Tuning the microstructural and magnetic properties of CoFe₂O₄/SiO₂ nanocomposites by Cu²⁺ doping

Abstract

Co–Cu ferrite is a promising functional material in many practical applications, and its physical properties can be tailored by changing its composition. In this work, Co_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.3) nanoparticles (NPs) embedded in a SiO₂ matrix were prepared by a sol–gel method. The effect of a small Cu²⁺ doping content on their microstructure and magnetic properties was studied using XRD, TEM, Mössbauer spectroscopy, and VSM. It was found that single cubic Co_1−xCu_xFe₂O₄ ferrite was formed in amorphous SiO₂ matrix. The average crystallite size of Co_1−xCu_xFe₂O₄ increased from 18 to 36 nm as Cu²⁺ doping content x increased from 0 to 0.3. Mössbauer spectroscopy indicated that the occupancy of Cu²⁺ ions at the octahedral B sites led to a slight deformation of octahedral symmetry, and Cu²⁺doping resulted in cation migration between octahedral A and tetrahedral B sites. With Cu²⁺ content increasing, the saturation magnetization (M_s) first increased, then tended to decrease, while the coercivity (H_c) decreased continuously, which was associated with the cation migration. The results suggest that the Cu²⁺ doping content in Co_1−xCu_xFe₂O₄ NPs plays an important role in its magnetic properties.

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1. Introduction

Cobalt ferrite (CoFe₂O₄) with moderate saturation magnetization, high coercivity and Curie temperature, as well as excellent chemical stability has gained increasing attention in technological applications, such as magnetic recording, catalysis, bio-targeted drug delivery, magnetic resonance imaging, and spintronics.^1–7 In general, CoFe₂O₄ possesses a cubic inverse spinel structure with the Fd3̄m space group,in which Co²⁺ ions predominantly occupy octahedral B sites and Fe³⁺ ions are almost equally distributed between tetrahedral A and octahedral B sites. However, cation distribution between the A and B sites varies with the chemical composition and synthesis procedure. Designing the composition through the incorporation of divalent metal ions (such as Zn²⁺, Mn²⁺, Cu²⁺, and Ni²⁺) serves as a flexible strategy to tune the cation distribution of CoFe₂O₄ nanoparticles (NPs), which may be beneficial to further modify their physical properties or introduce novel functionalities.^8–10

Recently Co–Cu ferrite, prepared through doping Cu²⁺ in CoFe₂O₄ NPs has been widely exploited for a variety of technological applications. Venkateshwarlu et al.⁸ reported that the increasing Seebeck coefficient was observed in CoFe₂O₄ after doping with Cu²⁺ ions. The enhanced effect of Cu²⁺ doping on photocatalytic degradation efficiency of CoFe₂O₄ was reported by Sundararajan et al.⁴ They also found that with Cu²⁺ content increasing, the saturation magnetization (M_s) decreased monotonously while the coercivity (H_c) first increased then decreased. Sanpo et al.¹¹ demonstrated the substitution of Cu²⁺ ions into CoFe₂O₄ could improve the antibacterial property on against multidrug-resistant E. coli and Staphyloc occus aureus. These experimental results suggest that Cu²⁺ doping content in CoFe₂O₄ significantly influences their physical property. However, it is well known that copper ferrite (CuFe₂O₄) can exist in face-centered cubic and face-centered tetragonal phases due to obvious Jahn–Teller distortion of Cu²⁺ ions.¹³ Thus, when larger content of Cu²⁺ ions was doped in CoFe₂O₄ lattice, the crystal structure can transfer from cubic to tetragonal phase.^12–14 Balavijayalakshmi et al. have reported that as the Cu²⁺ doping content x was >0.6, tetragonal CuFe₂O₄ can be observed in cubic Co_1−xCu_xFe₂O₄ NPs prepared by co-precipitation method.¹² With small content of Cu²⁺ ions doping in CoFe₂O₄ NPs, the crystal microstructure and physical properties can be tailored and investigated without undesired phase transformation. To date, a limited extent of work has been found in the literature on the microstructural investigation of Co–Cu ferrites with small Cu²⁺ doping content.

