High Exchange Bias in Fe3âÎ´O4@CoO Core Shell Nanoparticles

Article pubs.acs.org/JPCC

High Exchange Bias in Fe3−δO4@CoO Core Shell Nanoparticles Synthesized by a One-Pot Seed-Mediated Growth Method Walid Baaziz,†,‡ Benoît P. Pichon,*,† Christophe Lefevre,† Corinne Ulhaq-Bouillet,† Jean-Marc Greneche,§ Mohamed Toumi,‡ Tahar Mhiri,‡ and Sylvie Bégin-Colin*,† †

Institut de Physique et de Chimie des Matériaux de Strasbourg, UMR CNRS-UdS 7504, 23 rue du Loess BP 43, 67034 Strasbourg Cedex 2, France ‡ Faculté des Sciences de Sfax, Laboratoire de l’Etat Solide, Route de la Soukra km 3.5 BP 1171, 3000 Sfax, Tunisia § LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans IMMM, UMR CNRS 6283, 72085, Le Mans Cedex 9, France S Supporting Information *

ABSTRACT: Core−shell nanoparticles (NPs), which consist in a ferrimagnetic (FIM)/antiferromagnetic (AFM) interface and result in exchange bias coupling, became recently of primary importance in the field of magnetic nanoparticles. The enhancement of some applications such as hyperthermia or magnetic storage media based on the miniaturization of devices is correlated to the size reduction of NPs, which results in the decrease of the magnetocrystalline anisotropy and of the blocking temperature. We present here the synthesis of Fe3−δO4@CoO core−shell NPs by a one-pot seedmediated growth process based on the thermal decomposition of metal complexes at high temperature. A 2 nm thick CoO shell was grown homogeneously from the starting Fe3−δO4 core surface. The Fe3−δO4@CoO core−shell NP structure has been deeply investigated by performing XRD and advanced techniques based on TEM such as EELS and EFTEM. The high quality of the core−shell interface resulted in the large exchange bias coupling at 5 K (HE ≈ 4.1 kOe) between the FIM and the AFM components. In comparison to starting Fe3−δO4 NPs, the dramatic enhancement of the magnetic properties such as a high coercive field (at 5 K, HC ≈ 15 kOe) were measured. Furthermore, the core−shell structure resulted in the enhancement of the magnetocrystalline anisotropy and the increase of the blocking temperature to 293 K.

1. INTRODUCTION Magnetic properties of nanoparticles (NPs) are highly dependent on their shape, size, high surface volume ratio, composition, and interfacial characteristics. The NP size, which influences directly the magnetic moment and the magnetocrystalline anisotropy energy, is critical for applications such as magnetic recording media and biomedicine, which require room temperature blocked nanomaterials.1 Such NPs are characterized by a blocking temperature (TB), which corresponds to the ferro(i)magnetism to superparamagnetism crossover and depends on the NP size.1−3 Below a critical size, monodomain NPs become superparamagnetic at room temperature, that is, the orientation of their magnetic moment becomes unblocked against the thermal fluctuations.4,5 Although superparamagnetic NPs are suitable for biomedical applications,6 they cannot fulfill all the requirements necessary for spintronic and magnetic recording applications.1,7 Since the discovery of exchange bias coupling in Co@CoO NPs by Meiklejohn and Bean in 1956,8 NPs combining a ferro(i)magnetic (FIM) core and an antiferromagnetic (AFM) shell were revealed to be a very promising way to push back the superparamagnetic limit.9 Exchange bias coupling originates © XXXX American Chemical Society

from the coupling of the magnetic moments of the reversible FIM phase at the interface with the ones of the irreversible AFM phase below the Néel temperature (TN).9−12 Interfacial spins of the AFM phase, which are featured by a very high anisotropy, remain frozen while reversing the magnetic field in the opposite direction. It results in the modification of the magnetocrystalline energy to which is ascribed a supplementary term that refers to the FIM/AFM interface. So far, core−shell magnetic NPs represent a very promising way to increase the effective magnetic anisotropy energy of the raw NPs and to favor blocked NPs at room temperature with the smallest size as possible. While first investigations went on partially oxidized metallic NPs, core−shell NPs have been further synthesized to generate and to study exchange bias coupling leading to enhanced coercive field and blocking temperature.11,13 In recent years, the production of a variety of core−shell nanoparticles resulted in the better understanding on the structure−properties relationReceived: March 22, 2013 Revised: May 3, 2013

