Enhancement of magnetic properties in hard/soft CoFe₂O₄/Fe₃O₄ nanocomposites

Abstract

CoFe₂O₄/Fe₃O₄ nanocomposites with different concentrations of soft ferrite (Fe₃O₄) were fabricated using a co-precipitation method. Crystal structure, phase composition, morphology and magnetic properties of as-synthesized nanocomposites were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), Mössbauer spectroscopy (MS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). Results from XRD and MS reveal that the nanocomposites are composed of two phases (CoFe₂O₄ and Fe₃O₄), both of which have a spinel structure. SEM and TEM show that the average particle size of the nanocomposites is about 20 nm. VSM indicates that all hysteresis loops exhibit a single-phase-like behaviour. The remanence ratio (M_r/M_s), coercivity (H_c) and maximum magnetic energy product (BH)_max are significantly enhanced in all the nanocomposites compared with pure hard ferrite (CoFe₂O₄). The enhancement of the magnetic properties may be attributed to the exchange coupling interaction in the system.

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1. Introduction

Hard–soft magnetic composites have attracted great scientific and technological interest in recent years due to their many applications such as in microwave devices, permanent magnets, and data-storage systems.^1–5 They have excellent performance with respect to magnetic properties, which is based on an exchange spring mechanism. Kneller and Hawig first proposed the concept of the exchange spring in hard–soft composite magnets in the early 1990s.⁶ According to this concept, the exchange coupled composites can combine high saturation magnetization of the soft magnetic phase with large coercivity of the hard magnetic phase, which is superior to those of single-phase magnets. Hence, they exhibit large maximum magnetic energy products (BH)_max. Previous studies of the exchange spring behaviour have mainly focused on metallic systems especially in Nd–Fe–B and Sm–Co based composites with high (BH)_max.^7–14 But the cost of preparation is pretty high and corrosion resistance is poor.^15,16 In contrast, nanocomposites consisting of hard magnetic ferrite and soft magnetic ferrite can overcome these drawbacks to some extent and therefore are considered as promising advanced permanent magnetic materials.^17,18 Song et al. synthesized SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite microfibers with high saturation magnetization and large coercivity by sol–gel process.¹⁹ Lin et al. fabricated BaFe₁₂O₁₉/Y₃Fe₅O₁₂ composites with giant enhancement of (BH)_max by microwave sintering.²⁰ Poorbafrani et al. have observed the exchange spring behaviour in the Co_0.6Zn_0.4Fe₂O₄/SrFe_10.5O_16.75, achieving an enhancement of 53% in (BH)_max compared with the hard phase (SrFe_10.5O_16.75).²¹ Fei et al. prepared CoFe₂O₄/Fe₃O₄ nano-composite ceramics by spark plasma sintering (SPS) to obtain the effective exchange coupling when sintering temperature reached 500 °C.²² Fan gained CoFe₂O₄/Fe₃O₄ with different weight ratios by SPS at 500 °C and found that the remanence and coercivity decreased with the increase of the soft phase in these composites.²³ It was proposed that the decreases of remanence and coercivity were caused by dipolar interaction and appearance of magnetic vortex state. However, some recently reported synthesis methods of CoFe₂O₄/Fe₃O₄ composites require high sintering temperature and this may lead to an increased grain size, and deterioration of the exchange coupling between the soft and hard phases.^24,25 On the contrary, a good chemical homogeneity and narrow size distribution of nanoparticles can be attained by co-precipitation method.^26,27 Besides, to our best knowledge, few investigations on the enhancement of (BH)_max of CoFe₂O₄/Fe₃O₄ composites have been reported.

In this work, hard–soft CoFe₂O₄/Fe₃O₄ nanocomposites have been synthesized by co-precipitation method. Significant enhancements of M_r/M_s ratio, H_c and (BH)_max have been achieved in the CoFe₂O₄/Fe₃O₄ nanocomposites compared with single hard ferrite (CoFe₂O₄).