Magnetic CoFe₂O₄ NPs prepared by chemical method are prone to agglomerate, which makes it quite difficult to exploit their unique physical properties for practical applications. Two strategies have been developed to stabilize and reduce nanoparticle agglomeration, obtaining single phase ferrite. One is coating CoFe₂O₄ NPs with a uniform and stable ultrathin layer to form core–shell NPs. Since the thickness of the coating layer (such as ultrathin phosphate layer¹⁵ and silicon carbide layer¹⁶) is only of a few nanometers, the magnetic properties of the CoFe₂O₄ core are not compromised after capping. The other is dispersing CoFe₂O₄ NPs in non-magnetic matrix to form nanocomposites, for example, dispersing CoFe₂O₄ in amorphous SiO₂, i.e. CoFe₂O₄/SiO₂ nanocomposites.^17–20 For SiO₂-based nanocomposites, SiO₂ network can not only provide spatial nucleation sites for CoFe₂O₄ NPs, promote the formation of single-phase spinel, but also minimize the surface roughness and spin disorder, thereby enhance the magnetic properties of nanocomposites.^21,22

In this work, we prepared Co_1−xCu_xFe₂O₄/SiO₂ nanocomposites (0 ≤ x ≤ 0.3) using sol–gel method, in which SiO₂ was used to obtain monophasic Co–Cu ferrites. With small amount Cu²⁺ ion doping, the crystal microstructure and physical properties were tailored without phase transformation. The goal of the present work is to study the effect of the small amount of Cu²⁺ doping on the microstructure, the hyperfine interaction, and magnetic properties of Co_1−xCu_xFe₂O₄ by using X-ray diffractometer (XRD), Mössbauer spectroscopy, and vibrating sample magnetometer (VSM) at room temperature. The result shows that the crystallite size of Co_1−xCu_xFe₂O₄ increases with Cu²⁺ content. The Cu²⁺ doping in CoFe₂O₄ induces a slight deformation of octahedral symmetry and change in cation distribution, which in turn modifies the values of M_s and H_c.

2. Experiments

2.1 Synthesis of Co_1−xCu_xFe₂O₄/SiO₂ nanocomposites

The synthesis diagram for Co_1−xCu_xFe₂O₄/SiO₂ nanocomposites (70% wt. ferrite/30% wt. SiO₂) is presented in Fig. 1. Using cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O) as iron, cobalt, and copper sources, and tetraethyl orthosilicate (TEOS) as precursor of SiO₂, a series of Co_1−xCu_xFe₂O₄/SiO₂ nanocomposites (x = 0, 0.1, 0.2, and 0.3) were synthesized by sol–gel method. Firstly, the metal nitrates were weighted by the designed molar ratio and thoroughly dissolved in ethanol with magnetic stirring. Then, 1.5 mL ethylene glycol and 9.6 mL TEOS ethanol solution (volume ratio of 1 : 1) was injected into the solution, followed by adding 1 mL HNO₃ and continuously stirring for 5 h. Secondly, the solution was evaporated on a 60 °C water bath to form black brown sol. After that, the sol was dried at 100 °C for at least 24 h to form xerogel. Finally, the obtained gel was calcined at 1000 °C for 2 h in air and cooled to room temperature. The final collected product was taken for further investigation.

Fig. 1 Schematic diagram of the synthesis method for Co_1−xCu_xFe₂O₄/SiO₂ nanocomposites.

