Preparation of FeCo/Cu Core Shell Magnetic Nanoparticles

GSTF Journal of Engineering Technology (JET) Vol.4 No.2, March 2017

Preparation of FeCo/Cu Core Shell Magnetic Nanoparticles Shilabati Hembram, Angshuman Sarkar, Amitava Basu Mallick*

Pritam Deb Department of Physics Tezpur University Assam, India

Department of Metallurgy and Materials Engineering Indian Institute of Engineering Science and Technology Shibpur, Howrah, India Abstract— FeCo/Cu core shell structure with FeCo as a core and copper as the shell has been successfully synthesized by displacement reaction. The morphology, grain size, lattice strain, and magnetic properties of all the samples were examined by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). Annealing temperatures and its effect on magnetic properties of the FeCo/Cu core shell particles was investigated. A maximum coercivity (Hc) of 398.84 Oe was recorded for the sample heat treated in magnetic field. The blocking temperatures (TB) and squareness (Mr/Ms) of the sample gradually increased due to the influence of temperature and magnetic field.

nanoparticles to enhance their chemical stability leading to the formation of multifunctional material. Various coating techniques such as micro-emulsion, displacement reaction and high-temperature trans-metallation reaction were recommended for coating material surfaces. It has been reported that FeCo alloy nanoparticles exhibit excellent soft magnetic properties such as high saturation magnetization Ms with negligible magnetocrystalline anisotropy and low coercivity which restrict their usage in high coercivity applications. Therefore, at present investigation is focused on enhancing magnetic anisotropy to attain the highest magnetic ordering and higher coercivity in this FeCo system.

Keywords- Core/shell; FeCo alloy; displacement reaction; coercivity; blocking temperature; squareness; TEM; VSM.

I.

The current study aims at synthesizing FeCo magnetic nanoparticle with non-magnetic copper (Cu) as an outer layer, which gives stability to the magnetic FeCo nanoparticles by preventing oxidation in the ambient environment, furthermore it helps to enhance the coercivity of the material by encapsulating the single domain FeCo alloy particles and reducing the domain interactions. A displacement method was used to develop a Cu coating over the surface of FeCo nanoparticles. Annealing with and without magnetic field was carried out to investigate the effect of temperature and magnetic field on physical and magnetic properties of the core/shell FeCo/Cu nanoparticles.

INTRODUCTION

This Iron group magnetic nanoparticles of Fe [1], Co [2], Ni [3] and CoNi [4], FeNi [5] and FeCo [6-8] binary alloys have been the subject of extensive research due to their usage in developing advanced micro electronic devices [9], microwave absorbing materials [6], as catalyst [7], in medicinal biology and biomedical [8] for past two decades. Among this class of magnetic materials, the ferromagnetic FeCo nanoparticles have gained special attention due to their unique magnetic properties including high Curie temperatures, saturation magnetization, and permeability, which are desired for many applications [9].

II.

EXPERIMENTAL

A.

Synthesis of core/shell FeCo/Cu powder The FeCo/Cu core shell nanocrystalline powders were prepared by high energy ball milling technique using high purity (99.9%) elemental Iron metal powder [Fe, Loba Chemie, India] and Cobalt metal powder [Co, Loba Chemie, India] Constituent elemental powder of Fe and Co, total mass of 40 grams for FexCo1-x (x = 0.5) were taken. The powder blend was ball milled for 35 hours in a planetary ball mill [P6 Fritsch, Germany] with ball to powder ratio of 10:1. Toluene [C7H8, Merck, India] was used as a process control agent to reduce agglomeration of the powders inside the vial and to protect it from oxidation during milling. The as prepared ball milled FeCo sample was then dried in a vacuum oven at 120°C for 5 hours to remove the residual toluene from the sample. The

Moreover, in the literatures, it has been suggested that most of these novel properties arise due to their small particle size and high surface to volume ratio. Thus, synthesis and characterization of magnetic FeCo nanoparticles have received immense importance in the recent years [10]. However, the binary FeCo alloy system in nano metric length scale shows some severe limitations. It has been stated in many reports that the surface of FeCo nanoparticles is prone to oxidation in ambient environments [11]. The formation of toxic oxide layer over the FeCo nanoparticles weakens its magnetic properties and limits their usage in biomedical applications. In this context, several research efforts have been directed towards the development of inert layer such as silica [12], carbon [7], gold [13], silver [14], copper [8] and polymer [15] to coat FeCo