2. Experimental

2.1. Materials

Cobalt chloride (CoCl₂·6H₂O, 99.0%), ferric chloride (FeCl₃·6H₂O, 99.0%), ferrous sulfate (FeSO₄·7H₂O, 99.0%), and sodium hydroxide (NaOH, 99.9%) were purchased from Sinopharm Reagent Co., Ltd. All chemicals were of analytical grade and used without any further purification.

2.2. Preparation of CoFe₂O₄/Fe₃O₄ nanocomposites

CoFe₂O₄/Fe₃O₄ nanocomposites were prepared by conventional co-precipitation method.²⁸ Firstly, different mass ratios of CoCl₂·6H₂O, FeCl₃·6H₂O and FeSO₄·7H₂O were dissolved into deionized water with constant stirring. Then, different amounts of NaOH were added slowly into the obtained solution. And the solution was kept at 90 °C for three hours under vigorous stirring. Finally, the precipitates were thoroughly washed with deionized water, and dried at 80 °C for 24 hours to obtain the final samples. All samples with different labels are shown in Table 1. The pure CoFe₂O₄ and Fe₃O₄ were mechanically mixed with a mass ratio of 1 : 0.25. This sample is named as mix-CFO-FO-1.

Table 1 Mass ratios for different CFO/FO nanocomposite samples

Samples	Composition (mass ratio)
CFO	Pure CoFe2O4
CFO-FO-1	CoFe2O4/Fe3O4 = 1 : 0.25
CFO-FO-2	CoFe2O4/Fe3O4 = 1 : 1
CFO-FO-3	CoFe2O4/2Fe3O4 = 1 : 2
FO	Pure Fe3O4
Mix-CFO-FO	CoFe2O4/Fe3O4 = 1 : 0.25

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2.3. Characterization

Structural characterization was performed by X-ray diffraction (XRD; Bruker D8, GER). Morphological characterization of all samples was carried out using transmission electron microscopy (TEM, JEM-2100F, JPN) and field emission scanning electron microscopy (FE-SEM, JSM-7600F, JPN). The magnetic properties were measured by physical property measurement system (PPMS, PPMS-9T, USA) under an applied magnetic field in the range from −3 to +3 T at 77 K. The ⁵⁷Fe Mössbauer spectrum (MS; Wissel 550, GER) were measured at room temperature using ⁵⁷Co as the γ-ray source. The velocity scale was calibrated using an α-Fe metal foil at room temperature. All the Mössbauer spectra were fitted with Lorentzian lines via the least squares method. Elemental analysis of the samples was carried out using a sequential viewed in inductively coupled plasma optical emission spectrometer (ICP; PE Optima 7000, USA).

3. Results and discussion

3.1. Structural studies

Fig. 1 shows the XRD patterns of the CFO/FO nanocomposites. All the observed peaks of samples come close to the characteristic peaks in the JCPDS cards of cobalt ferrite (no. 01-1121) or magnetite (no. 75-0449). The crystallite size (D) was estimated according to Scherrer's equation:

D = kλ/β cos θ

where D, k, λ, θ and β are grain size in nm, Scherrer constant of 0.89, wavelength of X-ray radiation (1.5406 Å), Bragg diffraction angle and full width at half maximum of the (311) peak, respectively, the calculated average grain sizes of the CFO and FO are 23.2 and 19.8 nm, respectively.

Fig. 1 XRD patterns of CFO/FO nanocomposites with different concentrations of FO.

3.2. Morphology analyses

Fig. 2 displays SEM and TEM micrographs of the CFO/FO samples. CFO/FO nanocomposites are composed of near-spherical nanoparticles except a small number of large particles with square-like shape. SEM and TEM images show no obvious morphology difference in CFO-FO-1, CFO-FO-2, CFO-FO-3. The average nanoparticle size in all the samples is about 20 nm by SEM and TEM images, which is consistent with the results determined by XRD.

Fig. 2 SEM and TEM images of all samples: (a and f) CFO, (b and g) CFO-FO-1, (c and h) CFO-FO-2, (d and i) CFO-FO-3, (e and j) FO.

