CoFe2O4

Article pubs.acs.org/cm

Synthesis of Hard Magnetic Ordered Mesoporous Co3O4/CoFe2O4 Nanocomposites Harun Tüysüz,† Elena Lorena Salabaş,†,§ Eckhard Bill,‡ Hans Bongard,† Bernd Spliethoff,† Christian W. Lehmann,† and Ferdi Schüth*,† †

Max-Planck Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr/Germany Max-Planck Institut für Bioanorganische Chemie, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr/Germany

‡

S Supporting Information *

ABSTRACT: The nanocomposite Co3O4/CoFe2O4 heterostructured mesoporous material was produced via a simple solid−solid reaction of an iron precursor with ordered mesoporous Co3O4 that had been prepared via nanocasting from mesoporous silica as hard template. The magnetic behavior of the exchange-coupled antiferromagnetic/ferrimagnetic (AFM/FM) system was investigated via superconducting quantum interference device (SQUID) magnetometry and 57Fe Mössbauer spectroscopy. The low-temperature magnetization loops of the Co3O4/CoFe2O4 heterostructure present exchange bias under cooling in an applied magnetic field. The antiferromagnetic ordering temperature of Co3O4 is increased due to the proximity of the hard magnetic CoFe2O4 phase. The nanocomposite Co3O4/CoFe2O4 behaves as an exchange coupled system with a cooperative magnetic switching. KEYWORDS: ordered mesoporous materials, nanocasting, magnetism, exchange bias

1. INTRODUCTION Nanoscaled materials are currently of high scientific and technological interest in a variety of different fields. Among them, mesoporous silica and nonsilicon oxides are useful in various applications, such as catalysis, sorption, photonics, electronics, and drug delivery.1−5 Ordered mesoporous materials have narrow pore size distribution, high surface area, and high pore volume compared to nonordered porous materials. These kinds of materials can be prepared via soft (cooperative assembly) and hard templating (nanocasting) pathways. Ordered mesoporous materials are covered in several reviews.6−9 Following the cooperative assembly route, ordered mesoporous silicas, such as MCM-41,10 SBA-15,11 and KIT-6,12 and also nonsilica materials, such as TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, and WO3 mesoporous materials, were successfully synthesized.13 A series of mesoporous lanthanide oxides,14 Cr2O3,15 and γ-Al2O3,16 was also prepared via the soft templating approach. However, even if transition metal oxides are accessible via soft templating, it is more difficult to control the hydrolysis and polymerization of transition-metal alkoxides compared to the silicon counterparts. Metal oxides that are prepared via soft templating usually exhibit poor structural ordering and low thermal stability after removal of the surfactant templates. Ryoo and co-workers introduced the nanocasting route to synthesize ordered mesoporous materials.17 This hard templating approach has been applied to produce different kinds of metal oxides such as Co3O4,18−20 Cr2O3,21,22 CeO2,23 MgO,24 α-Fe2O3,25 2-line ferrihydrite,26 CuO,27 NiO,28 and aluminosilicate.29 Terasaki and colleagues © 2012 American Chemical Society

reported an alternative hard templating strategy for the fabrication of shape- and size-controlled ordered mesoporous Pt nanoparticles by utilizing silica as a hard template and ascorbic acid as a reducing agent at room temperature.30 Kleitz et al. described the synthesis of a series of crystalline mesoporous mixed metal oxides (e.g., NiFe2O4, CuFe2O4, Cu/CeO2) via a one-step-impregnation hard templating route.31 The preparation of spinel CoFe2O4 had been also described by using cubic and hexagonally ordered mesoporous silica as a hard template.32 Another possibility to prepare ordered mesostructured metal oxides is the reduction or oxidation of other ordered mesoporous metal oxides. Jiao et al. prepared cubic ordered mesoporous Fe3O4 by reduction of α-Fe2O3 with H2 as a reducing agent. Then, they converted Fe3O4 to γ-Fe2O3 by oxidation retaining the same ordered mesostructure.33 The same group also described the synthesis of Mn3O4 by first synthesizing ordered mesoporous Mn2O3, then reducing it to Mn3O4 with H2 as the reducing agent.34 We previously reported that pseudomorphic reduction of ordered mesoporous metal oxide can also be achieved by treatment with alcohol− water vapor at elevated temperatures. Following this process, we could prepare highly ordered mesoporous CoO and Fe3O4 by reducing Co3O4 and ferrihydrite, respectively.35,36 Shi et al. introduced a new ammonia nitridation approach through a Received: February 15, 2012 Revised: June 1, 2012 Published: June 15, 2012 2493

