Co7Fe3(Co) composites

Journal Pre-proofs Simultaneous enhancements of magnetization and remanence in sufficiently exchange-coupled Co0.8Al0.2Nd x Fe2– x O4/Co7Fe3(Co) composites Qingzhao Li, Xuehang Wu, Shaoping Ye, Wenwei Wu, Jiuyang Xia, Kaiwen Zhou, Yizhong Huang PII: DOI: Reference:

S0304-8853(19)33345-1 https://doi.org/10.1016/j.jmmm.2019.166150 MAGMA 166150

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

22 September 2019 31 October 2019 13 November 2019

Please cite this article as: Q. Li, X. Wu, S. Ye, W. Wu, J. Xia, K. Zhou, Y. Huang, Simultaneous enhancements of magnetization and remanence in sufficiently exchange-coupled Co0.8Al0.2Nd x Fe2– x O4/Co7Fe3(Co) composites, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.166150

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Simultaneous

enhancements

remanence

in

of

sufficiently

magnetization

and

exchange-coupled

Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites Qingzhao Lia, Xuehang Wua,*, Shaoping Yea, Wenwei Wua,b,, Jiuyang Xiaa, Kaiwen Zhoub, Yizhong Huangc a

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University, Nanning 530004, PR China c Guangxi Zhuang Autonomous Region Center for Analysis and Test Research, Nanning 530022, PR China b

Abstract：Hard/soft magnetic Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites are synthesized by the ball-milling-assisted ceramic process, followed by 5% H2-Ar reduction. The effects of the reduction time on their structure and magnetic properties are studied at room temperature. XRD patterns indicate that Co0.8Al0.2NdxFe2–xO4 sample, calcined at 900 oC for 3 h, contains the main spinel CoFe2O4 phase in combination of a small amount of foreign Fe2O3 and/or FeNdO3 phases. After being reduced at 550 oC for 30 min, surface magnetic particles of spinel Co0.8Al0.2NdxFe2–xO4 ferrite samples are transformed from the hard magnetic Co0.8Al0.2NdxFe2–xO4 to the soft magnetic Co7Fe3(Co). The magnetic measurement suggests that the specific saturation magnetization and remanence of spinel Co0.8Al0.2NdxFe2–xO4 decrease with increasing Nd3+ content. After being reduced at 550 oC, specific saturation magnetization and remanence of the composites, Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co), increase markedly. The hysteresis loop of the composites shows a single-phase magnetization

*Corresponding author at: School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China. E-mail addresses: [email protected] (X. Wu), [email protected] (W. Wu), [email protected] (W. Wu). 1

behavior, implying that the magnetic hard (Co0.8Al0.2NdxFe2–xO4) phase and soft [Co7Fe3(Co) alloy] phase in Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites are well exchange-coupled, which is confirmed by the switching field distribution curves further. Keywords: Hard/soft magnetic composites, oxide materials, magnetic measurements, magnetic behavior, exchange–coupling

1. Introduction Since the concept of exchange-spring (ES) behavior was put forward in the early 1990s by Kneller et al. [1], exchange-spring systems has been paid great attention by researchers due to their intriguing applications in the improvement of magnetic properties [2–5]. In the exchange-spring magnet, the composites will possess both large coercivity (Hc) of the hard phase and high specific saturation magnetization (Ms) of the soft phase simultaneously, resulting in high the maximum energy product (BH)max, which behaves both a much smaller switching field and thermal stability – a property that is vital for information storage [6,7]. In the last few decades, a lot of effort has been made in order to obtain materials with a high magnetic energy product. It was found that alloy composites-based permanent magnets like FePt, Nd-Fe-B, and Sm-Co [8–10] and some ferrite composites [11–13] exhibited high magnetic energy product. Due to the high cost of alloy-based permanent magnets, ferrite composites with exchange–spring behavior have been considered to be candidate alternatives to alloy-based permanent magnets. At present, many ferrite composites with exchange-spring behavior

have

been

Ni0.5Zn0.5Fe2O4/SrFe12O19

synthesized,

including

[16,17],

Ni0.5Zn0.5Fe2O4/BaFe12O19

Ni0.6Zn0.4Fe2O4/SrFe12O19

[14,15], [18,19],

(Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x [20], BaFe12O19/CoFe2O4 [11,21], CoFe2O4/Fe3O4 [22], 2