3.3. Mössbauer analyses

Fig. 3 presents the Mössbauer spectra of CFO/FO nanocomposites. The black dots and red solid lines represent the experimental data points and least square fit of the spectra, respectively. The parameters extracted from the fitting of spectra are listed in Table 2. The spectra of the samples CFO and FO both consist of three sextets, and samples CFO-FO-1, CFO-FO-2 and CFO-FO-3 are composed of five sextets. As shown in Table 2, the two magnetic sextets of 48.51–49.15 and 45.28–46.44 T match perfectly with the reported data of magnetite with typical spinel crystal structure.²⁹ The B_hf values of (A) and (B) patterns in all samples are within 47.51–48.91 and 41.47–43.86 T, respectively. And this can be assigned to the spinel structure of cobalt ferrite.³⁰ The relaxed spectra of C1, C2, C3, C4 and C5 are owing to small particle size distribution.^31,32 In addition, no other impurities in all the samples such as Fe₂O₃ are observed.³³ The co-existence phase of CFO and FO in the nanocomposites is confirmed from the results of Mössbauer spectra. This is well consistent with the results of XRD. Moreover, ICP results are in good agreement with the designed composition of all the samples (Table 1). From Table 2, the B_hf values of subspectra (A, B) in FO and CFO are (48.51 T, 45.28 T) and (47.51 T, 43.49 T), respectively.

Fig. 3 Room temperature Mössbauer spectra of the nanocomposites: (a) CFO, (b) CFO-FO-1, (c) CFO-FO-2, (d) CFO-FO-3, (e) FO.

Table 2 Isomer shift (IS), quadrupole splitting (QS), line width (LWD), hyperfine field (B_hf) and subspectral area (S) for the CFO/FO nanocomposites

Samples	Subspectrum	IS (mm s^−1)	QS (mm s^−1)	LWD (mm s^−1)	Bhf (T)	S (%)	Phase composition
CFO	A1	0.31	0.00	0.28	47.51	42.5	CoFe2O4
	B1	0.30	0.03	0.45	43.49	44.4
	C1	−0.42	0.35	0.84	30.84	13.1
CFO-FO-1	A21	0.50	0.01	0.11	49.15	6.3	Fe3O4
	B21	0.31	0.00	0.24	46.44	16.8
	A22	0.27	0.01	0.21	48.91	32.8	CoFe2O4
	B22	0.36	0.12	0.48	43.86	33.7
	C2	0.61	−0.06	0.44	18.10	10.4
CFO-FO-2	A31	0.45	0.01	0.22	48.72	13.7	Fe3O4
	B31	0.36	0.00	0.38	45.72	33.7
	A32	0.25	0.02	0.17	48.55	22.3	CoFe2O4
	B32	0.45	0.19	0.51	41.47	24.0
	C3	0.08	0.04	0.55	22.45	6.3
CFO-FO-3	A41	0.20	0.06	0.16	48.81	16.8	Fe3O4
	B41	0.35	0.01	0.27	45.52	16.5
	A42	0.42	0.01	0.22	48.71	28.7	CoFe2O4
	B42	0.36	0.07	0.57	42.52	29.6
	C4	0.22	0.02	0.55	20.35	8.4
FO	A5	0.33	0.02	0.24	48.51	35.0	Fe3O4
	B5	0.51	0.07	0.49	45.28	59.1
	C5	−0.86	−0.62	0.51	29.83	5.9

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The values of magnetite in CFO-FO-1, CFO-FO-2 and CFO-FO-3 are (49.15 T, 46.44 T), (48.72 T, 45.72 T) and (48.81 T, 45.52 T), respectively. The B_hf values of cobalt ferrite phase are obtained in CFO-FO-1 (48.91 T, 43.86 T), CFO-FO-2 (48.55 T, 41.47 T) and CFO-FO-3 (48.71 T, 42.52 T). The values of B_hf in CFO/FO nanocomposites increase compared with those in CFO and FO, which can be attributed to mutual exchange coupling between hard and soft magnetic phases at the interface.

