Bimagnetic urchin-like Co₃O₄/CoFe₂O₄ nanocomposites: synthesis and magnetic properties

Abstract

A heterostructured Co₃O₄/CoFe₂O₄ nanocomposite material is synthetised via the solid–solid reaction of an iron precursor with urchin-like Co₃O₄ which was prepared via hydrothermal treatment. The coupling of Co₃O₄ to the hard ferrimagnet CoFe₂O₄ leads to reinforcement for the magnetic interaction of the interfaces and the mismatched surface spin in the Co₃O₄/CoFe₂O₄ nanocomposite is greatly reduced. The magnetic performance of the exchange-coupled anti-ferromagnetic/ferrimagnetic (AFM/FM) system is investigated with superconducting quantum interference device magnetometry. In an applied magnetic field, the low-temperature magnetization loops of the nanocomposite heterostructure exchange bias under cooling. The antiferromagnetic ordering temperature of Co₃O₄ is increased owing to the adjacent hard magnetic CoFe₂O₄ phase. The Co₃O₄/CoFe₂O₄ nanocomposite behaves as an exchange coupled system with cooperative magnetic switching.

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Introduction

As an important class of synthetic nanoscale materials, magnetic nanocrystals have excited wide interest in the fundamental understanding of nanomagnetism and its technological application.^1,2 Many technological devices require high stability magnetization, such as magnetic storage media and permanent magnets.^3–5 It is well known that if magnetic materials approach the nanoscale regime, their properties will be markedly different from their bulk counterparts. With a change in particle size and superparamagnetism, magnetic properties vary greatly, which is a typical case for size-dependent behavior in the nanoscale. However, a decrease in the size of single phase nanocrystals results in a superparamagnetic limit, which imposes a lower threshold in the grain size to maintain the thermal stability of the magnetization, thus limiting the development of miniaturized devices.^6–8 One of the most effective solutions to solve this problem is to generate an exchange anisotropy by interfacing ferromagnetic (FM) with antiferromagnetic (AFM) phases, since the exchange anisotropy can overcome the thermal energy that is created by pinning the spins of FM and compensated spins of AFM.⁹ In order to increase the effective magnetic anisotropy of the compound, interface exchange coupling is exploited.^10–12 Coupling of the interface exchange adjusts the magnetic anisotropy between multifarious magnetic materials structures and different magnetic anisotropies, which has been researched on bimagnetic core/shell nanoparticles in the last decade. Additionally, decomposition of the organometallic precursor at high temperatures has to be focused on. Thus, the development of this method makes it possible to generate superior bimagnetic core/shell nano-compositions.^13–16 Over the years, inverted core–shell NPs have been researched in which AFM is the core and FM is the shell. It is found that the magnetic properties of inverted core–shell NPs, with special interest toward anisotropy, are enhanced with respect to their single phase counterparts.

As one of the most important and widely utilized magnetic materials, CoFe₂O₄ consists of both magnetically hard materials with high magnetocrystalline anisotropy (K_eff = 1.8 × 10⁷ erg cm⁻³), high coercivity (H_c = 5.4 kOe), and moderate saturation magnetization (80 emu g⁻¹).¹⁷ The magnetic moment of bulk Co₃O₄ is derived from the tetrahedrally coordinated Co²⁺ ions, meanwhile the Co³⁺ ions are diamagnetic. For bulk materials, a switching process from paramagnetic to antiferromagnetic occurs at around 40 K.¹⁸ For nano-materials their magnetic behavior is different from the bulk. Unusually, nanostructured Co₃O₄ shows singular magnetic properties due to its uncompensated surface spins, which are exchange-coupled to the antiferromagnetic core.^19–21 The close contact two different magnetic phases can generate curious and exceptional magnetic properties. It is worth noting that the magnetism of magnetic materials is not only dependent on the intrinsic magnetic material themselves, but also is affected by the interaction between magnetic nanoparticles. The intrinsic magnetic performance can be revealed from uniform nanoparticles. While for nanocomposites, magnetic interactions are difficult to interactions from the magnetism for the uncontrollable interparticles distance.²² A direct comparative study of the two nanoparticle systems provides a better basic understanding of the correlations between magnetic nanoparticles and the magnetic couplings of their lattice sites. Magnetic nanomaterial composites with controllable microstructures are a great challenge for the synthetic technology available. Jiao et al. provided an alternative hard template strategy for the synthesis of shape and size controlled cubic ordered Fe₃O₄ by the reduction of α-Fe₂O₃. After that, they transform the ordered Fe₃O₄ to γ-Fe₂O₃ by oxidation reaction with the same micromechanism.²³ Schüth et al. reported the fabrication of nanocrystal mixed CoFe₂O₄ via a one-step-doping of nano Co₃O₄ as a hard template, and measured its magnetic exchange bias effect at a low temperature.²⁴ Magnetic microstructures with a ferrimagnetic core and antiferromagnetic shell (or contrary) display the exchange bias phenomenon and show hysteresis loops along the magnetic field axis displacement.

