Effects of Al3+ Substitution on Structural and Magnetic Behavior of CoFe2O4 Ferrite Nanomaterials

A sol-gel autocombustion method was used to synthesize Al3+ ion-substituted cobalt ferrite CoAlxFe2−xO4 (x = 0–1.5). According to X-ray diffraction analysis (XRD), cobalt ferrite was in a single cubic phase after being calcined at 1000 °C for 3 h. Moreover, the lattice constant decreased with increase in aluminum substituents. When the sample was analyzed by Scanning Electron Microscopy (SEM), we found that uniformly sized, well-crystallized grains were distributed in the sample. Furthermore, we confirmed that Al3+ ion-substituted cobalt ferrite underwent a transition from ferrimagnetic to superparamagnetic behavior; the superparamagnetic behavior was completely correlated with the increase in Al3+ ion concentration at room temperature. All these findings were observed in Mössbauer spectra. For the cobalt ferrite CoAlxFe2−xO4, the coercivity and saturation magnetization decrease with an increase in aluminum content. When the annealing temperature of CoAl0.1Fe1.9O4 was steadily increased, the coercivity and saturation magnetization initially increased and then decreased.


Introduction
Ferrite is an important magnetic material. Cobalt ferrite is a hard ferromagnetic material, and its characteristic properties are as follows: moderate saturation magnetization (80 emu/g), high coercivity (5000 Oe), high Curie temperature T C (520 • C), large anisotropy constant (2.65 × 10 5 -5.1 × 10 5 J/m 3 ) [1,2]. Cobalt ferrite has the following properties: high electromagnetic performance, large magneto-optic effect, excellent chemical stability, and excellent mechanical hardness [1][2][3]. Because cobalt ferrite is a hard ferromagnetic material, it is used as a high-density recording medium [4]. Cobalt ferrite substituted nonmagnetic Al 3+ ions; such material has low magnetic coercivity and large resistivity. Soft ferrite is the core material in power transformers that are used in the field of electronics and telecommunication. Singhal et al. [5] used the aerosol route for substituting Fe 3+ ions in cobalt ferrite with Al 3+ ions. The magnetic hyperfine field decreases; the ratio of Fe 3+ (oct.)/Fe 3+ (tet.) ions increases with an increase in Al 3+ ions. Chae et al. [6] synthesized Al x CoFe 2−x O 4 ferrite powders, and they determined magnetic properties of the sample. In Al x CoFe 2−x O 4 ferrite powders, saturation magnetization and coercive force decrease with increasing concentration of Al. In a study conducted by Kumar et al. [7], it was found that crystallite size of cobalt ferrite increased when they were doped Figure 1 illustrates XRD patterns for CoAl x Fe 2−x O 4 (x = 0-1.5) ferrites, which were calcined at 1000 • C. The XRD spectrum shows that all the samples have a single-phase structure. An impurity peak was not observed in these samples. Table 1 and Figure 2 prove that the lattice constant can be decreased by increasing the concentration of Al 3+ ions. The decrease in lattice parameter is probably attributed to the radius of Al 3+ ions (0.50 Å), which is smaller than Fe 3+ ions (0.64 Å) [5,6]. X-ray density was determined from the following equation [5,8]: where a is the lattice constant; M is the relative molecular weight; and N is the Avogadro number. Table 1 and Figure 2 show that density decreases with an increase in Al 3+ ion content. Because the atomic weight of Fe is greater than that of Al, the relative density constant decreases with increasing Al 3+ ion substitution. X-ray density decreases under the following condition: the relative decrease in molecular mass is greater than the negligible decline in the lattice parameter. The average crystallite size decreases with an increase in the concentration of Al 3+ ions. This phenomenon has been attributed to the size mismatch of Al 3+ and Fe 3+ ions, increasing strain and stress in the sample [7].   