La3+ substitution impact on structural, magnetic, and electrical properties of CoFe2O4 synthesized via sucrose auto-combustion

Lanthanum (La)-substituted cobalt ferrites CoFe2-x La x O4 (x = 0.00–0.09) were successfully prepared using the sucrose sol-gel auto-combustion route. La-substitution effects on the structural, magnetic, and electrical/dielectric characteristics were investigated using different techniques. X-ray diffraction (XRD) results displayed a deviation from the single-phase structure at La-content ≥ 0.04 because of the formation of orthorhombic LaFeO3 at the boundaries of the grains. Both XRD and Fourier transform infrared (FT-IR) measurements showed that the crystal structure had not been significantly affected by La-substitution attributed to the crystallization out of this LaFeO3. Vibrating sample magnetometry (VSM) measurements revealed gradual decreases in the magnetization as the La-content increases, which could be referred to as the decrease in the super-exchange interaction in the octahedral sites because of the preferential substitution of large ionic radius La3+ ions in the octahedral positions. As the antiferromagnetic LaFeO3 formed, a severe reduction in the magnetization appeared. The increase in the magnetization at 0.09 was attributed to the cationic redistribution among sublattices. On the other hand, the coercivity data indicated the hard magnetic characteristics of all the samples. The electrical conductivity results showed the semiconducting character of all samples with obvious decreases with increasing La-content. According to Verwey’s hopping mechanism, these decreases were attributed to the preferential occupation of La3+ by octahedral positions. Dielectric results versus temperature indicated anomalies relaxations around 460 K with the successive addition of lanthanum attributed to the electrical inhomogeneity that occurred due to oxygen vacancy created or the Maxwell-Wagner mechanism.


Introduction
Ferrites are considered an important class of magnetic ceramic spinels with superior magnetic, electrical and optical properties. Their chemical formula, MFe 2 O 4, indicated M as a divalent cation and Fe in a tri-valence state, which could be substituted by any other trivalent cation. The different characteristics of these ferrites depend on various parameters, including synthesis route, composition, calcination temperature, and cation distribution [1]. Among the ferrites, CoFe 2 O 4 received enormous investigations in the last decades due to its fantastic features, such as coercivity, resistivity, magneto-crystalline anisotropy, and chemical stability. Therefore, they are used in extensive scientific and technological applications, including magnetic storage, drug delivery, antimicrobial, photocatalytic dye removal, ferrofluids, magnetic refrigeration systems, and microwave devices [2][3][4][5][6][7][8].
CoFe 2 O 4 is classified as a hard magnetic material because of its high coercivity; It always crystallizes in inverse spinel structures where the Co 2+ ions occupy the octahedral B-sites [9]. It is also one of the tunable magnetic systems in which the magnetic characteristics could be altered by substituting either Co 2+ or Fe 3+ ions with various transition or non-transition elements. The drawback with CoFe 2 O 4 is to control its crystal size and high nucleation rate during its preparation, which hinders obtaining tiny-sized material suitable for different technological applications. This could be achieved by substituting Fe 3+ ions with other larger ions [10]. In recent Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
publications [11][12][13][14][15][16][17], the substitution of Fe 3+ ions with different cations, such as La 3+ , Gd 3+ , Al 3+ , Y 3+ , and Nd 3+ , induced different electromagnetic properties. Generally, doping with rare Earth elements (Re 3+ ) could be promisingly modified in its electromagnetic properties since the magnetism carriers will be the 4f electrons, and the spin coupling will be between 3d and 4f electrons via the super-exchange interaction [12]. Therefore, the non-magnetic La 3+ substitution in the CoFe 2 O 4 could bring new properties or alter the original properties of the CoFe 2 O 4 .
