Preparation and Magnetic Properties of Multilayer Films Based on Self

Oct 28, 2008 - Fabrication of Magnetic Thin Films by a Layer-by-Layer Self-Assembly Approach. Weihong Lin , Weilin Sun , Jun Yang , Qihang Sun and ...
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J. Phys. Chem. C 2008, 112, 18217–18223

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Preparation and Magnetic Properties of Multilayer Films Based on Self-Assembly Weihong Lin, Weilin Sun,* Jun Yang, and Zhiquan Shen Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: July 14, 2008; ReVised Manuscript ReceiVed: September 7, 2008

The preparation and magnetism of four multilayer films, (DABT/Ni2+/PAA)15×2, (DABT/Cu2+/PAA)15×2, (PAA/ DABT)20×2-Ni2+, and (PAA/DABT)20×2-Cu2+, which were obtained by self-assembly of 2,2′-diamino-4,4′bithiazole (DABT), poly(acrylic acid) (PAA), and transition metal ions (Cu2+ or Ni2+) on polyethylene (PE) substrate, were described. The driving force for building up the multilayer film was identified by infrared spectroscopy. UV-vis spectra and AFM images were applied to characterize these films and indicate the uniform assembling process. The magnetic behavior was examined as a function of magnetic field strength at 4 K and as a function of temperature (4-300 K). All films display good soft ferromagnetic properties, even affected by the diamagnetic substrate. It is found that (PAA/DABT)20×2-Ni2+ exhibits a fairly high value of relative saturation magnetization (Ms ) 37.3 emu/g), while (DABT/Ni2+/PAA)15×2 exhibits a high Curie-Weiss temperature (θ ) 242 K). The magnetic results show that different assembling processes can infect the alignment of adjacent paramagnetic spins and induce different magnetic phenomena. 1. Introduction In recent years the development of new concepts for the rational design of larger, more complex molecules has led to much progress in supramolecular chemistry.1 Self-assembly of supramolecular structures is a spontaneous process by which molecules and nanophase entities may materialize into organized aggregates or networks.2 Above all, self-assembled ultrathin films have received considerable interest because they allow fabrication of supramolecular assemblies with tailored architecture and properties.3 This research can be traced back to the pioneering work of Iler in 1966,4 who reported for the first time the construction of multilayers by alternating deposition of positively and negatively charged colloidal particles. In addition to the electrostatic interaction, a layer-by-layer (LBL) film also could be fabricated by other noncovalent molecular forces including hydrogen-bonding bridges, coordination interactions, charge-transfer complexes, hydrophobic interactions, etc. Simplicity and universality of the LBL assembly makes it easily available for the preparation of light-emitting diodes, resists, sensors, catalysis, electrically conductive films, optical devices, and separation membranes.5-11 However, among so many compounds used for assembly, magnetic polymers have received little attention so far.12 Kotov and co-workers prepared core-shell magnetite nanoparticles containing Fe3O4 nanoclusters.13 Talham et al. had fabricated some kinds of homogeneous thin films containing Prussian blue.14 They showed interesting magnetic properties by subtracting the diamagnetic signal of the substrate from the total registered signal. Organic polymers containing paramagnetic species may provide a new kind of magnetic material owing to the magnetically long-range ordering of unpaired electrons through spin-spin interactions.15 Much attention has been focused on the quest for organic and polymeric magnets in the past two decades because of their advantages compared with traditional magnets, such as the diversity of structures, low density, low magnetic * Corresponding author. Telephone: +86 571 8795 3727. Fax: +86 571 8795 3727. E-mail: [email protected].