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dx.doi.org/10.1021/jp402823h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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ship.11 For instance, the shell thickness and the hollow structure in Co@CoO NPs have been shown to modulate the exchange coupling.14 In addition, two studies on CoFe 2 O 4@ZnFe2 O 4 15 and CoO@CoFe 2 O 416 NPs have shown that a larger anisotropy energy of the AFM phase than the one of the FIM phase and than the interface exchange energy is required to maintain the coupling of spins at the FIM/AFM interface upon reversing the magnetic field. Masala et al. also reported on the importance of the FIM/AFM interface quality in CoFe2O4@MnO NPs.17 Inverted structures such as FeO@Fe3−δO4,18 CoO@CoFe2O4,16 and MnxFe3−xO4/ FexMn3−xO4 have been also synthesized and favor exchange coupling at the core−shell interface. Recently, magnetically coupled exchange core−shell NPs, which consist in two different ferrites containing Co, Fe, or Mn, have been demonstrated to be efficient for magnetic thermal induction.19 Nevertheless, the preparation of core−shell NPs based on metal oxide with strong coercive field and TB higher than room temperature still remain a challenge. This can be achieved by favoring large exchange bias coupling at room temperature. Herein, we report on the design of core−shell nanoparticles synthesized by a one-pot synthesis approach. Nanoparticles consist in a ferrimagnetic iron oxide core surrounded by an antiferromagnetic cobalt oxide shell. Such a structure results in large exchange bias coupling and remarkably enhanced coercive field at low temperature.

HAADF mode with a 0.12 nm probe. EELS spectra were recorded in the diffraction mode and EFTEM images were taken on the Fe and Co edges with a 20 eV window, which gives a 1.5 nm resolution imaging. EDX were performed with a JEOL Si(Li) detector. Granulometry measurements based on dynamic light scattering (DLS) were performed on the suspension of NPs in chloroform using a nanosize MALVERN (nano ZS) apparatus. The NPs were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance equipped with a monochromatic copper radiation (Kα = 0.154056 nm) and a Sol-X detector in the 27−65° 2θ range with a scan step of 0.03°. High purity silicon powder (a = 0.543082 nm) was systematically used as an internal standard. Profile matching refinements were performed through the Fullprof program20 using the modified Thompson−Cox−Hasting (TCH) pseudo-Voigt profile function.21 Such a procedure enabled us to calculate cell parameters and crystal sizes. Moreover, no peaks related to Co3O4 were observed, indicating that no further oxidation of CoO took place. FTIR spectroscopy was performed using a Digilab Excalibur 3000 spectrophotometer (CsI beamsplitter) in the wavenumber range 4000−400 cm−1 on samples diluted in KBr pellets. The mass in metal oxide vs oleic acid was determined from dried powder samples by thermogravimetric analysis (TGA) using a SETARAM TGA 92 from 20 to 600 °C at a heating rate of 5 °C·min−1 under air. Mössbauer spectra were performed at 300 and 77 K using a conventional constant acceleration transmission spectrometer with a 57Co(Rh) source and a bath cryostat. The sample consists in powder with about 15 mg of Fe. The spectra were fitted by means of the MOSFIT program involving asymmetry and lines with Lorentzian profiles, while an α-Fe foil was used as the calibration sample. 2.3. Magnetic Characterization of NPs. Magnetic measurements were recorded on samples in the powder state by using a Superconducting Quantum Interference Device (SQUID) magnetometer (Quantum Design MPMS-XL 5). Zero-field cooled (ZFC) and field cooled (FC) magnetization curves as a function of the temperature were recorded as follows: the sample was introduced in the SQUID at room temperature and cooled down to 5 K with no applied field after applying a careful degaussing procedure. A magnetic field of 7.5 mT was applied, and the ZFC magnetization curve was recorded upon heating from 5 to 400 K. The sample was then cooled down to 5 K under the same applied field, and the FC magnetization curve was recorded upon heating from 5 to 400 K. Magnetization to saturation values were given as a function of the mass in iron oxide measured by TGA. Magnetization curves as a function of an applied magnetic field (M(H) curve) have been measured at 5, 10, 20, 50, 100, 200, 300, and 400 K on a different sample than the one used for M(T) measurements. The sample was also introduced in the SQUID at high temperature and cooled down to 5 K with no applied field (ZFC curve) after applying a subsequent degaussing procedure. The magnetization was then measured at constant temperature by sweeping the magnetic field from +7 T to −7 T, and then from −7 T to +7 T. To evidence exchange bias effect, FC M(H) curves have been further recorded after heating up at 400 K and cooling down to 5 K under a magnetic field of 7 T. The FC hysteresis loop was then measured by applying the