2.2 Characterization

The crystal structure, morphology, and magnetic properties of the as-prepared Co_1−xCu_xFe₂O₄/SiO₂ were investigated by Rigaku D/max-2500 X-ray diffractometer (XRD, λ = 1.5406 Å), JEM-2100HR transmission electron microscope (TEM), and LakeShore7407 vibrating sample magnetometer (VSM, B = 1.5 T), respectively. The crystallite size of Co_1−xCu_xFe₂O₄ was estimated by using Scherrer’s formula. The room temperature Mössbauer spectra were collected on a FAST Comtec Mössbauer system in transmission mode, using a ⁵⁷Co(Pd) source and a conventional constant acceleration mode. The Mössbauer spectra of the samples were fitted using Lorentzian lines via the least square method.

3. Results and discussion

3.1 Structure and morphology analysis

XRD patterns of the as-prepared Co_1−xCu_xFe₂O₄/SiO₂ samples are shown in Fig. 2. The diffraction peaks from (111), (220), (311), (222), (321), (400), (422) and (511) are consistent with the standard spectrum of cubic spinel CoFe₂O₄ (JCPDS no. 22-1086), which demonstrates the formation of Co–Cu ferrite with no detectable impurity phases. No reflection from SiO₂ can be detected in XRD patterns due to the low content of amorphous SiO₂. With increasing Cu²⁺ content, the diffraction peak (311) shifts from 35.455° to 35.374° with a small Δθ (0.081°) accompanied by increasing peak intensity and the narrower peak width.

Fig. 2 XRD patterns of the as-synthesized Co_1−xCu_xFe₂O₄/SiO₂ with different Cu²⁺ content.

Fig. 3 presents the variation of lattice parameter and crystallite size of Co_1−xCu_xFe₂O₄ with Cu²⁺ doping content. The lattice parameter was determined from the X-ray data with MDI Jade 6.5 software using the high-purity silicon powders as a standard sample. It can be seen that the lattice parameter a_o of 8.383 Å for the sample with x = 0 is in agreement with the reported value of pure CoFe₂O₄.²³ As Cu²⁺ content increases from 0 to 0.3, the lattice parameter a_o slightly increases from 8.383 to 8.389 Å. The increase in lattice parameter can be attributed to the difference in ionic radius of Co²⁺ (0.74 Å) and Cu²⁺ (0.76 Å).^4,24 Furthermore, the average crystallite size, calculated with Scherrer equation is found to increase with increasing Cu²⁺ content (18, 26, 35 and 36 nm for Co_1−xCu_xFe₂O₄ with x = 0, 0.1, 0.2, and 0.3, respectively). This indicates that the Cu²⁺ doping in CoFe₂O₄ NPs favors the grain growth rate during sol–gel preparation process. Similar phenomenon in crystallite size has been also observed by Ashour et al. and Dippong et al.^25,26

Fig. 3 Plot of lattice parameter and crystallite size of Co_1−xCu_xFe₂O₄/SiO₂ as a function of Cu²⁺ content.

TEM images of Co_1−xCu_xFe₂O₄/SiO₂ samples with x = 0 (Fig. 4a) and x = 0.2 (Fig. 4b) are shown in Fig. 4. It can be seen that near-spherical Co–Cu ferrites are environed by amorphous SiO₂ without obvious agglomerate. The average sizes are estimated to be 19 ± 5 nm (x = 0) and 39 ± 9 nm (x = 0.2), respectively, which are consistent with the results determined by XRD. Fig. 4c presents the selective area electron diffraction (SAED) pattern for the x = 0.2 sample. The diffraction rings are indexed as lattice plane (111), (220), (311), (400), (511), and (440) for spinel Co_0.8Cu_0.2Fe₂O₄, which is in agreement with the XRD result. The high resolution TEM (HRTEM) image of Co_0.8Cu_0.2Fe₂O₄ in Fig. 4d confirms that the sample is of good crystalline quality, and the clear space fringe with an interplanar spacing of 0.221 nm agrees with the (400) planes of CoFe₂O₄NPs.

