Polymer

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Synthesis of Highly Transparent Lithium Ferrite Nanoparticle/Polymer Hybrid Self-standing Films Exhibiting Faraday Rotation in the Visible Region Koichiro Hayashi,† Rintaro Fujikawa,‡ Wataru Sakamoto,† Mitsuteru Inoue,‡ and Toshinobu Yogo*,† DiVision of Nanomaterials Science, EcoTopia Science Institute, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan and Department Electrical and Electronics Engineering, Toyohashi UniVersity of Technology, Tenpaku-cho, Toyohashi, 441-8580, Japan ReceiVed: March 10, 2008; ReVised Manuscript ReceiVed: May 30, 2008

A lithium ferrite nanoparticle/ethyl(hydroxyethyl)cellulose (EHEC) hybrid was synthesized via in situ hydrolysis of prepolymerized iron(III) 3-allylacetylacetonate (IAA) and lithium acrylate (LA) in the presence of EHEC below 100 °C. The hybrid film was flexible and self-standing and showed high transmittance in the visible region. The crystallite size of lithium ferrite in the hybrid increased from 2.7 to 3.8 nm with decreasing amounts of EHEC. Nanosized lithium ferrite particle/EHEC hybrid showed a BH curve with no remanence at room temperature. The hybrid was superparamagnetic with a blocking temperature of 13 K. The absorption edge of the hybrid film was blue-shifted compared with bulk lithium ferrite. The blue-shift increased with decreasing crystallite size, which was controlled by the amount of EHEC. The self-standing film exhibited a Faraday rotation depending upon the magnetic field. The curve for the specific Faraday rotation (F) versus magnetic field corresponded to that for magnetization versus magnetic field. The figure of merit of the hybrid film was about 3.5 at 700 nm, which was higher compared with those of reported ferrite composite at shorter wavelengths, due to its high transparency. 1. Introduction Transparent magnetic materials have attractive applications in optical isolators, modulators, optical switches and magnetic sensors based on the Faraday effect.1-4 The Faraday effect is evaluated by F/R, where F is the specific Faraday rotation and R is an absorption coefficient.5 Good magnetic properties require a high content of a magnetic phase in a material, which results in the increase in absorption coefficient (R), leading to a decrease in transparency and the resulting Faraday effect. Because nonmagnetic materials, such as polymers and glasses, are transparent in the visible region, their magnetic composites are one of the possible candidates for transparent magnets. Ziolo et al. first synthesized transparent magnetic polymer composites dispersed with γ-Fe2O3 from an ion-exchange resin and iron chloride.6 Transparent and magnetic γ-Fe2O3 and ZnFe2O4/SiO2 composites have been synthesized by sol-gel6-9 and impregnation methods.5 A limited number of papers reported the Faraday rotations of sol-gel-derived γ-Fe2O3/SiO2 composites.5,8,10 Although the silica matrix loses flexibility after solidification, the polymer has the merits of flexibility and processability, combined with micro- and nanopatterning by photolithography and molding as well as spinning.11 Transparent and flexible magnets have potential use in optically responsive elastic magnets.12 Hybrid nanocomposites uniformly dispersed with nanosized particles can be transparent, since the particle size is smaller than the wavelength of visible light.13 A key for the synthesis of transparent composites is a uniform dispersion of nanoparticles in the matrix. However, the uniform dispersion of magnetic * To whom correspondence should be addressed. E-mail: yogo@ esi.nagoya-u.ac.jp; phone: 81-52-789-2750; fax: 81-52-789-2121. † Nagoya University. ‡ Toyohashi University of Technology.