As shown in Figure 3, X-ray patterns (XRD) of CoAl0.1Fe1.9O4 were sintered at different temperatures. An average CoAl0.1Fe1.9O4 crystallite size increase by increasing the calcining temperature is observed in Table 2. All the samples were single-phase structures of spinel ferrite, which indicates the absence of an additional phase. No significant changes were observed in the lattice parameter of all samples. The average crystallite size of CoAl0.1Fe1.9O4 increased with an increase in calcination temperature [5].     As shown in Figure 3, X-ray patterns (XRD) of CoAl0.1Fe1.9O4 were sintered at different temperatures. An average CoAl0.1Fe1.9O4 crystallite size increase by increasing the calcining temperature is observed in Table 2. All the samples were single-phase structures of spinel ferrite, which indicates the absence of an additional phase. No significant changes were observed in the lattice parameter of all samples. The average crystallite size of CoAl0.1Fe1.9O4 increased with an increase in calcination temperature [5]. As shown in Figure 3, X-ray patterns (XRD) of CoAl 0.1 Fe 1.9 O 4 were sintered at different temperatures. An average CoAl 0.1 Fe 1.9 O 4 crystallite size increase by increasing the calcining temperature is observed in Table 2. All the samples were single-phase structures of spinel ferrite, which indicates the absence of an additional phase. No significant changes were observed in the lattice parameter of all samples. The average crystallite size of CoAl 0.1 Fe 1.9 O 4 increased with an increase in calcination temperature [5].    Figure 4 shows SEM micrographs of CoAlxFe2-xO4 (x = 0, 0.1) samples, which were annealed at 1000 °C for 3 h. Uniformly-sized, well-crystallized grains of CoAlxFe2-xO4 were obtained. Figure 5 illustrates the grain-size distribution of CoAlxFe2-xO4 (x = 0, 0.1) ferrites. The average grain size of CoFe2O4 and CoAl0.1Fe1.9O4 was about 137.5 nm and 130.5 nm, respectively. The average grain size decreased when aluminum substituents were increased. The XRD pattern confirms that the average crystallite size tends to decrease with increasing Al content. The average grain size was greater than a nanoparticle (100 nm), and the sintering temperature of the sample was very high because grain size increased with increasing annealing temperature [9].    Figure 4 shows SEM micrographs of CoAl x Fe 2−x O 4 (x = 0, 0.1) samples, which were annealed at 1000 • C for 3 h. Uniformly-sized, well-crystallized grains of CoAl x Fe 2−x O 4 were obtained. Figure 5 illustrates the grain-size distribution of CoAl x Fe 2−x O 4 (x = 0, 0.1) ferrites. The average grain size of CoFe 2 O 4 and CoAl 0.1 Fe 1.9 O 4 was about 137.5 nm and 130.5 nm, respectively. The average grain size decreased when aluminum substituents were increased. The XRD pattern confirms that the average crystallite size tends to decrease with increasing Al content. The average grain size was greater than a nanoparticle (100 nm), and the sintering temperature of the sample was very high because grain size increased with increasing annealing temperature [9].    Figure 4 shows SEM micrographs of CoAlxFe2-xO4 (x = 0, 0.1) samples, which were annealed at 1000 °C for 3 h. Uniformly-sized, well-crystallized grains of CoAlxFe2-xO4 were obtained. Figure 5 illustrates the grain-size distribution of CoAlxFe2-xO4 (x = 0, 0.1) ferrites. The average grain size of CoFe2O4 and CoAl0.1Fe1.9O4 was about 137.5 nm and 130.5 nm, respectively. The average grain size decreased when aluminum substituents were increased. The XRD pattern confirms that the average crystallite size tends to decrease with increasing Al content. The average grain size was greater than a nanoparticle (100 nm), and the sintering temperature of the sample was very high because grain size increased with increasing annealing temperature [9].   