A method has been developed to prepare cobalt ferrite particle core polymer shell nanoparticles. These engineered nanoparticles can be further embedded into a polymer host matrix to develop highly transparent polymer based magneto-optic materials. A proof-of-principle all-optical magnetometer has been constructed based on the cobalt ferrite core polymer shell based nanocomposite material. A noise equivalent magnetic field sensitivity of 50nT/√Hz was observed using a 3μT 500Hz control magnetic field.
©2012 Optical Society of America
Magnetic field sensors (i.e., magnetometers) have been an intensive area of research over the last few decades and optical magnetometers have reached sensitivities that make them interesting candidates for applications such as magneto-encephalography and magneto-cardiography [1,2]. Extensive magneto-optic (MO) studies have been performed of magnetic field sensing based on inorganic rare-earth materials such as yttrium iron garnet, Tb+3 doped paramagnetic glass , and gallium gadolinium garnet in non-cryogenic conditions . Lately, the design, synthesis and study of MO nanocomposite materials with magnetic nanoparticles embedded in a non-magnetic host matrix have also attracted significant attention, as these mesoscopic materials are expected to exploit the magnetic and optical properties of the nanoparticles alongside the ease of processability of the host. Due to relatively low production cost, reliable commercial availability and a variety of available magnetic materials, these nanoparticles are interesting candidates for highly efficient Faraday materials compatible with photonic integrated circuits (PIC). Furthermore, nanoparticle polymer composite materials present opportunities to tune the resultant magnetic properties by using external stimuli, such as thermal and magnetic fields, to suitably control the interparticle distance; within the Rayleigh regime, the scattering cross section can be reduced by judicious choice of the refractive indices of the host materials. We have previously shown that the Faraday rotation of these highly transparent nanocomposite materials strongly depends on the nanoparticle size, their shape, and spectral properties; further, the MO properties appear to be synergistic with the nanoparticle concentration . This polymer-based approach has several advantages due to the inherent low cost and easy processability, along with the possible tunability of the MO properties, refractive indices and mechanical properties, increasing their appeal as potential candidates for high sensitivity magnetic field sensing. Typically such a polymer composite is prepared by mixing the nanoparticles into a polymer solution or melt and is subsequently processed via melt processing, drop casting, spin coating, layer-by-layer assembly, etc [5,6]. However, magnetic nanoparticles are known to aggregate under external magnetic fields as small as that of Earth’s magnetic field and tend to lose optical transparency over time due to increased scattering and phase separation.
In this communication we report on a novel method of synthesizing nearly monodisperse cobalt ferrite nanoparticles, a nanoparticle core - polymer shell composite and its optical and MO properties. As an example of the versatility of our MO materials, we also present a magnetic field sensing system based on this highly transparent and low scattering polymer nanocomposite and demonstrate a noise equivalent magnetic field sensitivity of 50nT/√Hz.
2. Materials synthesis and nanocomposite morphology
In a typical synthesis, 5mL of ethylene glycol (EG) and 25mL of hexafluorophosphate salt of 1-butyl-3-methylimidazolium (IL) were introduced into a 100mL round bottom flask and heated to 325°C. A mixture of EG and IL was used as the solvent. A solution of 2.7mg of FeCl3.6H2O and 1.2mg of CoCl2.6H2O in another 5mL of IL was injected into the hot solvent mixture. After refluxing for 6hr, the resulting brown solution was cooled down to 225°C and aged at that temperature for another 14hr. After aging, the mixture was cooled down and diluted with 100mL of deionized water. The entire solution was centrifuged at 8g and washed with deionized water repeatedly to remove excess ionic liquid. The particles were then dispersed in chloroform using oleyl amine. All chemicals were purchased from Sigma-Aldrich and were used as received without further purification.
Figure 1 shows a transmission electron microscope (TEM) image of a collection of resultant nanoparticles with a mean diameter of 19.8 ± 0.24nm. Compositional analysis using energy dispersive X-ray spectroscopy shows a 1:2 atomic ratio of Co to Fe as expected. A high resolution TEM image of the single nanoaprticle shows the nearly single crystalline nature of the particles. The selected area electron diffraction (SAED) pattern collected on the particles could be readily indexed to cobalt ferrite particles. The rings in the SAED pattern also indicate averaging of the crystalline anisotropy within the sampled region, with the same crystal plane in different particles oriented randomly. The indexed SAED pattern indicates an inverse spinel type structure for the synthesized CoFe2O4 particles where oxygen atoms make up an FCC lattice and with one half of Fe3+ ions occupying the tetrahedral A sites and the other half, together with Co2+ ions, located at the octahedral B sites.
