Structure and magnetic properties evolution of cobalt–zinc ferrite with lithium substitution

Materials Science in Semiconductor Processing 41 (2016) 162–167

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Structure and magnetic properties evolution of cobalt–zinc ferrite with lithium substitution Yuan Zhou a, Xuehang Wu a, Wenwei Wu a,b,n, Xusheng Huang c, Wen Chen a, Yulin Tian a, Dan He a a

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China Guangxi Colleges and Universities Key Laboratory of Applied Chemistry Technology and Resource Development, Nanning 530004, PR China c Guangxi Zhuang Autonomous Region Metallurgical Products Quality Supervision and Test Station, Nanning 530023, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 June 2015 Received in revised form 13 August 2015 Accepted 14 August 2015

LixCo0.5Zn0.5
xFe2O4 (0.0 rx r 0.3) is obtained by calcining precursor oxalates at 900 °C in air. The precursor and its calcined products are characterized by thermogravimetry and differential scanning calorimetry, X-ray powder diffraction, scanning electron microscopy, and vibrating sample magnetometer. A high-crystallized LixCo0.5Zn0.5
xFe2O4 with a cubic structure is obtained when the precursor is calcined at 900 °C in air for 3 h. Lattice parameters decrease with the increase of Li þ addition amount. The magnetic properties of LixCo0.5Zn0.5
xFe2O4 depend on Li þ doped amount and calcination temperature. Li0.3Co0.5Zn0.2Fe2O4 obtained at 900 °C has the highest specific saturation magnetization value, 70.24 emu/g. However, Li0.3Co0.5Zn0.2Fe2O4 obtained at 800 °C has the highest remanence (8.29 emu/g) and coercivity value (97.8 Oe). & 2015 Elsevier Ltd. All rights reserved.

Keywords: Magnetic materials Chemical synthesis X-ray diffraction Magnetic properties

1. Introduction Cobalt ferrite (CoFe2O4) with inverse spinel structure is a wellknown hard magnetic material, which has many unique properties, such as high coercivity, large magnetocrystalline anisotropy, moderate specific saturation magnetization, high mechanical hardness, high chemical stability, and high Curie temperature (~520 °C). Thus, CoFe2O4 has been widely used as audio and videotape, high-density digital recording disks, magnetic separation, ferrofluids, catalysts, magnetic resonance imaging, gas sensor, and drug targeting [1–11], etc. The doped CoFe2O4 can improve its magnetic performance. Therefore, doped CoFe2O4 caused great concern. Various synthetic approaches have been pursued to prepare spinel CoFe2O4 and doped CoFe2O4 with different particle sizes and morphological features, including solid-state reaction at low temperatures [1–3], sol–gel synthesis [4,6,12–14], co-precipitation [5,15,16], hydrothermal treatment [7,17], ball milling method [8,18,19], ceramic method [20,21], citrate precursor method [22,23], solvothermal method [24,25], microwave combustion method [26,27], and polyol process [28]. The crystallite diameter, morphology, and crystalline phases of CoFe2O4 associated with its n Corresponding author at: School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China. Fax: þ 86 771 3233718. E-mail addresses: [email protected], [email protected] (W. Wu).

http://dx.doi.org/10.1016/j.mssp.2015.08.024 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