loss, process of preparation without metallurgy at high temperature, etc. The design and preparation of organic ferromagnets is one of the great challenges in the field of molecular-based magnetic materials.16 Hoffmann et al. proposed that polymers built from five-membered rings containing sulfur, carbon, and nitrogen would theoretically display magnetic ordering,17 but none have been synthesized yet. Sun and co-workers have synthesized a variety of polymers containing bithiazole rings and the corresponding complexes with transition metal ions or rare earth metal ions.18 Most of them show interesting magnetic behavior, not only because of the good coordination ability due to two nitrogen atoms in the 2,2′-diamino-4,4′-bithiazole forming a stable five-membered ring with metal ions, but also the amino groups might provide cations in the process of assembly. The crystal structure of metal complex of 2,2′-diamino-4,4′-bithiazole (DABT) has been characterized by X-ray diffraction.19 We investigated the use of DABT to fabricate magnetic films through self-assembly for the first time. Here, we report on the preparation of polymeric multilayer films by layer-by-layer deposition on plastic substrates (polyethylene films) from DABT, poly(acrylic acid) (PAA), and transition metal ions. All of the films obtained display soft ferromagnetic properties without subtracting the diamagnetic signal of the PE from the total registered signal. The relative saturation magnetization can reach up to 37.3 emu/g, much higher than the previous report,20 and the Curie-Weiss temperature is as high as 242 K. As is well-known, PAA is a weak acidic polyelectrolyte. It can form polyelectrolyte complexes not only by electrostatic interactions with an oppositely charged polyelectrolyte in a specific solution, but also by hydrogen bonds.21 Furthermore, the polyethylene films have the often desirable characteristics of being inexpensive, chemically versatile, and sometimes elastic or biodegradable.22 The multilayer films show soft magnetic phenomena, which would create great information for the investigating of electrical, magnetic, and optical properties.

10.1021/jp806196s CCC: $40.75  2008 American Chemical Society Published on Web 10/29/2008

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SCHEME 1: Simplified Schematic Representation of the First Eight Steps in Assembling the Multilayer Film (DABT/M2+/PAA)n×2, Including Possible Structure in the Film (M ) Ni2+, Cu2+)

Figure 1. Photograph of the HDPE film and the magnetic multilayer film (DABT/Ni2+/PAA)15×2. The film was cut as a 1 × 2 cm2, and the film thickness was about 0.02 mm as measured.

2. Experimental Section 2.1. Materials. All the chemicals used were of AR grade. Tetrahydrofuran (THF) was distilled over CaH2 before use. 2,3Butyl dione, bromine (liquid), benzoyl peroxide (BPO), and thiourea were purchased from Shanghai Chemical Reagent Co. Nickel sulfate hexahydrate (NiSO4 · 6H2O) and copper sulfate pentahydrate (CuSO4 · 5H2O) were manufactured by Ningbo Chemical Reagent. 2,2′-Diamino-4,4′-bithiazole was prepared according to the literature. The polymerization of acrylic acid was carried out in THF with benzoyl peroxide at 67 °C for 1 h. The product was purified by fractional precipitation in chloroform and dried under vacuum. The yield was 80%, [η] ) 11.5 mL/g (1,4-dioxane, 30 °C), and the Mw was 2.3 × 104.23 The high-density polyethylene (HDPE) sample, having a density of 0.962 g/cm3, was used as the substrate. Pellets were pressed into 0.02 mm thick films at 170 °C. The HDPE substrate was modified according to the literature.24 The PE-CO2H film was prepared by immersing the PE film in a chromic acid solution (H2SO4/H2O/CrO3 ) 29/42/29, w/w) at 72 °C for 5 min and refluxing in concentrated nitric acid (70%) at 50 °C for 15 min.