Fig. 4 TEM images of the as-synthesized Co_1−xC_uFe₂O₄/SiO₂ with (a) x = 0 and (b) x = 0.2. (c) SAED pattern and (d) HRTEM image for x = 0.2 sample. Insets in panel (a) and (b) show the average particle size distribution obtained by approximate 50 nanoparticles, respectively.

3.2 Mössbauer spectroscopy

Mössbauer technique serves as one of the most powerful tools for probing the atomic and electronic configuration of Fe atoms, thus, the hyperfine interaction of Co_1−xC_uFe₂O₄ was investigated through Mössbauer spectra. Fig. 5 shows the experimental Mössbauer spectra and fitting lines of Co_1−xC_uFe₂O₄/SiO₂ with different Cu²⁺ doping contents, and Table 1 presents the correspondingly fitting parameters. These spectra are decomposed into two Zeeman sextets, demonstrating that Co_1−xCu_xFe₂O₄ NPs in the obtained samples are ferromagnetically ordered. The values of isomer shifts (IS) are in the range of 0.26–0.40 mm s⁻¹, suggesting that Fe ions in the present Co_1−xCu_xFe₂O₄ NPs are in high spin Fe³⁺ charge state. Among two sextets, one with smaller IS and hyperfine field (H_in) arises from the tetrahedral Fe³⁺ ions, and the other with larger IS and H_in can be ascribed to the octahedral Fe³⁺ ions. It is well known that the value of IS is dependent on s-electron density of Fe³⁺ nucleus. Owing to the larger bond length of Fe³⁺–O²⁻ at octahedral B sites, the orbital overlapping of Fe³⁺ and O²⁻ is smaller, hence the IS at octahedral B sites is larger than that of tetrahedral A sites. With increasing Cu²⁺ doping content, the IS_A value decreases while the IS_B increases, suggesting that the Cu²⁺ doping behavior can affect the s-electron distribution of Fe³⁺ ions at tetrahedral A and octahedral B sites due to Jahn–Teller effect of Cu²⁺ ions.²⁷

Fig. 5 Mössbauer spectra of Co_1−xCu_xFe₂O₄/SiO₂ samples. Symbols represent the experimental data and the continuous line corresponds to the fitting data.

Table 1 Mössbauer parameters of Co_1−xCu_xFe₂O₄/SiO₂ samples^a

Sample	Component	IS (mm s^−1)	QS (mm s^−1)	Hin (T)	FWHM (mm s^−1)	S (%)	SA/SB
a IS = isomer shift; QS = quadruple split, Hin = hyperfine field, S = relative absorption area, FWHM = the half width at half maximum.
x = 0	Sextet (A)	0.300 ± 0.004	0.027 ± 0.008	47.6 ± 1.1	0.296 ± 0.012	46.7	0.876
	Sextet (B)	0.324 ± 0.003	0.007 ± 0.002	49.6 ± 0.9	0.279 ± 0.011	53.3
x = 0.1	Sextet (A)	0.292 ± 0.003	0.022 ± 0.004	48.0 ± 0.8	0.248 ± 0.010	44.2	0.792
	Sextet (B)	0.346 ± 0.007	0.014 ± 0.003	49.8 ± 0.8	0.326 ± 0.021	55.8
x = 0.2	Sextet (A)	0.280 ± 0.005	0.037 ± 0.001	48.4 ± 0.7	0.222 ± 0.007	43.8	0.779
	Sextet (B)	0.369 ± 0.010	0.023 ± 0.003	50.0 ± 0.8	0.383 ± 0.011	56.2
x = 0.3	Sextet (A)	0.269 ± 0.006	0.039 ± 0.002	48.4 ± 0.6	0.217 ± 0.013	43.7	0.776
	Sextet (B)	0.399 ± 0.011	0.039 ± 0.003	49.8 ± 0.9	0.434 ± 0.011	56.3