performances highly depend on the synthesis method, calcination temperature, and doping elements. Lopes-Moriyama et al. [29] synthesized nano-octahedral grains of cobalt ferrite (CoFe2O4) with size around 20 nm by a hydrothermal route. Jia et al. [24] synthesized Co1
xZnxFe2O4 nanorods by the solvothermal annealing method. Co1–xZnxFe2O4 has the maximal specific saturation magnetization value, 43.0 emu g
1. Singhal et al. [14] prepared Co0.5Zn0.5AlxFe2
xO4 nanoparticles via sol–gel route. The results showed that Co0.5Zn0.5Fe2O4 obtained at 1000 °C had the highest specific saturation magnetization value, 64.33 emu/g; Co0.5Zn0.5AlFeO4 had the highest coercivity value, 162 Oe. Although many researchers have made great efforts to prepare single phase Co1
xZnxFe2O4 with high performance, facile and scalable synthesis of Co1
xZnxFe2O4 with high specific saturation magnetization, higher coercivity, and lower remanence values is still a significant challenge. Therefore, it is highly desirable and necessary to explore new synthetic methods for the preparation of Co1
xZnxFe2O4 and/or doped Co1
xZnxFe2O4. To the best of our knowledge, the synthesis and magnetic properties of LixCo0.5Zn0.5
xFe2O4 by thermal decomposition of oxalates has rarely been reported in previous studies. This study aims to prepare LixCo0.5Zn0.5
xFe2O4 by calcining oxalates in air and study effect of composition and calcination temperature on magnetic properties of LixCo0.5Zn0.5
xFe2O4. Our results clearly show that the magnetic properties, in particular the specific magnetizations (Ms) and coercivity (Hc) of LixCo0.5Zn0.5
xFe2O4, can be precisely tailored by controlling Li þ

Y. Zhou et al. / Materials Science in Semiconductor Processing 41 (2016) 162–167

doped amount and calcination temperature.

163

precursor is 0.5:0.5:2.00. 3.2. TG/DSC/DTG analysis of the precursor

2. Experimental Fig. 1 shows the TG/DSC/DTG curves of the precursor at a heating rate of 10 °C min
1. The TG/DSC/DTG curves show that the thermal transformation of 0.5CoC2O4–0.5ZnC2O4–2FeC2O4  7.32H2O below 900 °C occurred in five well-defined steps. The first step started at about 126.2 °C and ended at 157.1 °C, which can be attributed to the dehydration of the one water from 0.5CoC2O4–0.5ZnC2O4–2FeC2O4  7.32H2O (mass loss: observed, 3.22%; theoretical, 3.16%). The second transformation step started at 157.1 °C and ended at 220.1 °C, attributed to the dehydration of the 6.32 waters from 0.5CoC2O4–0.5ZnC2O4–2FeC2O4  6.32H2O (mass loss: observed, 20.18%; theoretical, 19.98%). The third transformation step started at 220.1 °C and ended at 265.2 °C, attributed to the reaction of 2FeC2O4 with 1.5O2 into Fe2O3 and the four CO2 molecules (mass loss: observed, 21.62%; theoretical, 22.47%). The fourth transformation step started at 265.2 °C and ended at 316.5 °C, attributed to the reaction of 0.5CoC2O4 with 1/3O2 into 0.5/3Co3O4 and the one CO2 (mass loss: observed, 6.16%; theoretical, 5.85%). The fifth transformation step started at 316.5 °C and ended at 392.9 °C, attributed to the reaction of 0.5/3Co3O4 and 0.5ZnC2O4 with 1/6O2 into 0.5CoO, 0.5ZnO, and one CO2 (mass loss: observed, 6.96%; theoretical, 6.79%).