particles in an organic matrix is difficult via conventional mechanical mixing, because magnetic nanoparticles agglomerate as a result of their magnetic moments and van der Waals forces. On the other hand, in situ synthesis of magnetic nanoparticles in an organic matrix is characterized by a uniform dispersion of particles because the aggregation of in situ-formed nanoparticles is prevented due to the organic matrix. The dispersed particles have a uniform size, which is controlled by the selection of the hydrolysis conditions of the starting metallorganics.14-16 Ferrimagnetic garnet oxide crystals such as Y3Fe5O12 (YIG) and Bi-substituted YIG (Bi1.8Y1.2Fe5O12, Bi-YIG) are used in magneto-optical devices, due to their large Faraday rotation.2,3 However, the use of YIG and Bi-YIG is limited in the range of 1000-1550 nm, because YIG has a large absorption coefficient below 1000 nm.17 YIG thin films are synthesized using laser ablation,18 sputtering,2 and liquid phase epitaxy3,17 on singlecrystal substrates. The high cost in the syntheses of YIG and Bi-YIG is also one of the problems. In recent years, growing attention has focused on wavelength division multiplexing optical communication systems for broadband communication.19 Therefore, Faraday rotators capable of exhibiting the Faraday rotation over wide bands are required. These applications are indispensable components for a high-speed optical network system. Lithium ferrite, Li0.5Fe2.5O4 or LiFe5O8, has large diagonal and off-diagonal elements of the dielectric tensor, which results in the magneto-optical effect in the visible region.20,21 Moreover, lithium ferrite is low-cost and has a high Curie temperature and high saturation magnetization.22 This paper describes the in situ synthesis of transparent lithium ferrite particle/polymer hybrid self-standing films, with flexibility below 100 °C. Natural polymers of cellulose consist of a fibril structure of linked glucose and have a mechanical strength comparable with synthetic polymers. Ethyl(hydroxy-

10.1021/jp802103d CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

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ethyl)cellulose (EHEC) is a derivative of cellulose and is used as a polymer matrix because of its solubility in organic solvents. The formation conditions of lithium ferrite particles in a cellulose matrix were studied. The magnetic and magneto-optical properties of the hybrid films were investigated. 2. Experimental Section 2.1. Synthesis of Li0.5Fe2.5O4 Nanoparticle/EHEC Hybrid. Iron allylacetylacetonate (IAA) was prepared by the method described in the literature.23 Commercially available lithium acrylate (LA) was purified from methanol. Ethanol was dried over magnesium ethoxide and then distilled before use. Commercial methylhydrazine was used as received. The hybrid was synthesized by prepolymerization of IAA and LA, and then hydrolysis. Prepolymerization was conducted to promote crystal growth of nanoparticles.24 IAA and LA were prepolymerized by sealed polymerization using 2,2′-azobis(isobutyronitrile) (AIBN) as a radical initiator for polymerization. IAA (300 mg, 0.317 mmol), LA (9.88 mg, 0.063 mmol) and 3.0 wt % AIBN were dissolved in ethanol in a glass capsule. The capsule containing the reaction mixture was immersed in liquid nitrogen at -190 °C and then evacuated to 10 Pa. The frozen product was then melted at 20 °C at 10 Pa. After the freezing-melting treatment was repeated 5 times to remove oxygen, the glass capsule was sealed at 10 Pa. The sealed capsule was heated at 80 °C for 20 h, yielding the IAA-LA oligomer. The solution including the IAA-LA oligomer was transferred to a round-bottled flask. An ethanol solution of EHEC (Tokyo Kasei, Tokyo, Japan, MW ) 1.3 × 105) was added dropwise to the prepolymerized IAA-LA solution, and then the mixture was ultrasonicated at 60 °C for 5 h. The amount of EHEC was 30, 50, or 70 wt % relative to the total weight of IAA, LA, and EHEC. A mixture of methylhydrazine (MH, 140 mg, 4 equiv to IAA-LA) and water (548 mg, 40 equiv to IAA-LA) dissolved in ethanol was added dropwise to the solution of prepolymerized IAA-LA and EHEC at room temperature. The reaction mixture was then refluxed at 80 °C for 24 h. A film-like solid product was obtained after removal of the solvent and drying under vacuum at room temperature. 2.2. Synthesis of EHEC-added Li0.5Fe2.5O4 Nanoparticle/ Organic Hybrid. The prepolymerized mixture was hydrolyzed with 4 equiv of MH and H2O and then refluxed at 80 °C for 24 h, yielding an ethanol solution of Li0.5Fe2.5O4 nanoparticle/ organic hybrid. The ethanol solution of EHEC was added dropwise to the hydrolyzed solution, and then the mixture was ultrasonicated. The mixture was evaporated in vacuo, affording a brown solid (EHEC-added LFO nanoparticle/organic mixture). 2.3. Fabrication of Self-standing Film. The hybrid solid was easily dissolved in ethanol at room temperature. The solution was cast on a Teflon plate and was left for several hours in air to slowly remove ethanol. A self-standing film was obtained after removal of the solvent. 2.4. Characterization. The products were analyzed using an Fourier transform infrared (FT-IR) spectrometer. The organics in the hybrid were measured by differential thermal analysisthermogravimetry. The crystalline phase in the hybrid was analyzed by X-ray diffraction (XRD) using Cu KR radiation with a monochromator. The crystallite size was estimated using the (311) reflection of spinel oxide based on the Scherrer equation.25 The elemental analysis of the products was conducted with an inductively coupled plasma (ICP) spectrometer. The magnetization was measured with a vibrating sample magnetometer at room temperature and a superconducting

