Chemical Functionalization, Self-Assembly, and Applications of Nanomaterials and Nanocomposites 2014View this Special Issue
Research Article | Open Access
The Structural and Magnetic Properties of Gadolinium Doped CoFe2O4 Nanoferrites
Gadolinium substituted cobalt ferrite CoGdxFe2−xO4 (x = 0, 0.04, 0.08) powders have been prepared by a sol-gel autocombustion method. XRD results indicate the production of a single cubic phase of ferrites. The lattice parameter increases and the average crystallite size decreases with the substitution of Gd3+ ions. SEM shows that the ferrite powers are nanoparticles. Room temperature Mössbauer spectra of CoGdxFe22−xO4 are two normal Zeeman-split sextets, which display ferrimagnetic behavior. The saturation magnetization decreases and the coercivity increases by the Gd3+ ions.
Cobalt ferrite is a hard magnetic material, and it has moderate saturation magnetization of about 80 emu/g, high coercivity of 5000 Oe, high Curie temperature of 793.15 K (520°C), high anisotropy constant of 2.65 × 106~5.1 × 106 erg/cm3, and high magnetostrictive of −225 × 10−6 [1, 2]. Moreover, cobalt ferrite exhibits high electromagnetic performance, large magneto-optic effect, excellent chemical stability, and mechanical hardness [3, 4]. In the materials containing 3d transition metals, the magnetism carriers are the electrons from the 3d shell that are considered to migrate from one atom to another. In rare earth (RE) metals, the magnetism carriers are the 4f electrons which are protected by the 5s25p6 shells, so their magnetic moments are well localized at individual atoms [5–7]. Small amounts of RE element gadolinium can affect the magnetic properties and the magnetic coercivity of Co ferrites. Peng et al.  and Rana et al.  investigated the effect of Gd3+ substitution on dielectric properties and saturation magnetization of nanocobalt ferrite.
In this paper, nanoferrites CoGdxFe2−xO4 () were prepared by a sol-gel autocombustion method. The aim of this study is to investigate variation structural and magnetic properties of cobalt ferrite powders by replacement of small amounts gadolinium.
2.1. Sample Preparation
RE ions substituted cobalt ferrite CoGdxFe2−xO4 () powders were prepared by a sol-gel autocombustion method. The analytical grades Co(NO3)2·6H2O, Fe(NO3)3·9H2O, Gd(NO3)3·6H2O, citric acid (C6H8O7·H2O), and ammonia (NH3·H2O) were used as raw materials. The molar ratio of metal nitrates to citric acid was taken as 1 : 1. The metal nitrates and citric acid were, respectively, dissolved into deionized water to form solution. Ammonia was added to the solution of metal nitrates to change the pH value from 7 to 9. The mixed solution was poured into a thermostat water bath and heated at 80°C under constant stirring to transform into a dried gel . Citric acid was dropped continually in the process of heating. The gel was dried at 120°C in a dry-oven for 2 h, being ignited in the air at room temperature, and the dried gel burnt in a self-propagating combustion way to form loose powder. The powder was ground and annealed at temperatures 800°C for 3 h.
The crystalline structure was investigated by X-ray diffraction (Rigaku D/max- 2500V/PC) with Cu Kα radiation (λ = 0.15405 nm). The micrographs were obtained by scanning electron microscopy (NoVa Nano SEM 430). The Mössbauer spectrum was performed at room temperature, using a conventional Mössbauer spectrometer (Fast Com Tec PC-mossII), in constant acceleration mode. The γ-rays were provided by a 57Co source in a rhodium matrix. Magnetization measurements were carried out with superconducting quantum interference device (MPMS-XL-7, Quantum Design) at room temperature.
3. Results and Discussion
3.1. XRD Patterns Analysis
Figure 1 shows the XRD patterns of CoGdxFe2−xO4 () ferrites calcined at 800°C for 3 h. The XRD patterns show that all the samples are single phase and all diffraction peaks can be indexed to the cubic spinel structure of CoFe2O4 (JCPDS card number 22-1086). No impurity peak was detected in these samples. Table 1 indicates that the lattice constant of Co ferrite substituting the Gd3+ sample is larger than that of the pure cobalt ferrite; it is due to the fact that the ionic radius of Gd3+ ions (0.938 Å) is larger than that of Fe3+ ions (0.645 Å) [8–11]. However, lattice parameter does not increase monotonously by increasing the gadolinium content, and it may be related to doping gadolinium having a larger radius in CoFe2O4 which leads to the lattice distortion .
