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Journal of Nanoscience and Nanotechnology Vol. 16, 7949–7954, 2016 www.aspbs.com/jnn

Structural and Magnetic Characterization of Copper Ferrites Prepared by Using Spray

Co-Precipitation Method

Nguyen Kim Thanh

¹^∗

, Nguyen Phuc Duong

¹

, Do Quoc Hung

²

, To Thanh Loan

¹

, and Than Duc Hien

¹

1International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology, Hanoi 100000, Vietnam

2Faculty of Physics and Chemical Engineering, Le Quy Don Technical University, Hanoi 100000, Vietnam

Crystal structure, cation distribution and magnetic properties of CuFe2O4 nanoparticles prepared by using spray co-precipitation method with different annealing temperature (Ta=600, 700, 800 and 900C) were studied systematically by synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). The crystal structure symmetry of the samples was confirmed to be cubic (space group Fd3m). Mean particle size of the samples varies in the range of 10–300 nm. The lattice parameter, particle size and saturation magnetization were found to increase with increasing annealing temperature. Rietveld refinement performed on SXRD data reveals that the distribution of Cu²⁺in crystallographic sites depends on annealing temperature. The variation of saturation magnetization and Curie temperature were discussed and explained based on cation distribution of Cu²⁺, surface and finite-size effects.

Keywords: Copper Ferrite, Spray Co-Precipitation, Crystal Structure, Cation Distribution, Magnetic Properties.

1. INTRODUCTION

Magnetic nanoparticles of ferrites MFe₂O₄ (M =Ni, Mn, Zn, Cu…) have recently attracted attention of many authors because they can be used for wide applications in different fields such as biomedicine,¹ catalyst² and magnetic recording media.³ Their advantages are high saturation magnetization, superparamagnetism, stability of properties at high frequencies, mechanical and chemical durability.⁴Among them, copper ferrite is one of the most important ferrites with interesting physical properties and potential applications. CuFe₂O₄is known to exist in tetragonal and cubic structures. Copper ferrites can have different cation distributions (Cu_xFe₁₋_x)^A [Cu₁₋_xFe₁₊_x]^BO₄ depending on heat-treatment conditions.⁵For bulk samples prepared by conventional ceramic method, on slow cool- ing from 900 C to room temperature, x is as small as 0.03 and the crystal structure has tetragonal symmetry due to Jahn-Teller distortion but by quenching rapidly from

∗Author to whom correspondence should be addressed.

900 C to room temperature, x≈02 and the structure becomes cubic.⁶This cation distribution greatly influences on the properties of copper ferrite, especially its magnetic properties. There are numerous methods for fabricating copper ferrite nanoparticles such as milling method, co- precipitation method, hydrothermal method^7–9 for many applications such as information storage, gas sensors, catalyst production for clean energy and environmental treatment.^10–12 However, for CuFe₂O₄ nanoparticles prepared by using soft chemical methods investigation of cation distribution is still limited.

In this work, we present the result of studying structural and magnetic characterization of copper ferrite synthesized by using spray co-precipitation method and annealed at different temperature (T_a=600, 700, 800 and 900C). The crystal structure, morphology, saturation magnetization and Curie temperature of the samples were determined.

The cation distribution in tetrahedral and octahedral sites was estimated by analysis of synchrotron X-ray diffraction (SXRD) data using the Rietveld refinement method and magnetic measurements.

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Figure 1. Synchrotron X-ray diffraction patterns of CuFe₂O₄annealed at 600C, 700C, 800C and 900C.

2. EXPERIMENTAL DETAILS 2.1. Process of Nanoparticles

Spray co-precipitation method was employed to pro- duce CuFe₂O₄nanoparticles. Mixed solution of Cu(NO₃)₂ 0.1 M and Fe(NO₃)₃ 0.2 M was prepared from the salt Cu(NO₃)₂·4H₂O and Fe(NO₃)₃·6H₂O. The solution NaOH 0.8 M was adulterated from NaOH. Mixed salt and NaOH solutions were sprayed into the vessel that con- tains NaOH 10⁻⁴ M to keep constant pH=10 of solution. A reddish-brown precipitate in the colloidal form was obtained from this reaction. Precipitate was washed by deionized water to pH=7–8. The precipitate was collected and heated at 100 C for 24 hours. The as- synthesized powders were sintered at various temperatures in the range of 600–900C during 5 hours then quenched from that high temperature into ice–melting water immedi- ately. Nanoparticles were obtained by a permanent magnet and drying at 60 C for 5 hours. We denote the samples which were annealed at different temperatures 600, 700, 800, 900 C, respectively, as CF600, CF700, CF800, and CF900.