Figure 1. Fundamental building block of EHEC.

quantum interference device (SQUID) from 5 to 300 K. The magneto-optical properties were evaluated using a magnetooptical spectrometer. 3. Results and Discussion 3.1. Synthesis of Li0.5Fe2.5O4 Nanoparticle/EHEC Hybrid. IAA and LA were used as starting compounds because of their high solubility in ethanol. IAA has terminal vinyl groups for polymerization and chelated Fe-O bonds. The Fe-O bond was reported to be hydrolyzed yielding iron oxide particles.14 LA also has a terminal vinyl group, which is used for polymerization with IAA. The authors previously reported the synthesis of the Li0.5Fe2.5O4 nanoparticle/polymer hybrid using IAA and LA.24 In this study, EHEC was selected as a polymer matrix. EHEC is a linear polymer of glucose rings bound through acetal linkages as shown in Figure 1. The OH groups are partly substituted for OCH2CH2OCH2CH3 in EHEC. The polymer chains interact via hydrogen bonding, affording a stiffness or rigidity to the cellulose. Li0.5Fe2.5O4 nanoparticles were synthesized in situ from prepolymerized IAA-LA in the presence of EHEC. Li0.5Fe2.5O4 nanoparticles were formed in a solution of prepolymerized ligands and EHEC. The product is described as an in situsynthesized Li0.5Fe2.5O4 (LFO) nanoparticle/EHEC hybrid. For comparison, EHEC was added to the LFO nanoparticle/organic hybrid. The hybrid was synthesized beforehand via hydrolysis of prepolymerized IAA-LA, in which lithium ferrite nanoparticles had already formed. All hydrolysis was conducted with 4 equiv of MH and 40 equiv of H2O and refluxed at 80 °C for 24 h. 3.2. Characterization of Products. Figure 2 shows the FT-IR spectra of EHEC, hydrolyzed EHEC, EHEC-added Li0.5Fe2.5O4 (LFO) nanoparticle/organic mixture, and the in situsynthesized Li0.5Fe2.5O4 (LFO) nanoparticle/EHEC hybrid. The hydrolyzed EHEC was obtained under the same conditions as those of the in situ-synthesized LFO nanoparticle/EHEC hybrid. The absorption bands at 1577 (solid circle) and 610 cm-1 (square) shown in Figure 2d are attributed to diketonate and spinel structure, respectively.26 Therefore, spinel particles coordinated by diketonate ligands are formed in the hybrid. The

Lithium Ferrite Nanoparticle/Polymer Hybrid Film

Figure 2. FT-IR spectra of (a) EHEC, (b) hydrolyzed EHEC, (c) 50 wt % EHEC-added LFO nanoparticle/organic mixture, and (d) LFO nanoparticle/EHEC hybrid with 50 wt % EHEC.