Figure 6 shows the Mösbauer spectra of CoAlxFe2-xO4 acquired at room temperature.The hyperfine parameters, isomer shift (I.S.), magnetic hyperfine field (Hhf), quadrupole shift (Q.S.), relative area (A0), and line width (Г) , were obtained by fitted spectra using Mösswinn 3.0 software (FAST Corporation , Oberhaching, Germany), and calibration was relative to a 25 μm thick sample of high-purity alpha iron. The characteristic features of the spectra were as follows: there were two Zeeman-splitting sextets; one sextet was assigned to Fe 3+ ion at the tetrahedral site, while the other sextet was attributed to Fe 3+ ions at the octahedral site. This proved the ferromagnetism of the samples. The first sextet had a larger value of isomer shift, and it was assigned to octahedral B site. The second sextet had a smaller value of isomer shift, and was assigned to tetrahedral A site. Compared to the tetrahedral A-site ions, the bond separation of Fe 3+ ions was greater in the octahedral B site of the Fe 3+ -O 2− complex (Table 3). This minimized the overlapping of orbits of Fe 3+ ions at the octahedral B-site; the larger isomeric shift was attributed to smaller covalency at octahedral B site [6].   Figure 6 shows the Mösbauer spectra of CoAl x Fe 2−x O 4 acquired at room temperature. The hyperfine parameters, isomer shift (I.S.), magnetic hyperfine field (H hf ), quadrupole shift (Q.S.), relative area (A 0 ), and line width (Г), were obtained by fitted spectra using Mösswinn 3.0 software (FAST Corporation, Oberhaching, Germany), and calibration was relative to a 25 µm thick sample of high-purity alpha iron. The characteristic features of the spectra were as follows: there were two Zeeman-splitting sextets; one sextet was assigned to Fe 3+ ion at the tetrahedral site, while the other sextet was attributed to Fe 3+ ions at the octahedral site. This proved the ferromagnetism of the samples. The first sextet had a larger value of isomer shift, and it was assigned to octahedral B site. The second sextet had a smaller value of isomer shift, and was assigned to tetrahedral A site. Compared to the tetrahedral A-site ions, the bond separation of Fe 3+ ions was greater in the octahedral B site of the Fe 3+ -O 2− complex (Table 3). This minimized the overlapping of orbits of Fe 3+ ions at the octahedral B-site; the larger isomeric shift was attributed to smaller covalency at octahedral B site [6].  It is well known that the values of isomeric shift are in the range of 0.6-1.7 mm/s for Fe 2+ (S = 2) ions; the values of isomeric shift are in the range of 0.1-0.5 mm/s for Fe 3+ (S = 1/2, 3/2, 5/2) ions [10]. As shown in Table 3, the values of I.S. indicate that iron is in Fe 3+ state. By increasing the aluminum content, the values of the magnetic hyperfine field decreased at tetrahedral A and octahedral B sites. This is because magnetic ions (Fe 3+ ions) are substituted by nonmagnetic ions (Al 3+ ions), affecting the supertransferred hyperfine fields [5]. For all samples, the quadrupole shift value was very small for the magnetic sextet at the A and B site. This indicates that spinel ferrites have local cubic symmetry. The spectra of CoAlxFe2-xO4 (0.6 ≤ x ≤ 0.8) included the magnetic sextet of B site; the magnetic sextet of A site vanished. This indicates that Fe 3+ ions existed only in the octahedral B site. When the spectrum of CoAlxFe2−xO4 (composition with x = 0.9 and 1.0) was analyzed, a single sextet and a central paramagnetic doublet were observed; this indicates relaxation effects. When the nonmagnetic Al content was increased in CoAlxFe2-xO4, the samples changed into a superparamagnetic character. The behavior of the sample went from a completely magnetic state to a mixed state of magnetic and superparamagnetic order [11,12]. For samples with x = 1.5, Mössbauer spectra consisted only of a central doublet; this exhibits a superparamagnetic character. The central doublet was attributed to the nearest nonmagnetic neighbors of magnetically isolated Fe 3+ ions. This leads to the deficiency of long-range magnetic ordering [13,14].