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Among Mössbauer parameters, quadrupole splitting (QS) is related to the crystal symmetry. As seen from Table 1, the value of QS_B gradually increases with Cu²⁺ content, while the values of QS_A do not exhibit a specific tendency. This phenomenon reveals that the local symmetry of octahedral B site Fe³⁺ ions is modified during Cu²⁺ doping process, suggesting that the Cu²⁺ ions preferentially occupied octahedral B sites in the as-prepared Co–Cu ferrites. Owing to Jahn–Teller effect of Cu²⁺ ions at octahedral B sites, they form dsp² orbital hybridization and produce strain in Co_1−xCu_xFe₂O₄ crystals, inducing the octahedral symmetry to deform slightly without disrupting the lattice symmetry.²⁸

As a consequence, hypothesizing that all Cu²⁺ ions locate at octahedral B sites, it is possible to give an estimate of cation distribution for Co_1−xCu_xFe₂O₄ NPs as (Cu_σFe_1−σ)_A[Co_1−x−σCu_xFe_1+σ]_B, where x is Cu²⁺ content and the value of σ can be determined by:

Here, assuming the recoilless fraction f_A and f_B to be same, the relative area ratio S_A/S_B thus directly corresponds to the ratio of the number of Fe³⁺ ions at tetrahedral A and octahedral B sites.²⁷ Based on the Mössbauer fitting data, the ratio S_A/S_B for the x = 0 sample is 0.876, thus the cation distribution can be written as (Co_0.066Fe_0.934)_A [Co_0.934Fe_1.066]_B, that is to say, 93.4% of Co²⁺ ions resides at octahedral B sites. Sawatzky et al.²⁹ reported that the ratio of octahedral Co²⁺ ions in CoFe₂O₄ depended on the heat treatment. They estimated that 96% and 79% of Co²⁺ ions presented in the slowly cooled and quenched CoFe₂O₄ NPs, respectively. When Cu²⁺ ions is doped in CoFe₂O₄, the ratio of S_A/S_B for x = 0.1 sample becomes 0.792. The cation distribution is represented as (Co_0.116Fe_0.884)_A[Co_0.784Cu_0.1Fe_1.116]_B, demonstrating that Cu²⁺ doping results in the relocation of small amount of Co²⁺ from B to A sites concomitantly with some Fe³⁺ ions migrated from A to B sites, although Cu²⁺ ions locate at the octahedral B sites. Further increasing Cu²⁺ content to 0.2 and 0.3, it is found that the concentration of Fe³⁺ ion in A and B sites almost unchanged (S_A/S_B = 0.779 for x = 0.2 and 0.776 for x = 0.3 sample), revealing that Cu²⁺ ions only replace octahedral Co²⁺ ions, and make no effect on Fe³⁺ distribution. The cation distribution can be written as (Co_0.124Fe_0.876)_A [Co_0.676Cu_0.2Fe_1.124]_B for the sample with x = 0.2, and (Co_0.126Fe_0.874)_A [Co_0.574Cu_0.3Fe_1.126]_B for the sample with x = 0.3.

From the 7th column of Table 1, we find that the half width at half maximum (FWHM) of A and B lines varies with Cu²⁺ content. In cubic spinel lattice, each A-site Fe³⁺ ion is surrounded by 12 nearest B-site ions neighbors and each B-site Fe³⁺ ion is surrounded by 6 nearest A-site ions neighbors, thus B-site Fe³⁺ is more sensitive to the change in the surrounding cation distribution than the A-site Fe³⁺ ions. According to the cation distribution, for the sample with x = 0, each Fe³⁺ ion in A and B sites is surrounded by approximately 6 nearest Fe³⁺ ions, therefore, the line width is comparable but relatively narrow. When Cu²⁺ ions are doped in CoFe₂O₄ lattice, some Fe³⁺ ions migrate from A to B sites, hence the A site Fe³⁺ ions get more nearest Fe_B³⁺ neighbors. This leads to a reduction in the total super-exchange strength of B-site Fe³⁺ ions while an increase in A-site Fe³⁺ ions.³⁰ Consequently, broadened B line and narrowed A line are observed in Co–Cu ferrite. In addition, Table 1 also presents the same increasing trend of the hyperfine field (H_in) for tetrahedral A and octahedral B sites with increasing Cu²⁺ content. The weighted average values of H_in are 48.7, 49.0, 49.3, and 49.2 T for the Co_1−xCu_xFe₂O₄ with x = 0, 0.1, 0.2, and 0.3, respectively. The increase in H_in can be attributed to the increasing crystallite size, since the fluctuation of magnetization vectors close to easy direction of magnetization can give rise to a size dependent magnetic hyperfine field.³¹