BaFe12O19@Fe3O4 BaFe12O19/Y3Fe5O12

[23],

BaFe12O19@Co3O4 [26]

[24],

BaFe12O19/NiFe2O4

Mn0.6Zn0.4Fe2O4/Sr0.85Ba0.15Fe12O19

[25], [27],

Li0.3Co0.5Zn0.2Fe2O4/SrFe12O19 [28], and SrFe12O19/ZnFe2O4 [29], et al. Studies showed that the larger the contact area between particles of

two magnets, the stronger the exchange

coupling effect. Hard/soft magnetic composites with core/shell structure can increase two phases contacted sufficiently. Among ferrites, spinel cobalt ferrite (CoFe2O4) is a good hard magnetic phase for studying the exchange-coupling behavior owing to its moderate specific saturation magnetization, large magnetocrystalline anisotropy and coercivity. In recent years, CoFe2O4/CoFe2 [30–33] and Co0.5LaxFe2.5–xO4/Fe7Co3(Co) [34] composites as a new type of nanocomposites have attracted much attention because they behave strong exchange-spring (ES) behavior and can be easily prepared by reducing cobalt ferrites. ES property of CoFe2O4/CoFe2 and Co0.5LaxFe2.5–xO4/Fe7Co3(Co) composite powders strongly depends on the synthesis conditions and powder composition. Therefore, it is worthy of researching on this topic further. In this paper, we prepared Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites by reducing Co0.8Al0.2NdxFe2–xO4 in 5% H2-Ar at 550 oC. The effects of the reduction time on the structure and magnetic properties of the composites were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Exchange-coupled effect between magnetic hard (Co0.8Al0.2NdxFe2–xO4) phase and soft [Co7Fe3(Co) alloy] is confirmed by the switching field distribution curves.

2. Experimental Synthesis of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites includes two parts. The first is 3

a ball-milling-assisted ceramic process, described in detail elsewhere [34], to generate Co0.8Al0.2NdxFe2–xO4 particles, and the second is a reduction process to partially reduce Co0.8Al0.2NdxFe2–xO4 particles into Co0.8Al0.2NdxFe2–xO4/Fe7Co3(Co) composites. Analytical grade CoC2O4·2H2O, Al(OH)3, Nd2(C2O4)3·10H2O, and FeC2O4·2H2O (purity > 99.7%) were used as raw materials and 5% H2-Ar was used as reduction gas. In brief, ethanol and raw materials were mixed at first, and then milled in a stainless steel ball-milling tank of 100 mL for 40 min with an angular velocity of 350 rpm and a ball-to-power weight ratio of about 15:1. The mixed powder was dried at 75 oC in air for 4 h in an oven. After drying, the mixture was calcined at 900 oC for 3 h in a muffle at a heating rate of 3 oC/min and cooled in the furnace down to room temperature to obtain cubic Co0.8Al0.2NdxFe2–xO4 particles. Co0.8Al0.2NdxFe2– xO4/Fe7Co3(Co)

composites were obtained by reducing Co0.8Al0.2NdxFe2–xO4 at 550 oC using

5% H2-Ar as reduction gas. To control the reduction level of Co0.8Al0.2NdxFe2–xO4 particles, same flow speed of reducing gas was used. Table 1 shows stoichiometry of chemicals used in the synthesis of Co0.8Al0.2NdxFe2–xO4. Table 1 Stoichiometry of chemicals used in the synthesis of Co0.8Al0.2NdxFe2–xO4. Quantity (mol) Composition, x