2. EXPERIMENTAL SECTION 2.1. Synthesis of NPs. In a typical synthesis, a two-necked round-bottom flask was charged with 1.38 g (2.22 mmol) of Fe(stearate)2 (9.47% Fe, Strem Chemicals), 1.254 g (4.44 mmol) of oleic acid (99%, Alfa-Aesar), and 20 mL of octyl ether (97%, Fluka, bp 287 °C) used as solvent. The mixture was sonicated and stirred at 120 °C for 10 min to dissolve the reactants until a clear solution was obtained. The solution was then heated to boiling temperature (∼287 °C) with a heating rate of 5 °C/min and kept at this temperature for 120 min under air. The resultant black solution was then cooled to 100 °C. 10 mL of solution was taken for characterization of Fe3O4 nanoparticles. In a second step, 0.67 g (2.22 mmol) of Co(stearate)2 (9−10% Co, Strem Chemicals) dissolved in 20 mL of octadecene (90%, Alfa Aesar, bp 318 °C) was added to the remaining solution kept at 100 °C, and the mixture containing both octyl ether and octadecene was heated again to reflux for 3 h under argon (heating rate of 1 °C/min). After cooling down to room temperature, each type of nanoparticle (Fe3O4 and Fe3O4@CoO nanoparticles) was precipitated by the addition of an excess of acetone and washed 3 times by a mixture of hexane/acetone (1/3) followed by centrifugation (14 000 rpm, 10 mn). Finally, the Fe3O4 and Fe3O4@CoO nanoparticles were easily suspended in chloroform. 2.2. Structural Characterization of NPs. The sizes and morphologies of NPs have been observed by transmission electron microscopy (TEM) with a TOPCON 002B microscope operating at 200 kV (point resolution 0.18 nm). The structure of NPs was investigated in the high-resolution mode (HR-TEM) and by electron diffraction. The size distribution of NPs was calculated from the size measurements of more than 300 NPs by using image J software. The composition of NPs was investigated by using a CS corrected JEOL 2100F electron microscope working with a 200 kV accelerating voltage equipped with a GATAN GIF 200 electron imaging filter. STEM images were taken in high-angle annular dark field B



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same field sweep as for the ZFC curve. The coercive field (HC) and the MR/MS ratio were measured from ZFC M(H) curves. The exchange bias field (HE) was measured from FC M(H) curves. AC measurements were performed to measure the susceptibility under an alternating magnetic field of 3.5 Oe at a frequency of 1 Hz) from 5 to 400 K after having cooled the sample in zero field.

peaks confirms the formation of an iron oxide spinel structure. Profile matching applied to the XRD pattern (Figure 1d) by using Fullprof software20 showed a lattice parameter (a = 8.372(1) Å), which is intermediate to that of stoichiometric magnetite Fe3O4 (8.396 Å) and that of maghemite γFe2O3 (8.338 Å). As reported in earlier studies,4,22−24 due to the high sensitivity of Fe2+ cations to oxidation at the nanoscale, the initial magnetite nanoparticles are partially oxidized at their surface.22,24 These results were further confirmed by Mö ssbauer spectrometry (see Supporting Information). The spectrum recorded at 300 K displays a hyperfine structure with only an extremely broad single line, which is characteristic of very fast superparamagnetic relaxation effects originating from isolated noninteracting nanoparticles. The hyperfine structure was refined by using a Lorentzian line, which allows estimating the average value of the isomer shift. Such a parameter is relevant to probe the mean valency state of Fe species. Its interpolation for a large variety of maghemite and magnetite phases resulted in the mean value of 0.42 ± 0.01 mm/s, which corresponds to a mean composition of Fe2.79O4 at 300 K.24 To schematize the oxidation rate of the nanoparticles, we have assumed a core−shell structure. Therefore, we expect nanoparticles to consist in a magnetite core surrounded by an oxidized shell, which is maghemite. Assuming spherical particles, the composition of nanoparticles has thus been calculated to be 38% of magnetite and 62% of maghemite. However, it is physically more realistic to consider a gradient of oxidation from the surface to the inner part of nanoparticle than a core−shell morphology.