2.1. Reagent and apparatus All chemicals used are of reagent-grade purity (purity 499.9%). The TG/DSC measurements were conducted using a Netzsch Sta 409 PC/PG thermogravimetric analyzer under continuous flow of air (35 mL min
1). The sample mass was approximately 12 mg. X-ray powder diffraction (XRD) was performed using a X′pert PRO diffractometer equipped with a graphite monochromator and a Cu target. The radiation applied was Cu Kα (λ ¼ 0.15406 nm), operated at 40 kV and 50 mA. The XRD scans were conducted from 5° to 70° in 2θ, with a step size of 0.01°. The morphologies of the synthesis products were observed using a S-3400 scanning electron microscope (SEM). The specific saturation magnetizations (Ms) of the calcined sample powders were carried out at room temperature using a vibrating sample magnetometer (Lake Shore 7410). 2.2. Preparation of LixCo0.5Zn0.5
xFe2O4 The LixCo0.5Zn0.5
xFe2O4 (x ¼0, 0.1, 0.2, and 0.3) samples were prepared by calcining precursor oxalates in air using Li2C2O4, CoC2O4  2H2O, ZnC2O4  2H2O, and FeC2O4  2H2O as raw materials. In a typical synthesis (Co0.5Zn0.5Fe2O4), CoC2O4  2H2O (4.62 g), ZnC2O4  2H2O (4.78 g), and FeC2O4  2H2O (18.15 g) were placed in a mortar, and the mixture was thoroughly ground by hand with a rubbing mallet for 35 min. The strength applied was moderate. The resulting material was determined to be 0.5CoC2O4–0.5ZnC2O4–2FeC2O4  7.32H2O by TG and inductively coupled plasma atomic emission spectrometry. A similar synthesis procedure was used to synthesize other LixCo0.5Zn0.5
xFe2O4 precursor. Cubic LixCo0.5Zn0.5
xFe2O4 was obtained by calcining the precursor at 900 °C in air for 3 h.

3.3. XRD analyses of the calcined products Fig. 2 shows the XRD patterns of calcined samples from different calcination temperatures for 3 h. Fig. 2a shows that part of characteristic diffraction peaks of cubic ZnFe2O4 appeared when LixCo0.5Zn0.5
xFe2O4 precursor (x¼ 0 and 0.1) was calcined at 700 °C. Characteristic diffraction peaks of cubic ZnFe2O4 become strong and those of impurity (Fe2O3, ZnO, and/or Co3O4) become weak and/or disappear with the increase of calcination temperature. When the precursor was calcined at 900 °C, all other diffraction peaks in the pattern agreed with those of cubic ZnFe2O4 with space group Fd-3m (227) from PDF card 22-1012 except for one weak diffraction peak of rhombohedral Fe2O3 at 33.05 for 2θ. No diffraction peaks of crystalline CoFe2O4 were observed, which implied that ZnFe2O4 and CoFe2O4 formed a solid solution. Fig. 2c and d shows that single phase CoFe2O4 with cubic structure [space group Fd-3m (227)] can be obtained when LixCo0.5Zn0.5
xFe2O4 (x ¼0.2 and 0.3) precursor was calcined at 900 °C. No diffraction peaks of crystalline ZnFe2O4 and/or Li0.5Fe2.5O4 were observed, implying that ZnFe2O4, Li0.5Fe2.5O4, and CoFe2O4 formed a solid solution. The lattice parameters of the sample were refined by the Rietveld analysis using MDI Jade (ver. 5.0) software. The refined lattice parameters of LixCo0.5Zn0.5
xFe2O4 obtained at 900 °C were a¼ b¼c ¼0.843901 nm for x¼ 0; a ¼b¼ c¼0.840565 nm for x¼0.1;

3. Results and discussion 3.1. Composition analysis of the precursor 0.0310 g precursor sample was dissolved in 10 mL 50 vol% HCl solution, then diluted to 100.00 mL with deionized water. Cobalt (Co), zinc (Zn), and ferrum (Fe) in the solution were determined by inductively coupled plasma atomic emission spectrometry (ICPAES, Perkin Elmer Optima 5300 DV). The results showed that the Co, Zn, and Fe mass percentage were 5.17%, 5.74%, and 19.60%, respectively. In other words, molar ratio of Co:Zn:Fe in the

0.1 100

100

191 C

0.0

149 C

0

TG (%)

-100 -200

70

-300

60

-400

50

-500 40 -600

230 C

30 150

300

450

600

Temperature ( C)

750

900

DSC (mW/min)

287 C 379 C

80

-0.1

DTG (mg/min)

90

-0.2

145 C 282 C 376 C

-0.3 -0.4 -0.5 191 C

-0.6 -0.7 226 C

-0.8 150

300

450

600

750

Temperature ( C)

Fig. 1. TG/DSC/DTG curves of 0.5CoC2O4–0.5ZnC2O4–2FeC2O4  7.32H2O at a heating rate of 10 °C/min in air.