Mössbauer Spectroscopy
The cation distribution of CoAlxFe2−xO4 ferrite can be written as follows: It is well known that the values of isomeric shift are in the range of 0.6-1.7 mm/s for Fe 2+ (S = 2) ions; the values of isomeric shift are in the range of 0.1-0.5 mm/s for Fe 3+ (S = 1/2, 3/2, 5/2) ions [10]. As shown in Table 3, the values of I.S. indicate that iron is in Fe 3+ state. By increasing the aluminum content, the values of the magnetic hyperfine field decreased at tetrahedral A and octahedral B sites. This is because magnetic ions (Fe 3+ ions) are substituted by nonmagnetic ions (Al 3+ ions), affecting the supertransferred hyperfine fields [5]. For all samples, the quadrupole shift value was very small for the magnetic sextet at the A and B site. This indicates that spinel ferrites have local cubic symmetry. The spectra of CoAl x Fe 2−x O 4 (0.6 ≤ x ≤ 0.8) included the magnetic sextet of B site; the magnetic sextet of A site vanished. This indicates that Fe 3+ ions existed only in the octahedral B site. When the spectrum of CoAl x Fe 2−x O 4 (composition with x = 0.9 and 1.0) was analyzed, a single sextet and a central paramagnetic doublet were observed; this indicates relaxation effects. When the nonmagnetic Al content was increased in CoAl x Fe 2−x O 4 , the samples changed into a superparamagnetic character. The behavior of the sample went from a completely magnetic state to a mixed state of magnetic and superparamagnetic order [11,12]. For samples with x = 1.5, Mössbauer spectra consisted only of a central doublet; this exhibits a superparamagnetic character. The central doublet was attributed to the nearest nonmagnetic neighbors of magnetically isolated Fe 3+ ions. This leads to the deficiency of long-range magnetic ordering [13,14]. The cation distribution of CoAl x Fe 2−x O 4 ferrite can be written as follows: 4 (2) Based on the above cation distribution, the absorption-area ratio of A sites to B sites can be written as follows [12]: where f A and f B are the recoil-free fractions of Fe 3+ ions in tetrahedral A sites and octahedral B sites, respectively. The Mössbauer absorption area is proportional to the distribution of iron ions of A sites and B sites. In the current study, we assumed that f A and f B are equal [12]. Table 4 shows the cation distribution of all samples, and it was calculated using Equation (3).  Figure 7 illustrates the hysteresis loops of CoAl x Fe 2−x O 4 samples at room temperature. For all the samples, magnetization reached saturation when the strength of the magnetic field was 10,000 Oe. Table 5 shows that saturation magnetization decreased with an increase in Al 3+ ion content. The saturation magnetization can be expressed with the following equation [12]:

Magnetic Analysis
where n B is the magnetic moment and M is the relative molecular mass. The relative molecular mass of CoAl x Fe 2−x O 4 decreased with an increase in Al content. The change in magnetic moment n B was determined by Néel's theory of magnetism. The magnetic moment of Al 3+ , Co 2+ , and Fe 3+ ions was 0 µ B , 3 µ B , and 5 µ B [15][16][17], respectively. Néel's theory of magnetism was used to develop two sublattice models, which were then used to explain cation distribution in the Mössbauer spectra (Table 4). Magnetic moment n B is expressed by Equation (5) [15,16]: where M B and M A are magnetic moments of the B and A sublattices, respectively. Figure 8 illustrates the changes in experimental and calculated magnetic moments, with changes in Al 3+ ion content. Figure 8 illustrates that the experimental and calculated magnetic moment decreases with an increase in Al content (x ≤ 0.1). According to Equation (4), calculated saturation magnetization decreased with an increase in Al 3+ ion substitution. The change trend of experimental and calculated saturation magnetization was similar for x ≤ 0.1, and there was deviation between experimental and calculated saturation magnetization, which can be attributed to the actual situation of ion distribution being more complicated than that obtained from the Mössbauer spectra. For the substituents (x ≥ 0.5), there was a big difference between calculated saturation magnetization and experimental saturation magnetization, and the experimental value was smaller than the calculated value for saturation magnetization [18][19][20]. This can be explained by the three-sublattice model of Yafet-Kittel (YK) [16]. It is reasonable that the spin-canting arrangement of the magnetic moment appeared on B sites of the sample when the content of nonmagnetic Al 3+ ion substituents was too high in cobalt ferrite samples. This led to a decrease in A-B interaction and an increase in B-B interaction, which subsequently decreased magnetization.    