UDK 537.622:549.73 Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 47(6)845(2013)
VARIATION IN MAGNETIC PROPERTIES OF SOL-GEL-SYNTHESIZED COBALT FERRITES
SPREMINJANJE MAGNETNIH LASTNOSTI KOBALTOVIH FERITOV, SINTETIZIRANIH PO SOL-GEL POSTOPKU
Anuchit Hunyek, Chitnarong Sirisathitkul
Molecular Technology Research Unit, School of Science, Walailak University, 80161 Nakon Si Thammarat, Thailand
schitnar@wu.ac.th
Prejem rokopisa — received: 2012-11-28; sprejem za objavo - accepted for publication: 2013-04-10
Single-phase cobalt ferrite (CoFe2O4) was synthesized with a sol-gel reaction between 10.59 g of cobalt nitrate and 29.40 g of iron nitrate using 480 mL of 10 % polyvinyl alcohol (PVA) in water. Their magnetic squareness and coercive field were slightly reduced with the increase in the annealing at 800 °C from 2 h to 6 h. To examine the reproducibility of ferrite products of the PVA sol-gel method, the synthetic condition was repeated 28 times. After annealing for 4 h, CoFe2O4 samples from different batches exhibit variations in the magnetic properties. The coercive field has a large distribution because of its sensitivity to the particle agglomeration, whereas the squareness fluctuates in a narrower range from 0.22 to 0.29. Since the squareness is the ratio of the remanence to the saturation magnetization, the particle agglomeration tends to increase both values while keeping this ratio rather unchanged.
Keywords: cobalt ferrites, PVA sol-gel, annealing time, magnetic squareness, coercive field
Enofazni kobaltov ferit (CoFe2O4) je bil sintetiziran s sol-gel reakcijo med 10,59 g kobaltovega nitrata in 29,40 g železovega nitrata z uporabo 480 mL 10-odstotne raztopine polivinil alkohola (PVA) v vodi. Njihova četverokotna magnetna oblika in koercitivno polje sta se rahlo zmanjšala s podaljšanjem žarjenja na 800 °C iz 2 h na 6 h. Da bi preiskali ponovljivost feritnih proizvodov iz PVA sol-gel metode, je bila sinteza ponovljena 28-krat. Po žarjenju 4 h so kazali vzorci CoFe2O4 iz različnih serij razliko v magnetnih lastnostih. Koercitivno polje kaže velik raztros zaradi občutljivosti za aglomeracijo delcev, medtem ko se četverokotnost oblike spreminja v ožjem področju od 0,22 do 0,29. Ker je četverokotnost razmerje med remanenco in nasičeno magnetizacijo, aglomeracija delcev povečuje obe vrednosti, medtem ko razmerje ostaja skoraj nespremenjeno.
Ključne besede: kobaltov ferit, PVA sol-gel, čas žarjenja, magnetna četverokotna oblika, koercitivno polje
1 INTRODUCTION
Cobalt ferrite (CoFe2Ü4) is under research and development for its applications in magneto-optical recording media, magnetic refrigerants, microwave absorbers and stress sensors.1,2 Bulk CoFe2Ü4 has an inverse spinel structure consisting of a cubic close-packed (fcc) arrangement of oxide anions, O2-. The tetrahedral and octahedral interstitial sites in the lattice are partially occupied by the Co2+ and Fe3+ cations, respectively.12 Like the other spinel ferrites (e.g., NiFe2O4, MnFe2O4, ZnFe2O4, CuFe2O4) and barium ferrites, CoFe2O4 nano-particles can be synthesized with the sol-gel method.3-17 In a sol-gel reaction, homogeneous CoFe2O4 nanoparti-cles are produced in-situ with a controlled decomposition of precursors, while the chelating gel is dried during the heat treatment at a relatively low temperature. Gatelyte et al.7 demonstrated that the characteristics of CoFe2O4, NiFe2O4, ZnFe2O4, YFeO3 and Y3Fe5O12 produced with aqueous sol-gel reactions under the same synthesis condition were comparable.