CoC2O4·2H2O

Al(OH)3

Nd2(C2O4)3·10H2O

FeC2O4·2H2O

0.00 0.04 0.08 0.12

0.04206 0.04142 0.04075 0.04019

0.01051 0.01036 0.01020 0.01005

0.00000 0.00104 0.00204 0.00301

0.10515 0.10147 0.09791 0.09445

The measurement of X-ray diffraction (XRD) patterns was carried out by X′pert PRO X-ray diffractometer using Cu-Kα radiation (λ = 0.15406 nm) in a 2θ range of 10-80o with a step size of 0.010o. Crystallite size of the sample is calculated using the Scherrer’s formula. Scanning electron microscopy (SEM) was used to analyze the morphology and particle size. 4

The Fourier transform infrared (FT-IR) spectra of samples were recorded using a spectrometer (Nexus 470) in the wavelength range of 200–1200 cm−1 with a resolution of 0.96 cm−1. Magnetic properties were checked at room temperature using a vibrating sample magnetometer (VSM, Lake Shore 7410) in an applied field up to 20 kOe. The specific saturation magnetization (Ms), coercivity (Hc), remanence (Mr), and squareness (Mr/Ms) were obtained from magnetization–hysteresis loops.

3. Results and discussions 3.1. Structural analysis Fig. 1a shows the XRD patterns of the Co0.8Al0.2NdxFe2–xO4 (0.00 ≤ x ≤ 0.12) samples calcined at 900 oC for 3 h. The XRD patterns of the Co0.8Al0.2Fe2O4 sample, calcined at 900 oC,

show the presence of cubic CoFe2O4 and α-Fe2O3 impurity phases. By contrast, after

being doped Nd3+, α-Fe2O3 and FeNdO3 phases are observed in addition to cubic CoFe2O4 phase. Diffraction peak intensity of FeNdO3 phase increases with the increase in Nd3+ content. Fig. 1b shows the local magnification of XRD patterns. The diffraction peak (311) is found to be shifted towards lower angles. The shifting of the peak to the lower angle side is attributed to the fact that the ionic radius of Nd3+ ion (0.099 nm) [35] is larger than that of the Fe3+ ion (0.067 nm) [35], resulting in increase in the lattice parameter. In other words, Nd3+ ions successfully replace the Fe3+ ions in all the samples. The diffraction peak intensity of FeNdO3 impurity phase increases with the increase in Nd3+ content, attributed that the ionic radius of Nd3+ ion is much larger than that of the Fe3+ ion and only a very limited amount Nd3+ ion can enter the lattice of ferrite, resulting in redundant Nd3+ ions form FeNdO3 phase with Fe3+ ions. XRD patterns of the sample were refined by the Rietveld analysis using MDI Jade (ver. 5.0) 5

software. Values of lattice constant and other structural parameters are extracted (listed in Table 2). From Table 2, it is found that the lattice constant (a) of Co0.8Al0.2NdxFe2–xO4 increases after doping Nd3+ ions, attributed that the replacement of Fe3+ ions in octahedral B sites by Nd3+ ions with larger ionic radius would cause the expansion of the unit cell, resulting in larger lattice constants. Similar phenomenon was also observed for CoFe2–xNdxO4 [36]. In this study, lattice constant a [0.83619(3) nm] of Co0.8Al0.2Fe2O4 sample is smaller than that (0.83861 nm) of Co0.5Fe2.5O4 prepared by the ball-milling-assisted ceramic process [34] and that (0.83861 nm) of CoFe2O4 prepared by the conventional solid state chemical reaction method [37]. This is because the ionic radius of Al3+ ion (0.051 nm) [38] is smaller than that of the Fe3+ ion (0.067 nm) [35]. Fig. 2 shows XRD patterns of the Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites reduced under 5% H2-Ar flux for 20 and 30 min at 550 oC. At a doping Nd3+ content of x = 0 and 0.04, characteristic diffraction peaks of Co7Fe3 and Co phases appear. For a higher doping level (x = 0.08 and 0.12) and reduction time of 30 min, a characteristic diffraction peak of CoO at 44o appears in addition to those of Co7Fe3 and Co. Besides, the strength of the peaks of Co7Fe3 phase increases obviously with increasing the reduction time and Nd3+ content, indicating a great increase in the amount of Co7Fe3 phase, while those of Co phase weakens with increasing Nd3+ content, suggesting that amount of Co phase decreases with the increase in Nd3+ content. The evolution of crystallite size and crystallinity of Co0.8Al0.2NdxFe2–xO4 as a function of the Nd3+ content (x) is shown in Fig. 3 and Table 2, respectively. The trend of crystallite size of Co0.8Al0.2NdxFe2–xO4 increases with the increase in Nd3+ content (Fig. 3a). The evolution of the crystallite size of Co0.8Al0.2NdxFe2–xO4 with Nd3+ content (x) can be explained as follows: 6