3.3 Magnetic properties analysis

Fig. 6a shows the magnetic hysteresis loops of Co_1−xCu_xFe₂O₄/SiO₂ samples measured at room temperature. Clearly, these loops show the typical characteristics of ferromagnetic materials. At the applied field intensity (15 kOe), saturation state cannot be reached yet, thus the saturation magnetization M_s was estimated by fitting the high-field part of the magnetization curves using the relation here H is the field strength, a and b are constant determined by the fitting procedure.³² The fitted M_s and the measured coercivity H_c are plotted as functions of Cu²⁺ content x in Fig. 6b. The M_s for the pure CoFe₂O₄ is 24.7 emu g⁻¹, which is close to the reported value for 10–15 nm CoFe₂O₄/SiO₂ (30–50% SiO₂) prepared sol–gel method.¹⁹ The low M_s for pure CoFe₂O₄ sample can be attributed to the existence of amorphous SiO₂ matrix, which modifies the magnetic behavior through minimizing the particle interactions between ferrite particles.^33,34 The value of M_s first increases to 34.3 emu g⁻¹ when Cu²⁺ content is 0.1, and then reduces to 27.1 emu g⁻¹ as Cu²⁺ content further increases to 0.3. Two factors are possibly responsible for the higher M_s values for Cu-doping CoFe₂O₄ comparing with pure CoFe₂O₄. For x = 0.1 sample, the Mössbauer analysis indicates that doping Cu²⁺ ions with magnetic moment 1 μ_B results in the migration of Fe³⁺ ions from tetrahedral A to octahedral B sites. This behavior leads to the magnetization of the octahedral B sites and hence the M_s increases.³⁵ For the samples with x = 0.2 and 0.3, more Cu²⁺ ions occupied B-sites decreases the B-sublattice magnetization, thereby the enhanced M_s can be attributed to the increasing crystallite sizes with Cu²⁺ content. Noted that the M_s of 34.3 emu g⁻¹ for the x = 0.1 sample is about 38.9% larger than pure CoFe₂O₄.

Fig. 6 (a) Hysteresis loops of Co_1−xCu_xFe₂O₄/SiO₂, (b) plot of M_s and H_c of samples as a function of Cu²⁺ content.

Considering the Neel' two sub-lattice collinear model of ferrimagnetism, the magnetic moment η^Neel_B per unit formula in Bohr magneton can be estimated by η^Neel_B = M_B(x) − M_A(x).³⁶ Assuming the magnetic moment of Fe³⁺, Co²⁺ and Cu²⁺ to be 5, 3 and 1 μ_B, respectively, then using the obtained cation distribution from Mössbauer analysis, the magnetic moments η^Neel_B are calculated and summarized in Table 2. Meanwhile, Table 2 also provides the magnetic moment η^obs_B determined by the fitted M_s using the following formula:³⁷ η^obs_B = (M_w × M_s)/5585, where M_w is the molecular weight of the ferrite. As Table 2 indicates, the calculated values of η^obs_B are smaller than that of η^Neel_B, which suggests Neel's collinear model is not suitable for the obtained samples. Moreover, there is a significant canted spin arrangement in B-sites, which enhances the B–B interaction and in turn decreases the A–B interaction. According to the Yafet and Kittel’s three sublattice model, the spin-canting angle θ_YK (Yafet–Kittle angle) is calculated by:³⁸