DOI: 10.5176/2251-3701_4.2.194

© The Author(s) 2017. This article is published with open access by the GSTF

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dried FeCo nano particles were then dispersed in 40 ml of highly viscous commercial gum in a glass beaker. 30 ml of 0.1M CuSO4 solution was added in step to the viscous gum medium containing FeCo nanoparticles. The viscous mass turns reddish in color which confirms deposition of Cu on FeCo nanoparticles and completion of the displacement reaction. The Cu coated FeCo particles were removed from the gum by washing several times with deionized water. The collected sample was then thoroughly dried in a vacuum oven at 120°C for 10 hours. B. Synthesis of core/shell FeCo/Cu powder The as prepared Cu coated FeCo nanocrystalline powders were subjected to heat treatment with and without magnetic field (500 Oe) at 400°C temperature, with a soaking time of 15 mins in the furnace. The sample codes are detailed in Table I. The crystallographic phases present in the as-prepared FeCo/Cu and heat treated with and without magnetic field was studied by using x-ray diffraction (XRD) technique. XRD measurements for all the powder samples were carried out using a Philips PW 1830 diffractometer with Co-Kα radiation operated at 35 kV/25 mA with a step size of 0.02°/s. Magnetic properties of the samples were characterized by recording magnetic hysteresis loop at room temperature for all the samples with a Lakeshore 7400 series vibrating sample magnetometer at a field up to 12KG in room temperature. The temperature dependence magnetization was measured in an applied magnetic field of 100 G between 4 and 300 K is using zero-field-cooled (ZFC) and field-cooling (FC) procedures. The morphology of the Cu coated FeCo nanoparticle was also studied by JEOL 2010 model high-resolution transmission electron microscope operating at 200kV.

Figure 1. XRD pattern of (a) as dried FeCo/Cu, (b) H400 sample, and (c) MH400 sample

Fig. 1(a-c) shows the XRD patterns of as prepared FeCo/Cu, heat treated and magnetically heat treated samples. The experimental 2θ values calculated from XRD peak positions were compared with the standard JCPDS data of Cu (FCC) and FeCo (FCC) and were found to match reasonably well with the standard value which indicates that a displacement reaction has occurred and Cu coating is formed successfully over the FeCo samples. No additional peaks have been detected in the heat treated with and without magnetic field samples which suggest that the heat treatment and magnetic field did not affect the crystallographic phases present in the samples.

TABLE I. SAMPLES DESCRIPTIONS AS PER THEIR HEAT TREATMENT TEMPERATURES Without Heat Sample Id

treated As dried FeCo/Cu

Heat treated at 400°C In magnetic field of 500 Oe

MH400

Without a magnetic field

H400

Grain size and lattice strain of all the samples estimated by using single line profile analysis (SLPA) technique [16] were summarized in Table II. III.

RESULT AND DISCUSSION TABLE II. SUMMARY OF THE SLPA ANALYSIS FOR FECO/CU SAMPLES

A. Structural and morphological analysis

Grain size of Fe-Co (nm)

Lattice strain

8.3

0.013

89.00

0.0048

H400

10.63

0.080

315.17

0.0014

MH400

8.05

0.007

597.23

0.0008

Sample Id As dried FeCo/Cu

of Fe-Co

Grain size of Cu (nm)

Lattice strain of Cu

It is found that the grain size of FeCo does not vary much with heat treatments in comparison to the significant grain growth of Cu, because of the Cu shell restricts the growth of FeCo core. However, the lattice strain of the Cu decreases due Centre of Excellence (COE), TEQIP-II, IIEST Shibpur, Howrah- 711103, India.


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to the fact that the stress was released from Cu lattices after heat treatment. It is also worthwhile to mention that the grain size and lattice strain in magnetically heat treated sample decreases due to the recrystallization of the FeCo powder at 400°C. Fig. 2 shows the TEM image with the SADP pattern of the Cu coated FeCo powder sample. It is evident from the TEM images, that the particles are agglomerated possibly due to the in-situ formation of a Cu coating on FeCo powder. The selected area diffraction pattern (SADP) analysis (shown inset in Fig. 2) of Cu coated Fe-Co powder revealed that the computed ‘d’ values from SADP matches reasonably well with the standard JCPDS data of Cu and FeCo alloy. Table III depicted the summary of the SADP analysis of FeCo/Cu core shell powder.

Figure 3. The temperature dependent magnetization plots of (a) as dried FeCo/Cu, (b) MH400 and (c) H400 samples.

Fig. 3 (a-c) shows temperature dependent magnetization plots for all three samples. Calculation of the blocking temperature (Tb), the transition temperature between the ferromagnetic and the super-paramagnetic state, has been carried out by measuring the maximum of the ZFC curve. It was noticed that the blocking temperature of the heat treated and magnetically heat treated samples shifted towards higher temperature region compared to the as prepared Cu coated sample, which is resulted because of the more dipolar interaction in Cu encapsulated ferromagnetic FeCo particles.