dx.doi.org/10.1021/cm3005166 | Chem. Mater. 2012, 24, 2493−2500

Chemistry of Materials

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Figure 1. Low (a) and wide (b) angle XRD patterns of Co3O4 and Co3O4/CoFe2O4. concentrated HCl (37%). n-Butanol (13.5 g) was added to the homogeneous solution at 35 °C. After 1 h stirring of the mixture, 29 g TEOS (tetraethoxysilane) was added to the solution, and stirring was continued at this temperature for another 24 h. After that, the mixture was aged at 40 °C for 24 h. The white solid product was filtered, dried at 90 °C, and finally calcined at 550 °C for 6 h. KIT-6 was used as solid template to prepare ordered mesoporous Co3O4. In a typical synthesis, 0.5 g of KIT-6 was dispersed in 4 mL of 0.8 M Co(NO3)2.6H2O in ethanol and stirred for 1 h at room temperature. Subsequently, the ethanol was evaporated at 50 °C. The composite was calcined at 200 °C for 6 h. The composite was reimpregnated, followed by calcination at 450 °C for 6 h. According to theoretical calculations, after loading twice, 17% of the pore volume of KIT-6 was filled by Co3O4. The silica template was then removed by treatment with 2 M NaOH aqueous solution, followed by repeated washing with water and then drying at 50 °C. Co3O4/CoFe2O4 was produced via a solid−solid reaction of iron with mesoporous Co3O4. 0.5 mL of 0.8 M Fe(NO3)3·9H2O (in ethanol) was added to 0.2 g of mesoporous Co3O4, stirred for 1 h at room temperature. Then, the ethanol was evaporated at 50 °C overnight, and the sample was subsequently heated to 450 °C with a heating rate of 2 K/min and kept that temperature for 4 h. To prepare the Fe-enriched sample, the concentration of the iron solution was increased. X-ray diffraction (XRD) patterns of all the samples were recorded on a Stoe STADI P diffractometer operating in reflection mode with Cu Kα radiation. Nitrogen adsorption isotherms were measured with an ASAP 2010 adsorption analyzer (Micromeritics) at liquid nitrogen temperature. Prior to the measurements, the samples were degassed at a temperature of 150 °C for 10 h. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.97. BET (Brunauer−Emmett−Teller) surface areas were estimated from the relative pressure range 0.06 to 0.2. Pore size distribution (PSD) curves were calculated by the BJH (Barrett−Joyner−Halenda) method from the desorption branch. High resolution transmission electron microscopy (HR-TEM) images of samples were obtained with an HF2000 microscope (Hitachi) equipped with a cold field emission gun. The acceleration voltage was 200 kV. Samples were prepared on a copper grid coated with a lacey carbon film. High resolution scanning electron microscopy (HR-SEM) images and scanning transmission electron microscopy (STEM) images of the samples were taken with an S-5500 microscope from Hitachi. The microscope was operated at 30 kV. The sample was prepared on a lacey film supported by a copper grid. The magnetic properties were investigated by means of magnetization measurements using a superconducting quantum interference device (SQUID) magnetometer (MPMS-7 Quantum Design) in the 2−280 K temperature range, with the applied fields up to 50 kOe. The samples (30−35 mg) were measured in powder form in a plastic capsule. The magnetization was normalized to the total weight of the sample. Mö ssbauer data were recorded on a spectrometer with alternating constant acceleration. The minimum