3.4. Magnetic properties

M–H hysteresis loops of FO, CFO and CFO/FO nanocomposites at 77 K are shown in Fig. 4a. The corresponding remanence ratio (M_r/M_s), coercivity (H_c), and saturation magnetization (M_s) values are listed in Table 3. All the samples display ferromagnetic behaviour. The CFO and FO are correspondingly characterized with hard and soft magnetic materials, respectively (Fig. 4a). From Table 3, the H_c of CFO (6000 Oe) and FO (250 Oe) differs by orders of magnitude, which reveals intrinsic magnetic properties of hard and soft magnetic materials, respectively. The hysteresis loops of all the nanocomposites exhibit single-phase-like behaviour, although they consist of two phases. From Fig. 4a, the values of M_s in all CFO/FO nanocomposites are both increased compared with that in CFO (70 emu g⁻¹). Furthermore, the M_s of CFO-FO-1 sample reaches 91 emu g⁻¹, which is higher than that of FO (79 emu g⁻¹). The variations of M_r/M_s and H_c in CFO/FO nanocomposites with different compositions are shown in Fig. 4b and c, respectively. The M_r/M_s ratios of CFO-FO-1, CFO-FO-2 and CFO-FO-3 are 0.63, 0.64, and 0.60, respectively (Fig. 4b). Fig. 4c demonstrates that the H_c value of samples first increases with the concentrations of FO and reaches its maximum value of 7200 Oe when FO is 50 wt% and then decreases. It's also worth noting that the H_c of all the nanocomposites is much higher than that of the individual CFO (6000 Oe) and FO (250 Oe).

Fig. 4 (a) Magnetic hysteresis (M–H) loops, (b) variation of the M_r/M_s ratio, (c) variation of the H_c of CFO/FO nanocomposites with different concentrations of FO.

Table 3 The dependence of magnetic properties on the soft phase concentration in CFO/FO nanocomposites

Samples	Hc (Oe)	Ms (emu g^−1)	Mr/Ms
CFO	6000	70	0.58
CFO-FO-1	6450	91	0.63
CFO-FO-2	7200	73	0.64
CFO-FO-3	6700	78	0.60
FO	250	79	0.20

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Fig. 5a shows the magnetic hysteresis (B–H) loops of CFO, FO and CFO/FO nanocomposites. The magnetic hysteresis (B–H) loops were derived from the magnetic hysteresis (M–H) loops using B = μ₀H + 4πm.³⁴ And the corresponding values of (BH)_max were calculated according to the demagnetization parts of the (B–H) loops, as shown in Fig. 5b. It can be clearly observed that the values of (BH)_max for all the CFO/FO nanocomposites are much higher compared with that of the single hard phase (CFO). It is important to note that sample CFO-FO-1 exhibits a maximum (BH)_max value of 21.4 kJ m⁻³. A giant enhancement of 86% in (BH)_max can be achieved compared with that of CFO.

Fig. 5 (a) Magnetic hysteresis (B–H) loops, (b) variation of the (BH)_max with the concentrations of FO for CFO/FO nanocomposites.