We have previously shown a UV photoexcitation mediated polymer shell synthesis on magnetite nanoparticles . Using a similar methodology we have synthesized a polybenzylmethacrylate (PBMA) shell on the CoFe2O4 particle surfaces. In a typical synthesis, 10mg of particles, 0.2mL of benzylmethacrylate (BMA) in 10mL of chloroform was sonicated for 2hr at 60°C in a water bath before being illuminated with a 360-400nm photoexcitation source with nominal power of 10mW/cm2. Photoexcitation is continued for 90min, during which the sonication and refluxing of the chloroform dispersion is continued. The reaction is monitored by following the vibration band of –C = C-H at 1628cm−1 using an FTIR spectrometer and once the band disappears the reaction mixture is cooled down to room temperature and subsequently the solvent volume is reduced to 2–3mL. Figure 2 shows a scanning TEM image of the CoFe2O4 particles core PBMA shells with ~4nm thick shells. Under TEM, a few particles were seen to form a chain (shown as inset in Fig. 2) which is expected given the fact that CoFe2O4 is known to acquire ferromagnetic properties in particles with diameter >20nm which is close to the particle sizes obtained here. However, in DC Faraday rotation measurements reported below, we have not observed any hysteresis indicating that the ferromagnetic CoFe2O4 particles, if any, are small in number and below the sensitivity level of our Faraday rotation measurement setup. The detailed characterization and reaction rate of this shell formation process has been discussed in Ref. 7. To further probe the nature of this reaction, we carried out high resolution X-ray photoelectron spectroscopy (XPS) on a representative magnetite nanoparticles-BMA shell composite system. A monolayer of magnetite nanoparticles was created on top of perfluorosilanized SiO on a silicon substrate using a self-assembly technique. This assembly of particles were dipped into a BMA in chloroform solution and was the illuminated with a 400nm photoexcitation source. XPS spectra were recorded on the samples before and after the photoexcitation as shown in Fig. 3 . The C1s XPS peak at 283.4eV and the O1s doublet at 529eV indicate new Fe-C and Fe-O bond formation after photoexcitation of the particles. CoFe2O4 particles are expected to follow a similar reaction pathway. The shell formation on top of the particles helps prevent aggregation among particles and improves long term stability and optical quality of the composite. The bulk composite was synthesized following the protocol developed in Ref. 7.
3. Magneto-optic characterization and sensing performance
The nearly monodisperse distribution of the nanoparticles shown in the TEM and STEM images shown above is an indication of the expected optical quality of the CoFe2O4 nanocomposite, as scattering is directly related to the amount of clustering and nanoparticle aggregation. The nanocomposite as a whole was uniform and showed excellent optical transmission. The nanoparticle content of the CoFe2O4 nanocomposite was 4wt% as determined by thermogravimetric analysis. Figure 4 shows the absorption spectra of the CoFe2O4 nanocomposite, and Fig. 4(a) and Fig. 4(b) show a 170 μm thick film over a computer LCD display and a 100μm thick film with a 1 inch diameter directed at the street, showing details of both the screen pixels and of distant objects with excellent optical quality, respectively. Table 1 shows the Verdet constant values at 850nm, 980nm, 1310nm and 1550nm. The saturation Faraday rotation at 980 was 0.0011 °/μm and was measured using a single pass DC MO polarimeter with a Hinds Instruments photoelastic modulator PEM-100 to enable phase locked detection, and was calibrated using a two pass AC MO homodyne polarimetric configuration used for the Verdet constant measurement of a 75μm thick film of CoFe2O4 nanocomposite, shown in Fig. 5 inset [8,9]. The data shown in Fig. 5 was fitted with a modified Langevin shape previously used in Ref. 4 for this type of magnetic core polymer shell nanocomposite and the correlation factor was R2 = 0.996. The figure of merit at 980nm is 0.95 ° (FOM = 2θFS/α) which is comparable to the yttrium iron garnet FOM of 4.2° at the same wavelength. The high Verdet constant values and low absorption, particularly at 1550nm, make the CoFe2O4 nanocomposite an interesting alternative to the 5 wt% magnetite nanocomposite published in Ref. 4, as the ratio V/α of the CoFe2O4 nanocomposite is an order of magnitude higher at 1550nm.