2.2. Characterization

Synchrotron X-ray powder diffraction experiments were carried out at beamline SAXS of the Synchrotron Light Research Institute (Thailand)=154 Åto identify the crystal structure of the samples. SXRD patterns of the

Table I. Lattice parametera, unit cell volumeV , coordinate of oxygenxO, average crystallite sizeDand the refinement fitting quality of the CuFe₂O₄samples annealed at different temperatures.

T_a(C) a(Å) V(Å³) D(nm) x(O) R_wp% ²

CF600 600 8.357(4) 583.6(4) 9431 0.377(1) 11.5 1.22

CF700 700 8.366(2) 585.5(3) 11581 0.378(2) 12.3 1.29

CF800 800 8.381(1) 588.7(1) 2681 0.376(1) 13.2 1.20

CF900 900 8.376(2) 587.5(2) 2291 0.376(2) 11.4 1.71

Note: Statistical errors are indicated in the last significant digit.

Figure 2. Synchrotron diffraction pattern of CuFe₂O₄ annealed at 800C and processed by the Rietveld method. The experimental points as well as calculated and difference functions are indicated.

samples are shown in Figure 1. The data were processed to analyze using the Rietveld method with the help of FullProf program.¹³ The diffraction peaks were modeled by pseudo-Voigt function. A standard of silicon was used to determine instrument broadening. The refinement fitting quality was checked by goodness of fit (²) and weighted profile R-factor (R_wp), which are given in Table I. The calculated results are accepted with²should approach 1 and R_wp must be close to or less than 10%.¹⁴ One of the synchrotron diffraction patterns and its refinement are shown in Figure 2.

Transmission electron microscopy TEM (TEM 1010, JEOL) was used to examine the particle size and morphology.

Magnetic characteristics are studied with the vibrating sample magnetometer VSM (ADE Technology—DMS 5000) in temperature range 88–900 K and applied fields up to 10 kOe.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of

CuFe₂O₄Nanoparticles

It is well known that copper ferrite may exist in two symmetry modifications: cubic at high temperature and tetragonal at lower temperature.¹⁵ The tetragonal-cubic transition temperature depends on preparation methods

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Figure 3. Lattice parameteraand the average crystallite sizeDof the samples CuFe₂O₄as a function of annealing temperature.

and fabrication conditions.¹⁶ In the present work, the CuFe₂O₄ samples quenched from high temperature were only obtained in cubic phase at room temperature as seen in Figure 1. For the sample annealed at 600C, although we are unable to distinguish the tetragonal from cubic phase due to the significant peak broadening, the refinement result in the tetragonal phase shows a very small value of tetragonal distortion (c/a=1004). Hence, it can be concluded that all the samples are cubic and well

Figure 4. TEM image of samples annealed at 600C, 700C, 800C, 900C.

refined in space group Fd3m with atoms in positions O in 32ex x x, A-site in 8a1/81/81/8 and B-site in 16d1/21/21/2. The refined values of structural parameters including lattice parameter, unit cell volume, coordinate of oxygen and the average size of coherent scattering region of the samples are given in Table I. The average size of coherent scattering region D (usually called the crystallite size) was obtained by analysis of the peak broadening. In our work the average size of coherent scattering region was determined on applying Rietveld method using Fullprof program with condition that instrumental resolution function was provided.

A small amount of impurity was observed in X-ray diffraction pattern of the sample annealed at 900 C and analyzed via Rietveld refinement. The impurity phase is -Fe₂O₃ with concentration ∼5%. -Fe₂O₃ is rare Fe₂O₃ polymorph that usually exists in the form of nanostructures.¹⁷

The dependences of lattice parameter and crystallite size on annealing temperature are shown in Figure 3. It is seen that lattice parameter as well as crystallite size increase with increasing of annealing temperature from 600C to 800C. Similar trend was reported previously by Hoque et al.⁸ The increase of lattice parameter can be explained by improved crystallinity of the samples which is due to better atomic diffusion at higher annealing temperatures.