absorptions marked with arrows at 3480, 1420, and 650 cm-1 are ascribed to the OH bonds of cellulose.27 These absorptions are clearly observed in EHEC, hydrolyzed EHEC, and EHEC-added LFO nanoparticle/organic mixture. In contrast, the OH absorption at 3480 cm-1 almost disappears for the in situsynthesized LFO nanoparticle/EHEC hybrid. The broad absorption of the C-O stretching vibration is observed for EHEC from 1260 to 1000 cm-1. After hydrolysis the CO absorption of EHEC decreases in bandwidth, as shown in Figure 2b. This suggests the increase of low-molecular-weight components by hydrolysis under basic conditions and reflux at 80 °C for 24 h. The cleavage of acetal linkage in cellulose chains is reported to occur under similar hydrolysis conditions.28 Also, the absorptions at 1738 cm-1 marked with an open circle and 2830-2695 cm-1 are assigned to the CdO and C-H stretching vibrations of ring-opened aldehyde, respectively. EHEC itself includes the ring-opened aldehyde (HO-CH2-(HCOH)4C(H)dO) as shown in Figure 2a. Such absorptions almost disappear in the LFO nanoparticle/EHEC hybrid. This suggests that EHEC does not exist as a ring-opened aldehyde state but as a cyclic structure (Figure 1) in the LFO nanoparticle/EHEC hybrid. On the basis of these results, both the carbonyl group of the aldehyde form of EHEC and the OH groups in glucose rings are considered to coordinate to LFO nanoparticles. Also, the OH groups of cellulose are considered to react with the carbonyl of diketonate and acrylate ligands, affording acetal derivatives in the resultant hybrid. Figure 3 shows the XRD patterns of EHEC, LFO nanoparticle/EHEC hybrids, and an EHEC-added LFO nanoparticle/ organic mixture. LFO nanoparticle/EHEC hybrids shown from Figure 3b to 3d were synthesized in the presence of various amounts of EHEC from 30 to 70 wt %. The broad diffraction from 15 to 25° derives from EHEC. The reflections of spinel are observed from traces b-e in Figure 3. The crystallite size (D311) of the ferrite particles shown in Figure 3, traces b-e, were calculated to be 3.8, 3.1, 2.7, and 4.9 nm, respectively, based on the Scherrer equation. The crystallite size of the LFO nanoparticle/EHEC hybrid (Figure 3c) is lower than that of the EHEC-added mixture (Figure 3e) when the content of EHEC is 50 wt %. Without EHEC, the size of the LFO particles is larger than those synthesized in the presence of EHEC. When increasing EHEC, LFO particles decrease in crystallite size from Figure 3b to 3d. Therefore, the growth of spinel particles depends upon the amount of added EHEC.

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Figure 3. XRD patterns of EHEC, LFO nanoparticle/EHEC hybrids, and EHEC-added LFO nanoparticle/organic mixture. (a) EHEC, (b) LFO nanoparticle/EHEC hybrid with 30 wt % EHEC, (c) LFO nanoparticle/EHEC hybrid with 50 wt % EHEC, (d) LFO nanoparticle/ EHEC hybrid with 70 wt % EHEC, and (e) 50 wt % EHEC-added LFO nanoparticle/organic mixture.

Figure 4. Microstructure and SAED of LFO nanoparticle/EHEC hybrid with 50 wt % EHEC.