3. RESULTS AND DISCUSSION Fe3−δO4@CoO core−shell nanoparticles have been synthesized by a one-pot seed-mediated growth method (Scheme 1). Scheme 1. Schematic Illustration for the Synthesis of Fe3−δO4@CoO Core−Shell Nanoparticles from Fe3−δO4 NPs by Seed-Mediated Growth Based on the Thermal Decomposition Method

Spherical Fe3−δO4 NPs with a narrow size distribution (11.2 ± 0.9 nm) and high stability in organic solvent resulting from in situ coating have been synthesized first in octyl ether (bp 288 °C) as we reported previously (see Supporting Information).4 The XRD pattern shows broad peaks in agreement with the short-range order of hkl reflections and so to nanoparticles with sizes in the range of few nanometers. The indexation of XRD

Figure 1. Fe3−δO4@CoO NPs. (a) TEM micrograph. (b) Size distribution measured from TEM micrographs and hydrodynamic diameter measured by granulometry. (c) Electronic diffraction pattern. (d) X-ray diffraction patterns (black) of Fe3−δO4 (up) and Fe3−δO4@CoO core−shell (down) nanoparticles and profile matching refinement (red). Peaks are indexed to hkl reflections of Fe3−δO4 (up in black) and CoO (down in red). Stars show peaks corresponding to silicone, which is used as an internal standard. C



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surface of Fe3−δO4 NPs. Obviously, these values do not correspond to two separated lattices of CoO but to a unique CoO phase, which is strained at the interface with Fe3−δO4 and gradually relaxes as long as the surface is reached. We have already observed such a variation of the cell parameter due to lattice mismatch between lattices of FeOx and Fe3−δO4 in core− shell cubic NPs.18 Finally, the refinement of the XRD pattern enabled us to calculate the crystallite size of CoO, which is 2 nm. This value is in agreement with the NP size variation observed on TEM micrographs and confirms the growth of the CoO shell at the surface of the iron oxide core. Because Fe and Co have very close electron densities, no difference in contrast between the inner and the outer parts of each NP could be observed in conventional TEM modes. Therefore, the composition of NPs has been investigated more deeply by performing advanced analyses based on TEM with the aim to support the core−shell structure. High-resolution (HR) TEM micrographs of core−shell NPs (Figure 2a) exhibit