According to our previous sol-gel synthesis using polyvinyl alcohol (PVA) as the chelating agent,8 the magnetic properties of CoFe2O4 are influenced by PVA contents for two reasons. Firstly, the single-phase CoFe2O4 obtained in the case of sufficient PVA gives rise
to a relatively high coercive field because of its strong magnetic anisotropy. In the case of a diluted PVA solution, the second phase (e.g., a-Fe2Os) may also be present, reducing the overall coercive field. The other reason is the dependence of the cluster size on the PVA contents used in the sol-gel reaction. The cluster size, on the other hand, dictates the coercive field and magnetization of CoFe2O4.915 The role of the PVA gel is to cleave atoms during the reaction. An increase in PVA improves the crystallinity of CoFe2O4 without an excessive particle agglomeration.
The temperature of the heat treatment of the as-synthesized products also affects the magnetic properties of CoFe2O4. Our previous result agrees with the other experimental works indicating that both the coercive field and the magnetic squareness (the ratio of the remanence to the saturation magnetization) of CoFe2O4 are initially increased with an increase in the annealing temperature up to 700-800 °C and then reduced with further increases in the temperature.815-19 The growth and agglomeration of CoFe2O4 particles at high temperatures result in a decrease in the coercive field and an increase in the saturation magnetization. The squareness is, therefore, reduced due to this high-temperature regime. In addition, Toksha et al.15 reported a decrease in the coercive field and an increase in the saturation magne-
tization due to an increased particle size after prolonged annealing. Recently, Sajjia et al.14 employed the response surface methodology to simulate the optimum heat treatment to obtain single-phase CoFe2O4 and minimize the cost of electricity.
To implement a sol-gel synthesis of ferrites on a commercial scale, another aspect of concern is the reproducibility of the products from different batches. Although this issue is hardly addressed in research papers, it is particularly important since ferrites of the order of a hundred grams or less are usually obtained with a laboratory-scale sol-gel synthesis. In this research, the condition for preparing the single-phase CoFe2O4 from the previous work8 is reproduced in order to study the variation in magnetic properties observed after a repeated synthesis. The effect of the annealing time is also investigated.
2 EXPERMENTAL WORK
The PVA solution was prepared by dissolving PVA powders in distilled water (10 %). It was heated at 70—80 °C until the solution became clear (5-7 h). Then, 10.59 g of cobalt nitrate, Co(NOs)2 • 6H2O, and 29.40 g of iron nitrate, Fe(NO3)3 • 9H2O powders were mixed with the 480 mL PVA solution. The reaction mixture was stirred for a further 3 h. After that, it was heated at 80 °C for 10-12 h or until the gel was dry.
The morphology of the ferrite product was shown with scanning electron microscopy (SEM) and its structure was characterized with powder X-ray diffractometry (XRD). A copper target was used as an X-ray source (k«, X = 0.154058 nm) with 40 kV between the cathode and the copper target. The measurement was performed in the range of 20 angles from 10° to 80° with each rotating step accounting for 0.02°. The crystallite size
was calculated from the peak width using the Scherrer's formula:
0.9X
(1)
d =
P cos e
where fi is the broadening of the diffraction line measured at half the maximum intensity and X is the wavelength of K«.
To investigate the effect of the annealing time and reproducibility of sol-gel products, the time of annealing at 800 °C varied between (2, 4, 6) h and the same synthesis condition was repeated for 28 batches with the same, annealing 4 h. The magnetic properties of all the samples were characterized with vibrating sample magnetometry (VSM).
3 RESULTS AND DISCUSSION
The XRD pattern in Figure 1 shows the cubic spinel CoFe2O4 phase [JCPDS 22-1086] with the peaks at 30.0°, 35.4°, 43.0°, 56.9°, 62.6° corresponding to the (220), (311), (400), (511) and (440) planes, respectively. Other oxides or impurity phases are not detected and the CoFe2O4 peaks are particularly sharp. The crystallite size, calculated from the line broadening of the (311) diffraction peak is approximately 45 nm. These crystallites tend to agglomerate into microscale clusters that are quite common for sol-gel-synthesized ferrites.20 As seen in the SEM micrograph in the inset, these aggregates have flat surfaces and sharp edges. Compared to the ferrite products from the previous work,8 the cluster size tends to increase with a reduction of PVA in the sol-gel reaction.