Herein, we report the nanofabrication route for a magnetic 3D heterostructure composed of Co₃O₄ and CoFe₂O₄. The microstructural characteristics and magnetic properties of the nanocomposite are investigated. Magnetism is studied according to magnetic hysteresis loops and zero field cooling–field cooling (ZFC–FC) curves. The results exhibit the enhancement of effective anisotropy relative to single phase Co₃O₄ nanoparticles. The influence of the interface magnetic interactions on the magnetism of the CoFe₂O₄/Co₃O₄ nano-composites is discussed.

Experimental

In this study all the chemicals used were of analytical grade and obtained from Aladdin, and were used as received without further purification.

First, urchin-like Co₃O₄ nanocrystals were synthesized. 1.0 mmol of cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) and 4.0 mmol of urea were dispersed in 40 mL distilled water and stirred for 0.5 h at room temperature. Afterwards, the mixture was transferred to a 50 mL Teflon-lined autoclave. The autoclave was heated at 90 °C for 6 h. After the autoclave cooled to room temperature naturally, black precipitates (the precursors) were collected, washed with deionized water and absolute ethanol, followed by calcination at 350 °C for 2 h. In this step, Co₃O₄/CoFe₂O₄ was compounded via the solid–solid reaction of the iron(III) precursor with the urchin-like Co₃O₄. 0.4 mL of 0.4 M Fe(NO₃)₃·9H₂O (in ethanol) was added to 0.2 g of urchin-like Co₃O₄ ((Fe)/(Fe + Co) = 0.16), with stirring for 1 h at room temperature. After that, the solvent was evaporated at 60 °C for 12 h, and subsequently the product was heated to 700 °C at the rate of 1 °C min⁻¹ and kept at this temperature for 2 h. The concentration of the iron solution (3 M) was increased to obtain an Fe-enriched sample.

The morphology and structure of the urchin-like Co₃O₄/CoFe₂O₄ were characterized using a Tecnai G² F30 TEM, and high resolution TEM and selected area electron diffraction (SAED). X-ray diffraction (XRD) patterns for all the samples were obtained on a Stoe STADI P diffractometer with Cu Kα radiation. Field emission scanning electron microscopy (FE-SEM) was performed with a HELIOS NanoLab 600i microscope. Nitrogen absorption–desorption isotherms were measured on an ASAP-2020. Magnetic properties were determined on a superconducting quantum interference device (SQUID) magneto-meter (MPMS-7 Quantum Design) in the temperature range of 10–320 K with a 100 kOe maximum field.

Results and discussion

Fig. 1 presents the XRD patterns of the resultant urchin-like Co₃O₄ (a), and Co₃O₄/CoFe₂O₄ (b and c). The relative intensity and the position match well with the structure of the Co₃O₄ and CoFe₂O₄ phases. The corresponding reflection peaks are close to that of Co₃O₄ crystals according to JPCDS card number 42-1467 and CoFe₂O₄ according to JPCDS card number 22-1086. In Fig. 1a, the diffraction peaks at 10–90° can be indexed to the face centered cubic Co₃O₄ space group Fd3m with the lattice parameter a = 0.8056 nm. The crystal structures of the CoFe₂O₄ and Co₃O₄ phases are clearly depicted in Fig. 1b and c. As seen in Fig. 1b, the nanocompounds are crystalline, and diffraction peaks similar to the Co₃O₄ phase are observed, since the lattice constants of Co₃O₄ and CoFe₂O₄ are very similar to each other.²⁵ In comparison, with the addition of the iron(III) source and a second calcination process, the intensity of the peaks (Fig. 1b) decreased. The presence of the CoFe₂O₄ phase can be inferred from the (533) and (622) reflections at 74–75°, whereas the corresponding reflection for the binary Co₃O₄ spinel at 74.1° remains visible (see ESI Fig. S1† for the at the local magnified XRD pattern). The average crystallite size before and after iron impregnation can be estimated using the Scherrer equation.²⁴ The average diameter of the Co₃O₄ particles is 16 nm and the Co₃O₄/CoFe₂O₄ particles is 27 nm. However, upon increasing the iron(III) source concentration from (Fe)/(Fe + Co) = 0.16 to 0.60, the CoFe₂O₄ phase can be inferred, which is very legible and obvious with an increased the iron content (Fig. 1c). It is also reasonable that a higher concentration of the iron solution is more beneficial to the formation of the CoFe₂O₄ phase.