900

164

Y. Zhou et al. / Materials Science in Semiconductor Processing 41 (2016) 162–167

0.2510

32

0.2505

30

0.2500

(nm)

34

28 d

Crystallite diameters (nm)

Fig. 2. XRD patterns of LixCo0.5Zn0.5
xFe2O4: Co0.5Zn0.5Fe2O4 (a), Li0.1Co0.5Zn0.4Fe2O4 (b), Li0.2Co0.5Zn0.3Fe2O4 (c), and Li0.3Co0.5Zn0.2Fe2O4 (d).

26

0.2495 0.2490 0.2485

24 0.2480

22 0.00

700

750

800

850

900

Temperature ( C) Fig. 3. Dependence of crystallite diameters of LixCo0.5Zn0.5
xFe2O4 on calcination temperature and Li þ content.

0.05

0.10

0.15

0.20

0.25

0.30

Li content (x) Fig. 4. Dependence of interplanar spacing (d311) LixCo0.5Zn0.5
xFe2O4 on Li þ content.

Y. Zhou et al. / Materials Science in Semiconductor Processing 41 (2016) 162–167

the Bragg equation [36]:

100

d(311) =

Crystallinity (%)

90 80 70 60 50 700

750

800

850

900

Temperature ( C) Fig. 5. Dependence of LixCo0.5Zn0.5
xFe2O4 crystallinity on calcination temperature and Li þ content.

a ¼b¼ c¼0.839217 nm for x ¼0.2; and a¼ b¼c ¼0.839216 nm for x ¼0.3, respectively. That is, the lattice parameters decrease with the increase of Li þ addition amount. The lattice parameters decrease of the as-prepared ferrite samples can be explained on the basis of the radii of metal ions in the samples. The radius of Li þ ion (0.068 nm) [30] is smaller than that of Zn2 þ (0.074 nm) [31,32]. The replacement of Zn2 þ ions in tetrahedral A sites and/or octahedral B sites by Li þ ions would cause the contraction of the unit cell, resulting in smaller lattice parameters. Similar phenomenon was also observed for Li þ -doped manganese–zinc ferrites [33,34]. The crystallite diameter of LixCo0.5Zn0.5
xFe2O4 was estimated using the following Scherrer formula [34,35]:

D = Kλ /(β cos θ ),

165

(1)

where D is the crystallite diameter, K ¼0.89 (the Scherrer constant), λ ¼ 0.15406 nm (wavelength of the X-ray used), β is the width of line at the half-maximum intensity, and θ is the corresponding angle. From the position of the (311) peak (2θ(311)) in XRD patterns, the d(311) interplanar spacing is determined using

λ , 2 sin θ(311)

(2)

