The Structural and Magnetic Properties of Gadolinium Doped CoFe₂O₄ Nanoferrites

Abstract

Gadolinium substituted cobalt ferrite CoGd_xFe_2−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 CoGd_xFe_22−xO₄ are two normal Zeeman-split sextets, which display ferrimagnetic behavior. The saturation magnetization decreases and the coercivity increases by the Gd³⁺ ions.

1. Introduction

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. [8] and Rana et al. [9] investigated the effect of Gd³⁺ substitution on dielectric properties and saturation magnetization of nanocobalt ferrite.

In this paper, nanoferrites CoGd_xFe_2−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. Experimental

2.1. Sample Preparation

RE ions substituted cobalt ferrite CoGd_xFe_2−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 [9]. 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.

2.2. Characterization

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 CoGd_xFe_2−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 [8].

Table 1 

Lattice parameters, average crystallite size, and X-ray densities date of (, 0.04, 0.08) ferrites annealed at 800°C.

Figure 1 

Room temperature X-ray diffraction patterns of CoGd_xFe_2−xO₄ () annealed at 800°C.

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 CoGd_xFe_2−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].

(a)

(b)

(c)

(d)

Figure 2 

SEM micrographs depict CoFe₂O₄ ferrites with diameters of 1 μm (a), 2 μm (b), 5 μm (c), and 10 μm (d).

Figure 3 

Histogram of grain size distribution for CoFe₂O₄ ferrites annealed at 800°C.

The SEM micrographs of CoGd_xFe_2−xO₄ () are shown in Figure 4. The distribution of grains with almost uniform size, well crystallized for CoGd_xFe_2−xO₄ (), can be observed. Some particles are agglomerated due to the presence of magnetic interactions among particles [14].

(a)

(b)

Figure 4 

SEM micrographs of different part in CoGd_xFe_2−xO₄ () ferrites with diameters of 300 nm (a) and 500 nm (b).

3.3. Mössbauer Spectroscopy

The Mössbauer spectra recorded at room temperature are shown in Figure 5 for CoGd_xFe_2−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 [23]. 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 [24]. From Table 2, values for IS in this paper indicate that iron is in Fe³⁺ state.

Table 2 

Mössbauer parameters of isomer shift (IS), quadrupole splitting (QS), magnetic hyperfine field (), linewidth (Γ), and absorption area () for (, 0.04, 0.08) ferrites annealed at 800°C.

Figure 5 

Room temperature Mössbauer spectra of CoGd_xFe_2−xO₄ () ferrites annealed at 800°C.

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 [8].

Table 3 

Magnetic data for (, 0.04, 0.08) ferrites annealed at 800°C.

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 CoGd_xFe_2−xO₄ at room temperature. The magnetization of all samples nearly reaches saturation at the external field of 10000 Oe.

Figure 6 

Room temperature hysteresis loops of CoGd_xFe_2−xO₄ () annealed at 800°C.

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 [7]: where is the magnetic moment with Bohr magneton as the unit and is relative molecular mass. The relative molecular mass of CoGd_xFe_2−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[CoGd_xFe_1−x]_BO₄. According to Néel’s two sublattice models of ferrimagnetism, the magnetic moment n_B 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 .

Figure 7 

The variation in experimental and theoretical magnetic moment of CoGd_xFe_2−xO₄ ferrites with gadolinium 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 CoGd_xFe_2−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 [14]. 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].

4. Conclusion

The analysis of XRD patterns reveals the formation of single-phase cubic spinel structure for CoGd_xFe_2−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 CoGd_xFe_2−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.

Acknowledgments

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).