On the other hand, increasing the annealing temperature promotes the particle growth, thus leading to the increase

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Copyright: American Scientific Publishers of D. The increase of lattice parameter and crystallite

size with increasing the annealing temperatures were also observed in other spinel ferrite nanoparticle systems.^18–20 However, for the sample annealed at 900 C, both lattice parameter and crystallite size were found to decrease. This observation may be attributed to the presence of impurity as detected by SXRD analysis. The presence of -Fe₂O₃ impurity phase in this CF900 sample may lead to lattice distortion due to off-stoichiometry in the main phase and suppresses the growth of crystallites.

Size and shape of CuFe₂O₄nanoparticles are showed in TEM images (Fig. 4). The particle sizes of the samples annealed at T_a=600, 700 and 800C are in good agreement with the average crystallite size results in Table I.

The isotropic growth of nanoparticles was observed in all the samples. The particles are nearly spherical and have a uniform shape. It is also seen that the particles are adhered together due to annealing at high temperatures and also due to attractive forces between them such as those of magnetic dipole–dipole type. The particle size increases largely to about 300 nm for the sample annealed at 900C.

This can be explained by the fact that as the particle sizes are in the nanoscale the melting point of the material reduces therefore moderate annealing temperature is enough to facilitate the forming of larger particles from small nanocrystallites. It is also noted that the impurity phase separation in this sample observed via diffraction measurement may have influence on the average crystallite size which is about 30 nm (Table I) while the grain size is enlarged to submicron scale.

3.2. Magnetic Properties

The variation of the magnetization as a function of external magnetic field for all the samples were measured at different temperature form 88 K to 900 K. As examples, Figure 5 shows the M-H curves for the samples measured at 88 K and 293 K. Magnetization of the samples was found to increase with increasing annealing temperature. The samples CF600, CF700 are unsaturated with 10 kOe external magnetic field while the samples CF800, CF900 was saturated in approximately above 5 kOe magnetic field. Within the investigated magnetic field range, we could apply law of approach to saturation to determine the saturation magnetization M_s of the samples at each measuring temperature. The magnetization can be expressed as a function of magnetic field as follows:²¹²²

M=M_s1−a/H^1/2−b/H² (1) where the terma/H^1/2arises from defects in the particles and the termb/H² is attributed to the effective anisotropy energy of the samples. The fitting curves to the experimental data points according Eq. (1) are also presented in Figure 5 for the samples.

Figure 6 shows the temperature dependence of saturation magnetization M_s of the samples. At all measured

temperatures, the magnitude ofM_s decreases as one goes from CF900 to CF600 samples. The decrease inM_s is in accordance with the decrease of particle size as observed in Figure 4. This phenomenon clearly reflects the role of surface effect in the nanoparticles. Because of the small size of the nanoparticles, the ratio of the surface area to the particle volume becomes large. The cations Cu²⁺, Fe³⁺

in the surface shell have varied coordination numbers with respect to those further inside the particle therefore one may expect asymmetric and broken bonds in the surface region. As a consequence, the spins in the outer shell become disorder, leading to the decrease of the total magnetization of the particle. This effect is most pronounced in CF600 sample with the smallest particle size. In order to determined the saturation magnetization at zero Kelvin M_s0, the M_s data at temperature region below 300 K (Fig. 6) were fitted using the modified Bloch’s function M_sT =M_s01−BT whereB is a constant and is the Bloch exponent.²³ The extrapolatedM_s0results are given in Table II.

Figure 5. M-H curves measured at 88 K and 293 K for the samples annealed at T_a=600, 700, 800 and 900C. Dots are the experimental values and solid lines are fits to the experimental data according to Eq. (1).

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Figure 6. Temperature dependence of saturation magnetization of the samples with different annealing temperatures. Inset: Dependence of Curie temperature of the samples on the annealing temperature.

The variation of the Curie temperature T_C of the samples as a function of annealing temperature is shown in the inset of Figure 6. The Curie temperature of CF900 sample with large grain sizes is 773 K which is in agreement with that of bulk counterpart (780±20 K).²⁴The Curie temperature decreases significantly as the average particle size of the samples decreases. The magnetic order in the materials is decided by the total exchange energy which in turn depends on the volume of the samples. For small particles, the exchange energy is smaller compared to that of the bulk therefore the magnetic order in the nanoparticles can be overcome by thermal energy at lower temperatures. As shown previously for copper ferrites,⁸²⁵the Curie temperature can be affected by both finite-size effect and cation distribution. The influence of cation distribution in saturation magnetization and Curie temperature is discussed in the following section.

