The Structural and Magnetic Properties of Gadolinium Doped CoFe 2 O 4 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 Gd 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 Gd ions.


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 793.15K (520 ∘ C), high anisotropy constant of 2.65 × 10 6 ∼5.1 × 10 6 erg/cm 3 , 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 5s 2 5p 6 shells, so their magnetic moments are well localized at individual atoms [5][6][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 Gd 3+ substitution on dielectric properties and saturation magnetization of nanocobalt ferrite.
In this paper, nanoferrites CoGd  Fe 2− O 4 ( = 0, 0.04, 0.08) 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.

Sample Preparation. RE ions substituted cobalt ferrite
CoGd  Fe 2− O 4 ( = 0, 0.04, 0.08) powders were prepared by a sol-gel autocombustion method.The analytical grades , and ammonia (NH 3 ⋅H 2 O) 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.    1 indicates that the lattice constant of Co ferrite substituting the Gd 3+ sample is larger than that of the pure cobalt ferrite; it is due to the fact that the ionic radius of Gd 3+ ions (0.938 Å) is larger than that of Fe 3+ ions (0.645 Å) [8][9][10][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 CoFe 2 O 4 which leads to the lattice distortion [8].

Results and Discussion
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 Gd 3+ ions doping, which is in agreement with the results of literature [14][15][16][17][18].They pointed out that the larger the bond energy of Gd 3+ -O 2− as compared to that of Gd 3+ -O 2− , the more the energy needed to make Gd 3+ ions enter into the lattice and form the bond of RE 3+ -O 2− .Therefore, Gd 3+ 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 Xray density is tending to increase with Gd 3+ substitution.The atomic weight of Gd is greater than that of Fe, so the relative molecular mass increases with the substitution of Gd 3+ ions, and the lattice parameter of cobalt ferrite substituting the Gd 3+ has no significant changes.So the increase in X-ray density is attributed to the fact that the relative molecular mass increases.

Structures and Grain
Sizes.The SEM micrographs of CoFe 2 O 4 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  Fe 2− O 4 ( = 0) 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][20][21].
The SEM micrographs of CoGd  Fe 2− O 4 ( = 0) are shown in Figure 4.The distribution of grains with almost uniform size, well crystallized for CoGd  Fe 2− O 4 ( = 0), can be observed.Some particles are agglomerated due to the presence of magnetic interactions among particles [14].

Mössbauer Spectroscopy.
The Mössbauer spectra recorded at room temperature are shown in Figure 5 for CoGd  Fe 2− O 4 ( = 0, 0.04, 0.08).All samples have been analyzed using Mösswinn 3.0 program.For all samples, the spectra exhibit two normal Zeeman-split sextets due to Fe 3+ at tetrahedral and octahedral sites, indicating the ferromagnetic behavior of the samples.The sextet with the larger isomer shift is assigned to the Fe 3+ ions at B site, and the one with the smaller isomer shift is assumed to the Fe 3+ ions occupying A site.Maybe it is due to the difference in Fe 3+ -O 2− internuclear separations.For the bond separation being larger for B site Fe 3+ ions, in comparison with A site ions, smaller overlapping of orbits for Fe 3+ and O 2+ ions at B site occurs, resulting in smaller covalency and larger isomer shift for B site Fe 3+ ions [22,23].It is evident from   Table 2 that isomer shifts values show very little change with Gd 3+ substitution, which indicates that s electrons charge distribution of Fe 3+ is not much influenced by Gd 3+ substitution [23].It is reported that the values of IS (Isomer shift) for Fe 2+ ions lie in the range 0.6∼1.7 mm/s, while for Fe 3+ ions they lie in the range 0.1∼0.5 mm/s [24].From   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 Gd 3+ substitution.Maybe the Fe 3+ ions of samples at lattice site are substituted by Gd 3+ ions at B sites.Gd is the only RE element that has a Curie temperature   (293.2K) close to room temperature [16,25].Magnetic dipolar orientation of the RE exhibits a disordering form at room temperature; therefore, introducing rare-earth Gd 3+ ions in CoFe 2 O 4 seems like substituting magnetic Fe 3+ ions (in octahedral B site of spinel lattice) by nonmagnetic atoms [8].
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.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  B is the magnetic moment with Bohr magneton as the unit and  is relative molecular mass.The relative molecular mass of CoGd  Fe 2− O 4 increases as Gd content  increases.The change of magnetic moment  B can be explained with Néel's theory.The magnetic moment per ion for Gd 3+ , Co 2+ , and Fe 3+ ions is 7.94  B , 3  B , and 5  B [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 (Gd 3+ ) are considered as nonmagnetic ones at room temperature.Since Co 2+ prefer to occupy the octahedral site (B) in CoFe 2 O 4 material of inverse spinel structure [1,2], Gd 3+ ions occupy only the B sites for their large ion radii [10,16].That is, the cation distribution is (Fe)

Magnetic Property of Particles.
According to Néel's two sublattice models of ferrimagnetism, the magnetic moment n  is expressed as [7,11] where  B and  A 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 CoGd  Fe 2− O 4 .It indicates that the coercivity of Co ferrite substituting the Gd 3+ ions is larger than that of the pure cobalt ferrite.The phenomenon can be explained as follows.Like Co 2+ ions, rare earth ions (Gd 3+ ) have stronger s-l coupling and weaker crystal field, so they have stronger magnetocrystalline anisotropy [6, 10, 18, 25, 26].Furthermore, the radii of Gd 3+ ions are larger than that of Fe 3+ ions, and the symmetry of crystal will be decreased after the sample was substituted by Gd 3+ 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 Gd 3+ 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 Gd 3+ 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].

Conclusion
The analysis of XRD patterns reveals the formation of single-phase cubic spinel structure for CoGd  Fe 2− O 4 ( = 0, 0.04, 0.08) ferrite annealed at 800 ∘ C. The increase in lattice constant is due to replacement of smaller Fe 3+ ions by larger Gd 3+ 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  Fe 2− O 4 ( = 0, 0.04, 0.08) 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 Gd 3+ 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.

Figure 6
shows hysteresis loops of CoGd  Fe 2− O 4 at room temperature.The magnetization of all samples nearly reaches saturation at the external field of 10000 Oe.

Table 2 ,
values for IS in this paper indicate that iron is in Fe 3+ state.