These results have been fairly confirmed by performing the measurement at 77 K. One observes a sextuplet with broad and asymmetric lines originating from the occurrence of superparamagnetic relaxation phenomena. These lines were further described by means of a distribution of static magnetic hyperfine field, which is slightly correlated to that of the isomer shift.24 The mean composition can also be estimated from the mean isomer shift and is Fe2.77O4, which is in agreement with that estimated at 300 K. It corresponds to a proportion 70% maghemite and 30% magnetite. According to a core−shell (magnetite−maghemite) spherical model, we can estimate the thickness of the maghemite shell to be appoximalety 2 nm assuming that the Lamb−Mössbauer factor f of both phases are identical. Therefore, we expect Fe3−δO4 NPs to consist in a stoichiometric magnetite core surrounded by an oxidized layer of maghemite. A CoO shell was further grown at the surface of Fe3−δO4 NPs by performing the thermal decomposition of cobalt stearate. It is worthy to note that octylether was replaced by octadecene (bp 318 °C) because cobalt stearate decomposes at higher temperature than iron stearate (see TGA in Supporting Information). Therefore, cobalt stearate was disolved in octadecene and was injected slowly in the hot Fe3−δO4 NP suspension to favor the growth of CoO at the Fe3−δO4 NP surface. We observed the formation of CoO despite the presence of capping agents (oleic acid) on the iron oxide surface as reported in the literature for other systems.25 Transmission electron microscopy (TEM) clearly showed NPs with narrow size distribution centered at 14.2 ± 3.1 nm (Figure 1a). This value is larger than the one of Fe3−δO4 NPs and suggests that the growth of the CoO shell has been achieved on the Fe3−δO4 seeds. Fe3−δO4@CoO core−shell NPs are also highly stable in organic solvents such as chloroform. It is impeded to the coating of the NP surface by oleic acid as demonstrated by the presence of νCOO− and νC−H bands in the FTIR spectrum (see Supporting Information). Granulometry measurements show a narrow distribution with a hydrodynamic diameter centered to 17.2 ± 1.0 nm (Figure 1b), which confirms the high stability of the suspension and the nonaggregation of NPs. Such a higher value than the mean size distribution measured by TEM agrees with the presence of oleic acid at the NP surface. The crystalline structure of Fe3−δO4@CoO core−shell nanoparticles has been investigated very carefully by X-ray diffraction (XRD) (Figure 1d). While diffraction peaks well matched the Fe3−δO4 spinel structure, an extra peak at 2θ = 36.5° and the enlargements of (400) and (440) peaks, which refer to the (111), (200), and (220) peaks of the wüstite-type structure, demonstrated the presence of the CoO. The refinement of the XRD pattern by the Fullprof software20 was performed on the basis of the values calculated for the Fe3−δO4 NPs and brought interesting insights. After the growth of CoO, the XRD refinement allows to conclude that NPs consist in a Fe3−δO4 core, which is featured by a cell parameter a of 8.372(1) Å and a crystallite size of 11 ± 1 nm. These values demonstrate that the Fe3−δO4 phase is not affected by the CoO growth. However, while the growth/formation of CoO is clearly observed, the good refinement of the XRD pattern can only be achieved if the CoO phase consists in two components having different lattice parameters a1 = 4.260 Å and a2 = 4.218 Å. While the former agrees well with the one of bulk fcc CoO (4.261 Å), the latter is close to half of the parameter of maghemite and is ascribed to CoO grown by epitaxy at the

Figure 2. HRTEM micrographs of (a) a core−shell Fe3−δO4@CoO NP and (b) a Fe3−δO4 NP.

a homogeneous contrast and a crystal structure, which is indicative of the well-crystallized nature of NPs (Figure 2b). The homogeneity of the core−shell crystal structure was checked further by calculation of Fast Fourier Transform on several areas of a HRTEM micrograph (see Supporting Information). The HRTEM micrograph of NPs after CoO growth is very similar to the one observed for Fe3−δO4 NPs before the growth of CoO. The latter consists in facets, which can be indexed to the (111) and (001) reflections of the spinel structure. Although we would have expected a different structure of the shell, it is also worthy to note that the highresolution TEM micrograph cannot allow one to distinguish if a CoO layer has been grown at the NP surface due to the relatively thin thickness of the shell. The formation of a layer of CoO on the Fe3−δO4 NPs could be partly evidenced by means of the XRD analysis as reported above. Moreover, the crystal structure of the CoO shell probably matches the one of the Fe3−δO4 core, so that NPs appear to be single crystal and that no interface could be observed although it exists. These observations confirm the epitaxial matching of crystal structures of both core and shell. A better understanding of the core−shell structure was highlighted by analysis modes. Energy filtering mode (EFTEM) was performed on several NPs at Fe and Co edges with a resolution of about one nanometer (Figure 3).26 The EFTEM mapped images showed that Fe and Co are homogeneously distributed. Nevertheless, the composite micrograph (Figure 3c) illustrates unambiguously that iron atoms are not located at the NP surface, which means that they are located in the core. In contrast, cobalt atoms are located at D



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Figure 3. EFTEM micrographs of Fe3−δO4@CoO NPs (a) at Fe edge, (b) at Co edge, and (c) composite of Co (green) and Fe (red) edges.