The crystallite diameter (D) of LixCo0.5Zn0.5
xFe2O4 from calcining the precursor at different temperatures and d(311) interplanar spacing of LixCo0.5Zn0.5
xFe2O4 obtained at 900 °C are shown in Figs. 3 and 4, respectively. From Fig. 3, it can be seen that after the addition of Li þ ions, the average crystallite diameter of samples at 900 °C decreases notably and reaches the lowest value (29.1 nm) when x is 0.3, which means that Li þ ions can lower the crystallite diameter of ferrite. This can be attributed that the binding energy of Li þ –O2
is larger than that of Zn2 þ –O2
. When Li þ ions enter into the lattice to form the Li þ –O2
bonds, the crystal nucleation and growth of Li þ substituted (Co,Zn)Fe2O4 ferrite (CZFO) will consume more energy, which results in a smaller average particles size for substituted CZFO ferrite. The d(311) values of the obtained samples in Fig. 4 reveal that the interplanar spacing decreases slightly with the addition of small amount of Li þ ions, which is consistent with the XRD result in Fig. 2e. It can be clearly seen from Fig. 2e that the (311) peak shifts slightly to a higher degree with the increase of Li þ content. The decrease in interplanar spacing of samples can be explained on the basis of the radii of metal ions. The radius of Li þ ion (0.068 nm) is smaller than that of Zn2þ ion (0.074 nm). The replacement of Zn2þ ions in octahedral B sites and/or tetrahedral A sites by Li þ ions could cause the contraction of the unit cell, resulting in the decrease of interplanar spacing. \The crystallinity of LixCo0.5Zn0.5
xFe2O4 can be calculated by MDI Jade (ver. 5.0) software. The crystallinity of LixCo0.5Zn0.5
xFe2O4 (x¼0, 0.1, 0.2, and 0.3) obtained at different temperatures is shown in Fig. 5. The crystallinity of LixCo0.5Zn0.5
xFe2O4 increases with the increase of calcination temperature. The crystallinities of LixCo0.5Zn0.5
xFe2O4 (x¼0.2 and 0.3) obtained at 900 °C are approximately 100%. Lattice strains of the LixCo0.5Zn0.5
xFe2O4 were determined using the Williamson-Hall formula [37,38]:

Fig. 6. SEM images of the products calcined at 900 in air for 3 h: Co0.5Zn0.5Fe2O4 (a), Li0.1Co0.5Zn0.4Fe2O4 (b), Li0.2Co0.5Zn0.3Fe2O4 (c), and Li0.3Co0.5Zn0.2Fe2O4 (d).

166

Y. Zhou et al. / Materials Science in Semiconductor Processing 41 (2016) 162–167

Fig. 7. M–H (magnetization–hysteresis) loops of LixCo0.5Zn0.5
xFe2O4 samples obtained at 800 and 900 °C in air for 3 h.

where B is the full width at half of the maximum (in radian) of the peaks, θ is the peak position, and ε is the lattice strain of the structure. Lattice strains of LixCo0.5Zn0.5
xFe2O4 obtained at 900 °C were 0.274% for x ¼0; 0.293% for x ¼0.1; 0.305% for x ¼0.2; and 0.364% for x ¼0.3, respectively. That is, the lattice strain of LixCo0.5Zn0.5
xFe2O4 increases with the increase of Li þ content and/or decrease of Zn2 þ content. It is attributed that the radius of Li þ ion (0.068 nm) is smaller than that of Zn2 þ (0.074 nm). The replacement of Zn2 þ ions in tetrahedral A sites and/or octahedral B sites by Li þ ions would cause the contraction of the unit cell, resulting in the increase of lattice strain in LixCo0.5Zn0.5
xFe2O4.

70 800 C 900 C

Ms(emu/g)

60 50 40 30 20 10 0.00

0.05

0.10

0.15

0.20

0.25

0.30

3.4. SEM analyses of the calcined products

Li content Fig. 8. Dependence of specific saturation magnetization of LixCo0.5Zn0.5
xFe2O4 on Li þ content and calcination temperature.

β , 4 tan θ

(3)

100

8 C

C

80 Hc (Oe)

6 Mr(emu/g)

ε=

The morphologies of the calcined products at 900 °C are shown in Fig. 6. LixCo0.5Zn0.5
xFe2O4 (x¼ 0, 0.1, and 0.2) sample obtained at 900 °C is composed of approximately spherical grains, there is a soft agglomeration phenomenon among the particles of LixCo0.5Zn0.5
xFe2O4, and the particle sizes are mainly between

4 2

60 40 20

0

0

0.00

0.05

0.10

0.15

0.20

Li content

0.25

0.30

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Li content

Fig. 9. Dependence of remanence (Mr) and coercivity (Hc) of LixCo0.5Zn0.5
xFe2O4 on Li þ content and calcination temperature.