Fig. 1 XRD patterns of urchin-like Co₃O₄ and Co₃O₄/CoFe₂O₄ nano-composites. (a) Co₃O₄, (b) Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.16), and (c) Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.60).

The morphology of Co₃O₄ and Co₃O₄/CoFe₂O₄ were further investigated by FESEM and TEM. Fig. 2 displays FESEM images of the as-fabricated samples at different reaction steps, which represent the panoramic and locally enlarged images of the Co₃O₄ (Fig. 2a and b), Co₃O₄/CoFe₂O₄ (Fe)/(Fe + Co) = 0.16 (Fig. 2c and d), and Co₃O₄/CoFe₂O₄ (Fe)/(Fe + Co) = 0.60 (Fig. 2e and f) urchin-like samples. It is found that the Co₃O₄ sample is composed of 3D urchin-like structures, which are constructed of many rough nanorods with lengths ranging from 2 to 3 μm and widths ranging from 50 to 80 nm. It can be clearly observed that some nanorods align together to form the same central spheroidal morphology. From Fig. 2c and d, it is noticed that after iron(III)nitrate impregnated is into Co₃O₄ ((Fe)/(Fe + Co) = 0.16) and calcined at 700 °C the morphology of the corresponding samples remains unchanged. Although, the diameter of the urchins and the nanorods grow thicker (from 80 nm to 100 nm), due to the fusion of the nanocrystals at a higher annealing temperature and the decomposition of CoFe₂O₄ at this temperature.^25,27 However, with an increase in the iron content ((Fe)/(Fe + Co) = 0.60), the morphology of Co₃O₄/CoFe₂O₄ becomes agglomerated clusters of the disorderly nanorods (Fig. 2e and f), and the morphology of the urchin-like structures change gradually.

Fig. 2 HR-SEM for the panoramic and locally enlarged images of the (a and b) Co₃O₄, (c and d) Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.16) and (e and f) Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.60).

Fig. 3 displays the FE-TEM, HR-TEM images and the corresponding SAED patterns of Co₃O₄, Co₃O₄/CoFe₂O₄ nanocomposite. Fig. 3a shows the FE-TEM image of the designed single Co₃O₄ urchin-like nanoparticles. The individual Co₃O₄ nanorod is composed of interconnected nanoparticles with a diameter of 10–20 nm, which confirms the XRD observations and shows a porous structure ranging from 2–6 nm in length (inset Fig. 3a). The measured lattice fringe with the interplane spacing of 0.24 nm corresponds well to the (311) plane of Co₃O₄ (Fig. 3b). Fig. 3c shows a low magnification TEM image of THE urchin-like Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.16) nanocomposite after impregnation of the iron(III) precursor and calcination. Compared with the pure Co₃O₄, the Co₃O₄/CoFe₂O₄ nanorod is composed of interconnected nanoparticles with a diameter of 20–30 nm and shows a porous structure ranging from 3–11 nm in length (inset Fig. 3c). The high-magnification TEM (HRTEM) images (Fig. 3d) of the Co₃O₄@CoFe₂O₄ nanorods unambiguously exhibit that the Co₃O₄ nanoparticles (5–10 nm) are highly crystalline and encapsulated by crystalline CoFe₂O₄ shells, although there are some plane stacking faults. The interlayer distance in the HRTEM image is calculated to be 0.30 nm for the shell lattice fringes and 0.24 nm for the core lattice fringes, which agree well with that of the (220) lattice plane of CoFe₂O₄ and the (311) lattice plane of Co₃O₄, and thus confirm the XRD analysis. Fig. 3e shows a TEM image of the Co₃O₄/CoFe₂O₄ ((Fe)/(Fe + Co) = 0.60) sample. The increase in the concentration of the iron solution increased the diameter of the Co₃O₄/CoFe₂O₄ nanorod, and the diameter of the Co₃O₄/CoFe₂O₄ nanorods is not uniformly distributed and there are no pores on the nanorods (inset Fig. 3e). The ranges of particle sizes are the same as the FE-SEM observation. In order to obtain structural information, HR-TEM imaging was also done on the edges of the nanorods (Fig. 3f), and the obtained neighboring interplanar distances are consistent with the (311) planes for the CoFe₂O₄ phase. SAED patterns of the urchin-like Co₃O₄ and urchin-like Co₃O₄/CoFe₂O₄ nanocomposite samples are shown in Fig. 3 (inset in panels (b), (d) and (f)). The ring patterns are consistent with the standard diffraction of the Fm3m Co₃O₄ and Fd3m CoFe₂O₄ phases. The planes match with that of the XRD patterns very well. The elemental mapping images (EDS) in Fig. 4 reveal the distribution of a randomly selected single urchin-like of the Co₃O₄/CoFe₂O₄ nanocomposite ((Fe)/(Fe + Co) = 0.16). The distribution of the Fe element covers the location of the Co elements, which displays the uniform distribution of CoFe₂O₄ on the surface of the Co₃O₄ nanorods.