2.2. Instrumentation. FT-IR spectra were recorded on a Bruker Vector 22 spectrometer. UV-vis spectra were collected by a UV-1601 UV-visible spectrophotometer. The atomic force microscopy (AFM) measurements were performed on a Seiko SPI3800N station (Seiko Instruments, Inc., Japan) in the tapping mode. The resonance frequency of the Si cantilever (NSG10, NT-MDT) was about 300 kHz. Typically, the image was obtained with a scan rate of 1 Hz and a resolution of 256 × 256 pixels. The element analysis for C, H, and N was performed by using a Flash EA 1112 element analyzer (Thermo Finnigan). The magnetic measurements were carried out by a Physical Properties Measurement System (PPMS-9T) magnetometer (Quantum Design). The measured temperature ranged from 4 to 300 K, and the intensity of magnetic field was measured from -50 to 50 kOe. 2.3. Preparation of the Multilayer Films. DABT and PAA were synthesized according to the literature methods. Two kinds of multilayer films have been prepared (Scheme 1): (i) A 20bilayer film was made by 20 times alternate deposition of DABT and PAA on the acid surface substrate and then reacted with transition metal ions (Cu2+ or Ni2+), for the construction of the films (PAA/DABT)20×2-Cu2+ and (PAA/DABT)20×2-Ni2+. (ii) The molecular complex of DABT and transition metal ions was first formed via metal-ligand chelation, and later PAA was deposited. Thus, the films (DABT/Cu2+/PAA)15×2 and (DABT/ Ni2+/PAA)15×2 have been prepared by repeating these steps in a cyclic fashion 15 times. Synthesis of 2,2′-Diamino-4,4′-bithiazole (DABT). 2,2′Diamino-4,4′-bithiazole (DABT) was prepared from thiourea and 1,4-dibromobutanedione.25,26 1,4-Dibromobutanedione (0.135 mol) and thiourea (0.276 mol) were refluxed at 80 °C for 1 h in 300 mL of absolute ethanol. The resulting solution was poured into 600 mL of hot water and ammonia solution was added until a precipitate appeared. The precipitate was filtered and then recrystallized from a H2O-EtOH (1:1 in volume) mixture. Brown needle crystals were obtained, which were filtered and dried at 40 °C for 24 h in a vacuum. The yield was 95%, and the decomposition temperature was 240 °C. The monomer was characterized by 1H NMR and FT-IR spectroscopy. FT-IR (KBr, cm-1): ν 3447 (m), 3288 (m), 3127 (w), 1598 (s), 1527 (s), 1460 (m), 1297 (s), 1035 (m). 1H NMR (DMSO-d6): δ (ppm) 6.9 (4H, NH2), 6.64 (2H, thiazole H). Anal. Calcd for C6H6N4S2: C, 36.35%; N, 28.26%; H, 3.05%. Found: C, 36.35%; N, 28.18%; H, 3.24%. Preparation of Multilayer Films (DABT/Ni2+/PAA)15×2 and (DABT/Cu2+/PAA)15×2. Freshly cleaned PE-CO2H was immersed into an ethanolic solution of 2,2′-diamino-4,4′-bithiazole (DABT) (1 mg/mL) for 3 h. After rinsing by dipping into three beakers with absolute ethanol for 5 min each and drying under a cool wind, the substrate was immersed into an aqueous

Magnetic Properties of Multilayer Films

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Figure 3. FT-IR spectra of (a) purified, high-density polyethylene, (b) oxidized polyethylene (PE-COOH) having a clear carbonyl band at 1710 cm-1, (c) film (PAA/DABT)20×2, (d) film (PAA/ DABT)20×2-Cu2+, and (e) film (DABT/Ni2+/PAA)15×2 in the region from 1500 to 1800 cm-1.

rinsed with deionized water, and dried with a cool wind. An alternating (DABT/Ni2+/PAA)15×2 multilayer film can be obtained by repeating these three steps in a cyclic fashion 15 times. The film (DABT/Cu2+/PAA)15×2 can be prepared by a similar process. Preparation of the Film (PAA/DABT)20×2-Cu2+. Freshly cleaned PE-CO2H was immersed into an ethanolic solution of DABT (1 mg/mL) for 3 h. After rinsing with absolute ethanol and drying under a cool wind, the substrate was dipped into a solution of PAA (1 mg/mL) for 1 h, then rinsed with deionized water, and dried with a cool wind. An alternating DABT/PAA multilayer film can be obtained by repeating these two steps in a cyclic fashion 20 times. The (PAA/DABT)20×2-Cu2+ film was obtained by immersing a (PAA/DABT)20×2 multilayer film into an aqueous solution of cupric sulfate (1 mg/mL) for 7 days at room temperature, followed by rinsing to remove excess CuSO4 (examined by BaCl2 solution). The film (PAA/ DABT)20×2-Ni2+ can be prepared by a similar process.

Figure 2. UV-vis absorption spectra of the following films on PE substrate: (a) 1-6 bilayer film (from lower to the upper curves) of PAA and DABT; (b) films (PAA/DABT)20×2 and (PAA/ DABT)20×2-Cu2+; (c) 1-6 bilayer film (from lower to the upper curves) of the multilayer film DABT/Ni2+/PAA. The inset shows the absorbance at 360 nm versus the number of bilayers.

solution of NiSO4 (1 mg/mL) for 12 h at room temperature, then rinsed three times with deionized water for 40 min each (examined by BaCl2 solution), and dried. The resulting substrate was dipped into the solution of PAA (1 mg/mL) for 1 h, then