The bond energy of Nd3+–O2– is much smaller than that of Fe3+–O2– due to the larger ionic radius of Nd3+ ion than that of Fe3+ ion. When Nd3+ ion enters the cubic lattice to form the Nd3+–O2– bond, the crystal nucleation and growth of Nd3+ substituted Co0.8Al0.2Fe2O4 will consume less energy, resulting in the increases of crystallite size with the increase in Nd3+ content (x) [39]. By contrast, after being reduced, crystallite size of Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composites markedly decrease, attributed that surface particles of spinel

Co0.8Al0.2NdxFe2–xO4 samples are reduced to metal phase Co7Fe3(Co). Crystallinity of Co0.8Al0.2NdxFe2–xO4 decreases with the increase in Nd3+ content (Fig. 3b). This is because the ionic radius of Nd3+ ion is much larger than that of Fe3+ ion. The amount of Fe3+ ions replaced by Nd3+ ions is limited, resulting in the redundant Nd3+ ions to form FeNdO3 phase. The density and lattice strains of Co0.8Al0.2NdxFe2–xO4 vary nonlinearly with the increase in Nd3+ content.

Fig. 1. XRD patterns of Co0.8Al0.2NdxFe2–xO4 (0.00 ≤ x ≤ 0.12) calcined at 900 oC for 3 h (a) and local magnification (b).

7

Fig. 2. XRD patterns of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites (0.00 ≤ x ≤ 0.12) reduced at 550 oC. Table 2 Structural parameters of Co0.8Al0.2NdxFe2–xO4 calcined at 900 oC. Composition, x

a (nm)

Crystallite size (nm)

Density (g·cm–3)

Lattice strains (%)

0.00

0.83619(3)

93.74 ± 2.34

5.330(4)

0.1087(9)

0.04

0.83862(6)

100.81 ± 2.52

5.284(2)

0.1023(2)

0.08

0.83725(4)

99.22 ± 2.48

5.310(2)

0.1089(4)

0.12

0.83870(5)

106.93 ± 2.67

5.282(7)

0.1023(3)

Fig. 3. Effect of Nd3+ content (x) on crystallite size (a) and crystallinity (b) of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. 8

3.2. SEM and FT-IR analyses of samples SEM images of the samples, calcined at 900 oC, are presented in Fig. 4. As can be observed, calcined products show approximate spherical shape morphology, and there is a soft agglomeration phenomenon in the particles of Co0.8Al0.2NdxFe2–xO4 sample, attributed to a long range magnetic dipole-dipole particles interaction [40]. Samples contain particles having a distribution of small particles (70–150 nm) and large particles (150–600 nm). Besides, Nd3+ content does not have significant effect on the particle size and morphology of the samples. SEM images of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) particles reduced by Ar/H2 (95/5 vol%) at 550 oC for different reduction time are shown in Fig. 5. It can be seen that the Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) particles are roughly spherical and highly agglomerated. Particle size of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites is slightly smaller than that of Co0.8Al0.2NdxFe2–xO4 obtained at 900 oC.