The average crystallite size of the investigated samples is found to be around 31.6 to 55.6 nm by using Scherrer’s formula [10, 12, 13]. The decreasing average crystallite size is with Gd3+ ions doping, which is in agreement with the results of literature [14–18]. They pointed out that the larger the bond energy of Gd3+-O2− as compared to that of Gd3+-O2−, the more the energy needed to make Gd3+ ions enter into the lattice and form the bond of RE3+-O2−. Therefore, Gd3+ substituted ferrites have higher thermal stability relative to pure Co ferrite, and more energy is needed for the substituted samples to complete crystallization and grow grains.
The X-ray density was calculated using the following relation [10, 11, 17]:where is relative molecular mass, is Avogadro’s number, and “” is the lattice parameter. Table 1 shows that the X-ray density is tending to increase with Gd3+ substitution. The atomic weight of Gd is greater than that of Fe, so the relative molecular mass increases with the substitution of Gd3+ ions, and the lattice parameter of cobalt ferrite substituting the Gd3+ has no significant changes. So the increase in X-ray density is attributed to the fact that the relative molecular mass increases.
3.2. Structures and Grain Sizes
The SEM micrographs of CoFe2O4 ferrites annealed at 800°C for 3 h are shown in Figure 2. The distribution of grains with almost uniform size, well crystallized for the sample, can be observed. Figure 3 shows the histogram of grain size distribution of CoGdxFe2−xO4 () ferrites. The average grain size estimated by a statistical method is approximately 96.26 nm. The average grain size is slightly larger than the average crystallite size determined by XRD. This shows that every particle is formed by a number of crystallites [19–21].
The SEM micrographs of CoGdxFe2−xO4 () are shown in Figure 4. The distribution of grains with almost uniform size, well crystallized for CoGdxFe2−xO4 (), can be observed. Some particles are agglomerated due to the presence of magnetic interactions among particles .
3.3. Mössbauer Spectroscopy
The Mössbauer spectra recorded at room temperature are shown in Figure 5 for CoGdxFe2−xO4 (). All samples have been analyzed using Mösswinn 3.0 program. For all samples, the spectra exhibit two normal Zeeman-split sextets due to Fe3+ at tetrahedral and octahedral sites, indicating the ferromagnetic behavior of the samples. The sextet with the larger isomer shift is assigned to the Fe3+ ions at B site, and the one with the smaller isomer shift is assumed to the Fe3+ ions occupying A site. Maybe it is due to the difference in Fe3+-O2− internuclear separations. For the bond separation being larger for B site Fe3+ ions, in comparison with A site ions, smaller overlapping of orbits for Fe3+ and O2+ ions at B site occurs, resulting in smaller covalency and larger isomer shift for B site Fe3+ ions [22, 23]. It is evident from Table 2 that isomer shifts values show very little change with Gd3+ substitution, which indicates that s electrons charge distribution of Fe3+ is not much influenced by Gd3+ substitution . It is reported that the values of IS (Isomer shift) for Fe2+ ions lie in the range 0.6~1.7 mm/s, while for Fe3+ ions they lie in the range 0.1~0.5 mm/s . From Table 2, values for IS in this paper indicate that iron is in Fe3+ state.
Table 3 shows that the values of magnetic hyperfine field at A site have no significant changes, and the magnetic hyperfine field at B site is tending to decrease by Gd3+ substitution. Maybe the Fe3+ ions of samples at lattice site are substituted by Gd3+ ions at B sites. Gd is the only RE element that has a Curie temperature (293.2 K) close to room temperature [16, 25]. Magnetic dipolar orientation of the RE exhibits a disordering form at room temperature; therefore, introducing rare-earth Gd3+ ions in CoFe2O4 seems like substituting magnetic Fe3+ ions (in octahedral B site of spinel lattice) by nonmagnetic atoms .