The synthesis of rare Earth-doped CoFe 2 O 4 suffers from many difficulties, such as segregation of orthoferrites (ReFeO 3 ), hematite (Fe 2 O 3 ) formation, or metal monoxides even with using a very low concentration of the Re 3+ . The main reason for these difficulties is that the larger ionic sizes of these rare Earth elements prefer the substitution of Fe 3+ ions located in the octahedral positions, resulting in lattice strains [12]. Many various techniques have been employed in the synthesis of rare Earth-substituted CoFe 2 O 4, such as sol-gel [13], co-precipitation [18], hydrothermal [19], and solid-state reaction [14] methods. Recently, the sol-gel route was found to afford size control besides its low cost and easy production with a well-controlled distribution of substituents [20].
Hence, the present study aims to tune the different cobalt ferrite physical characteristics, especially the electric and magnetic ones using La 3+ substitution in the system CoFe 2-x La x O 4 prepared using sucrose-assisted auto-combustion. X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were utilized to investigate the structural properties. Fourier-transform infrared (FT-IR) was used to investigate the chemical functional groups. The expected magnetic dilution due to non-magnetic La 3+ substitution was analyzed using a vibrating sample magnetometer (VSM). Further, electrical/dielectric properties were investigated. The impacts of La 3+ substitution on the structural, magnetic, and electrical/dielectric characteristics were thus discussed. To the best of my knowledge, such integrated studies on this ferrite type and using the entire auto-combustion route have not yet been studied in the literature.   11 . In the standard experiment, the calculated nitrate ratios were added to distilled water (∼100 ml). After complete dissolving, a 100 ml solution of sucrose-containing 12 g solid was added under vigorous stirring at about 60°C; ammonium hydroxide (NH 4 OH) was utilized to adjust the pH solution to nearly 7. A solution in the form of gel was formed and further heated at the maximum hot plate temperature until the auto-combustion process was initiated. Then, the dry gel formed was swelled into a powder with the emergence of dense gases. The outcome   powder was collected and stored as it is. The ferrite formation via the auto-combustion process could be suggested in the following equation for x = 0.02 as an example:

Experimental
The synthesized powders' phase formation and crystal structure were explored using XRD, model D8 Advanced (Bruker) (CuK α1 , 40 kV, 25 mA) in the 2θ from 10°to 70°. Perkin-Elmer FT-IR, model Spectrum 3 FT-IR (PerkinElmer) was utilized with the KBr sampling technique to obtain the spectra of the entire ferrites in the spectral range 800-300 cm −1 . Powder morphologies were examined using TEM, model JEOL-2010 (JEOL). At room temperature, the magnetic characteristics were characterized using VSM, model VSM-8600M (Lakeshore) up to the magnetic field of 1 kOe. Discs were made to measure electrical/dielectric properties; the diameter and thickness of the discs were 1 cm and 1.5 mm, respectively. The discs' faces were rubbed with a thin layer of silver. The measurements were recorded in the frequency range of 100 Hz-5MHz, and temperatures up to 723 K using an LCR bridge, model high tester 3531 (Hioki).

Results and discussion
The obtained XRD patterns for the prepared CoFe 2-x La x O 4 system are exhibited in figure 1. According to the ICDD card no. 79-1744, the diffraction peaks were indexed as (220), (311), (222), (400), (422), (511), and (440); these diffracted peaks were assigned to rhombohedral CoFe 2 O 4 . By increasing La 3+ ions concentrations, a deviation from the single-phase structure appeared for La 3+ concentrations (x) 0.04. This deviation is attributed to the appearance of a very small peak at about 2θ = 32.5°corresponding to the orthorhombic LaFeO 3 secondary phase (JCPDS card no. 79-1744). The larger ionic radius of La 3+ (1.032 Å) compared to that of Fe 3+ in the octahedral sites (0.645 Å) [21] limited the amount of substituted Fe 3+ relative to that of La 3+ . It thus resulted in the redundancy of La 3+ at the grain boundaries to form the LaFeO 3 phase [15].