Figure 4 shows the microstructure and selected area electron diffraction (SAED) of the LFO nanoparticle/EHEC hybrid including EHEC 50 wt %. The bright field image shows that black particles are dispersed in the organic matrix. The SAD displays a pattern of several rings. The d values of the SAD were in agreement with those of Li0.5Fe2.5O4.29 However, the ring pattern suggests that the crystallinity of particles is low. The mean particle size of lithium ferrite is 3.8 nm, which is slightly larger than the crystallite size (3.1 nm) estimated from the Scherrer equation. The size distribution is narrow with a relative standard deviation of 5.3%. The hydroxy groups of EHEC lead to the formation of intramolecular and intermolecular hydrogen bonds. Prepolymerized IAA-LA is considered to interact with the intricate array of EHEC through hydrogen bonding. IAA-LA is hydrolyzed yielding metal hydrous hydroxide, which undergoes crystal growth upon aging at 80 °C. The diffusion and growth of metal hydrous oxides are hindered by the presence of cellulose networks. Thus, the size of lithium ferrite particles decreases with increasing EHEC from 30 to 70 wt %. Figure 5 shows photographs of the LFO nanoparticle/EHEC self-standing film including 50 wt % EHEC. The thickness was

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Figure 6. (a) Change of magnetization with applied field for a selfstanding film of LFO nanoparticle/EHEC hybrid with 50 wt % EHEC measured at 100, 200, and 300 K. (b) H/T superposition of isothermal magnetization data from 100 to 300 K for the hybrid shown in panel a. (c) Change of magnetization with applied field at 5 K for the hybrid shown in panel a.

Figure 5. Photographs of the self-standing film: (a) film on the paper, (b) film held up with a magnet, and (c) folded film.

TABLE 1: Amounts of Ferrite, Polymer, and Magnetization for the Hybrids with Different EHEC Concentrations

sample hybrid with 30 wt % EHEC hybrid with 50 wt % EHEC hybrid with 70 wt % EHEC

corrected amount amount magnetization magnetization of ferrite of polymer at 20 kOe and at 20 kOe and 300 K (emu/g) 300 K (emu/g) (%) (%) 30

70

9.2

31

12

88

2.8

23

7

93

1.5

21

measured to be 40 µm by SEM. Because the self-standing film is highly transparent, it is possible to view characters through a folded film (Figure 5c). Additionally, the film has a sufficient magnetization to be held up with a Nd-Fe-B system magnet (Figure 5b). Furthermore, this film is flexible as shown in Figure 5c. On the other hand, when EHEC was added to the LFO nanoparticle/organic hybrid synthesized beforehand, only opaque film was prepared from the resulting mixture solution. The castability of the EHEC-added mixture was better than that of the in situ-synthesized LFO nanoparticle/EHEC hybrid. However, the mixing of EHEC and LFO nanoparticle/organics was not uniform in the former. Insufficient mixing at the nanometer level is the reason for the opaque film. 3.3. Magnetic Properties. Table 1 summarizes the amount of ferrite and polymer, the magnetization at 20 kOe and 300 K, and the corrected magnetization for the hybrids with 30, 50, and 70 wt % EHEC, respectively. The amount of ferrite was estimated from the weight after burn-out of the organics by thermogravimetry. Magnetization curves of the LFO nanopar-

ticle/EHEC hybrid with 50 wt % EHEC at various temperatures are shown in Figure 6a. The magnetization at 20 kOe and 300 K is 2.8 emu/g, which is corrected to be 23 emu/g based on the amount of ferrite in the hybrid. The low crystallinity of lithium ferrite nanoparticles is considered to be the reason for the low magnetization compared to that of bulk lithium ferrite. Decreasing temperature from 300 to 100 K increases the magnetization from 2.8 to 5.0 emu/g. The magnetization of the hybrid does not exhibit saturation even in a strong magnetic field of about 20 kOe. This behavior is explained by the core-shell morphology of the nanoparticles.30,31 The shells are enriched with defects, such as broken bonds, broken symmetry, topological disorder, and compositional gradients. The outer dead layer of the core-shell structure reduces overall magnetization. Stronger fields are required to magnetize the surface region, because the region has highly disordered spin states. Thus, the very gradual increase in magnetization is observed up to very strong fields. As shown in Figure 6a, the hybrid reveals neither remanence nor coercivity above 100 K. The magnetization of fine particle magnetic solid M is written as the Langevin equation:

