas mixture of Fe(CO)5 and TiCl4.3 Laser pyrolysis is a gas-phase method, which is based on the overlapping of the emission line of the laser with an absorption line of one or more precursors. The experimental apparatus was described previously.4, 5 C2H4 was used both as sensitizer (as absorbant of the CO2 laser radiation) and as carrier for the reactant vapors to be introduced within the reaction zone. Air was used as oxidizer. A continuous wave of CO2 laser (maximum output power about 100 W) was directed through a NaCl window onto a small reaction central reaction volume defined by the crossing of the laser beam and the effusive gases exiting through a nozzle system. The nucleated nanoparticles were entrained by the Ar confinement flow and the flow of exhaust gases toward a collection chamber provided with a filter in the direction of the rotary pump.

The properties of these nanoparticles were determined by different analytical methods,3 such as electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction. Transmission electron microscopy studies (TEM) were carried out by a Philips CM 200 FEG instrument. For the TEM measurements, the nanoparticles were deposited on TEM carbon grids.

Composites based on γ-doped iron oxides were obtained by mixing these nanoparticles with a solution of styrene-isoprene-styrene block copolymer (from Aldrich) in high-purity toluene (from Aldrich). For a good homogenization of nanoparticles within the polymer, the solution containing the nanoparticles was sonicated for about 2 days at very low power. The sonication conditions (extremely low power of the order of 10 W and very long sonication time) were selected to prevent polymer degradation. DSC measurements confirmed that the glass and melting transition temperatures of pristine films (containing no magnetic nanoparticles) are not shifted by sonication. Thin films were obtained by solvent evaporation, in an oven, at 80 °C for several days. The as-obtained nanocomposite films were measured by SQUID. Ferromagnetic resonance measurements were done by using a Bruker SP 300 spectrometer, operating in X band (ca. 9.5 GHz). The temperature dependence of FMR spectra has been investigated.

RESULTS AND DISCUSSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

Titanium-doped γ-iron oxide samples with increasing titanium content were synthesized by varying the TiCl4 vapor flow.3 Chemical analysis performed on some of the obtained of Ti-doped samples revealed that the magnetic nanoparticles contained about 0.84 wt % Ti. TEM micrographs of Ti-doped iron nanoparticles are shown in Figure 1(A). A relatively narrow distribution of magnetic particles diameters, with an average diameter of about 5 nm [see Fig. 1(B)] is observed. Selected area electron diffraction (SAED) patterns [see Fig. 1(C)] indicate the presence of γ Fe2O3 (maghemite). X-ray diffractograms3 proved the formation of γ Fe2O3 and β FeO(OH) (akaganeite) phases. Diffraction methods cannot distinguish between maghemite, magnetite, and titanium maghemite.5 The akaganeite is produced in Cl2 atmosphere resulted by the decomposition of TiCl4.3

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Figure 1. The TEM micrograph of titanium-doped iron nanoparticles (A). The particle size distribution of titanium-doped iron nanoparticles (B). The in situ small angle electron diffraction patterns of titanium-doped iron nanoparticles (C).

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High-resolution TEM (HRTEM) images are shown in Figures 2(A,B). The morphology of this dual nanostructure exhibits large size particles [Fig. 2(A)], which present interplanar distances with values close to maghemite iron oxide [ICSD 29-129]. The small size particles can be identified as akaganeite [see Fig. 2(B); ICSD 67-119]. It is worthwhile noticing that the incorporation of titanium in the iron oxide lattice leads most probably to the formation of different quantities of titanium maghemite [ICSD 84-1595] whose structure is difficult to determine from X-Ray diffraction data due to many coincident reflections with γ-Fe2O3. However, the doping with Ti was confirmed by X-ray energy dispersive analysis––EDAX [see the inset of Fig. 2(A)].

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Figure 2. HRTEM data showing the presence of both akaganeite (A) and maghemite or titanium-doped maghemite (B) phases. The inset of left panel proves the presence of titanium in the magnetic nanoparticles as revealed by X-ray dispersive analysis.

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The hysteresis loops of titanium-doped iron oxide dispersed in styrene-isoprene-styrene block copolymer recorded at different temperatures during the cooling of the sample in a magnetic field of 1 T are shown in Figure 3. A detailed analysis of hysteresis loops reveals the shift of hysteresis loops. The unidirectional exchange anisotropy at the interface between two magnetic structures (usually ferromagnetic/antiferromagnetic or ferromagnetic/ferromagnetic) shifts the hysteresis loop against the applied field by an exchange bias field, HB, without a significant change of the coercive field, HC. Recent data pointed toward a small increase of the coercivity due to unidirectional exchange anisotropy effects.6, 7 The temperature dependence of coercive and exchange bias fields is represented in Figure 4(A) (the samples were cooled in an applied magnetic field of 1 T). Although the exchange bias effect requires an interface between ferromagnetic and antiferromagnetic (or ferromagnetic) materials, recent experimental data8 proved the occurrence of exchange bias field effects in the hysteresis loops of single-phase magnetic nanoparticles, such as FeOOH.8 Hence, the observed exchange bias effect reflects the presence of FeOOH, as indicated by X-ray diffraction studies.

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Figure 3. Hysteresis loops of titanium-doped iron oxide–styrene-isoprene-styrene nanocomposites at various temperatures (SQUID data).

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Figure 4. The temperature dependence of the coercive field (left axis, circles) and exchange bias field (right axis, triangles) (A). The temperature dependence of the magnetization for the sample cooled in an applied external magnetic field of 1 T (left axis, triangles) and in zero (actually 0.1 mT) magnetic field (right axis, circles) (B).

