Elsevier

Materials Research Bulletin

Volume 95, November 2017, Pages 9-16
Materials Research Bulletin

Synthesis and magnetic properties of CoFe2/CoFe2O4 nanoparticles diluted in the MgO matrix

https://doi.org/10.1016/j.materresbull.2017.07.015Get rights and content

Highlights

  • The structurally reverse CoFe2(core)/CoFe2O4(shell) nanostructure was prepared.

  • We introduced an useful method to discuss the magnetic properties.

  • The correlation of Mr/Ms∝1/lgHdip was observed.

Abstract

Well-dispersed CoFe2O4 nanoparticles were synthesized and coated by MgO, followed by reduction in a H2/N2 gas mixture to prepare CoFe2/MgO. The CoFe2/MgO was oxidized in the open air at 500 °C and 900 °C to prepare CoFe2(core)/CoFe2O4(shell)/MgO nanostructures with the CoFe2/CoFe2O4 mass ratios of 25.4% and 4.7%, respectively. It was found that the magnetization of the nanostructure with CoFe2(core)/CoFe2O4(shell) was not the simple sum of individual magnetizations of CoFe2 and CoFe2O4 due to the dipolar interaction (interparticle) and the coupling between CoFe2 and CoFe2O4 (intraparticle). Furthermore, the remanence (Mr) to saturation magnetization (Ms) ratio Mr/Ms, mainly determined by the magneto-crystalline anisotropy in the bulk materials, is sensitive to the surface spins and dipolar interaction in the nano-sized materials. It was observed that Mr/Ms is proportional to the inverse of the logarithm of maximum dipolar field Hdip, i.e. Mr/Ms  1/lgHdip, independent to the moment and concentration of magnetic particles.

Graphical abstract

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Well-dispersed 16-nm CoFe2O4 particles (a) were synthesized and then some nanoparticles were coated by MgO (b) followed by the reduction in a H2/N2 gas mixture to prepare CoFe2/MgO (c). CoFe2/MgO was oxidized in the open air at 500 °C and 900 °C to prepare CoFe2(core)/CoFe2O4(shell)/MgO (d) with the CoFe2/CoFe2O4 mass ratio being 25.4% and 4.7%, respectively. By comparing the experimental and calculated data, the magnetic properties were discussed in detail. Our results revealed that the correlation between the remanence Mr to saturation magnetization ratio Mr/Ms and maximum dipolar field Hdip roughly follows Mr/Ms∝1/lgHdip (e and f), independent to the moment and concentration of magnetic particles.

Introduction

Spinel cobalt ferrite (CoFe2O4) has been extensively investigated because of its high saturation magnetization (Ms) and magnetic anisotropy, which give rise to desirable magnetic behavior at room temperature [1], [2], [3]. In particular, nanoscale CoFe2O4 particles often exhibit novel properties that differ from those of their bulk polycrystalline counterparts due to the finite size effects [4], [5], [6]. Recently, many researchers have focused on the nano-composite with CoFe2O4 core and Co-Fe shell, aiming to improve the magnetic performance of CoFe2O4 [7], [8], [9], [10], [11], [12], [13], [14]. The Co-Fe alloy is a typical soft magnet and has the highest Ms value of ∼230 emu/g among all ternary alloys. Therefore, the magnetic performance of CoFe2O4 core/Co-Fe shell nanostructure is expected to be superior to that of individual CoFe2O4 or Co-Fe, provided the exchange-coupling interaction occurs at the interface between hard CoFe2O4 and soft Co-Fe. However, the effect of exchange-coupling has not been fully realized up to date. It is well-known that the distribution of hard and soft magnetic phases plays an important role in the magnetic properties. Therefore, in this work, the reverse CoFe2O4/Co-Fe nanostructure, i.e. the Co-Fe(core)/CoFe2O4(shell) nanostructure was synthesized and the magnetic properties were investigated.