the NP surface. Green halos (shown by arrows) are also observed on the edge of NPs, which confirm the homogeneous coating of CoO on the initial Fe3−δ04 NPs. Moreover, a better insight on the distribution of Fe and Co atoms was obtained by performing electron energy loss spectroscopy (EELS). EELS line scans performed on a single NP show the evolution of the Fe and Co atoms along the cross-section of a single NP (Figure 4). The Co signal is almost observed along the whole section

Figure 4. EELS measurements of Fe3−δO4@CoO NPs. (a) Fe (full line) and Co (dashed line) core−loss profiles across a NP shown on (b) the HAADF STEM micrograph. (c) Average EELS spectrum extracted from the line scan on the NP shown in panel b.

with a lower amount than the Fe signal, which shows unambiguously the growth of a CoO shell on the surface of the Fe3−δO4 core. Finally, the relationship between the structure and the magnetic properties of NPs has been investigated by using a superconducting quantum interference device (SQUID) magnetometer. Magnetization (M(H)) curves for Fe3−δO4@ CoO core−shell NPs (Figure 5a), which were recorded at 400 and 300 K, correspond to NPs in the superparamagnetic state. In contrast, hysteresis loops are observed below 300 K with increasing coercive field (HC) and squareness (MR/MS) as long as the temperature decreases (Figure 5c). Remarkably high values of HC (15 015 Oe) and MR/MS (0.59) at 5 K were obtained with respect to standard 11 nm sized Fe3−δO4 NPs (HC = 376 Oe and MR/MS = 0.28) (Figure 6a). Such a behavior is directly correlated to a larger anisotropy due to the FIM/ AFM core−shell structure of Fe3−δO4@CoO NPs. The low saturation magnetization at 5 K of Fe3−δO4@CoO NPs (37 emu·g−1) in comparison to 11 nm sized Fe3−δO4 NPs (55

Figure 5. Magnetic measurements of Fe3−δO4@CoO core−shell NPs. Magnetization M(H) curves at different temperatures from 400 to 5 K (a) after cooling without applying any magnetic field (ZFC) and (b) after cooling under a 7 T magnetic field (FC). (c) Coercive fields (HC) and exchange field (HE) plotted against temperature.

emu·g−1)27 also refers to different nanostructures. Indeed, CoO has a low magnetization (15 emu·g−1 at 1.7 K for 4.5 nm sized CoO NPs),28 which decreases the overall magnetization of core−shell NPs. In order to demonstrate the exchange interaction at the FIM/AFM interface, magnetization was also recorded against magnetic field after cooling from 400 to 5 K under a 7 T field (FC curves) (Figure 5b). All hysteresis loops recorded below 300 K exhibit a horizontal shift from the origin, which increases when the temperature decreases. Below the Néel temperature of CoO (TN = 293 K in bulk),29 unidirectional anisotropy exists unlike the nonobservation of this shift above 300 K. Although E



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reported values and confirm the good quality of the obtained Fe3−δO4@CoO NPs. Magnetization measurements as a function of temperature (zero-field cooled, ZFC, and field cooled, FC, curves) recorded under a 10 mT field (Figure 6b) give complementary insights in the exchange bias coupling with regard to TN. While Fe3−δO4 NPs display a ZFC maximum centered to 150 K, it is dramatically shifted to 314 K for Fe3−δO4@CoO NPs. The ZFC maximum being usually correlated to the blocking temperature TB, the exchange bias coupling at the FIM/AFM interface results in NPs blocked at a temperature close to room temperature. Nevertheless, the nonobservation of a hysteresis loop in M(H) curves at 300 K (Figure 5a) disagrees with the value of TB suggested by the ZFC maximum. However, ZFC maximum at higher temperature than TN has been also reported for exchange biased core−shell structures, which does not fit to theory.16,18,36,37 Indeed, the accurate value of TB corresponds to the maximum of the imaginary part of the susceptibility χ″, which can be determined more precisely by performing measurements under an alternative field (1 Hz frequency).27 A TB value of 293 K was measured for Fe3−δO4@ CoO NPs (Figure 6b, inset) which is perfectly correlated to TN for CoO.29 Therefore, Fe3−δO4@CoO NPs are blocked at a temperature below 293 K. This value being similar to the one reported for the bulk state, it is not influenced by the nanoscale. Moreover, the gradient in CoO parameter lattice, which is expected to induce strains at the interface with iron oxide, does not seem to influence the exchange bias coupling. The design of 13.8 nm sized Fe3−δO4@CoO NPs results in the overcrossing of the superparamagnetic limit in the range of room temperature. In comparison, Fe3−δO4 NPs with a slightly larger diameter of 16 nm are featured by a blocking temperature of only 200 K.27