Y. Zhou et al. / Materials Science in Semiconductor Processing 41 (2016) 162–167

100 and 300 nm (Fig. 6a–c). However, Li0.3Co0.5Zn0.2Fe2O4 is composed of approximately platelet grains, and the particle sizes are between 300 nm and 900 nm (Fig. 6d). The average crystallite sizes of the calcined samples determined by X-ray diffraction were significantly smaller than the values determined by SEM, which can be attributed to the fact that the values observed by SEM have the size of the secondary particles. In addition, the X-ray line broadening analysis disclosed only the size of a single crystallite [34]. 3.5. Magnetic properties of LixCo0.5Zn0.5
xFe2O4 Hysteresis loops of LixCo0.5Zn0.5
xFe2O4 samples calcined at 800 and 900 °C are shown in Fig. 7. Fig. 8 shows the dependence of specific saturation magnetization on Li þ content and calcination temperature. The specific saturation magnetization increases with the increase of Li þ concentration in the sample and calcination temperature, which can be attributed that crystallinity of LixCo0.5Zn0.5
xFe2O4 increases and/or weak magnetic Fe2O3 [39,40] and Co3O4 particles decrease with the increase of Li þ concentration in the sample and calcination temperature. On the other hand, the inclusion of Li þ in the lattice promotes a cation arrangement between octahedral and tetrahedral sites, resulting in higher Ms values [32]. Among LixCo0.5Zn0.5
xFe2O4 (x ¼0, 0.1, 0.2, and 0.3), Li0.3Co0.5Zn0.2Fe2O4 obtained at 900 °C has the highest specific saturation magnetization value, 70.24 emu/g. Compared to magnetic properties of CoFe2O4 [41], ZnFe2O4 [42,43], and Co0.5Zn0.5Fe2O4 obtained at the same temperature, the Li0.3Co0.5Zn0.2Fe2O4 exhibits higher specific saturation magnetizations than ZnFe2O4, CoFe2O4, and Co0.5Zn0.5Fe2O4, which implies that Li þ , Co2 þ , and Zn2 þ ions in Li0.3Co0.5Zn0.2Fe2O4 have a synergistic effect in improving the specific saturation magnetization of Li0.3Co0.5Zn0.2Fe2O4. Similar phenomenon was also observed for Li þ -doped manganese–zinc ferrites [32,33] and Li þ -doped manganese–nickel ferrites [34]. Dependence of remanence (Mr) and coercivity (Hc) on Li þ content is shown in Fig. 9. Li0.3Co0.5Zn0.2Fe2O4 obtained at 800 °C has the highest remanence (8.29 emu/g) and coercivity value (97.8 Oe). LixCo0.5Zn0.5
xFe2O4 has higher specific saturation magnetization, higher coercivity, and lower remanence, which is a very desirable characteristic for recording media [44].

4. Conclusions LixCo0.5Zn0.5
xFe2O4 (x ¼0, 0.1, 0.2, and 0.3) was successfully synthesized by calcining precursor oxalates in air. The XRD analysis suggests that a cubic LixCo0.5Zn0.5
xFe2O4 with space group Fd-3m(227) is obtained by calcining LixCo0.5Zn0.5
xFe2O4 precursor oxalates at 900 °C in air for 3 h. Magnetic characterization indicates that magnetic properties of LixCo0.5Zn0.5
xFe2O4 depend on Li þ doped amount and calcination temperature. Specific saturation magnetization value increases with the increase of calcination temperature and Li þ concentration in the sample. Li0.3Co0.5Zn0.2Fe2O4 obtained at 900 °C has the highest specific saturation magnetization value, 70.24 emu/g. However, Li0.3Co0.5Zn0.2Fe2O4 obtained at 800 °C has the highest remanence (8.29 emu/g) and coercivity value (97.8 Oe). LixCo0.5Zn0.5
xFe2O4 has higher specific saturation magnetization, higher coercivity, and lower remanence, which is a very desirable characteristic for recording media.

167

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant no. 21161002) and the Guangxi University Student Innovation Foundation of China (Grant no. SYJN20130356).

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