gas−solid reaction for the synthesis of ordered mesoporous CoN and CrN.37 The synthesis of ordered mesoporous LiCoO2 was achieved by solid−solid reaction of silica-free Co3O4 with LiOH·H2O.38 Bulk CoFe2O4 is ferrimagnetic below 860 K and has an inverse spinel structure with Co2+ ions distributed on B sites and Fe3+ ions distributed on A and B sites. Cobalt ferrite is a hard magnetic material with very high cubic magnetocrystalline anisotropy (Keff = 1.8 × 107 erg/cm3), high coercivity (Hc= 5.4 kOe), and moderate saturation magnetization (80 emu/g).39 These properties make it a promising material for permanent magnets. Bulk Co3O4 is a normal spinel having the cation distribution (Co 2+)[Co3+2]O4. Its magnetic moment arises only from the tetrahedrally coordinated Co2+ ions, while the Co3+ ions are diamagnetic. In the bulk, a transition from the paramagnetic state to antiferromagnetic ordering occurs around 40 K.40 For nanoscale materials, the role of the surface becomes increasingly important, and the magnetic properties can be different with respect to the bulk. Particularly, nanostructured Co3O4 reveals interesting magnetic behavior due to the uncompensated surface spins, which are exchange-coupled to the antiferromagnetic core.41−43 Bringing in close contact two different magnetic phases can lead to interesting and novel magnetic properties. For example, when an antiferromagnetic material is in contact with a ferromagnetic material, the exchange coupling at the interface gives rise to a unidirectional anisotropy called exchange bias. The exchange bias effect is a cornerstone of the design and operation of spin valve devices, and it is a simple way to combat the superparamagnetic limit in magnetic recording media.44 In this paper, we demonstrate the versatility of the nanocasting route for the example of the preparation of a mesostructured magnetic heterostructure composed of Co3O4 and CoFe2O4.45 The structural and magnetic properties of the nanocomposite system were investigated. Our studies have shown a reduced Néel temperature for a 6 nm Co3O4 nanostructure, while the heterostructure of Co3O4 with CoFe2O4 led to an increase of the antiferromagnetic ordering temperature. The coupling of Co3O4 to the hard ferrimagnetic layer influences the ordering temperature of the AFM layer.

2. EXPERIMENTAL SECTION Cubic ordered mesoporous KIT-6 was prepared according to the literature.12 Briefly, 13.5 g of surfactant (Pluronic 123, EO20PO70EO20) was dissolved in a solution of 487.5 g distilled water and 26.1 g 2494



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experimental line width was 0.24 mm s−1 (full width at half-height). The sample temperature was maintained constant in an Oxford Instruments Variox cryostat. The 57Co/Rh source (1.8 GBq) was at room temperature; isomer shifts are quoted relative to iron metal at 295 K.

decomposed cobalt ferrite with an excess amount of cobalt.48 It was found that the formation of single or two spinel phase regions depends on temperature and the relative quantity of iron and cobalt. In the temperature range between 400 and 900 °C, a single Co3O4 spinel phase was observed when the (Fe)/ (Co+Fe) atomic ratio was below 0.1. Increasing amounts of iron in the composite cause the formation of the inverse spinel CoFe2O4 phase in addition to the Co3O4. In the present study, a calcination temperature of 450 °C and an Fe/(Co + Fe) atomic ratio of 0.14 was chosen. According to the phase diagram of the Fe−Co−O ternary system in air, under these conditions, the formation of CoFe2O4 and Co3O4 phases would be expected. Thus, the appearance of the CoFe2O4 reflection at higher iron loading, the ferromagnetic behavior, and the reported phase diagram suggest that after impregnation of iron species into the nanocasted cobalt oxide and heat treatment, small domains of the CoFe2O4 are formed in the Co3O4 matrix. Texture parameters of Co3O4 and Co3O4/CoFe2O4 were investigated by nitrogen physisorption. The nitrogen adsorption−desorption isotherms and the pore size distributions for cubic ordered mesoporous Co3O4 and Co3O4/CoFe2O4 are given in Figure 2. The isotherms are of type IV of the IUPAC