The nanostructures with metallic core and metal-oxide shell possess many advantages in practical applications. The metallic core has a high Ms value, which is beneficial for effectively separating, collecting and driving in the environment of the magnetic field when the metallic particles are used for photocatalytic and biomedical fields. However, bare metallic particles have unstable magnetization. To stabilize the high magnetic moment, some robust coating strategies have been applied. For example, a layer of graphitic shell was coated onto Co-Fe nanoparticles (NPs) to protect their high magnetic moment from fast decay [15], [16]. Recently, a more facile method was developed in which Fe/ferrite and Co-Fe/ferrite core/shell nanostructures were synthesized through the oxidation of Fe and Co-Fe NPs in air atmosphere [17], [18], [19]. These core/shell nanostructures have better stability and the ferrite shell is biocompatible, therefore they are expected to be promising materials for various bio-sensing applications.

However, little attention has been paid to the magnetic properties of the Fe/ferrite and Co-Fe/ferrite core/shell nanostructures until now. Del Bianco et al. reported the effects of particle size on the magnetic properties of Fe/Fe oxide granular system [20]. By oxidizing CoFe in the open air for nearly 48 h, FeCo/CoFe2O4 was prepared and the room temperature magnetic hysteresis was measured [19]. In our recent work, the CoFe2 alloy was oxidized at 500 °C for 1 h in a flow of 100 sccm pure O2 to prepare the CoFe2, Fe2O3 and CoFe2O4 composite [21], which displayed unusual magnetizing behavior at low temperature. In this work, well-dispersed uniform CoFe2O4 NPs were synthesized. The as-prepared CoFe2O4 NPs were reduced in a H2/N2 gas mixture to obtain the CoFe2 alloy. Then, the CoFe2 alloy was oxidized separately at 500 °C and 900 °C in the open air to prepare two samples of CoFe2 core/CoFe2O4 shell nanostructures. The mass ratio of CoFe2/CoFe2O4 was 25.4% for the sample oxidized at 500 °C and 4.7% for the sample oxidized at 900 °C. To prevent the agglomeration of magnetic particles during the reduction and oxidation processes, they were coated by the MgO matrix. Then the magnetic properties of these as-prepared nanostructures were investigated in order to obtain interesting and valuable results.

Section snippets

Preparation of CoFe2O4 NPs

CoFe2O4 NPs were prepared by the thermal decomposition of Co(acac)2 (97%, acac is acetylacetonate) and Fe(acac)3 (98%) in the high-boiling-point organic solvent mixture of benzyl ether (97%), oleic acid (90%) and oleylamine (80–90%). The detailed process has been described elsewhere [22].

Dilution of CoFe2O4 NPs in a MgO matrix

Firstly, Mg(acac)2 (98%), benzyl ether (97%), oleic acid (90%) and oleylamine (80–90%) were mixed in a 1000 ml three-necked round-bottom flask by magnetic stirring under a flow of nitrogen (99.999%). The mixture

Morphology and structure

The SEM image in Fig. 1(a) shows that the as-prepared CoFe2O4 NPs sample consists of dispersed spherical NPs. Fig. 2(b) shows the TEM image of CoFe2O4 NPs. The diameters of one hundred and twenty particles were measured, and the distribution of particle size is shown in the histogram in the inset of Fig. 2(b). Then this histogram was fitted by the Gaussian function (solid line). The peak of the Gaussian fitting curve locates at 16 nm, which is defined as the average particle size. Therefore, the

Conclusions

The CoFe2/MgO sample was synthesized by reduction of CoFe2O4/MgO in a H2/N2 gas mixture. By oxidization of CoFe2/MgO in the open air at 500 °C (CoFe2/500/MgO) and 900 °C (CoFe2/900/MgO), CoFe2(core)/CoFe2O4(shell)/MgO nanostructures were prepared and their corresponding CoFe2/CoFe2O4 mass ratios were calculated to be 25.4% and 4.7%.

The magnetic properties of CoFe2O4 NPs, CoFe2/MgO, CoFe2/500/MgO and CoFe2/900/MgO were investigated by measurements of hysteresis loops at 10, 50, 100, 150, 200, 250,

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51471001, 11174004 and 11304001).

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