Table 2 Magnetic parameters of Co_1−xCu_xFe₂O₄/SiO₂ at room temperature

Sample	Ms (emu g^−1)	Mr (emu g^−1)	Hc (Oe)	η^obsB (μB)	η^NeelB (μB)	θYK (degree)
x = 0	24.7	7.6	1525	1.48	3.26	38.6
x = 0.1	34.3	11.1	1430	2.06	3.37	33.6
X = 0.2	30.4	10.8	1405	1.83	3.10	33.1
X = 0.3	27.1	9.35	1265	1.63	2.91	33.4

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The results are given in Table 2. It should be noted that the values of θ_YK is 38.6° for x = 0.1 sample, comparable to the reported value for CoFe₂O₄/SiO₂ with 30% silica in ref. 39. However, the θ_YK decreases to ∼33° for Cu-doping ferrites (Table 2), which indicates the presence of Cu²⁺ ions at B sites reduces the degree of spin canting. Using high field Mössbauer spectra, Peddis et al.^39,40 confirmed that the spin canting mainly located in the octahedral B sites. Owing to the high anisotropy energy of Co²⁺ ions,⁴¹ the non-collinear canting spin mainly occurs in B-site Fe³⁺ magnetic moment.^42,43 The similar θ_YK values observed in as-prepared Co_1−xCu_xFe₂O₄ with 18–36 nm sizes indicate that the spin canting is not a surface phenomenon but an effect throughout the volume of the particles, including surface spin and core spin.⁴¹

On the other hand, the coercivity H_c decreases continuously from 1525 to 1265 Oe as Cu²⁺ doping content increases from 0 to 0.3. The change in H_c with Cu²⁺ content may be related to crystallite size, cation distribution, and magneto crystalline anisotropy constant. It is well known that the H_c of magnetic particle with single domain should increase with crystallite size in principle. In the present case, the average crystallite sizes of Co_1−xCu_xFe₂O₄ NPs are lower than the single domain critical size (40 nm) of CoFe₂O₄ NPs. Therefore, the decrease in H_c should be attributed to the cation distribution and magneto-crystalline anisotropy constant. Since Co²⁺ ion at octahedral B site has larger anisotropy (+850 × 10⁻²⁴ J per ion) than that at tetrahedral A site (−79×10⁻²⁴ J per ion),⁴⁴ the octahedral Co²⁺ ions can be responsible for the high magneto-crystalline anisotropy of CoFe₂O₄.^45,46 The replacement of octahedral Co²⁺ by Cu²⁺ ion results in the reduction in the percentage of Co²⁺ in B sites, and thus decreases the anisotropy constant.

4. Conclusions

To summarize, we have studied the effect of Cu²⁺ doping content on the microstructural and magnetic properties of Co_1−xCu_xFe₂O₄/SiO₂ (0 ≤ x ≤ 0.3) nanocomposites. Although all the obtained Co_1−xCu_xFe₂O₄ NPs have cubic spinel structure, the substitution of Cu²⁺ for Co²⁺ ions can bring change in the crystallite size, cation distribution, and magnetic properties. The crystallite size increases with Cu²⁺ doping content. The preferred occupancy of Cu²⁺ ions at octahedral B sites results in slight deformation of octahedral symmetry and Fe³⁺ ions migration from tetrahedral A to octahedral B sites. Moreover, the values of M_s and H_c are strongly dependent on Cu²⁺ doping content, which can be attributed to the cation migration between both sublattices (A and B). The relatively large spin-canting angle θ_YK reveals that the spin canting mainly occurs in the octahedral Fe³⁺ throughout the particles. The results suggest that the Cu²⁺ doping content in Co_1−xCu_xFe₂O₄ NPs can play an important role in tuning their physical properties, which may be of great significance in to exploit novel applications in high density information storage, electronic devices and biomedicine.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21371071), Foundation of Science and Technology of Jilin, China (Grant No. 201205075).

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