3. RESULTS AND DISCUSSION Texture parameters of the hard template play an important role in determining the structure of nanocast materials. Cubic ordered mesoporous silica (KIT-6) has two mesoporous channel systems that are connected to each other through micropores. This mesostructured silica is characterized by a low angle XRD pattern that typically shows the (211) and (220) reflections. If the interconnectivity between two mesopore channels of the silica hard template via the micropores is high enough, nanocast metal oxides possess a perfect replica structure with identical symmetry that shows the same reflections. However, if KIT-6 prepared at a lower aging temperature (normally below 60 °C) is used as hard template, the result is a decreased degree of interconnectivity between the two mesopore systems of the gyroid structure. In this case, a replica with lower symmetry is formed, and the low angle XRD patterns of the nanocast materials show an additional (110) reflection.20 Low angle XRD patterns of template free Co3O4 and the Co3O4/CoFe2O4 replica, prepared via the nanocasting pathway, are shown in Figure 1a. The (110) and (211) reflections are clearly visible; after impregnation with iron(III) nitrate and the calcination process, the (211) peak lost some intensity, but the cubic ordered structure can still be identified. The unit cell parameter of the nanocomposite was estimated to be 19.3 nm from the position of the (211) reflection. The average crystallite sizes of the sample before and after iron doping were calculated from the wide angle XRD patterns by using the Scherrer equation. Values of 9 and 10 nm, respectively, were obtained. Wide angle XRD patterns of mesostructured Co3O4 and the composite sample after iron loading are shown in Figure 1b. As seen from the figure, the materials are highly crystalline, and in both cases, only one phase is observed, which corresponds to the Co3O4 spinel structure. Since the unit cell parameters of Co3O4 and CoFe2O4 are very close to each other, one can not distinguish these two phases with XRD analysis, particularly not for small nanoparticles with strongly broadened reflections. However, at higher iron concentration (increased from (Fe)/(Fe+Co) = 0.14 to a ratio of 0.41), the presence of the CoFe2O4 phase can be inferred from the (440) reflection at 63° (2Θ), which is very broad and increases in intensity with iron content, whereas the corresponding reflection for the binary Co3O4 spinel at 65° (2Θ) remains visible (see Supporting Information Figure S1 for the wide angle XRD pattern). In this study, Fe (NO3)3·9H2O was directly impregnated into the pores of ordered Co3O4 to prepare a Co3O4/CoFe2O4 composite. During the calcination process, a solid−solid reaction occurs between the iron species and cobalt oxide to generate the ferrimagnetic CoFe2O4 spinel phase, probably dispersed within the antiferromagnetic Co3O4 matrix. After impregnation with the iron precursor and calcination, the obtained black composite sample showed ferromagnetic behavior, the sample was easily dragged using a magnet, which also indicates the formation of CoFe2O4. Early studies on the pseudobinary Co3O4/CoFe2O4 system in air showed that there is a miscibility gap in the spinel-structure region.46,47 Takahashi et al. reported the phase diagram of spinodally

Figure 2. Nitrogen isotherms and the pore size distribution (in inset) calculated from desorption branch, for cubic ordered mesoporous Co3O4 and Co3O4/CoFe2O4.

classification. They show the typical hysteresis loop of mesoporous materials prepared by the hard-templating method. Co3O4 and Co3O4/CoFe2O4 have a BET surface area of 162 and 122 m2g−1, and a pore volume of 0.527 and 0.418 cm3g−1, respectively. Figure 2 inset indicates a bimodal pore size distribution, which was calculated from the desorption branch of the isotherm by the BJH method. Small pores for Co3O4 and Co3O4/CoFe2O4 are around 6 nm, while the big pores are around 11 nm for both materials. KIT-6 is composed of two channel systems and early studies indicated that when metal oxides grow only in one of the channels of KIT-6, the nanocast metal oxides show a bimodal pore size distribution.20,28 After introduction of the iron species, the surface area and the pore volume of the material decreased slightly; however, the pore size distribution did not change significantly. That could be due to the relatively low loading of the iron and rather similar bulk density values of Co3O4 (6.1 g.cm−3) and CoFe2O4 (5.3 g.cm−3). The morphology and pore topology of the Co3O4 and Co3O4/CoFe2O4 were further investigated by HR-SEM and 2495



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Figure 3. HR-TEM and STEM images for Co3O4 (a, b) and Co3O4/CoFe2O4 (c, d).

Figure 4. HR-SEM images for Co3O4 (a) and Co3O4/CoFe2O4 composite (b).

mesoporous structure of the composite can still be seen in the TEM and BF-STEM projections in Figure 4c and d. No bulk iron oxide was detected in the Co3O4/CoFe2O4 sample by TEM analysis. This suggests the successful fabrication of a Co3O4/CoFe2O4 ordered mesoporous nanocomposite. As seen from Figure 4, HR-SEM images of Co3O4 and Co3O4/CoFe2O4 indicate predominantly an uncoupled subframework structure. In the regions where the uncoupled subframework structure is observed, the material has a pore size around 11 nm, in good agreement with N2-sorption measurements.