9

Fig. 4. SEM images of Co0.8Al0.2NdxFe2–xO4 (0.00 ≤ x ≤ 0.12) calcined at 900 oC.

10

Fig. 5. SEM images of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites (0.00 ≤ x ≤ 0.12) reduced at 550 oC for different time. Fig. 6a-b depicts FT-IR spectra of Co0.8Al0.2NdxFe2–xO4 and its reduced product, respectively. For Co0.8Al0.2NdxFe2–xO4 sample (Fig. 6a), the band observed at 325 cm–1 (ν2) is attributed to the vibration of Fe-O, Co-O, and Al-O in the octahedral position. The band at 538 cm−1 (ν1) is attributed to the vibration of Fe-O, Co-O, and Al-O in the tetrahedral position [36,41], which supports the formation of the cubic spinel structure for all the samples [34]. The absorption band ν1 is greater than ν2, attributed to the shorter bond length of tetrahedral cluster and longer bond length of octahedral cluster [36]. It can be seen that there is no obvious difference in infrared spectra of samples with different Nd3+ content, indicating that 11

amount of Fe3+ ions substituted by Nd3+ ions is very limited. Similar FT-IR spectra was also observed for Co0.5LaxFe2.5–xO4 obtained using the by the ball-milling-assisted ceramic process [34] and CoFe2–xNdxO4 synthesized by starch-assisted sol-gel auto-combustion method [36]. FT-IR spectra of samples reduced at 550 oC for 30 min, Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co), are shown in Fig. 6b. It can be observed that the absorption bands at 325 and 538 cm−1 disappear or weaken, attributed that the surface particles of spinel cobalt-based ferrites are reduced to metal phase, which has been confirmed by XRD analysis.

Fig. 6. FT-IR spectra of Co0.8Al0.2NdxFe2–xO4 (a) and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites (b).

3.3. Magnetic properties Fig. 7 and Fig. 8 show the hysteresis loops of the Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. It can been observed that the hysteresis loops of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites show a single-phase magnetization behavior, implying that the magnetic hard (Co0.8Al0.2NdxFe2–xO4) phase and soft [Co7Fe3(Co) alloy] phase in Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites are well exchange–coupled. Their magnetic parameters are shown in Figs. 9, 10, 11, and Table 3, respectively. 12

Fig. 7. Magnetic hysteresis loops for Co0.8Al0.2NdxFe2–xO4 (0.00 ≤ x ≤ 0.12).

Fig. 8. Magnetic hysteresis loops for Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites (0.00 ≤ x ≤ 0.12).

Fig. 9. Effect of Nd3+ content (x) on specific saturation magnetization (Ms) (a) and remanence (Mr) (b) of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. 13

From Fig. 9a, it can be seen that the specific saturation magnetization of Co0.8Al0.2NdxFe2–xO4 markedly decreases with the increase in Nd3+ content. The evolution in specific saturation magnetization of the as-prepared ferrites can be explained as follows: The magnetic moment of per ion for Co2+, Fe2+, Fe3+, Al3+ and Nd3+ ions is 3 μB, 4 μB, 5 μB, 0 μB, 3 μB, respectively [34,35,42]. When part of Fe3+ ions in Co0.8Al0.2Fe2O4 are replaced by Nd3+ ions, Nd3+ ions (0.099 nm) [35] with larger ionic radius prefer the octahedral site occupation (B-site), resulting in the decrease in the net magnetic moment of Co0.8Al0.2NdxFe2–xO4 in octahedral sites (B-site) and specific saturation magnetization. By contrast, after being reduced at 550 oC for 20 min, the specific saturation magnetization of composites is markedly improved, but slowly decreases with the increase in Nd3+ content. When reduction time increases to 30 min, the specific saturation magnetization of composites varies nonlinearly with the increase in Nd3+ content. Besides, reduced product, Co0.8Al0.2Nd0.04Fe1.96O4/Co7Fe3(Co) reduced for 20 min, has the highest specific saturation magnetization value (78.65 emu/g), which is higher than that of Co0.5Fe2.5O4 (78.48 emu/g) synthesized by ball-milling-assisted ceramic process [34], that of CoFe2O4 (about 72.13 emu/g) synthesized by auto-combustion method [43], and that of CoFe2–xNdxO4 (0 ≤ x ≤ 0.10) (between 12.85 and 18.21 emu/g) synthesized by starch-assisted sol-gel auto-combustion method [36]. Co0.8Al0.2Nd0.04Fe1.96O4/Co7Fe3(Co) composites