M ⁄ Ms ) coth(µH ⁄ κT) - (κT ⁄ µH)

(1)

where Ms is the saturation magnetization, H is the field, T is the absolute temperature, µ is the magnetic moment, and κ is Boltzmann’s constant. Figure 6b shows the change of H/T with magnetization of the LFO nanoparticle/EHEC hybrid, and the 50 wt % EHEC is shown in Figure 6a. The H/T values from 300 to 100 K are plotted against H on the same curve. The curves measured at different temperatures satisfy the Langevin equation above 100 K. The hybrid, therefore, showed a similar magnetic behavior to Langevin paramagnetism.32 The magnetization curve measured at 5 K for the hybrid is shown in Figure 6c. The curve shows a hysteresis with a remanent magnetization and a coercivity.

Lithium Ferrite Nanoparticle/Polymer Hybrid Film

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Figure 7. ZFC and FC magnetization curves measured at an external field of 200 Oe for the hybrid shown in Figure 6. Figure 9. Faraday rotation curves of self-standing film shown in Figure 6 measured at at 0, 2.5, 5.0, and 10 kOe.

Figure 8. Transmittance spectra of self-standing films of LFO nanoparticle/EHEC hybrids with various amounts of EHEC, (a) 70, (b) 50, and (c) 30 wt % EHEC.

Figure 7 shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for the LFO nanoparticle/EHEC hybrid with 50 wt % EHEC. The curve was measured at an external field of 200 Oe. The FC magnetization increases monotonically with decreasing temperature. However, the ZFC increases with decreasing temperature to a cusp around 13 K, and then decreases with further cooling. The cusp in the ZFC magnetization curve corresponds to the blocking temperature (TB). Magnetizations versus H/T curves are superimposed on the same curve for various values of T (Figure 6b). Also, the magnetic materials show no remanence above a blocking temperature of 13 K. Because the hybrid satisfies the abovementioned conditions, it is superparamagnetic.33 The blocking temperature of the current LFO nanoparticle/EHEC hybrid is much lower than that without EHEC (75 K). The TB value is known to decrease with decreasing particle size, which corresponds to the decrease in interparticle interaction.34,35 Hence, the lower TB value in the present hybrid is attributed to the smaller size of LFO particles (3.1 nm) than that without EHEC (4.7 nm). 3.4. Optical Properties. Figure 8 shows the change in transmittance and absorption coefficient with wavelength for self-standing films containing 30, 50, and 70 wt % EHEC. The absorption coefficient (R) is estimated from Figure 8 using the Lambert-Beer law.8 These films exhibit relatively high transmittance in the visible region. The transmittance increases with increasing EHEC from Figure 8c to 8a. The absorption edges

of films with 30, 50, and 70 wt % EHEC are 598, 536, and 512 nm, respectively. The absorption edge of the film was taken from the minimum of the second derivatives of the spectra.36 The edge was shifted to shorter wavelength with a decrease of particle size estimated using the Scherrer equation, as shown in Figure 3. The absorption edge was blue-shifted compared to that of a lithium ferrite single crystal (650 nm).37 The blueshift extends the absorption edge to shorter wavelength, leading to high transmittance in the short wavelength region below 700 nm. The blue-shift of the absorption edge corresponds to the increase in the band gap between the valence and conduction bands with decreasing particle size. The size of particles in the hybrid was below 4 nm, as shown in Figure 3. Magnetite particles below 5 nm are reported to show an increase in band gap due to the quantum size effect.38 The smaller size of lithium ferrite below 4 nm with almost uniform diameter is considered to be the reason for the quantized effect. Also, the change in absorption edge can be brought about by possible change in composition. Thus, the metal compositions of the products were analyzed by ICP. The weight percentages of iron and lithium for the hybrid synthesized with 30 wt % EHEC were 65.7 and 1.63 wt %, which corresponded to an atomic ratio of Fe/Li ) 5.01/1.00. Similarly, the Fe/Li ratios for the hybrids with 50 wt % EHEC and 70 wt % EHEC were 4.98/1.00 and 5.06/1.00, respectively. Because the composition of lithium ferrite is actually constant for the hybrids with different EHEC, the shift is considered to result from the change in crystallite size. 3.5. Magneto-optical Properties. Figure 9 shows the specific Faraday rotation of the self-standing film from 600 to 1000 nm, measured at 0, 2.5, 5.0, and 10 kOe. The film is an LFO nanoparticle/EHEC hybrid including 50 wt % EHEC with a thickness of 40 µm. The specific Faraday rotation (F) is defined as the Faraday rotation angle (θ) per unit path length (l). The specific Faraday rotation increases negatively with increase in applied field and decrease in wavelength. The rotation angle (θ) of the plane of polarization is in proportion to the pathlength, l, and to the field H. The relation is expressed in eq 2,