Generally, exchange coupling interaction and dipolar interaction play a vital role for the determination of magnetic properties of the composite magnets.^6,35 The grain size of as-synthesized CFO/FO nanocomposites is about 20 nm from results of XRD, SEM and TEM, which is favorable for the strong exchange coupling interaction.³⁶ Moreover, the exchange coupling can be manifested by the high M_r/M_s ratios (>0.5) in all CFO/FO nanocomposites according to the Stoner–Wohlfarth theory (Fig. 4b).^6,37 In order to further illustrate the exchange coupling behaviour in CFO/FO nanocomposites, magnetic hysteresis loops at 77 K of sample mix-CFO-FO-1 and CFO-FO-1 are compared in Fig. 6. The sample CFO-FO-1 exhibits a single-phase-like behaviour. This is due to the fact that the hard (CFO) and soft (FO) magnetic phases are well exchange coupled to each other and the magnetization of both phases reverses cooperatively.^38,39 However, a typical “bee waist” type hysteresis loop is observed in sample mix-CFO-FO-1, which reveals that the two phases are independent of each other. There is no exchange coupling behaviour existing in the physically mixed sample. The enhanced magnetic properties (H_c, M_s and M_r) of the CFO/FO nanocomposites can be explained on the basis of the exchange-coupling interaction. All the CFO/FO nanocomposites show a single-phase-like behaviour, which suggests that the hard phase (CFO) and the soft phase (FO) are well exchange-coupled at intergranular.^38,39 In CFO/FO nanocomposites, three types of magnetic interaction between the soft and hard grains can be taken into consideration. The major one is the interaction between FO and CFO in the nanocomposites while other two are weak interaction between CFO and CFO or between FO and FO.⁴⁰ In the absence of the soft phase, direct coupling among hard grains is governed which tends to deviate magnetic moments from easy axis by magnetostatic energy. By inserting the soft grains among the grains of the hard phase, direct coupling of the hard grains decreases. Moreover, at the low concentrations of soft phase, the exchange interaction on the soft magnetic moments excreted by the hard phase is strong, which results in the increase of H_c. However, with the increasing concentrations of soft phase, the exchange force on the soft grains would be enervated. Therefore the reverse domains in the soft phase with low nucleation field could be nucleated easily, which leads to the decrease of H_c.^20,41 This behaviour is consistent with what observed in Fig. 4c for the variation of H_c. More interestingly, the M_r of the nanocomposites is higher than that of CFO (Fig. 4a). In CFO/FO nanocomposites, since the CFO has a high magnetocrystalline anisotropy, it is difficult to achieve magnetization reversal with a low applied field. However, the FO can be easily aligned in the direction of the applied field due to the smaller magnetocrystalline anisotropy energy compared to the CFO. When CFO and FO are sufficiently exchange coupled, the CFO will couple the magnetic moments in the FO by the strong exchange interaction and try to align in its own easy direction. So the moment direction in the nanocomposites becomes parallel and results in a higher value of magnetization.¹⁹ Besides, it is worth noting that all the H_c, M_r and M_r/M_s ratio are much higher than those of the single phase CFO for all CFO/FO nanocomposites (Fig. 4). It seems to imply that the exchange interaction is not suppressed by the dipolar interaction of the soft phase to some extent and is always dominating for the CFO/FO nanocomposites even with high FO concentration. This may be related to the grain size, particle shape, and distribution in the nanocomposites. So a giant enhancement of 86% in (BH)_max has been achieved, which is mainly due to the exchange spring behaviour. Based on the above analyses, the enhancement of (B_hf, H_c, M_r), large M_r/M_s ratio and a single-phase-like behaviour in all nanocomposite samples indicate that the hard and soft magnetic phases are well exchange coupled to each other. These results are significant for the design of oxide composite magnetic materials with high (BH)_max.

Fig. 6 Magnetic hysteresis (B–H) loops of sample CFO-FO-1 and mix-CFO-FO-1.

4. Conclusions

In conclusion, magnetic nanocomposites of CFO/FO were fabricated by co-precipitation method. XRD and Mössbauer results confirm that spinel cobalt ferrite and magnetite phases coexist in CFO/FO nanocomposites. The average grain size of CFO/FO nanocomposites is about 20 nm by XRD, SEM and TEM. Moreover, these nanocomposites exhibit an enhanced magnetic properties, mainly represented by the increases of H_c, M_r and M_r/M_s ratio compared with pure CFO. This enhancement can be attributed to the excellent exchange coupling of the hard phase (CFO) and the soft phase (FO). A giant 86% enhancement of (BH)_max can be achieved in exchange coupled CFO/FO nanocomposites in comparison with pure CFO, which will be of technological significance in the future.

Notes and references

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