3. Results and Discussion 3.1. Structural, Optical, and Morphological Characterization. In the deposition process, the film color can turn deeper with the increase of layers. From Figure 1, we can find that the color changed from colorless of the PE film to brown red of the multilayer film (DABT/Ni2+/PAA)15×2. The multilayer films were characterized by UV-visible spectroscopy, IR spectroscopy, and AFM. All the results of these measurements verified that well-organized close-packed multilayers are formed on the substrate. UV-visible spectroscopy was used to monitor the assembling process of 20-bilayer PAA/DABT multilayer thin films, (PAA/DABT)20×2-Cu2+ and (DABT/Ni2+/PAA)15×2 (Figure 2). Figure 2a shows the UV-vis absorption spectra of a multilayer film of PAA and DABT with different numbers of bilayers on a PE-CO2H slide. The absorbance of the band at 360 nm due to the contribution of bithiazole rings increases linearly with the increasing number of bilayers, thus indicating a uniform assembling process for the multilayer film (PAA/ DABT)20×2 (Figure 2a, inset). The (PAA/DABT)20×2 multilayer film was immersed into an aqueous solution of metallic sulfate where the metal ions diffused into the multilayer and chelated with the DABT and PAA to form a (PAA/DABT)20×2-M2+

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Figure 5. Temperature dependence of χT and reciprocal magnetic susceptibility (χ-1) as a function of temperature (T) for the oxidized polyethylene (PE-COOH) film at an applied field magnetic filed of 30 kOe.

Figure 6. Temperature dependence of magnetization (M) for the films (PAA/DABT)20×2-Ni2+ and (DABT/Cu2+/PAA)15×2 at H ) 30 kOe.

Figure 4. AFM images of (a) oxidized polyethylene (PE-COOH) film, (b) film (DABT/Ni2+/PAA)15×2, and (c) film (PAA/DABT)20×2-Cu2+.

film. Compared with (PAA/DABT)20×2, the characteristic absorption of bithiazole rings shows an obvious red shift (∼10 nm) of the (PAA/DABT)20×2-Cu2+ film, and a new absorption appeared at about 220 nm (Figure 2b). These changes result from the Cu2+ chelating not only with bithiazole rings to form stable five-membered rings but also with excess carboxyl of the PAA.27 As shown in Figure 2c, the increase of red shift and absorbance at 360 and 220 nm bands from one to six bilayers (DABT/Ni2+/PAA)15×2 become larger with the film layer increase. This also means that, chelating with Ni2+, the conjugation system and coplanar function strengthens between the carboxyl groups and bithiazole rings. The linear increase

of the absorbance at 360 nm with increasing number of bilayers also indicates a process of uniform assembling. The driving force for building up the multilayer film was identified by infrared spectroscopy. Figure 3 shows the IR spectra of the films PE, PE-CO2H, (PAA/DABT)20×2, (PAA/ DABT)20×2-Cu2+, and (DABT/Ni2+/PAA)15×2. After the PE film was oxidized, the sample had an absorption peak at 1710 cm-1, which could provide a hydrophilic surface for assembly (Figure 3b). In Figure 3c, the absorption in the range 1633-1623 cm-1 can be assigned to the vibration of bithiazole rings and the formation of ammonium groups (NH3+).28 The broad absorption band at around 1540 cm-1 is attributed to the carboxylate (COO-) asymmetrical deformation modes. On the other hand, the CdO stretching vibration appeared at around 1720 cm-1, which implies that carbonyl group is in a less associated state owing to the formation of hydrogen bonding. Combining the O-H stretching vibration at 2535 and 1949 cm-1, we can infer that the driving force includes electrostatic interaction and hydrogen bonding. Meanwhile, the residues of the COOH in PAA also exist in the film because of the steric hindrance effect of the DABT and the flexibility of the PAA. After reaction with CuSO4, the skeletal vibration of the

Magnetic Properties of Multilayer Films

Figure 7. χT and χ-1 as a function of T at 30 kOe (a) and hysteresis loop at T ) 4 K for the multilayer film (PAA/DABT)20×2-Cu2+ (b). (inset) Expanded view of the region from -20 to 20 Oe.

bithiazole ring takes an obvious red shift to 1612 cm-1, and the absorption peak of carboxyl at 1715 cm-1 becomes very weak (Figure 3d). These phenomena also prove that the complexation has taken place not only with the bithiazole ring but also with the residual COOH. The phenomenon of red shift in FT-IR may also be attributed to the electron cloud around the CdN bond of the bithiazole ring and the CdO bond of the PAA flow to the complex ions, and the corresponding electron cloud density decrease compared with that in the pure multilayer film. This result is consistent with that of the UV-vis spectra. We believe that this kind of electron motion is conducive to the regular alignment of the spin sources, thereby inducing strong magnetic exchange. For the multilayer film (DABT/Ni2+/ PAA)15×2 (Figure 3e), the skeletal vibration of bithiazole ring appeared at 1616 cm-1 while the broad absorption of the carboxylate was at about 1544 cm-1. The morphology of the film was investigated by atomic force microscopy (AFM), as shown in Figure 4. From Figure 4a, we can find that the surface of the PE-CO2H film is quite rough and visibly etched after oxidization.24 In contrast, the images of the surface of the multilayer films (DABT/Ni2+/PAA)15×2 and (PAA/DABT)20×2-Cu2+ show obvious densely packed nanogranules. However, the surface of the film (DABT/Ni2+/ PAA)15×2 is more smooth, and has a more ordered arrangement (Figure 4b). Meanwhile, we can see the cluster of nanoparticles of the film (PAA/DABT)20×2-Cu2+ obviously. That phenom-