Fig. 3 HR-TEM images of the Co₃O₄ (a and b) and Co₃O₄/CoFe₂O₄ nanoparticles ((c and d) (Fe)/(Fe + Co) = 0.16 and (e and f) (Fe)/(Fe + Co) = 0.16). The inset in the images shows a highly magnified portion and electron diffraction pattern of the nanoparticle system.

Fig. 4 Energy-dispersive X-ray spectrometry (EDS) mapping analysis for the Co₃O₄/CoFe₂O₄ nanoparticles ((Fe)/(Fe + Co) = 0.16). (a) TEM, (b) Fe and (c) Co.

The N₂ adsorption–desorption isotherms of Co₃O₄ and the Co₃O₄/CoFe₂O₄ nanocomposite are exhibited in Fig. 5. The isotherms of the Co₃O₄ and Co₃O₄/CoFe₂O₄ powders can be classified as type IV, which show the mesoporous characteristics of the sample.²⁶ The H-3 hysteresis loop proves that asymmetric, interconnected, slit-like mesoporosity exists in the samples. The BET specific surface area of the urchin-like Co₃O₄/CoFe₂O₄ is measured to be about ∼112.3 m² g⁻¹, which is higher than that of the urchin-like Co₃O₄ (∼55.8 m² g⁻¹). The higher surface area of the urchin-like Co₃O₄/CoFe₂O₄ originates from the presence of the CoFe₂O₄ layer around the Co₃O₄ nanoparticles, except for the intrinsic pores of the Co₃O₄ phase. With an increase in CoFe₂O₄ particles, the particle growth increases the partial closure of the relatively large pores and the generation of many small pores. Thus, the numerous micropores in the products result in a higher BET surface area rather than the mere presence of mesopores. The pore-size-distribution of Co₃O₄ (Fig. 5b) displays the fact that most of the pores are ∼4 nm. In comparison, the pore size distribution of the Co₃O₄/CoFe₂O₄ sample (3 nm and 8 nm) does not changed significantly. This may be attributed to the low doping of iron(III) and similar volume density of Co₃O₄ (6.1 g cm⁻³) and CoFe₂O₄ (5.3 g cm⁻³).^17,25

Fig. 5 (a) N₂ adsorption–desorption isotherms of Co₃O₄ and the Co₃O₄/CoFe₂O₄ nanocomposite, and (b) pore size.

It is found that the hydrothermal time plays a key role in the formation of the perfect cobalt hydroxy carbonate (precursor) urchin-like nanostructures. Fig. 6 shows the FE-SEM nano-structures of the prepared samples obtained by the hydrothermal method at 90 °C for (a) 2 h, (b) 4 h, (c) 6 h and (d) 8 h. From Fig. 6a, only 1D nanorods are obtained for 2 h. The diameter of the nanorods is ∼50 nm. When increased to 4 h, the sample is a mixture of nanorods and deficient urchin-like shapes with a loose center (Fig. 6b). The above results may imply that the deficient urchin-like precursor could be self-assembled with these nanorods. With an increase in the reaction time (6 h and 8 h), the flawless urchin-like morphology can be obtained. Subsequently, the nanorods with the urchin-like structure become thick and uneven (Fig. 6c and d). In comparison with Fig. 2a, it is found that the morphology of the samples remains unchanged after calcination at 350 °C.

Fig. 6 HR-SEM images for the starting precursor at different hydrothermal times. (a) 2 h, (b) 4 h, (c) 6 h and (d) 8 h.