The calculated lattice parameters (a) (table 1) indicated a tiny decrease while increasing La 3+ content; this result could be explained in the view of the relaxing strain that occurred at the octahedral sites because of the substitution of the larger ionic radius La 3+ (preferring octahedral site occupation) for, the smaller Fe 3+ ions [22]. Another suggested reason for this behavior is that some La 3+ ions can diffuse into grain boundaries. In contrast, others cannot and reside at the grains' boundary (forming LaFeO 3 ), and thus the small quantity of Fe 3+ gets replaced in the lattice by larger La 3+ [14,19].
The crystallite sizes (L) were estimated with Scherrer's equation [23]; they are listed in table 1. They exhibited the formation of nanocrystalline ferrites and indicated, in accordance with the calculated lattice parameters, a slight decrease by increasing lanthanum content. This behavior is supported by the evident reduction in the crystallinity of the particles with increasing lanthanum content, as shown by the peaks' broadening (figure 1). Due to the mismatch in size between Fe 3+ ions and La 3+ substituent, a crystalline anisotropy could be formed, which could create volume strain in the crystal. This volume strain is then relaxed via decreasing the crystallite size with increasing lanthanum substitution by forming the impurity phase, LaFeO 3, as discussed above [19,24]. The same observations were observed by Mariosi et al [2] for La-substituted CoFe 2 O 4 synthesized via the citrate sol-gel route.
X-ray densities (D x ) were calculated for the different samples, summarized in table 1, indicating a gradual increase with increasing lanthanum content. The larger atomic weight of La (138.906 g mol −1 ) compared to that of Fe (55.845 g mol −1 ), besides the relaxed strain in the volume with increasing substitution, could describe this obvious increase. Similar behaviors are already reported for La-substituted CoFe 2 O 4 [17]. Figure 2 exhibits the FT-IR spectra of the prepared substituted ferrites at different lanthanum content. The obtained spectra indicated that two strong absorption peaks in the FTIR spectral range of 750-300 cm −1 agreed well with the common feature of all spinel ferrites [25]. The observed absorption peaks did not show any dependence on the amount of the lanthanum substituent and showed the same behavior even at high lanthanum concentrations. The peaks' centers as a function of La-content are represented in table 1. The absorption peak The obtained peaks' positions agreed well with those reported for nanocrystalline CoFe 2 O 4 [20]. The absence of no changes in the band positions with La-substitution agreed well with the LaFeO 3 formation of the secondary phase on the boundary of grains without any inclusion in the crystal lattice at higher La-content. Generally, from XRD and FT-IR measurements, one can conclude that the La-substitutions show no significant impact on the crystal structure since most added lanthanum at higher concentrations crystallized out on the grain boundaries of CoFe 2 O 4 crystal.
TEM photomicrographs for the ferrites with La-contents of 0.00 and 0.09, as typical examples, are exhibited in figure 3. Most of the particles are spherically shaped and are densely agglomerated owing to the strong magnetic interaction between particles. The extent of agglomerations decreased by increasing La-substitution, which might be referred to as the decrease in magnetic interaction. The particle sizes were found to decrease from 28 nm in the unsubstituted sample to 23 nm in the highly substituted one, which appeared to be slightly smaller than those reported via XRD measurements (table 1).
The magnetic properties viz. saturation coercivity (H c ), remanent magnetization (M r ), and magnetization (M s ), for the entire studied system were investigated via hysteresis loops measurements ( figure 4). For all samples, the hysteresis displayed the s-shape behavior belonging to the ferromagnetic materials feature; at the same time, they exhibited unsaturated magnetization even at the maximum applied magnetic field. These unsaturation features might be referred to as the requirement of higher magnetic fields to saturate [26] or the antiferromagnetic interaction besides the ferromagnetic one in the materials [27].
The data in table 1 revealed that the magnetization values decreased gradually when the La-substitution increased. Our early publication on nano-sized CoFe 2 O 4 synthesized via sucrose auto-combustion route [23] indicated an inverse spinel structure ((Fe 0.95 Co 0.05 )[Co 0.95 Fe 1.05 ]O 4 ) in which Co 2+ is distributed among octahedral (B-site) and tetrahedral (A-site) sublattices. Besides, La 3+ is known to be restricted to the occupancy of the octahedral site because of their only +3 valence state and larger ionic radius [24]. In this context, the obtained magnetic values will be expected to have a relation with the cationic arrangement between the two sublattices, which can affect the unit cell net magnetic moment.