The 170 μm free standing film shown in Fig. 4(b) was used a sensor head MO element. The film was prepared by melt processing and subsequently attached to a gold coated mirror on a 250 μm thick glass substrate. The nanocomposite sensitivity was tested in the optical configuration shown in Fig. 6(c) with a 980nm laser. The sensing head also consisted of two tapered cylindrical magnetic flux concentrators made of Ferroxcube 3B1 MnZn ferrite materials, with an initial permeability of 900 at room temperature . The introduction of high magnetic permeability materials such as MnZn ferrite has been used to enhance the magnetic flux density at the sensing MO element [11,12]. The glass substrate was fixed to a solid tapered magnetic flux concentrator of 3B1 and a hollowed out concentrator that allows access for the optical probe was fixed to the exposed polymer surface. The outer diameter of the concentrators was 10mm and was tapered to 4mm and 2mm for the front and back concentrators, respectively at a 45° angle. Figure 6(b) shows the assembly of the concentrators and the MO sensing element. The front concentrator has a clear aperture of 2mm diameter. The high permeability of the concentrators enhances the magnetic flux density at the smallest diameter of the solid concentrator and channels to the front hollowed concentrator as it propagates through the MO nanocomposite film. Figure 6(a) shows the distribution of the magnetic flux density component parallel to the propagation vector k as simulated in COMSOL. The otherwise uniform magnetic field generated by the Helmholtz coil is sharply increased at the front of the solid concentrator, especially at the edges of the taper as it is the closest path towards the front concentrator.
The sensitivity was measured with a 3μT 500 Hz control magnetic field generated by a Helmholtz coil. The magnetic field was measured with a commercial gaussmeter hall probe. Figure 7 shows the sensitivity, signal to noise ratio (SNR), and minimum detectable field Bmin of the magnetometer at various measurement time constants. Point-to-point comparison of the sensitivity indicates that the magnetometer has noise equivalent magnetic field sensitivity of ~50nT/√Hz as seen by the solid squared red dotted line in Fig. 7. Using magnetic flux concentrators, the sensitivity of the sensor is enhanced by roughly a factor of 20 with respect to the probe without a concentrator surrounding the polymer nanocomposite. The sensitivity, SNR and Bmin in the absence of magnetic flux concentrators is shown in dashed and circled dotted lines in Fig. 7. The sensitivity can be further improved by taking advantage of the low scattering of the MO polymer, using a thicker film, employing magnetic field poling to increase the MO response of the nanocomposite, as well as exploring other nanoparticle systems with a higher magnetic permeability, as one of the most important factors that drive the magnetic field concentration is the contrast of magnetic permeability between the concentrators and the MO nanocomposite film.
The shot noise limited minimum detectable field sensitivity BSN of the sensor is influenced by the responsivity R of the detector, the saturation power P of the photoreceiver and the MO responsivity S, consisting of the product of the Verdet constant V and the thickness L of the film, according to Eq. (1), where e is the electron charge (1.602 × 10−19C), R is 0.75A/W at 980nm, and P is 0.5 × 10−3W. The autobalanced noise cancellation from the photoreceiver and and the two pass configuration, increase S by a factor of 200 and 2, respectively, in addition to the product of the Verdet constant and the film thickness to account for the MO response S = 816°/T = 14.2 rad/T. This corresponds to a shot noise limited minimum detectable field sensitivity BSN = 1 nT/√Hz. This calculated shot noise limit is shown in Fig. 7 illustrating the enhancement of the sensitivity by using the magnetic field concentrators to reduce the gap above the shot noise limit.
The relative frequency response was measured between 20Hz and 10kHz as shown in Fig. 8 , and indicated the sensor bandwidth is around 10kHz. The SNR as observed at 500Hz with the control field of 3μT was 41 dB (dB = 20log(SNR)) and showed a 1/f noise trend at low frequencies as shown in Fig. 7 inset. The frequency response of the sensing system at lower frequencies is influenced by several factors such as the limitation of the autobalancing loop speed in the photoreceiver, the attenuation of the 60 Hz filter in the lock-in amplifier and the overall dominance of 1/f noise. The higher frequency response of the sensing system is expected to be limited only by the Helmholtz coil secondary effects rather than the nanocomposite frequency response . The field was measured using a Lakeshore Hall probe gaussmeter for frequencies below 600Hz, as well as magneto-optically using BK7 glass. Since BK7 is a diamagnetic material and therefore its MO response is independent of the modulating frequency, it can be used to calibrate the magnetic control field from 600Hz to 10kHz.
A novel method of preparing nearly monodisperse cobalt ferrite nanoparticle-based polymer composites has been described. As a result of the low level of nanoparticle clustering and aggregation, the optical quality of the cobalt ferrite polymer nanocomposite shows an FOM of 0.95° at 980nm, comparable to YIG FOM of 4.2° at 980nm. A proof of principle all-optical magnetometer with a noise equivalent magnetic field sensitivity of 50 nT/√Hz indicates that these highly transparent and MO responsive materials may be used in magnetic field sensing systems where high sensitivity is required.
Research support of the ERC NSF Center for Integrated Access Networks under Grant No. EEC-0812072, the Defense Advanced Research Projects Agency under award number W31P4Q-09-C-0345, and the Air Force Office of Scientific Research under Grant No. FA9550-06-1-0039 are gratefully acknowledged.
References and links
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