behave

higher

specific

saturation

magnetization

compared

with

Co0.8Al0.2Nd0.04Fe1.96O4 and other cobalt ferrite [36,43], implying strong exchange-coupling between magnetic hard (Co0.8Al0.2NdxFe2–xO4) phase and soft [Co7Fe3(Co) alloy] phase. Fig. 9b shows effect of Nd3+ content (x) and reduction time on remanence (Mr) of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. Remanence (Mr) of 14

all samples decreases with the increase in Nd3+ content except for Co0.8Al0.2Fe2O4/Co7Fe3(Co) reduced for 20 min. Co0.8Al0.2Fe2O4/Co7Fe3(Co), reduced at 550 oC for 30 min, has the highest remanence value (29.16 emu/g). Fig. 10a shows effect of Nd3+ content (x) and reduction time on coercivity (Hc) of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. Coercivity of Co0.8Al0.2NdxFe2–xO4 increases after doping a small amount of Nd3+ (x = 0.04), and then linearly decreases with the increase in Nd3+ content. Coercivity of Co0.8Al0.2NdxFe2–xO4 decreases with the increase in Nd3+ content, attributed that the crystallite size of Co0.8Al0.2NdxFe2–xO4 increases with the increase in Nd3+ content and the coercivity is inversely proportional to the crystallite size of the material [34,44,45]. By contrast, after being reduced, coercivity of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites decreases, attributed that the formation of soft magnetic phase Co7Fe3(Co) with low coercivity in Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composites. The existence of soft magnetic phase Co7Fe3(Co) plays an

important role in the coercivity of ferrite. Co0.8Al0.2Nd0.04Fe1.96O4 has the highest coercivity value (1414.89 Oe).

Fig. 10. Effect of Nd3+ content (x) on coercivity (Hc) (a) and Mr/Mr (b) of Co0.8Al0.2NdxFe2– xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. Squareness (Mr/Ms) of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) 15

composites is shown in Fig. 10b. Mr/Ms value of Co0.8Al0.2NdxFe2–xO4, calcined at 900 oC, decreases with the increase in Nd3+ content. After being reduced, Mr/Ms value of composites markedly decreases. Based on the fact that Mr/Ms values of all samples are less than 0.5, the as-synthesized

Co0.8Al0.2NdxFe2–xO4

and

its

xO4/Co7Fe3(Co),

are multi-domain structures [34].

reduction

products,

Co0.8Al0.2NdxFe2–

Fig. 11. Effect of Nd3+ content (x) on effective anisotropy constants of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites. The effective anisotropy constant (Keff) of samples is calculated by the following formula [34]: K eff =

Hc  Ms 0.985

(1)

Fig. 11 shows the effect of Nd3+ content and reduction time on the effective anisotropy constant (Keff) of Co0.8Al0.2NdxFe2–xO4 and its reduction products. The results show that the Keff of all sample increases after doping a small amount of Nd3+ (x = 0.04), and then decreases with the increase in Nd3+ content. Besides, after being reduced, Keff value of Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composites decreases. Co0.8Al0.2Nd0.04Fe1.96O4 has the highest Keff value

(83011.67 erg/g).