The undoped CoFe 2 O 4 indicated a saturation magnetization value (M s ) of 73.3 emu g −1 . This value is gradually reduced while the subsequent substitution of La 3+ ions to 39.5 emu g −1 at a La-content of 0.04. This decrease could be because of the reduction of the super-exchange interaction and, consequently, the net magnetic moment on subsequent non-magnetic La 3+ substitution at the expense of magnetic Fe 3+ located in the octahedral sites. The severe decrease in the M s to about its half value by increasing La-content to 0.05 (21.7 emu g −1 ) could be related to the formation of antiferromagnetic LaFeO 3 [28], as discussed above in the XRD part. A similar decreasing trend was also reported by Vedavil et al [29] and Rotray et al [10] for La-substituted CoFe 2 O 4 prepared by co-precipitation and glycine nitrate methods, respectively. The unexpected increase in the M s by increasing La-content to 0.09 (31.6 emu g −1 ) might be referred to as the alteration in the substitution mechanism. Here, the La 3+ ions tend to substitute Co 2+ ions instead of Fe 3+ in the octahedral positions, which permits the migration of Co 2+ ions to tetrahedral positions accompanied by the moving of Fe 3+ in the opposite direction to octahedral positions. This will increase the net magnetic moment since the magnetic moment per ion of Fe3 + (5 BM) is higher than that of Co 2+ ions [9]. The same feature was observed by Demirci et al [22] for La-substituted cobalt ferrite prepared by the citrate method. The recorded Ms values were generally higher than those mentioned in the published works for similar systems [2,24].
From the magnetic measurements, the summarized coercivity values in table 1 indicated that the coercivity values of the prepared ferrites range between 841 and 1156 Oe. This obtained high coercivity indicated hard magnetic characteristics for all the studied ferrites. The coercivity values tend to increase up to La-content of 0.03, after which a gradual decrease is initiated. The coercivity is considered a microstructure character and depends on a lot of factors, including the presence of non-magnetic atoms, surface, defects, and strains [15]. Generally, the magnetic Fe 3+ replacement by non-magnetic La 3+ on the octahedral sites weakens the interactions between magnetic ions besides reducing the resistance of the domain-wall motion and thus decreases coercivity [2,18,24]. The coercivity increase in the present work by increasing La-content could be referred to as the hard ferrite characteristics, as Haque et al [19] reported for La-substituted cobalt ferrite prepared via the hydrothermal method. Another reason could be enhancing the magneto crystalline and magnetoelastic anisotropies by the larger ionic radius La 3+ ions, as reported by Kadam [11] for Gd 3+ substituted cobalt ferrite.
The presence of impurities on the grain boundaries or oxygen vacancy on the material's surfaces could break the super-exchange interaction between magnetic cations and go against domain wall displacement, thus inducing spin disorders [19,30]. Thus, the initiation of the antiferromagnetic LaFeO 3 formation at the grain boundaries of the entire prepared ferrites at the La-content of 0.04 would be responsible for the coercivity decrease by subsequent higher La-substitution. Similar results were observed by Haque et al [19] and Yadav et al [15] for lanthanum and neodymium-substituted cobalt ferrite, respectively. Figure 5 displays the electrical conductivity as a function of 1000/temperature (T) for all the prepared samples. Obviously, a semiconducting behavior was obtained for the entire system, where the ac-conductivity gradually increases with the rising temperature. Besides, the figure also showed an increase in the conductivities with growing frequency, indicating that the conduction is through the electron hopping between different valence cations. This increase could be due to the frequency force and facilitates the hopping of electrons [23]; it is like a pumping force. On the other hand, at higher temperatures, this frequency dependence vanishes due to the increase of the lattice vibrations generated by increasing thermal energy that scatters the hopping electrons and overcomes the frequency effect.