Figure 6. Comparison of the magnetic properties of Fe3−δO4@CoO and Fe3−δO4 NPs. (a) M(H) curves at 5 K and (b) ZFC and FC magnetization versus temperature curves and imaginary part of the susceptibility χ″ measured under a 3.5 Oe alternative field at 1 Hz for Fe3−δO4@CoO NPs (inset).

4. CONCLUSIONS Fe3−δO4@CoO core−shell nanoparticles have been designed by a one-pot seed-mediated growth based on the thermal decomposition method. The structural investigations were demonstrated on the core−shell structure of nanoparticles consisting of a 11 nm sized Fe3−δO4 core shelled by a 2 nm thick CoO layer. The mechanism formation of such a core− shell structure occurred by epitaxial growth of CoO on the maghemite surface of Fe3−δO4 seed and led to the high quality of the interface between FIM and AFM phases. Moreover, the high complementarity of magnetocrystalline anisotropies of both Fe3−δO4 and CoO gave rise to a remarkably large exchange bias field (HE = 4,125 Oe) and efficiently enhanced coercive field (HC = 15 015 Oe), which is up to 300% with respect to one of the Fe3−δO4 NPs. These results agree with the fact that FIM/AFM core−shell structures represent promising systems to enhance the magnetocrystalline anisotropy at the nanoscale.

the exchange bias field (HE) is very weak at 300 K (81 Oe), it remarkably increases up to 4,125 Oe when decreasing the temperature to 5 K (Figure 5c). Such a strong exchange bias coupling between the maghemite surface of the Fe3−δO4 core and the CoO shell can be explained by the high quality of the FIM/AFM interface.30 Moreover, the large difference between the effective anisotropy of the CoO phase (3 × 107 erg·cm3 for a similar shell)31 and the Fe3−δO4 phase (1.1 × 105 erg·cm3 for a similar core)27 also contributes to the efficient pinning of the FIM phase by the AFM phase. HC and HE values can be compared to the ones of core−shell nanoparticles reported recently. Similar magnetic core−shell systems such as CoFe2O4@MnO,17 γFe2O3@CoO, Co@ γFe2O3,32 MnO@γMn2O3, MnO@Mn3O4,33 Fe@Fe3O4,34 and MnxFe3‑xO4@FexMn3‑xO435 were reported to display coercive field values at 5 K ranging from 2 600 Oe to 9 500 Oe. Very recently, Lima et al. reported on Co@CoFe2O4 NPs with a remarkably high coercive field of 27 800 Oe at 5 K although the larger effective anisotropy of CoFe2O4 than that of CoO led to the strong coupling of both phases and thus resulted in a zero exchange field.16 In contrast, similar systems cited above display exchange fields that lie usually about 600− 2,300 Oe and depend on various parameters such as the core− shell interface quality, the particle size, the shell thickness, or the difference between effective anisotropies. Therefore, the HC and HE values we report in this study are among the highest

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ASSOCIATED CONTENT

S Supporting Information *

TEM micrographs, electronic diffraction, and size distribution of Fe3−δO4 NPs. Mössbauer and FTIR spectra, TGA, HRTEM, and HAADF micrographs for Fe3O4‑δ and Fe3O4‑δ@CoO NPs. This material is available free of charge via the Internet at http://pubs.acs.org. F



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.P.P.); sylvie.begin@ unistra.fr (S.B.-C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports were provided by the Agence Nationale de la Recherche (ANR), project ANR08-BLAN-NT09-459731, “MAGARRAY”.

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