16

Table 3 Magnetic properties of Co0.8Al0.2NdxFe2–xO4 and Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co). Composition, x

Time (min)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

Mr/Ms

ƞB (µB)

Keff (erg/g)

0.00

180

63.52

28.85

1156.66

0.4542

2.596

74589.89

0.04

180

57.79

24.03

1414.89

0.4158

2.398

83011.67

0.08

180

55.05

22.96

1285.78

0.4171

2.319

71860.09

0.12

180

50.48

20.60

1156.66

0.4081

2.159

59277.36

0.00

20

75.36

25.63

853.24

0.3401

65279.36

0.00

30

77.08

29.16

752.10

0.3783

58854.69

0.04

20

78.65

26.89

911.34

0.3419

72768.42

0.04

30

72.96

27.05

853.24

0.3708

63200.40

0.08

20

70.13

24.57

939.32

0.3503

66877.68

0.08

30

72.93

24.21

825.26

0.3320

61102.75

0.12

20

72.26

20.31

681.08

0.2811

49964.31

0.12

30

70.98

21.73

795.14

0.3061

57298.51

Calcine at 900 oC

Reduce at 550 oC

Intergranular exchange-coupling effect between hard and soft phases in Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composites is evaluated by the switching field distribution curve further,

which is the 1st derivative of the demagnetization curve (dM/dH of demagnetization curve). Fig. 12a and Fig. 12b show the demagnetization curves of magnetic hysteresis loops of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites and plots of dM/dH versus H, respectively. Plots of dM/dH versus H at room temperature (Fig. 12b) show a single peak at about –992 Oe for all composites, confirming the excellent exchange-coupling for all the Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composites. The stronger intensity of the peak shows the higher degree of

exchange-coupling

between

the

hard

and

soft

magnetic

phases

[28,46].

So,

Co0.8Al0.2Nd0.04Fe1.96O4/Co7Fe3(Co) composite has the highest degree of exchange-coupling.

17

Fig. 12. (a) Demagnetization curves and (b) dM/dH of demagnetization curves of Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites.

4. Conclusions Hard/soft magnetic Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co) composites are synthesized by a two-step process, including ball-milling-assisted ceramic process and reduction process in 5% H2-Ar. The effects of the reduction time on their structure and magnetic properties are studied at room temperature. XRD patterns indicate that Co0.8Al0.2NdxFe2–xO4 sample, calcined at 900 oC

for 3 h, contains the main spinel CoFe2O4 phase in combination of a small amount of

foreign Fe2O3 and/or FeNdO3 phases. After being reduced at 550 oC for 30 min, surface magnetic particles of spinel Co0.8Al0.2NdxFe2–xO4 ferrite samples are transformed from the hard magnetic Co0.8Al0.2NdxFe2–xO4 to the soft magnetic Co7Fe3(Co). The magnetic measurement suggests that the specific saturation magnetization and remanence of spinel Co0.8Al0.2NdxFe2–xO4 decrease with increasing Nd3+ content. By contrast, after being reduced at 550

oC,

specific saturation magnetization and remanence of the composites,

Co0.8Al0.2NdxFe2–xO4/Co7Fe3(Co), increase markedly. The hysteresis loop of the composites shows a single-phase magnetization behavior, implying that the magnetic hard (Co0.8Al0.2NdxFe2–xO4) phase and soft [Co7Fe3(Co) alloy] phase in Co0.8Al0.2NdxFe2– 18

xO4/Co7Fe3(Co)

composites are well exchange-coupled, which is confirmed by the switching

field distribution curves further. Co0.8Al0.2Nd0.04Fe1.96O4/Co7Fe3(Co) composite, reduced at oC

550

for

20

min,

has

the

highest

degree

of

exchange-coupling.

Co0.8Al0.2Nd0.04Fe1.96O4/Co7Fe3(Co) composite has higher specific saturation magnetization, moderate values for the coercivity and remanence, which make Co0.8Al0.2NdxFe2– xO4/Co7Fe3(Co)

composite a promising candidate for applications such as high density data

storage devices and microwave-absorbing materials.

Acknowledgements This study was financially supported by Key Program Projects of Research and Development of Guangxi (Grant no. AB19110024), Open Foundation of Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University (Grant no. 2019GXYSOF05).

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