The value of quadrupole shift of the magnetic sextet is very small in all the samples indicating that the local symmetry of the ferrites obtained is close to cubic.
3.4. Magnetic Property of Particles
Figure 6 shows hysteresis loops of CoGdxFe2−xO4 at room temperature. The magnetization of all samples nearly reaches saturation at the external field of 10000 Oe.
It is observed from Table 3 that saturation magnetization decreases as Gd content increases. The saturation magnetization could be expressed by means of the following relation :where is the magnetic moment with Bohr magneton as the unit and is relative molecular mass. The relative molecular mass of CoGdxFe2−xO4 increases as Gd content increases. The change of magnetic moment can be explained with Néel’s theory. The magnetic moment per ion for Gd3+, Co2+, and Fe3+ ions is , , and [15, 16], respectively. As previously mentioned, magnetic dipolar orientation of the rare earth exhibits a disordering form at room temperature. Hence, in this paper, it may be reasonable that rare earth ions (Gd3+) are considered as nonmagnetic ones at room temperature.
Since Co2+ prefer to occupy the octahedral site (B) in CoFe2O4 material of inverse spinel structure [1, 2], Gd3+ ions occupy only the B sites for their large ion radii [10, 16]. That is, the cation distribution is (Fe)A[CoGdxFe1−x]BO4. According to Néel’s two sublattice models of ferrimagnetism, the magnetic moment nB is expressed as [7, 11]where and are B and A sublattice magnetic moments, respectively. Figure 7 shows the change in experimental and theoretical magnetic moment with Gd content .
From Figure 7, the experimental and theoretical magnetic moment decreases as Gd content increases, and according to relation (2) the theoretical saturation magnetization decreases with Gd content . The variation of the experimental and theoretical saturation magnetization is in a good agreement with each other for all samples.
It is observed from Table 3 that the variation of coercivity with Gd content increases for CoGdxFe2−xO4. It indicates that the coercivity of Co ferrite substituting the Gd3+ ions is larger than that of the pure cobalt ferrite. The phenomenon can be explained as follows. Like Co2+ ions, rare earth ions (Gd3+) have stronger s-l coupling and weaker crystal field, so they have stronger magnetocrystalline anisotropy [6, 10, 18, 25, 26]. Furthermore, the radii of Gd3+ ions are larger than that of Fe3+ ions, and the symmetry of crystal will be decreased after the sample was substituted by Gd3+ ions and hence may distort the lattice or crystalline field and generate an internal stress [13, 14]. Moreover, it is known that the grain boundary increases with decreasing crystallite size. In this study, Gd substituted ferrites have a decrease in crystallite size with the substitution of Gd3+ ions. The area of disordered arrangement for ions on grain boundaries may fix and hinder the domain walls motion; thus the coercivity of the samples increases with Gd3+ ions substituted cobalt ferrite . However the coercivity does not increase monotonously by increasing the gadolinium content, and it may be related to the coercivity which is influenced by many factors, such as crystallinity, microstrain, magnetic particle morphology and size distribution, anisotropy, and magnetic domain size [14, 27, 28].
The analysis of XRD patterns reveals the formation of single-phase cubic spinel structure for CoGdxFe2−xO4 () ferrite annealed at 800°C. The increase in lattice constant is due to replacement of smaller Fe3+ ions by larger Gd3+ ions. SEM results indicate the distribution of grains and morphology of the samples. Some particles are agglomerated due to the presence of magnetic interactions among particles. The ferrite powers are nanoparticles. Room temperature Mössbauer spectra of CoGdxFe2−xO4 () ferrites are two normal Zeeman-split sextets. It displays ferrimagnetic behavior for the samples. The saturation magnetization decreases and the coercivity increases with the substitution of Gd3+ ions. The decreases of the saturation magnetization can be explained with Néel’s theory. The variation of coercivity is attributed to magneto-crystalline anisotropy, microstrain, and grain boundary.
Conflict of Interests
The authors declared that they have no conflict of interests to this work.
This work was financially supported by the National Natural Science Foundation of China (no. 11364004, 11164002) and Innovation Project of Guangxi Graduate Education under Grant (no. 2010106020702M47).
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