An illustration of the synthetic process of the Co₃O₄/CoFe₂O₄ heterocomposite is depicted in Scheme 1. In this study, the starting urchin-like precursor was first prepared via the traditional hydrothermal treatment. Under the mild hydrothermal reaction conditions at 90 °C, urea hydrolyzes gradually and releases NH₃ and CO₂. Soon afterwards, they become OH⁻ and CO₃²⁻ ions. Compared with the release of NH₃, the solubility of CO₂ is relatively low in aqueous solution, thus the solution is weak alkaline.^27,28 The alkaline medium is believed to create heterogeneous nucleation of the Co species. The Co²⁻ of the cobalt salt cooperates with the CO₃²⁻ and OH⁻ anions to form the nuclei of cobalt hydroxyl carbonate. The Co²⁺ exposed facets prior adsorb OH⁻ ions. Consequently, dipolar interactions dominated by 3D self-assembly is achieved via hydrogen bonding and electrostatic force.²⁹ In this system the reactions involved are listed as follows:

CO(NH₂)₂ + xH₂O → 2NH₃ + CO₂ + (x − 1)H₂O (1)

NH₃ + H₂O → NH₄⁺ + OH⁻ (2)

CO₂ + H₂O → CO₃²⁻ + 2H⁺ (3)

Co²⁺ + OH⁻ + 0.5CO₃²⁻ + 0.11H₂O → Co(CO₃)_0.5(OH)·0.11H₂O (4)

Scheme 1 Illustration of the synthetic process of the Co₃O₄/CoFe₂O₄ heterocomposite.

Initially NH₃ and CO₂ are present in the form of bubbles in the solution, which act as soft templates.²⁸ Then, the cobalt hydroxyl carbonate nanoparticles surround the bubbles to form a loosely packed hierarchical structure. By Ostwald ripening, the precursor particles grow along a certain crystallographic axis forming 1D nanorods. Then, by prolonging the hydrothermal time (6–8 h), the imperfect nanorods assemble to form an urchin-like structure. Subsequently, the samples are calcined in laboratory air at 350 °C for 2 h to generate the intermediate product, which consists of urchin-like Co₃O₄.

6Co(CO₃)_0.5(OH)·0.11H₂O → 2Co₃O₄ + 3CO₂ + 1.66H₂O (5)

Finally, Fe(NO₃)₃·9H₂O is directly impregnated into the urchin-like Co₃O₄ to generate the Co₃O₄/CoFe₂O₄ composite. Under the conditions of the calcination process, the iron source and cobaltosic oxide compound CoFe₂O₄ by a solid–solid reaction. The ferrimagnetic CoFe₂O₄ probably wraps around the antiferromagnetic Co₃O₄ matrix. Early studies on the pseudobinary Co₃O₄/CoFe₂O₄ system show that there is a miscibility gap in the spinel-structure region.^30,31 In the report by Takahashi et al., the single or two phase regions are attributed to the calcination temperature and the atomic ratio of iron and cobalt.³² Thus, at a temperature greater than 400 °C but less than 900 °C, the single Co₃O₄ spinel phase is observed when the (Fe)/(Co + Fe) ratio is below 0.1. As the iron(III) concentration increases, the inverse spinel CoFe₂O₄ phase is formed in addition to Co₃O₄. In this paper, we chose the atomic ratio of 0.16 (Fe/(Co + Fe)) and calcination temperature of 700 °C. According to the conditions given by the phase diagram, the product components can be preliminarily controlled. It is suggested that a small amount of CoFe₂O₄ is gained along the Co₃O₄ matrix with the calcination process and iron species impregnation into the cobalt oxide.