Figure 2. TEM image with corresponding SADP pattern

TABLE III. SUMMERY OF SADP ANALYSIS OF FECO/CU CORE SHELL STRUCTURE Sample Id

Diameter of ring

Phase identification

Plane

1.284

Copper (Cu)

(220)

0.754

Iron (Fe)

(321)

(A ) As dried FeCo/Cu

B. Magnetic analysis

Figure 4. Hysteresis loops of (a) as dried FeCo/Cu, (b) MH400 and (c) H400 samples. Inset demagnetization curve of all three samples was plotted.


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Hysteresis loops recorded at room temperature for the as prepared and heat treated samples are shown in Fig. 4. The change in magnetic properties includes coercivity (Hc), saturation magnetization (Ms), remnant magnetization (Mr) and the squareness ratio (Mr/Ms) for all the samples were calculated and are tabulated in Table IV. It can be seen that the magnetic properties of the heat treated and magnetically heat treated FeCo/Cu samples were improved significantly. The higher value of coercivity, magnetic saturation and the squareness (Mr/Ms) were obtained for the annealed and magnetically annealed samples as compared to as dried sample. This mainly attributed to the fact that the applied magnetic field during annealing forces the magnetic dipoles to align. Though the magnetic saturation of all the samples significantly lowered compared to the theoretical value of equiatomic FeCo alloy, but enhanced coercivity and higher squareness will be pushing this material towards potentially more useful application in future.

ACKNOWLEDGMENT The financial assistance received from Center of Excellence, TEQIP-II, IIEST, Shibpur is acknowledged. The authors also acknowledge the characterization facilities availed from Tezpur University, Assam, India. REFERENCES [1]

[2]

[3]

[4] TABLE IV. MAGNETIC PROPERTIES OF FECO/CU SAMPLES Sample Id As dried

Coercivity (Hc) (in Oe)

Retentivity (Mr) (in emu/gm)

302.52

4.936

Saturation magnetization (Ms) (in emu/gm) 26.516

Squareness ratio Mr/Ms

[5]

0.19 [6]

FeCo/Cu H400

397.59

6.505

28.190

0.23

MH400

398.84

6.823

31.524

0.22

IV.

[7]

CONCLUSIONS

The following conclusions can be drawn from the present investigations: 







[8]

The adapted displacement reaction technique has resulted in the successful synthesis of FeCo/Cu core/shell structure. The XRD and TEM results are consistent with the encapsulation of FeCo nanoparticles by Cu and confirm the formation of the coating of Cu over FeCo magnetic particles.

[9]

[10] [11]

The grain size and lattice strain determined for FeCo/Cu core/shell nanocrystalline powders by SLPA technique have shown that heat treatment exerts significant influence on grain size and lattice strain on the Cu coated FeCo samples.

[12]

[13]

The M-H curve of Cu coated FeCo samples indicates that, coercivity increases with the increase in heat treatment temperature; whereas magnetic field induced heat treatment gives an additional effect on coercivity of the samples. The highest value of coercivity ~398.84 Oe was recorded for the magnetically annealed sample.

[14]

[15]

The blocking temperatures of the heat treated and magnetically heat treated samples shifted towards higher temperature compared to the as prepared Cu coated sample.

[16]

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Authors' Profile

Shilabati Hembram is a PhD scholar at IIEST Shibpur. She recived her M.Tech degree from Department of Metallurgy and Materials Engineering at IIEST, Shibpur in 2014. Her research interest involves synthesis of nanomaterials and composites.

Dr. Pritam Deb is Associate Professor in the Department of Physics, Tezpur University. His area of research is nanostructured functional materials and magnetic materials.

Angshuman Sarkar did his M.Tech in Nanoscience and Technology from Pondicherry Central University. He joined as senior research fellow at Centre of Excellence, IIEST, Shibpur in 2013. He started his PhD in 2014 at Department of Metallurgy and Materials Engineering Department, IISET, Shibpur. His research involves in the development and fabrication of high energy density magnetic materials.

Dr. Amitava Basu Mallick is a Professor in the Department of Metallurgy and Materials Engineering, IIEST Shibpur. He is presently associate dean (academics) and coordinator of Centre of Excellence, TEQIP-II. He has published more than 80 research papers in various internationally reputed journals with 950 and above citations. His research interest includes metal matrix composite, hybrid materials, nanostructured materials and magnetic materials.


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