Effect of Cobalt Ion Concentration and Thermal Annealing Temperature on Structural and Magnetic Properties of CoFe 2 O 4 nanoparticles

Exploring physical properties of magnetic nanoferrites for applications in data storage media and biomedicine is a crucial step, providing new insights into the physics of nanostructured materials. Here, the focus is on studying the effect of cobalt ion concentration and thermal annealing temperature on structural and magnetic properties of cobalt ferrite (CoFe 2 O 4 ) nanoparticles (NPs) synthesized using a co-precipitation method. To this end, Co 1 − x (Fe 2 O 4 ) x (x = 0.25, 0.5, and 0.75) NPs are initially prepared and then thermally annealed at different temperatures (T = 400 ºC–800 ºC). X-ray diffraction patterns along with eld-emission scanning electron microscopic images indicate the formation of inverse cubic spinel structure with different crystallite sizes and NP size distributions when changing the cobalt ion concentration. Based on hysteresis loop measurements, magnetic parameters such as saturation magnetization (M s ) and coercivity (H c ) show increasing trends from 5.641 emu/g and 146.246 Oe to 8.936 emu/g and 1789.555 Oe when decreasing the cobalt ion concentration. By performing the annealing process, magnetic properties are signi�cantly enhanced in the case of x = 0.25 and 0.5 at T = 400 ºC and 600 ºC, achieving M s = 129.954 emu/g and H c = 1137.697 Oe. Meanwhile, rst-order reversal curve (FORC) diagrams are employed to map magnetostatic interactions and coercivity distributions as a function of cobalt ion concentration for NPs annealed at T = 400 ºC, manifesting magnetically soft and hard phases. It is found the maximum FORC distribution shifts to higher H c values with decreasing the cobalt ion concentration.


Introduction
Magnetic plasmonic nanoparticles are of tremendous interest in biomedicine due to their numerous potential uses [1], including as in drug delivery, magnetic hyperthermia, photo thermal treatment, and molecular imaging [2].Metallic-nonmetallic structures are the most common type of magnetic-plasmonic heterodimer due to their superior qualities, particularly magnetic properties and chemical/biological stability.Where it is considered CoFe 2 O 4 Nps, one of the types nonmetallic magnetic transition metal oxides used to form magnetic-plasmonic nanoparticles [3].
Cobalt ferrite (CoFe 2 O 4 ) is one of hard magnetic materials and have recently been of interest due to its high coercivity (H c = 5400 Oe) at room temperature, moderate saturation magnetization (M s = 80 emu/g), and good mechanical and chemical stability [4,5].Also, cobalt ferrite possesses high hardness, electrical insulation, photomagnetism, magnetocrystalline anisotropy, wear resistance, and electromagnetic performance [6][7][8].In turn, these features make CoFe 2 O 4 a promising candidate for various applications in magnetic devices such as high-density recording media and spintronics [9,12], as well as in magnetic uids for biomedical purposes [13].
CoFe 2 O 4 is considered a member of the family of inverse spinel structures, where cobalt ions (Co + 2 ) occupy half of octahedral sites of the lattice structure.The other half of octahedral sites as well as all the tetrahedral sites are occupied by iron ions (Fe + 3 ).Accordingly, the ferrimagnetic structure is created by the two anti-parallel sub lattices, being coupled by superexchange interactions through oxygen ions (O − 2 ) [14,15].NPs using different methods such as co-precipitation [17], hydrothermal [18], micro-emulsion [19], citrategel precursor [20], and sol-gel [21].However, several of these methods cannot be used for large scale synthesis purposes because they require costly and often toxic reagents, high reaction temperatures, cumbersome synthetic steps, and lengthy reaction times [22].Among the aforementioned methods, the co-precipitation is taken into account as one of the best and attractive approaches due to its simple and cost-effective experimental steps, while also providing small NP sizes at low annealing temperatures [23].
However, the effect of Co + 2 ion concentration and thermal annealing temperature on structural properties and magnetic behavior of CoFe 2 O 4 NPs needs to be better studied and understood.
As an important task, it is crucial to understand magnetic characteristics of ferrite NPs so as to develop their applicability in different research elds.In this respect, rst-order reversal curve (FORC) diagram technique can be utilized as a powerful approach to investigating magnetic properties in detail [24].In other words, FORC diagram technique can reveal different magnetic phases, in addition to determining magnetic states (e.g., single domain, superparamagnetic, and multi-domain states) of materials.This technique also enables the determination of magnetostatic interactions and coercive eld distributions, providing extra data compared to hysteresis loop measurements.Thus far, FORC diagram technique has been employed to study magnetic characteristics of various nanomaterials, including nanowires, NPs, and nanorods [25].
In this paper, a co-precipitation method is utilized to synthesize cobalt ferrite NPs with varied cobalt ion concentrations, followed by thermal annealing at different temperatures.Various characterization techniques are used to study structural, morphological, and compositional properties of the resulting NPs.
In particular, magnetic behavior of as-synthesized and annealed Co  1) and dissolved in 400 ml of deionized water.The resulting mixed solution was stirred at the temperature of ~ 75 ºC for 30 min in order to improve its homogeneity.
Afterwards, NaOH solution (6 M in 100 ml of deionized water) was dropwise added to the previous solution using a burette until reaching pH = 13.During the reaction process, the solution color changed from brown to dark brown, which indicated the formation of ferrite NPs.The precipitate was collected using a permanent magnetic and then rinsed several times with distilled water and ethanol.Finally, the rinsed precipitate was heated in an oven at 100 ºC for 6 h, followed by crushing it to powder.Figure 1 schematically depicts the co-precipitation synthesis process of CoFe 2 O 4 NPs.On the other hand, the effect of thermal annealing temperature on the properties of the CoFe 2 O 4 NPs was studied by placing them in a furnace at temperatures of 400 ºC, 600 ºC, and 800 ºC under mixed argon and hydrogen atmosphere (85% Ar and 15% H 2 ).