The conductivities measured at 360 K and 100 kHz are listed in table 1, indicating a slight change in the conductivity followed by an obvious decrease with increasing La-content. Based on Verwey's hopping mechanism [31,32], the conductions in spinel ferrites are mainly controlled by electrons or hole exchanges between cations with mixed valence states. Thus, three types of electron hopping could have proceeded in the sublattices: A-A, B-B, and A-B hoppings. Where the B-B hoping one will be the most predominant since mixedvalence iron cations (Fe 2+ and Fe 3+ ) are present in the octahedral B-sublattice. A-A hopping cannot participate since only Fe 3+ occupies this position, and no Fe 2+ is present. In addition, the considerable distance between A and B-sites hindered the proceeding of the electron hopping.
The successive substitution of La 3+ for Fe 3+ in the octahedral B-sites thus blocked Verwey's hopping mechanism, limited the degree of electrical conduction, and consequently decreased the conductivity at higher substitution. Similar behavior was observed by Kumar et al [18] for La-substituted cobalt ferrites prepared by the co-precipitation route.
The dielectric constant (ε′) at different frequencies for the synthesized ferrites is exhibited in figure 6. It can be seen that the ε′ indicated the presence of dielectric anomalies that appeared as broad peaks around 460 K with the successive addition of lanthanum. The apparent decrease in the peak's height with the rising frequency could indicate a relaxor-like characteristic [33]. According to the previously reported results on La-substituted cobalt ferrites [19], this type of anomaly could be referred to as the electrical inhomogeneity that occurred due to oxygen vacancy created or the Maxwell-Wagner mechanism.
The whole temperature dependence of the dielectric constant must be discussed further to understand the relaxations with lanthanum addition. There is no alteration in the dielectric constant at low-temperature ranges until a specific temperature of around 375 K. This independent behavior could be attributed to [14] the grain insulating-boundaries formation because of the La-substitution that could reduce Fe 3+ ions on the octahedral sites and consequently reduces the Fe 3+ ↔ Fe 2+ polarization. In addition, the large ionic radius La 3+ ions increase lattice-electron interactions, thus restricting charge carriers from moving. On attaining the temperature of 375 K, charge carriers could increase and initiate the space charge polarization, leading to the sudden maximization in the ε′ [14,19]. Figure 7 illustrates the ε′ as a function of temperature. The figure indicated high dielectric values at low frequency, which decreases steeply by further rising of frequency. This behavior is normal dielectric behavior and could be because of the inability of the Fe 3+ ↔ Fe 2+ electron exchange to follow up the fast change in the dipoles of the applied electric field [18]. Generally, ε′ values measured at 360 K and 100 kHz for the studied system (table 1) revealed a similar trend as that obtained for conductivity. Again, the reduction in the dielectric constant with rising the La-substitution could be referred to as the limitation of Fe 3+ ↔ Fe 2+ exchange.

Conclusions
The sucrose sol-gel auto-combustion route has led to the successful preparation of the CoFe 2-x La x O 4 system as obtained from XRD measurements. The La-substituted ferrites indicated a single-phase structure up to Lacontent of 0.03, after which a secondary phase of LaFeO 3 started to appear. Both XRD and FT-IR measurements concluded that the La-substitution has no significant effect on crystal structure since most of the added lanthanum is crystalized out on the grain boundaries of CoFe 2 O 4 . VSM measurements showed magnetization reduction values from 73.3 to 31.6 emu g −1 with rising La-substitution. This behavior was attributed to the cationic distribution and the presence of antiferromagnetic LaFeO3. In addition, the obtained coercivity values indicated the hard magnetic properties of the studied samples. The electrical conductivity measurements as a function of temperature showed semiconducting characteristics for all samples, with conductivity decreasing by increasing La-content. The substitution of La 3+ for Fe 3+ in the octahedral B-sites blocked Verwey's hopping mechanism and thus limited the degree of conduction. The dielectric constant (ε′) measurements indicated similar behavior to that of conductivity.