TEM. In Figure 3a−d, TEM and STEM images of Co3O4 and Co3O4/CoFe2O4 are depicted. The well-ordered mesoporous structure of Co3O4 is clearly visible in images a and b of Figure 3. The average primary particle size of the Co3O4 domains is estimated to be around 6 nm. This is slightly smaller than the value that was calculated from the XRD pattern by using the Scherrer equation. However, such a small difference is not unusual, since a particle size determination using the Scherrer equation gives volume averaged size while TEM analysis typically leads to a number average size. After impregnation of the iron precursor and calcination, the highly ordered 2496



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Figure 5. Line scan STEM-EDX elemental analysis of Co3O4/CoFe2O4 composite.

tured Co3O4 antiferromagnetic nanoparticles with cubic symmetry is shortly presented. Figure 6a shows the magnetic

The distribution of iron in the Co3O4 matrix was investigated by scanning electron microscopy combined with energydispersive X-ray spectroscopy (STEM-EDX). STEM-EDX is a technique in which X-ray signals are collected from the area scanned by STEM. Resolution depends on a number of factors, such as spot size of the electron beam and sample thickness; it can be estimated to a few nanometers for the samples studied. A line-scan STEM-EDX elemental analysis of a single layer of the ordered mesoporous Co3O4/CoFe2O4 composite is presented in Figure 5. As seen from the graph, the signal intensity patterns of iron and cobalt are similar and closely related to one another. This suggests that iron is dispersed in the cobalt oxide matrix homogenously at an atomic ratio of 1 to 6, which corresponds to the ratio used in the impregnation step. The EDX and elemental analyses of a large domain of the Co3O4/CoFe2O4 composite are presented in Supporting Information Figures S2 and S3. Similar to the line scan elemental analysis, the STEM-EDX elemental mapping also suggests a homogeneous distribution of iron (Co/Fe; 6/1 molar ratio) in the nanostructured mesoporous material. This can be interpreted as follows: Subsequent to the impregnation and drying process, most of the iron precursor resides in the pores of Co3O4. One might expect that, after impregnation of the Co3O4 mesostructure with iron species and calcination, a thin layer of CoFe2O4 coating the pore surface of Co3O4 would form. However, from the elemental analysis, there is no indication of a layer of CoFe2O4 formed on a Co3O4 core. However, due to the fact that also in the center of the struts there would be a CoFe2O4 layer above and below the Co3O4 core in that case and thus iron would be detected, and due to the work at the resolution limit, we can not fully exclude such a core−shell structure. However, during high temperature treatment, the iron species could also diffuse into the Co3O4 matrix (the required diffusion distances are only on the order of nanometers) and form the CoFe2O4 spinel distributed over the material, and thus, this interpretation appears to be more probable. A similar phenomenon was observed by Andrews et al. in the preparation of M2O3(ZnO)n (M = In, Ga, Fe) superlattice nanowires, in which ZnO nanowires had been doped with various metal clusters and calcined at high temperature.49 However, in the present study, the exact arrangement of the CoFe2O4 with respect to Co3O4 could not be established with certainty, in spite of intensive efforts, due to the resolution limit of the STEM-EDX analysis. Before discussing the magnetic properties of the composite heterostructure, the magnetic behavior of the pure mesostruc-

Figure 6. The magnetic behavior of Co3O4 nanoparticles. The variation of magnetization with the field of 100 Oe (at 5 K, a) and the temperature (b) are represented.

hysteresis loops, measured at 5 K, of the Co3O4 with a primary particle diameter of about 6 nm, after cooling in zero-field (see Supporting Information Figure S4 for field cooled curve). The inset of Figure 6a shows the lower field region of the hysteresis loop. The measurements at 5 K display a residual magnetization with a coercivity of about Hc = 280 Oe. For a bulk antiferromagnet below the Néel temperature, magnetization is expected to vary linearly with applied magnetic field and no remanent magnetization should be detected. However, small 2497