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Figure 8. χT and χ-1 as a function of T at 30 kOe (a) and hysteresis loop at T ) 4 K for the multilayer film (DABT/Ni2+/PAA)15×2 (b). (inset) Expanded view of the region from -20 to 20 Oe.

enon is mainly attributed to the different processes of assembly. The (DABT/Ni2+/PAA)15×2 film was formed by layer-by-layer assembly from beginning to end, but (PAA/DABT)20×2-Cu2+ film was formed by immersing the uniform (PAA/DABT)20×2 film into an aqueous solution of cupric sulfate. The access is not homogeneous in the process of Cu2+ diffusing into the multilayer film, which brings on an irregular agglomeration of nanogranules (Figure 4c), considering the different transition metal ions with different orbital arrangements {Cu2+ (3d9) and Ni2+ (3d8)}, ionic radii {Cu2+ (70 pm) and Ni2+ (72 pm)}, chelate effect, etc. These two different assembly processes are likely to have a significant influence on the magnetism. 3.2. Magnetic Characterization. The magnetization of the films was examined by a Physical Properties Measurement System (PPMS) magnetometer. The amount of each film used in magnetic measurement was about 200 mg, and the films were held by a specific tube. According to expectations, the polyethylene acid film shows a diamagnetic property due to the absence of paramagnetic sources just like other pure polymers.18b We can observe that its negative diamagnetic susceptibility (χ) is temperature-dependent at low temperatures (Figure 5), and is field-dependent from 50 to -50 kOe (Supporting Information). Therefore, the PE-CO2H film shows abnormal diamagnetic behavior incompatible with Landau diamagnetic theory. According to magnetic theory, the atoms could be a right distance apart so that the exchange force can cause the d-electron spins in one atom to align the spins in a neighboring atom.29

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TABLE 1: Magnetic Properties of the Multilayer Films