3.3. Cation Distribution

The cation distribution in tetrahedral and octahedral sites was estimated by the analysis of synchrotron X-ray diffraction data using the Rietveld refinement method and is given in Table II. The amount of Cu²⁺ in A site is found to increase with increasing annealing temperature.

The results show that at high temperatures, an amount of Cu²⁺can occupy A sites, the samples were then quenched rapidly to low temperature, the cation distribution state

Table II. Cation distribution of CuFe2O4samples annealed at different temperatures.

Ta(C) A site B site Msat 0 K^b(emu/g) Msat 0 K^a(emu/g) d(nm)

CF600 600 Cu₀₀₀Fe₁₀₀₄ Cu₁₀₀Fe₁₀₀₄ 23.26 19.40 0.27

CF700 700 Cu₀₀₇₃Fe₀₉₃₃ Cu₀₉₃₃Fe₁₀₇₃ 36.28 28.21 0.47

CF800 800 Cu₀₁₁₃Fe₀₈₉₃ Cu₀₈₉₃Fe₁₁₁₃ 43.73 42.75 0.11

CF900 900 Cu₀₁₅₃Fe₀₈₅₃ Cu₀₈₅₃Fe₁₁₅₃ 51.17 44.92 0.49

Notes: Statistical errors are indicated in the last significant digit.^aRietveld refinement method.^bMagnetic measurement.

at high temperature were “frozen” and kept at a room temperature. In bulk material, when the amount of Cu²⁺

located in B sites1−x≥08, the structure is tetragonal as a result of cooperative Jahn-Teller distortion.²⁶ How- ever, in small size particles as in the present study, the ratio between surface area to core volume is high, therefore cooperative distortion effect is cancelled out and all samples are found in form of cubic structure although the amount of Cu²⁺ located in B sites higher than 0.8.

The theoretical net magnetic moment values were also calculated for the samples using cation distribution results derived from Rietveld refinement analysis. In the calculation, the spin values of Fe³⁺ S=5/2 g=2, Cu²⁺

S=1/2 g=2 were used. The corresponding spontaneous magnetization values at 0 K were calculated from the theoretical magnetic moment values and are listed in Table II. It is seen that theM_s0determined from magnetization measurements are lower than those estimated by Rietveld refinement method. This effect can be explained reasonably based on the spin disorder in surface shell regions. Assuming the core–shell model applying to spherical nanoparticles in which the core orders ferrimagneti- cally and the shell is magnetically disordered, the thickness of the surface shell d is calculated from the formula²⁷ M_S=M_S0D/2−d/D/2³ whereDis the diameter of the nanoparticles,M_s0 is saturation magnetization of bulk counterpart. In the calculation,D values are taken as average crystallite size (Table I),M_s0 is estimated by Rietveld refinement method andM_sis determined by magnetization data (Table II). The results of thickness d are shown in Table II. The estimated surface shell thickness is less than a unit cell length.

Finally, we discuss the influence of cation distribution on the Curie temperature of the samples. It is known that Curie temperature of spinel ferrites depends on the strength of intersublattice exchange interaction J_AB and in copper ferrite J_AB is expected to be largest when the number of interaction pairs between iron ions in A and B sublattices is maximized. The cation distribution data of the samples (Table II) reveal that the Fe³⁺(A)–Fe³⁺[B]

pair number is largest in CF600 sample with x=0 and gradually decreases in CF700, CF800 and CF900 samples because of the increase in x. This observation is against the increase tendency ofT_Cwith increasing annealing temperature. It is therefore concluded that the variation inT_C of the samples is driven by the finite-size effect.

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Copyright: American Scientific Publishers 4. CONCLUSION

Spray co-precipitation is an efficient method to fabricate copper ferrite samples. All the samples were found to crys- tallize in cubic structure. By changing annealing temperature from 600–900C, average particle size can be varied from 11–300 nm. The lattice parameter, amount of Cu²⁺in A-site and saturation magnetization were found to increase with increasing annealing temperature. The spontaneous magnetization of the samples was found to be dependent on both cation distribution and reduction of particle size while the change in the Curie temperature is mainly gov- erned by the finite-size effect.

Acknowledgments: The current work was financially supported by the Vietnam National Foundation for Sci- ence and Technology under Grant 103.02-2012.07. The authors thank Dr. S. Soontaranon and Dr. N. Thammajak for their valuable experimental assistance at Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand.

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Received: 29 May 2015. Accepted: 31 August 2015.