Table 5 shows that saturation magnetization decreased with an increase in Al 3+ ion content. The saturation magnetization can be expressed with the following equation [12]:  where nB is the magnetic moment and M is the relative molecular mass. The relative molecular mass of CoAlxFe2−xO4 decreased with an increase in Al content. The change in magnetic moment nB was determined by Néel's theory of magnetism. The magnetic moment of Al 3+ , Co 2+ , and Fe 3+ ions was 0 μB, 3 μB, and 5 μB [15][16][17], respectively. Néel's theory of magnetism was used to develop two sublattice models, which were then used to explain cation distribution in the Mössbauer spectra (Table 4). Magnetic moment nB is expressed by Equation (5) [15,16]:

Magnetic Analysis
where MB and MA are magnetic moments of the B and A sublattices, respectively. Figure 8 illustrates the changes in experimental and calculated magnetic moments, with changes in Al 3+ ion content.  Figure 8 illustrates that the experimental and calculated magnetic moment decreases with an increase in Al content (x ≤ 0.1). According to Equation (4), calculated saturation magnetization decreased with an increase in Al 3+ ion substitution. The change trend of experimental and calculated saturation magnetization was similar for x ≤ 0.1, and there was deviation between experimental and calculated saturation magnetization, which can be attributed to the actual situation of ion distribution   [7]. The electron configuration of Co 2+ ions is 3d 7 [21]. The anisotropy is attributed to Co 2+ ions in the octahedral site, causing frozen orbital angular momentum and spin coupling [22]. The Al 3+ ions elicit zero angular momentum (l = 0), which does not affect magnetic anisotropy [23][24][25]. When Al 3+ ions were replaced with Fe 3+ ions, the spin-orbit coupling weakened and magnetocrystalline anisotropy decreased.
Equation (6) describes the relationship between the following parameters: coercivity H C , magnetic anisotropy K 1 , and saturation magnetization M S [7]: When magnetic anisotropy decreased with an increase in Al 3+ ions, it led to a decrease in coercivity. Figure 9 shows the magnetic hysteresis curves of an unsintered CoAl 0.1 Fe 1.9 O 4 sample at room temperature; magnetic hysteresis curves of CoAl 0.1 Fe 1.9 O 4 sample were also obtained after sintering them at 600 • C and 1000 • C, respectively. Table 6 shows that the saturation magnetization of CoAl 0.1 Fe 1.9 O 4 sample increased with an increase in sintering temperature; these changes were attributed to an increase in particle size [5]. There is no significant change in the saturation magnetization of the unsintered CoAl 0.1 Fe 1.9 O 4 sample; moreover, the CoAl 0.1 Fe 1.9 O 4 sample did not show any significant change even after being annealed at 600 • C. This confirms that the uncalcined sample has good crystallinity, which was further established by XRD. were located at the tetrahedral A sites and octahedral B sites. The magnetocrystalline anisotropy is primarily attributed to Co 2+ ions of octahedral sites, which are present in pure cobalt ferrite CoFe2O4 [7]. The electron configuration of Co 2+ ions is 3d 7 [21]. The anisotropy is attributed to Co 2+ ions in the octahedral site, causing frozen orbital angular momentum and spin coupling [22]. The Al 3+ ions elicit zero angular momentum (l = 0), which does not affect magnetic anisotropy [23][24][25]. When Al 3+ ions were replaced with Fe 3+ ions, the spin-orbit coupling weakened and magnetocrystalline anisotropy decreased. Equation (6) describes the relationship between the following parameters: coercivity HC, magnetic anisotropy K1, and saturation magnetization MS [7]: When magnetic anisotropy decreased with an increase in Al 3+ ions, it led to a decrease in coercivity. Figure 9 shows the magnetic hysteresis curves of an unsintered CoAl0.1Fe1.9O4 sample at room temperature; magnetic hysteresis curves of CoAl0.1Fe1.9O4 sample were also obtained after sintering them at 600 °C and 1000 °C, respectively. Table 6 shows that the saturation magnetization of CoAl0.1Fe1.9O4 sample increased with an increase in sintering temperature; these changes were attributed to an increase in particle size [5]. There is no significant change in the saturation magnetization of the unsintered CoAl0.1Fe1.9O4 sample; moreover, the CoAl0.1Fe1.9O4 sample did not show any significant change even after being annealed at 600 °C. This confirms that the uncalcined sample has good crystallinity, which was further established by XRD.  With a steadily increasing sintering temperature, the coercivity of CoAl0.1Fe1.9O4 sample initially increased and then steadily decreased. This may be attributed to variation in grain size. The coercivity of the single-domain region is given by the following equation: HC = g-h/D 2 . In the multidomain region, the relationship between coercivity and grain size is established by the following equation: HC = (a + b)/(D). Here, 'D' is the diameter and 'g, h, a, and b' are constants of the particle [5,26]. Hence, coercivity increased with increasing grain size in the single-domain region. In the multidomain region, coercivity decreased with an increase in particle diameter [27,28]. In our study, we determined  With a steadily increasing sintering temperature, the coercivity of CoAl 0.1 Fe 1.9 O 4 sample initially increased and then steadily decreased. This may be attributed to variation in grain size. The coercivity of the single-domain region is given by the following equation: H C = g-h/D 2 . In the multidomain region, the relationship between coercivity and grain size is established by the following equation: H C = (a + b)/(D). Here, 'D' is the diameter and 'g, h, a, and b' are constants of the particle [5,26]. Hence, coercivity increased with increasing grain size in the single-domain region. In the multidomain region, coercivity decreased with an increase in particle diameter [27,28]. In our study, we determined the grain size of CoAl 0.1 Fe 1.9 O 4 samples that were calcined at different temperatures; the grain size of CoAl 0.1 Fe 1.9 O 4 samples varied from the single-domain region to the multidomain region. With an increasing annealing temperature, the coercivity of CoAl 0.1 Fe 1.9 O 4 sample increased initially and then decreased.

Conclusions
XRD analysis reveals the single-phase structure of CoAl x Fe 2−x O 4 samples that were calcined at 1000 • C. The lattice constant decreased when smaller Al 3+ ions were replaced with larger Fe 3+ ions. The XRD spectra of CoAl 0.1 Fe 1.9 O 4 samples were obtained after sintering them at different temperatures; these samples were prepared with a sol-gel autocombustion method, so they had good crystallinity. SEM results indicate that well-crystallized particles of uniform size were present in the sample. We obtained the Mössbauer spectra of CoAl x Fe 2−x O 4 samples, which were calcined at 1000 • C. The Mössbauer spectra reveal that with an increase in aluminum concentration, CoAl x Fe 2−x O 4 samples undergo a transition from ferrimagnetic behavior to superparamagnetic behavior. Cation distribution was estimated from the Mössbauer data. The coercivity and saturation magnetization of CoAl x Fe 2−x O 4 samples decreased with an increase in Al content (x). The changes in saturation magnetization can be attributed to Néel's theory and the Yafet-Kittel model. Coercivity decreased with an increase in aluminum content, which is attributed to the weakening of magnetocrystalline anisotropy. The coercivity and saturation magnetization of CoAl 0.1 Fe 1.9 O 4 sample initially increased and then steadily decreased. Particle size increased with an increase in annealed temperature.