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example, 8.5 nm particles are ferrimagnetic below 300 K, whereas particles with a size of 4.5 nm block around 150 K. Figure 8 shows the ZFC and FC hysteresis loops of the Co3O4/CoFe2O4 heterostructure. The field-cooled hysteresis

antiferromagnetic nanoparticles can show a behavior that resembles weak ferromagnetism below the Néel temperature.50 This has been ascribed to the presence of uncompensated surface spins. It was suggested that the uncompensated spins at the surface of nanocast Co3O4 nanowires, with 2D hexagonal symmetry, behave similar to a spin-glass-like system and are exchange-coupled to the spins located in the antiferromagetic core,41 but the behavior can possibly be better interpreted as a dilute antiferromagnet in a field.42,43 A similar behavior has been observed in Co3O4 nanoparticles with cubic symmetry. A slightly larger field-cooled coercivity (Hc = 350 Oe) and a shifted hysteresis loop, Heb = −920 Oe, were observed for the sample cooled in a magnetic field of 5 kOe. The magnetic susceptibility as a function of temperature for Co3O4 nanoparticles is shown in Figure 6b. The ZFC curve was recorded by cooling the sample in zero field from room temperature to 2 K and then measuring the magnetization while increasing temperature stepwise in a field of 100 Oe. The magnetic susceptibility exhibits a maximum around 20 K, which is attributed to the Néel temperature. For bulk Co3O4, values between 30 K51 and 40 K for the Néel transition have been reported.40 A reduced value of the antiferromagnetic ordering temperature can be related to the small size of the particles. The reduction of the Néel temperature due to the finite-size effects has been observed in numerous studies.52−54 In all systems, the Néel temperature is reduced with decreasing particle size. The magnetic properties of the composite Co3O4/CoFe2O4 system are discussed next. Figure 7 shows the temperature

Figure 8. Magnetic hysteresis of Co3O4/CoFe2O4 composite nanostructure at T = 5K.

loops of the system were taken with increasing temperature, after field cooling in 50 kOe from room temperature down to 5 K. The ZFC hysteresis loop is symmetric about the origin and shows a typical ferrimagnetic behavior. The large coercivity Hc value (6.6 kOe) of the nanocomposite system originates from the low temperature hard magnetic properties of CoFe2O4. A complete saturation of magnetization is not achieved in magnetic fields up to 5 T; thus we do not have a complete alignment for the magnetic moments. From the analysis of the hysteresis loop, it can be seen that the magnetization changes smoothly with the applied magnetic field, which is similar to the behavior of a single-phase material. This indicates that the antiferromagnetic Co 3 O 4 and ferromagnetic CoFe2O4 are exchange-coupled. The evidence of an intimate contact between the Co3O4 and CoFe2O4 is supported by the evolution of the FC hysteresis loop. At 5 K, a shift of the hysteresis loop along the field axis, Heb = 1.1 kOe, has been measured. The exchange bias was calculated by averaging the positive and negative coercivities. Interestingly, the exchange bias vanishes at temperatures higher than 40 K. This finding suggests that the antiferromagnetic order of the Co3O4 plays an important role in the occurrence of the exchange bias. The hysteresis loops recorded at T = 80 K are illustrated in Figure 9. It can be seen that the Co3O4/CoFe2O4 composite system shows a superparamagnetic behavior and no evidence for an exchange bias system was observed at this temperature. The composite nature of the Co3O4/CoFe2O4 system is also corroborated by zero-field Mössbauer measurements, since down to 80 K the spectra consists of pure quadrupole doublets without any contribution from magnetically split sextet spectra (Figure 10). This indicates fast relaxation of the particle magnetic moments, compared to the time scale of Mössbauer spectroscopy (τM ≈10−9 s). In contrast, ferrimagnetic particles of CoFe2O4 with an average diameter of around 6 nm show almost exclusively sextet Mössbauer spectra at 80 K, due to effective blocking of the relaxation process. The transition from the sextet to the doublet pattern occurs in a range from about 60 to 150 K, distinguished by a changing superposition of both subspectra. For CoFe2O4

Figure 7. Temperature-dependent magnetic susceptibility (zero field cooled) for the Co3O4/CoFe2O4 composite measured in an external magnetic field of 100 Oe.