θ ) VHl constant.39

(2)

where V is the Verdet The path length is the film thickness of the hybrid, which is constant. Therefore, the increase in Faraday rotation with increasing magnetic field is explained by eq 2. The Faraday rotation depends upon the off-diagonal elements of the dielectric tensor and is proportional to the off-diagonal

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Hayashi et al. and exhibits a maximum around 700 nm. The absorption coefficient of the film is about 13 cm-1 from 650 to 750 nm, as shown in Figure 8. The small absorption coefficient of the film is responsible for the higher F/R value compared with those previously reported. The low absorption coefficient is one of the advantages of magnetic particle/polymer composites as shown by Ziolo et al.6 The present hybrid nanocomposite demonstrates a lightweight and processable magnetic material. 4. Conclusions

Figure 10. Change of Faraday rotation with applied field for the selfstanding film shown in Figure 6 measured at 633, 800, and 900 nm.

Novel magnetic, transparent, and flexible self-standing films were successfully synthesized via in situ processing of metallorganics in the presence of EHEC. The in situ synthesis of lithium ferrite in EHEC is essential for the uniform dispersion of nanosized particles. Uniform dispersion results in the size quantized effect and superparamagnetic properties of the transparent film. Thus, the magnetization and transmittance of self-standing films are controlled by crystal growth through hydrolysis in EHEC. Relatively high transmittance below 700 nm of such films resulted from the quantum size effect, which originated from nanosized lithium ferrite particles. The Faraday effect of the film increased with decreasing wavelength; therefore, the figure of merit below 800 nm was higher than reported values of γ-Fe2O3. The current magnetic hybrid material has potential applications in novel field-responsive materials based on its magnetic, optical, and elastic properties. References and Notes

Figure 11. Figures of merit of the film shown in Figure 6 and reported values.

elements.40 The off-diagonal elements are reported to increase with decreasing wavelength from 1000 to 500 nm for lithium ferrite.20,21 Hence, the increase in Faraday rotation with decreasing wavelength is considered to result from the increase of the off-diagonal element. Figure 10 shows the change of the specific Faraday rotation with magnetic field for the self-standing film shown in Figure 9. The specific Faraday rotation increases with increase in applied field. The curve shape is in agreement with that of the magnetization curve shown in Figure 6a, except that the sign is inverted with respect to magnetic induction. The gradual increase of Faraday rotation corresponds to the superparamagnetic behavior of the film. The maximum rotation is 25 °/cm at 633 nm, which is comparable to those reported for γ-Fe2O3/SiO2 nanocomposites given by Zayat et al.5 (26 °/cm) and Taboada et al.10 (29.6 °/cm at 810 nm). The Faraday effect is a tradeoff between magnetization and transparency. Therefore, a figure of merit, F/R, is used for the practical evaluation of the Faraday effect, where F is the specific Faraday rotation and R is an absorption coefficient. The data for the current film are compared with reported values in Figure 11.8,41 The figure of merit for MnBi film is shown in Figure 11, since it covers 300-900 nm. The figure of merit for the present film is higher than that of γ-Fe2O3/SiO2 below 850 nm,

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