Magnetic properties

For comparison, the hysteresis cycle of the urchin-like Co₃O₄ single phase and the bimagnetic system are exhibited in Fig. 7, in which both of the hysteresis loops at the lower field region are inserted. The single Co₃O₄ measurements linearly increase with the external magnetic field and there is no remanent magnetization, which indicates antiferromagnetic behavior.²⁴ This is ascribed to the weak exchange interaction between antiferromagnetism and ferromagnetism for the surface effect below the Néel temperature (T_N).³³ This is because of the existence of uncompensated surface spins.^19,21 However, the magnetic hysteresis loops of the CoFe₂O₄/Co₃O₄ nanocomposite in Fig. 7b indicate that the property of antiferromagnetism changes to ferromagnetism. Based on the above analysis results, the magnetization (M_s) of the CoFe₂O₄/Co₃O₄ increased with doping CoFe₂O₄ from 0.676 emu g⁻¹ to 1.023 emu g⁻¹ at 10 K. The exchange bias effect of the CoFe₂O₄/Co₃O₄ is shown at 10 K and 100 K for the interface interaction between the Co₃O₄ nanoparticles and CoFe₂O₄ nanoparticles. In conclusion, the coercivity (H_c) and exchange bias field (H_EB) of the CoFe₂O₄/Co₃O₄ nanocomposite are much larger than that of the Co₃O₄ nanoparticles under the same conditions. The formation of CoFe₂O₄ nanoparticles can enhance the magnetic exchange coupling effect between the interfaces of Co₃O₄ and CoFe₂O₄.

Fig. 7 Magnetic hysteresis loops of the urchin-like (a) Co₃O₄ and (b) CoFe₂O₄/Co₃O₄ nanocomposites. The insets are enlarged regions at low field.

The magnetic susceptibility as a function of temperature curves of both Co₃O₄ and CoFe₂O₄/Co₃O₄ for ZFC–FC are displayed in Fig. 8a and b, respectively. The samples for the ZFC curve were registered by cooling in the zero field to 10 K, then increasing the temperature to 320 K in a field of 100 Oe. Due to the antiferromagnetic transition, a distinctive characteristic broad peak in both ZFC curves appears around 37 K. This T_N is slightly less than Co₃O₄ in the bulk state (T_N-bulk = 40 K).^29–31 The principle of the T_N reduction has been argued to be due to a variety of factors, such as dimension change,^21,26 and surface and interface exchange coupling effects.^30–32 For ferrite nanoparticles, their lower T_N values are attributed to the increase of the super exchange interaction.^32,33 The M of Co₃O₄ in the FC curve increases with the decrease temperature and presents a corner near T ∼ 33 K in Fig. 7a. This inflection is due to surface and interface exchange coupling effects between adjacent nanoparticles due to the small interparticle distance.¹⁴ The FC curve for the urchin-like CoFe₂O₄/Co₃O₄ nanocomposite is very different with that of the urchin-like Co₃O₄ mesoporous nanoparticles (in Fig. 8b), which show that the magnetism of the CoFe₂O₄/Co₃O₄ composite changes a lot at low temperature. The CoFe₂O₄ nanoparticles in the pores of the urchin-like Co₃O₄ reduce the distance of surface spin coupling, and the more mismatched spin of the surface of the Co₃O₄ nanoparticles can recouple the CoFe₂O₄ nanoparticles. Thus, the mismatched surface spin of the CoFe₂O₄/Co₃O₄ nanocomposite decreases, and the influence of the surface effect is attenuated for the magnetism. Consequently, the kink in the FC curve of the urchin-like Co₃O₄ is present in the CoFe₂O₄/Co₃O₄ nanocomposite. With a decrease in temperature, the magnetization (M) of the FC curve decreases at a very low temperature. At the same temperature, the M of CoFe₂O₄/Co₃O₄ nanocomposite is larger than that of Co₃O₄ in the ZFC–FC curves.

Fig. 8 Temperature dependence of the zero field cooling (ZFC) and field cooling (FC) magnetization curves. (a) Co₃O₄ and (b) CoFe₂O₄/Co₃O₄.

Conclusions

It has been shown that magnetic nano-hetero-structures can be synthesized by the solid–solid reaction of iron species with urchin-like Co₃O₄ which were prepared via hydrothermal treatment. In the composite, the coupling of Co₃O₄ to the hard ferrimagnet CoFe₂O₄ leads to reinforcement for the interface magnetic interaction, and the mismatched surface spin in the Co₃O₄/CoFe₂O₄ nanocomposites are greatly reduced. The magnetic behavior of the Co₃O₄/CoFe₂O₄ nanocomposites makes a great difference with the urchin-like Co₃O₄ at low temperature. Magnetization increases with the CoFe₂O₄ compound and a decrease in temperature. At 100 K, the exchange bias effect increases with the doping of CoFe₂O₄. This shows that the Co₃O₄/CoFe₂O₄ nanocomposite behaves as an exchange coupled system with cooperative magnetic switching. The H_c of both samples hardly changes with temperature and H_EB increases with an increase in temperature up to 100 K and then decreases. Finally, this relatively simple synthetic method can be extended to other composition ranges and other elements.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (51372055 and 51321061).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18846j

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