dependence of the magnetization, after cooling in zero-field, for the CoFe2O4/Co3O4 composite, measured in a field of 100 Oe (see Supporting Information Figure S4 for the field cooled curve). From the peak of the ZFC curve, a transition temperature of about 40 K can be determined, which indicates that the ordering temperature of Co3O4 in the Co3O4/CoFe2O4 composite system is larger than the Néel temperature for the pure Co3O4 mesostructure. This enhancement of the Néel temperature can be attributed to the proximity effect of the CoFe2O4 spinel. The peak at 40 K can not be associated with a blocking temperature of the CoFe2O4 nanostructure. In general, for nanostructured CoFe2O4 systems, much larger values for the blocking temperature have been reported. For 2498



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electric field gradient tensor at the Mö ssbauer nucleus. Moreover, electric field gradients are much more affected by charge asymmetry and strain induced by surface effects in small particles than the scalar values of the isomer shift. Some microheterogeneity of the iron sites is indicated by the relatively large line width of the two subspectra of 0.66 mm/s (full-width at half-maximum), which may indicate a distribution of the electric Mössbauer parameters.

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CONCLUSIONS It was demonstrated that nanoscale magnetic heterostructures can be produced by solid−solid reaction of iron species with ordered mesoporous Co3O4 that had been prepared via the nanocasting route by using a novel method. For a mesostructure with 6 nm primary Co3O4 nanoparticles, a reduced Néel temperature compared to the bulk has been observed. In the composite, the coupling of Co3O4 to the hard ferrimagnet CoFe2O4 leads to an increase in the Néel temperature. This indicates that the coupling of Co3O4 to the hard ferrimagnet influences the ordering temperature of the antiferromagnet. Magnetic measurements showed that the nanocomposite Co3O4/CoFe2O4 behaves as an exchange coupled system with a cooperative magnetic switching. The occurrence of the exchange bias at low temperatures is driven by the antiferromagnetism of Co3O4 nanoparticles. On the Mössbauer time scale, the ferrimagnetic ordering occurs at temperatures below 80 K. Since the synthetic pathway is relatively simple, it could be possible to extend the method to other composition ranges and other elements. Moreover, by adjusting the annealing temperature, the heterogeneity of the samples might be controllable. This could allow the synthesis of magnetic heterostructures with tunable properties by coupling materials with different collective magnetic behavior.

Figure 9. Hysteresis loop of Co3O4/CoFe2O4 composite at 80 K after cooling from 300 K in a field of 50 kOe.

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Figure 10. Zero-field Mö ssbauer spectrum of Co3O4/CoFe2O4 composite taken at 80 K. The red line is a 1:1 superposition of the two subspectra (1) and (2) marked in green and blue with δ(1) = 0.39 mm/s, ΔEQ(1) = 0.61 mm/s, δ(2) = 0.46 mm/s, ΔEQ(2) = 0.95 mm/ s, and line width Γfwhm = 0.66 mm/s.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures, including XRD and SEM images and temperature-dependent magnetic susceptibility graphs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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particles of 6 nm size, the transition was characterized by a Mössbauer blocking temperature of TM B = 127 K (defined as the temperature at which doublets and sextets are having equal intensity).55 Because of the absence of any sextet contribution in our spectrum at 80 K, we infer that the Mössbauer blocking temperature for our composite system is below that value. This excludes the presence of significant CoFe2O4 precipitates in the structure. The asymmetric pattern of the Mössbauer doublets of the Co3O4/CoFe2O4 composite system could be fitted with two symmetric quadrupole doublets with Lorentzian line shape and isomer shifts and quadrupole splittings of δ(1) = 0.39 mm/s, ΔEQ(1) = 0.61 mm/s, and δ(2) = 0.46 mm/s, ΔEQ(2) = 0.95 mm/s at 80 K (intensity ratio fixed to 1:1). The parameters clearly indicate Fe(3+), and we assign the two subspectra to iron at A and B sites of the CoFe2O4 phase. Particularly, the isomer shifts compare well with those reported for A and B sites in particles of CoFe2O4 (δ(A) = 0.38 mm/s, δ(B) = 0.47 mm/s). The quadrupole shift in the magnetic spectra of that system at low temperatures is vanishingly small, but that is not the true quadrupole splitting, because the large anisotropic hyperfine field selects only an unknown component of the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Oerlikon Solar, Hauptstrasse 1a, 9477 Trübbach, Switzerland.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Göbels for the SQUID and B. Mienert for the Mössbauer spectroscopy measurements (Max-Planck Institute for Bioinorganic Chemistry, Mülheim an der Ruhr, Germany).

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REFERENCES

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