Characterizations
To
The average crystallite size was estimated using Scherrer's equation [29], taking into account the highly intense peak of (311) orientation.The lattice constant (a) was also calculated using the following equation: [30,31], where d is the interplanar distance and (hkl) are the Miller indices.The results obtained are presented in Table 2  It is worth noting that the mean diameter of the NPs matches well with the crystallite size obtained from the XRD analysis (see Table 2).The EDS spectra shown in Fig. 5 also con rm the formation of Co 1 − x (Fe 2 O 4 ) x NPs with the stoichiometric ratio.

Hysteresis loop measurements
The room-temperature hysteresis loop measurements of as-synthesized and annealed Co 1 − x (Fe 2 O 4 ) x NPs (x = 0.25, 0.5, and 0.75) are shown in Fig. 6.Based on these measurements, the values of M s , H c , remnant magnetization (M r ) and remanence ratio (M r /M s ) of the NPs were extracted, and the results are presented in Table 3.
From Fig. 6(a) and Table 3, magnetic properties of the as-synthesized NPs are noticeably observed to increase with decreasing the Co + 2 ion concentration.In general, an increase in M s can be justi ed based on the exchange interaction taking place between tetrahedral and octahedral sub-lattices and site occupation.The Neel model speci es three types of magnetic interactions between tetrahedral and octahedral sites for the spinel ferrite system, including both ions at the tetrahedral A site (i.e., A-A interaction), both ions at the octahedral B site (i.e., B-B interaction), and one ion at the tetrahedral A site and the other at the octahedral B site (i.e., A-B interaction).The strength of the (A-B) interaction is considerably higher than that of the other two magnetic interactions [31].Moreover, the increase in M s (from 5.641 to 8.936 emu/g) with the reduction in Co + 2 concentration may arise from the replacement of Co + 2 ions (having a magnetic moment of 3 µ B ) by stronger magnetic Fe + 3 ions (magnetic moment = 5 µ B ) in the octahedral B sites.
Alternatively, H c value is observed to increase by replacing Co + 2 ion in Fe + 3 octahedral site of CoFe 2 O 4 NPs. Essentially, H c of magnetic NPs is affected by several factors such as magnetocrystalline anisotropy, shape anisotropy, NP morphology, NP size distribution, and M s [32].In fact, it has been well documented that H c of ferrite NPs can be correlated with their size, so that single domain particles with larger sizes will possess a larger magnetic anisotropy [33].Herein, the increase in H c (from 146.246 to 1789.555Oe) with the reduction in Co + 2 concentration may be related to the increase in the mean diameter (from 9.89 to 24.03 nm), according to the FE-SEM results shown in Figs. 3 and 4.  Regarding the variation behavior of H c , an increase in size of annealed NPs may enhance the anisotropy energy, which in turn results in an increase in H c .The further increase in the thermal annealing temperature decreases H c , which could be related to one of the following two reasons.Firstly, it may be due to a transition from single domain to pseudo-single domain or multidomain state with increasing NP size.Secondly, a combination of surface anisotropy and thermal energies may occur due to the increased temperature [34].It is possible to acquire information about exchange interactions and magnetocrystalline anisotropy of ferrite nanomaterials based on M r /M s ratio [34].In this respect, non-interacting uniaxial single domain NPs are expected to have M r /M s of 0.5, as theoretically described by Stoner-Wohlfarth model [36].In the present study, M r /M s of annealed CoFe 2 O 4 NPs is found to be in the range of 0.485-0.081,indicating their magnetically interacting nature that can be evaluated using FORC diagram technique.