(DABT/Ni2+/PAA)15×2 (DABT/Cu2+/PAA)15×2 (PAA/DABT)20×2-Ni2+ (PAA/DABT)20×2-Cu2+

relative saturation magnetization, Ms/emu · g-1

remnant magnetization, Mr/ emu · g-1

coercivity, Hc/Oe

Curie-Weiss temperature, θ/K

35.5 22.9 37.3 15.2

0.0163 0.0104 0.0210 0.0063

15.6 18.5 19.5 17.5

242 115 63 131

That regular alignment of the internal spins, which minimizes electron-electron repulsions and holds a net magnetic moment, is the most stable state. In this regard, it is equivalent to a uniform three-dimensional network containing unpaired electrons for the film we prepared. The multilayer films would display good magnetic properties, even affected by the diamagnetic PE substrate. Figure 6 shows the temperature dependence of the magnetization of the films (DABT/Cu2+/PAA)15×2 and (PAA/ DABT)20×2-Ni2+ at an applied magnetic field of 30 kOe from 4 to 300 K. The positive magnetization decreased sharply with the temperature increase when the temperature was below 50 K and then decreased slowly until 300 K. The temperature dependence of the reciprocal magnetic susceptibility (χ-1) and χT product for the multilayer film (PAA/ DABT)20×2-Cu2+ was measured as shown in Figure 7a. The magnetic susceptibility follows the Curie-Weiss law, χ ) C/(T - θ), with the Curie-Weiss temperature θ ) 107 K in the range 300-225 K, indicating that there is effective paramagnetic behavior of magnetic material above its critical temperature, 225 K. The positive θ value indicates the multilayer film (PAA/ DABT)20×2-Cu2+ is a ferromagnet.30 When it is in the low temperature stage, the relationship between T and χ is not in accordance with the Curie-Weiss law; this suggests that the ferromagnetic coupling among spins in the long range overcomes the thermal energy owing to the parallel alignment of adjacent magnetic spins in the regular framework. The value of χT increases with decreasing temperature from 300 to 60 K. When the temperature decreased from 60 to 10 K, the χT changed slightly; this suggests that the ferromagnetic interaction among adjacent magnetic spins have been saturated in the wellordered three-dimensional network structure. The adjacent magnetic moments are prone to parallel alignment owing to the ordered arrangement in three dimensions of the polymeric complex on the PE film. The field-dependent magnetization, M(H), is characteristic of magnetic ordering and exhibits hysteretic behavior with a coercive field, Hc, of 17.5 Oe and remnant magnetization, Mr, of 0.0063 emu/g at 4 K (Figure 7b). The magnetization of the multilayer film (DABT/Ni2+/ PAA)15×2, shown in Figure 8, displays the different phenomena. From 300 K, the χT increases dramatically until reaching the rounded maximum at 15 K, while χ-1 decreases with the same trend of temperature. The relationship between T and χ deviates from the Curie-Weiss relationship below 280 K, with θ ) 242 K. The high positive θ value also indicates that the spins are in more appropriate positions in this multilayer film, inducing strong ferromagnetic coupling in the long range. An “S” shape hysteresis loop (Hc ) 15.6 Oe, Mr ) 0.0163 emu/g) was observed, and the phenomenon indicates that the multilayer film (DABT/Ni2+/PAA)15×2 is a soft ferromagnet (Figure 8b). The relative saturation magnetization (Ms) value can reach 35.5 emu/g. Seen from Table 1, the four multilayer films have different Curie-Weiss temperatures θ. The value decreases gradually from 242, 115, 107, to 63 K corresponding to the films (DABT/ Ni2+/PAA)15×2, (DABT/Cu2+/PAA)15×2, (PAA/DABT)20×2Cu2+, and (PAA/DABT)20×2-Ni2+. That may arise from the

different metal ions and assembling processes which can be concluded from the characterization results. The magnetic sources display different alignments in the regular threedimensional network multilayer films. Meanwhile, the spins are prone to arrive at the most suitable distance for forming effective ferromagnetic alignment even at high temperature in the film (DABT/Ni2+/PAA)15×2. However, for the (PAA/DABT)20×2Ni2+ film, it is more difficult to reach a homogeneous state in the process of Ni2+ diffusing into the film for the steric hindrance effect. The typical S shape and such low Hc and Mr values indicate that the multilayer films are organic soft magnets. All films have relative saturation magnetizations higher than those in the previous report,20,31 while the film (PAA/ DABT)20×2-Ni2+ has the highest value at 37.3 emu/g. All display good soft ferromagnetic properties which are different from their complementary partners such as the diamagnetic polyethylene, PAA, DABT, and the paramagnetic DABT-M2+. The exact nature of the arrangement and magnetism remains an interesting challenge. Further studies are in progress. 4. Conclusions In conclusion, we have fabricated four magnetic multilayer thin films by self-assembly and characterized them by UV-vis, IR, and AFM for the first time. The films show high specific magnetization, up to the Curie-Weiss temperature 242 K, with an S shape hysteresis loop. It is noteworthy that a fairly high value of relative saturation magnetization (Ms ) 37.3 emu/g) was observed for the film (PAA/DABT)20×2-Ni2+. Such behavior is mainly caused by the strong ferromagnetic interactions among paramagnetic spins in the regular three-dimensional framework. The differences observed in the ferromagnetic ordering temperature reflect the specific structural arrangements of the compounds on the substrate surface. These films are flexible and stable, showing good soft ferromagnetic properties, and would promote the development of organic magnetic applications. The studies of the magnetic properties of these multilayer films will not only contribute to the research concerning the relationship of structure and magnetism, but also provide some useful information for novel functional materials with electric and magnetic properties. Acknowledgment. The authors are grateful for financial support from the National Natural Science Foundation of China (Grants 20674071 and 20434020). Supporting Information Available: Magnetic measurements of the films polyethylene, (PAA/DABT)20×2-Ni2+and (DABT/Cu2+/PAA)15×2. Synthesis and magnetic properties of the organic complex DABT-Ni2+. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lin, X.; Doble, D. M. J.; Blake, A. J.; Harrison, A.; Wilson, C.; Schroder, M. J. Am. Chem. Soc. 2003, 125, 9476–9483. (2) (a) Huie, J. C. Smart Mater. Struct. 2003, 12, 264–271. (b) Khopade, A. J.; Mo¨hwald, H. AdV. Funct. Mater. 2005, 15, 1088–1094.

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