According to VSM hysteresis loops in Figure 2 all the CoFe2O4 samples after varying annealing times exhibit ferrimagnetic properties. The absence of super-paramagnetic behaviors is due to the particle agglome-
Figure 1: XRD pattern of sol-gel-synthesized ferrite products. Its SEM micrograph is shown in the inset.
Slika 1: XRD-posnetek feritnega proizvoda, sintetiziranega po sol-gel postopku. SEM-posnetek je vložen.
Figure 2: Hysteresis loops of CoFe2O4 from the sol-gel reaction after annealing at 800 °C for (2, 4 and 6) h
Slika 2: Histerezna zanka CoFe2O4, izdelanega po sol-gel reakciji, po žarjenju (2, 4 in 6) h na 800 °C
36-42	43-49	50-56
Coercive field range (kA/m)
Figure 3: Distribution of the coercive field of CoFe2O4 synthesized in 28 batches
Slika 3: Razporeditev koercitivnega polja CoFe2O4, sintetiziranega v 28 ponovitvah
ration. In the applied magnetic field of 440 kA/m, the magnetization does not reach the complete saturation. The highest magnetic moment per mass is around 60 A m2/kg, a value comparable to that of sol-gel-synthesized CoFe2O4 nanoparticles.9-1115 Both the coercive field and the squareness are slightly decreased with the increase in the annealing time from 2 h to 6 h. Longer annealing times apparently have a similar effect on the increase in the annealing temperature beyond 800 °C and the cluster size is, consequently, enhanced. It follows that the coercive field and the squareness are reduced.
The variation in the coercive field of CoFe2O4 is shown in the form of a histogram in Figure 3. Ranging from 31.2 kA/m to 63.2 kA/m, the distribution is approximately exponential with the coercive field of less than 40 kA/m in the majority of the samples. This implies that most particles are unevenly agglomerated, like those in Figure 1. The average coercive field of 28 batches is (39.8 ± 8.7) kA/m (compared to 34.4 kA/m in Figure 2). If an enhanced coercive field is aimed for, either an increase in the PVA contents or a decrease in the annealing temperature and time is recommended. In Figure 4, the squareness exhibits a narrower distribution, from
o
t+H
° 0.15 CI
0
1	0.10 £
0.05
			0.32	0.32
				
	0.21			
		0.11		
0.04				
				
0.20-0.21 0.22-0.23 0.24-0.25 0.26-0.27 0.28 -0.29 Magnetic squareness
Figure 4: Distribution of the magnetic squareness of CoFe2O4 synthesized in 28 batches
Slika 4: Razporeditev magnetne cetverokotne oblike CoFe2O4, sintetiziranega v 28 ponovitvah
0.22 to 0.29. In addition to its effect on the coercive field, the particle agglomeration also results in an increase in the magnetization. However, the squareness does not exhibit a large variation, because it is the ratio between the magnetizations. With the average value of 0.26 ± 0.03, a large fraction of the samples have a squareness beyond 0.25. Interestingly, the minimum squareness of 0.22 is obtained from the batch with the smallest cluster size, signified by the maximum coercive field.
4 CONCLUSION
The magnetic squareness and coercive field of the single-phase CoFe2O4 synthesized with a sol-gel reaction between Co(NO3)2 • 6H2O and Fe(NO3)3 • 9H2O exhibit a slight decrease with the increase in the annealing time from 2 h to 6 h at 800 °C. However, repeated syntheses show that the magnetic properties of CoFe2O4 obtained under the same synthetic and annealing conditions may vary due to non-uniform particle agglomerations. This must be taken into the account before implementing the sol-gel-synthesized ferrites on a large scale.
Acknowledgements
This work was funded by the Industry/University Cooperative Research Center (I/UCRC) in HDD Component, the Faculty of Engineering, the Khon Kaen University and the National Electronics and Computer Technology Center, the National Science and Technology Development Agency. The characterizations using XRD, SEM and VSM were carried out at the National Metal and Materials Technology Center (MTEC), the Chiang Mai University and the Kasetsart University, respectively.
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