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The gradual change from a magnetically ordered nanocomposite to a superparamagnetic behavior as the sample temperature is increased is reflected by the temperature dependence of the shape (coercivity) of the hysteresis loops, and it is confirmed by the temperature dependence of the magnetization during field cooling (in an external magnetic field of 1 T) and zero field cooling (actually the external field was about 0.1 mT) shown in Figure 4(B). From the temperature dependence of the magnetization during the zero field cooling, it is observed that the magnetic particles dispersed in polymer presents a blocking temperature of about 220 K[see Fig. 5(B)]. The glass-transition temperature of polystyrene is in the range of 200–210 K.9 While the changes in the local elasticity of the matrix as the temperature is raised above the glass-transition temperature of the soft phase may affect the reorientation of magnetic particles, these effects are negligible near the glass-transition temperature, because the ratio between the volume of magnetic nanoparticles and free volume is still very large (of the order of 1.000). From Figure 5(B), it is observed that the magnetization for zero field cooling and field cooling branches converge each to other, but are not equal at the high-temperature limit. This indicates that the magnetization within these nanoparticles is not fully averaged out by thermal reorientation; higher temperatures are required to achieve a full thermal averaging of the magnetization.

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Figure 5. The ESR spectra of titanium-doped iron nanoparticles dispersed in styrene-isoprene-styrene at various temperatures (A). The temperature dependence of ESR line position (HE, opened circle left axis) and of the resonance line width (HPP, field triangles, right axis) (B).

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The FMR spectra of the nanocomposite consist in a single broad resonance line [see Fig. 5(A)]. The FMR line of bulk maghemite in X band consists of a single resonance line while the FMR spectrum of maghemite nanoparticles consists of two overlapping lines corresponding to the easy magnetization axis parallel and perpendicular to the external magnetic field, respectively.10 However, the FMR spectrum of maghemite dispersions in ethanol at room temperature consists of a single broad line located near g = 2.00 and characterized by a line width of about 1270 Oe.10 The result was explained by the superposition of the two components of the resonance line of maghemite nanoparticles due to random jumps of the nanoparticle. As it is observed from Figure 5(A), the resonance spectrum of titanium-doped iron nanoparticles is in qualitative agreement with the aforementioned results. This proves that the diffusion of magnetic nanoparticles within the polymer contribute to the resonance line width. However, the resonance line width of magnetic nanoparticles dispersed in styrene-isoprene-styrene block copolymer is larger than the resonance line width of magnetic particles in methanol due to the higher viscosity of the block copolymer, compared with that of methanol. This indicates that the frequency of magnetic particle reorientation is smaller in the block copolymer than in methanol. The resonance line shape is well-fitted by a single Lorentzian line. The resonance line position and the resonance line width are temperature dependent. The temperature dependence of the resonance line position expresses the fact that the uncoupled electronic spin experiences the effect of two combined magnetic field, a molecular field, HM, reflecting the magnetic order and the external magnetic field HE. The resonance condition implies

  • equation image

In the ESR spectroscopy, the frequency of the electromagnetic field is constant. Taking into account that the local molecular field is proportional to the magnetization of the sample, the molecular field decreases as the temperature of the sample is increased [see Fig. 4(B)]. Accordingly, the external magnetic field at which the resonance is recorded, HE, will increase as the temperature of the sample is increased. For an X band experiment that involves noninteracting s = 1/2 electronic spins (HM = 0), the theoretical value of the resonance field above Curie temperature (i.e., in the paramagnetic phase where HE = HR) is roughly about 0.35 T. It is observed from Figure 5(B) that the external resonance field at which the resonance is observed increases asymptotically toward 0.35 T as the temperature of the sample is raised. This confirms the gradual transition from magnetic to superparamagnetic behavior, derived from SQUID measurements.

The temperature dependence of the resonance line width is shown in Figure 5(B). As the sample temperature is increased the frequency of the magnetization reorientation within magnetic nanoparticles increases, resulting in the narrowing of the resonance line. Taking into account that the polymeric phase presents a hard (frozen) phase (polystyrene) and a soft phase (polyisoprene), the narrowing of the recorded resonance spectra may also reflect the enhancement of segmental motions within the soft phase. As the ratio between the volume of magnetic nanoparticle and the free volume of polystyrene in this temperature range is of the order of 1000, the effect of segmental motions on the resonance line parameters is weak. Further details on the physical properties of magnetic nanoparticles dispersed in polymeric matrices are analyzed previously.2, 11, 12

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

The laser synthesis of titanium-doped γ-Fe2O3 nanoparticles and the preparation of titanium-doped iron oxide–styrene isoprene styrene composites was briefly described. The as-synthesized nanoparticles consisted of up to three phases, maghemite, akaganeite, and titanium-doped maghemite. The magnetic nanocomposite films were measured by SQUID and ESR spectroscopy. SQUID measurements revealed an exchange bias field that shifted the hysteresis loops. The temperature dependence of the magnetization indicated a blocking temperature of about 220 K (zero field cooling). The ESR line of this composite is a single broad line in the temperature range of 150–300 K. The resonance line position is shifted toward the theoretical “paramagnetic” value of about 0.35 T as the sample is heated toward room temperature. This reflects the gradual decrease of the magnetization as the temperature of the sample is increased. The resonance line width decreases as the sample temperature is increased due to the motional narrowing of resonance spectra. The main contribution comes from the thermal reorientation of the magnetization within magnetic nanoparticles.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

We acknowledge Prof. B. Rand for his help during the HRTEM analysis of Ti-doped nanopowders.

REFERENCES AND NOTES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES