1. Introduction

Amongst the most attractive properties of the magnetic transparent compounds are those related to the magneto-optical effects and their scientific and industrial applications in areas such as: optical fiber sensors, optical isolators, information storage, etc. One of these magneto-optical effects is the Faraday rotation, which is defined as the change produced in the plane of polarization of the light transmitted through a material when a magnetic field is applied. Although this phenomenon is not exclusive of ferromagnetic substances [1,2 ], the largest values of the Faraday rotation have been found in this type of materials. However, due to their high absorption (many ferromagnetic materials are metals) this effect is mainly measured in thin films [3]. For this reason, since the pioneering work of Ziolo et al. [4], the preparation of transparent nanocomposites has been envisaged as an interesting route to produce devices such as optical isolators, magnetic field and electric current sensors. In this regard, the Faraday rotation of γ-Fe₂O₃ based systems, a ferromagnetic and nearly transparent material, has been studied in different structures. Tepper et al. measured the Faraday effect in iron oxide thin films deposited by pulsed laser deposition [5] whereas Guerrero et al. [6] observed Faraday rotation in γ-Fe₂O₃/SiO₂ composites, in which the iron oxide particles were grown during the formation of the sol-gel silica matrix. In particular, a composite, with an 18% of γ-Fe₂O₃ weight content, showed a rotation Faraday value of 1.1·10⁴ °/m at 765 nm. Taboada et al. [7] obtained maghemite-silica aerogels through sol-gel chemistry and supercritical drying of the wet gel. The composite, with a 10.4 wt% of iron oxide, had a specific Faraday rotation of 29.6 °/cm at 810 nm. More recent works have shown a route to develop photonic crystals with combined optical and magnetic functionalities [8] and also that it is possible to enhance the Faraday rotation by gold coating maghemite nanoparticles (NPs) [9] or by preparing nanocomposite structures composed of spinel-type ferromagnetic nanoparticles and plasmonic metallic nanoparticles [10]. It is very important to note that a strong dependence of the Faraday rotation with the size of the γ-Fe₂O₃ particles has been recently [11] shown, which explains the differences found in the data published in the literature. Most of the works carried out so far have focused on the near IR Faraday effect of maghemite based nanocomposites (probably due to low transparency of maghemite based composites in the visible range). In order to expand the operative range to the visible, highly transparent matrices and an excellent distribution of the NPs are mandatory.

In the present work, the preparation of highly transparent polymer composites with well distributed maghemite NPs obtained by a simple and scalable processing route, and their Faraday activity is described. Generally speaking, obtaining homogeneous nano-particle composites is a complicate task which requires the use of specific additives and usually only small concentrations can be prepared. The herewith preparation approach relies on the incorporation of the maghemite NPs onto a micrometric carrier, sepiolite particles, which on its turn is easily distributed into a polymeric matrix with higher nanoparticle contents and processed by conventional methods (melt compounding). This approach has already been used to disperse Au and Ag NPs with plasmonic activity into polyethylene and polystyrene [12] and FeCo NPs into polystyrene [13] with excellent results. The success of this approach is based on, first, the possibility to support the metallic NPs onto the micrometric carrier [14,15 ], which avoids their aggregation when dispersed in the polymeric medium; second, on the ease to disperse the micrometric carrier sepiolite into polymers by simple melt compounding [16] and, finally, on the similarity of the refractive index of sepiolite with an important set of polymeric matrices [12] (in the present case: n_Sep = 1.53 and n_PS = 1.59). It is worth noting that these composites perform as thermoplastics, in view of their mechanical properties and flexibility as reported in Ref [12].

2. Experimental

2.1 Materials

A magnesium silicate clay (Sepiolite, Sep) has been used as a template to synthetize maghemite NPs on its surface due to its special structural properties. Sep powder, purified and micronized by a wet process was supplied by TOLSA, S.A (Spain). The defibrillated Sep used has a needle like structure and a particle size below 1 µm. Polystyrene (PS, 143E from BASF) was used as received.

2.2 Preparation of the maghemite nanoparticles

A water dispersion of Sep (10 wt%) was acidified with H₂SO₄ at pH close to 1 using high shear mixing. During this procedure, Sep structure is modified to achieve different acid centres where the maghemite NPs will grow, according to our previous work [17]. Sep dispersion was mixed with an aqueous solution of FeSO₄·7H₂O so that the final relative metal concentration into Sep was 50 wt%. Thereafter, the pH of the dispersions was adjusted with NaOH to stabilize the pH of the suspension at 8.0, in order to precipitate the corresponding metal oxide into the octahedral position of leached magnesium. Finally, the maghemite doped Sep (Mag-Sep) was isolated by vacuum filtration and washing with water.

2.3 Preparation of the PS/mag-sep composites

The composites were obtained by melt compounding in a Haake MiniLab extruder [18]. 4.95 g of polymer and 0.55 g of Mag-Sep were physically mixed and directly introduced into the machine. The processing temperature, shear rate and residence time in the extrusion method were 160 °C, 120 rpm, and 20 min, respectively. Finally, a subsequent extrusion step under the same conditions and lasting 10 min was performed to achieve a fine dispersion of the fibers. 60 µm thick films from the composite were obtained by compression moulded at 140 °C.

2.4 Characterization

The powder X-ray diffraction (XRD) patterns were collected on a Bruker AXSD* Advanced Powder X-ray diffractometer by using Cu-Kα radiation (λ = 0.15418 nm). The patterns were recorded in the 2-theta (2θ) range from 5° to 50°. The X-ray photoelectron spectrometer and a non-monochromatic Mg K_α X-ray source (hv = 1253.6 eV). The electron energy analyzer was operated with and energy pass of 20 eV for high resolution spectra. The sepiolite powder was glued onto an aluminium foil by means of double-sided conductive adhesive carbon tape. Before XPS measurements, the sepiolite power was outgassed at room temperature in the UHV chamber during 48 h. XPS measurements were performed at 5·10⁻⁹ hPa. In these complex samples, the adventitious C 1s peak position at 284.8 eV was used as a reference for charge correction. The microparticle dispersion in the composite was characterized by scanning electron (SEM, Philips XL30ESEM). Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100.

The hysteresis loops were measured using an alternate gradient magnetometer (AGM, MicroMag, Princeton Measurements Corporation) at room temperature with an applied magnetic field up to 5 kOe. The transmittance and Faraday spectra were acquired with a SOPRA GES E5 ellipsometer in the wavelength range 350-965 nm at normal incidence in the parallel beam mode. Concerning the Faraday spectra (constant magnetic field applied of ± 2 kOe applied by a set of Helmholtz coils), the polarizer was set to 0° (p-polarization) and transmittance measurements were carried out varying the analyzer between 86 and 94° in steps of 0.1°. These measurements were repeated, changing the sign of the applied field. For each wavelength, the difference in the analyzer positions that yield the minimum of transmission for positive and negative applied fields was calculated.

3. Results and discussion

Figure 1(a) shows the X-ray diffractogram corresponding to the Mag-Sep fibers. The diffraction peaks at 30, 35.5 and 42° correspond to the maghemite particles and the width of the peaks indicates that the size of the particles is in the nanometer range; the remaining peaks correspond to sepiolite. However, it is well known that it is difficult to distinguish between maghemite from magnetite (Fe₃O₄) by X-ray diffraction. Both compounds have a similar crystallographic structure. Even more, the small differences between both diffractograms are very hard to detect because of the X-ray fluorescence of iron atoms excited by the Cu kα radiation. In this regard, we have analyzed the XPS Fe2p spectra to achieve information about the iron valence state to discard the presence of magnetite Fe²⁺ cations. Figure 1(b) shows the XPS spectrum of Fe 2p core level of sepiolite. The spectrum clearly exhibits shake-up satellite structures that can be described as a strong configuration interaction in the final state involving significant ligand-metal charge transfer that results in an extra 3d electron compared to the initial state. The peak positions of Fe 2p_1/2 and Fe 2p_3/2 and shake-up satellites are very sensitive to the oxidation states. The Fe 2p_3/2 is located at about 711.1 eV and its satellite peak at 8.3 eV toward high binding energies. These results indicate that the iron in this sepiolite is mainly present as Fe³⁺ state [19].

 

Fig. 1 (a) X-Ray diffractogram corresponding to the sepiolite fibers-maghemite nanoparticles, Mag-Sep; (b) XPS Fe 2p core level spectrum of sepiolite.

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The SEM characterization (see Fig. 2(a) ) shows a homogenous dispersion of the sepiolite fibers (white spots) in the polymer matrix. No preferred spatial orientation of the fibers has been detected in this characterization. The particles nucleated homogeneously on the Sep fibers as shown in the TEM image (see Fig. 2(b)), analogously to other metallic and oxidic nanoparticles previously reported [20,21 ]. From the TEM images, the particle size distribution of the maghemite particles was obtained, with an average size of the particles around 10 nm (see inset in Fig. 2). However, a small fraction of particles (less than 2%) still have diameters of 20 nm or slightly larger. It is well known that when the volume of the particles is small enough, the thermal agitation beats the magnetic order, leading to superparamagnetism. It is considered that particles become superparamagnetic when K·V = 25K_B·T. Taking 2.5·10⁴ J·m⁻³ for K, the magnetic anisotropy of maghemite [22], the diameter below which the maghemite particles become superparamagnetic at room temperature can be estimated around 20 nm, that is, the magnetic behavior of the composite is expected to be a mixture of a dominant superparamagnetic contribution plus a minor ferromagnetic contribution due to the largest maghemite particles.

 

Fig. 2 (a) SEM image showing a uniform distribution of the sepiolite fibers in the polymer matrix; (b) TEM images of the Mag-Sep nanoparticles. Inset shows the particle size distribution.

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The magnetic hysteresis loops in Fig. 3 show a soft magnetic behavior of the polymer composites. Magnetic saturation is obtained at around 3 kOe with a ferromagnetic behavior, yet close to superparamagnetism. As shown in the inset, the coercive field is not exactly zero for these samples, but it takes values close to 20 Oe. This non-zero coercive field can be attributed to the residual presence of particles with diameters of around 30 nm. However, the average small size of the maghemite NPs still allows obtaining an almost linear behavior of the magnetization of the sample vs. the magnetic field in the range of ± 300 Oe, approx.

 

Fig. 3 Hysteresis loop of the PS/Mag-Sep composite.

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Once the total magnetization is known, and considering a value of Ms = 73.5 emu/cc (at room temperature) for maghemite [23], it is possible to calculate the vol. % of γ-Fe₂O₃ in the 2.2 mm x 3.8 mm x 60 µm samples used for the hysteresis loops. A simple calculation leads to a value of 0.9% vol. of γ-Fe₂O₃ in the polymer matrix composite.

The 60 µm thick films showed a high real in line transmittance (see Fig. 4 ; red line and inset showing a picture of the composite), particularly in the near IR range in which the transmitted intensity was close to 70%. In the visible range, the real in line transmittance was 50% around red wavelengths decaying to 10% around blue wavelengths. The processing route and a careful choice of the polymer and the NP carrier leads to a low scattering, which has allowed a full characterization in saturation of the Faraday spectrum of the composites. In the IR range the Faraday rotation (Fig. 4; black line) takes low values, ̴ 0.1°, corresponding to a specific maximum rotation of the order of 28 rad·m⁻¹, similar to those reported by other authors [24], increasing in the visible range to a maximum rotation around 460nm (2.7 eV) of 0.4°, which corresponds to a specific maximum rotation of 116 rad·m⁻¹. The Faraday rotation decays below 460 nm (2.7 eV) and the data become noisy below 410 nm (3 eV) due to the low transmittance of the system. At 460 nm, and considering the range in which the magnetization can be taken as linear with the external magnetic field (in the present case, between ± 300 Oe, approx.), the Verdet constant can be estimated to be 2.6·10³ rad·T⁻¹m⁻¹. At 620 nm (2 eV), the Verdet constant for this composite is close to 300 rad·T⁻¹m⁻¹, which is more than twice larger than the Verdet constant reported for commercial terbium gallium garnet (TGG) devices, used in optical isolators [25]. The reason for such a high Faraday rotation in the present composites lies on the fact that nanoparticles are homogeneously dispersed all along the sample and, therefore, all nanoparticles contribute to the Faraday effect, contrary to composites in which maghemite particles are agglomerated. Other studies performed on composites reported specific rotation values several times smaller than the one reported here even though the maghemite content is much smaller in the present composites. Considering that the filling factor in the present composites is 9·10⁻³, the values shown in this work are of the same order of magnitude than those reported by T. Tepper et al. [5] measured on iron oxide thin films deposited by pulsed laser. Contrary to thin films in which magnetic domains undergo strong magnetic interactions, the superparamagnetic character of the nanoparticles leads to a different hysteresis loop and, therefore, the range of magnetic fields in which thin films or this kind of composites may be used (in applications such as magnetic field sensing) is different. This way, it is shown that high real in-line transmittance values are obtained in the visible range accompanied by a high Faraday rotation. At this point it is worth noting that the polymer matrix simply acts as a support for the doped sepiolites; no additional Faraday activity can be assigned to a coupling between the polymer matrix and the doped sepiolites. Therefore, a route to improve the performance of the present composites would be to find a better matching in the refractive index between the matrix and the sepiolites, which would decrease light scattering and, as a consequence, increase the transparency of the system. Also, the specific rotation may be further raised in the composites by increasing the concentration of maghemite, up to an upper threshold limited by light scattering which would allow its use as optical isolators or magnetic field sensors.

 

Fig. 4 Faraday rotation and transmission spectra of the 60 µm thick PS/Mag-Sep composites.

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The Faraday effect is larger at the visible region than in the infrared as a consequence of the dependence of the Verdet constant with the variation of the refractive index vs. energy. Because the bandgap is located around 620 nm (2.0 eV), according to photodissolution studies [26], it is expected that both refractive index, and therefore the Faraday effect, will increase below this wavelength. However in Fig. 4, it seems that this energy is shifted to 560 nm (2.2 eV). This result can be explained considering theoretical calculations [27] that indicate that the density of states around the Fermi level is not symmetric in maghemite and that there are two populations of electrons with different band gaps (2.2 eV and 1.8 eV). It is likely that the largest variation of the refractive index will take place only when the energy is large enough to excite all the electrons in the conduction band. This explains the small Faraday rotation around 690 nm (1.8 eV) and the larger rotations below 560 nm (2.2 eV).

4. Conclusions

This work describes the preparation of highly transparent, both at the NIR and visible range, polymer-maghemite composites exhibiting a large Faraday rotation. Homogeneous 10-30 nm γ-Fe₂O₃ nanoparticles have been synthesized on magnesium silicate fibers and further incorporated into a polymeric matrix. Due to the small difference of refractive index between the magnesium silicate and the polymer matrix the light scattering is minimized and consequently the transmittance is quite good up to UV regions, allowing a full characterization of the Faraday spectrum in the near IR and the full visible range. The studied composites present a Verdet constant over twice larger than that reported for TGG based materials used in commercially available devices.