FORC diagrams
The FORC diagram technique was employed to acquire detailed and comprehensive information about magnetic behavior of the NPs [24].The FORC diagrams of

Conclusions
Co 1 − x (Fe 2 O 4 ) x NPs (x = 0.25, 0.5, and 0.75) have been synthesized by a co-precipitation method, followed by thermal annealing at T = 400 ºC, 600 ºC and 800 ºC.Various characterization techniques such as XRD, FE-SEM, EDS and VSM were utilized to investigate structural, morphological, compositional, and magnetic properties of the resulting NPs.Based on XRD patterns and FE-SEM images, crystallite size and mean diameter of spherical-like NPs showed increasing trends in the range of 7.077-23.559and 9.89-24.03nm, respectively, when decreasing the cobalt ion concertation (from x = 0.25 to 0.75).The VSM results comprised hysteresis loop and FORC diagram measurements of as-synthesized and annealed NPs.From the former measurement, a signi cant enhancement was observed in H c value up to 1789.555Oe, which was ascribed to the increase in the mean diameter of NPs.From the latter measurement, decreasing the cobalt ion concentration at T = 400 ºC was found to separate soft and hard phases with reduced magnetostatic interactions.Also, the FORC diagrams indicated dominant role of single domain NPs in determining the magnetic properties after performing the annealing process.

Declarations
Authors' contributions: Ameer F. Shamkhi wrote the main manuscript text and prepared all gures.Hashim Jabbar supervised the manuscript.All authors reviewed the manuscript.

Figure 3 (
Figure 3(a)-(c) shows FE-SEM images of Co 1 − x (Fe 2 O 4 ) x NPs synthesized using different cobalt ion concentrations (x = 0.25, 0.5, and 0.75).The NP size distributions were determined using Digimizer software, and the corresponding histograms are shown in Fig. 4. As observed, the NPs have spherical-like morphology with size distribution ranging from 7 to 30 nm.While the NPs possess homogeneous sizes, the agglomeration increases with decreasing the size.In other words, the mean diameter of the synthesized NPs increases with decreasing the concentration of Co 2+ ions, thereby reducing their agglomeration.

Figure 4 (
Figure 4(b)-(d) shows room-temperature hysteresis loops of Co 1 − x (Fe 2 O 4 ) x NPs (x = 0.25, 0.5, and 0.75) annealed at T = 400 ºC, 600 ºC, and 800 ºC.As can be seen, the hysteresis loop shape varies depending on the thermal annealing temperature.In the case of Co 0.25 (Fe 2 O 4 ) 0.75 NPs, M s increases with increasing the annealing temperature, whereas the corresponding H c is reduced.For Co 0.75 (Fe 2 O 4 ) 0.25 and Co 0.5 (Fe 2 O 4 ) 0.5 NPs, the maximum values of M s (49.385 and 129.954 emu/g) are achieved at T = 600 ºC.
Co 1 − x (Fe 2 O 4 ) x NPs (x = 0.25, 0.5, and 0.75) annealed at T = 400 ºC are shown in Fig. 7(a)-(c).From Fig. 7(a), a tear-drop con guration is observed in the FORC diagram, having broad coercive eld distributions with two peaks.The rst distribution is located around the origin of the diagram, featuring the contribution of magnetically soft NPs with almost zero FORC coercivity value ~ 0 Oe).The second distribution is elongated along the H c axis, arising from magnetically hard NPs with = 1185 Oe.From the hysteresis loop measurements, H c of Co 0.75 (Fe 2 O 4 ) 0.25 NPs was found to be about 1095 Oe, which is near to the corresponding value.This indicates the dominant contribution of single domain NPs in determining the magnetic properties.In the case of Co 0.5 (Fe 2 O 4 ) 0.5 NPs, shifts to a higher value (~ 1450 Oe) (see Fig. 7(b)), which is however accompanied with a slight increase in the hysteresis loop H c to about 1135 Oe.The reason for this variation can be ascribed to the narrower coercive eld distribution along the H c axis after annealing the NPs with the reduced cobalt ion concertation.By further reducing the cobalt ion concentration for the annealed Co 0.25 (Fe 2 O 4 ) 0.75 NPs, Fig. 7(c) demonstrates distinct magnetic soft and hard phases.In this case, of the hard phase is similar to that of annealed Co 0.5 (Fe 2 O 4 ) 0.5 NPs.Moreover, the coercive eld distribution is reduced along the H c axis, leading a decrease in hysteresis loop H c to about 1065 Oe.Concerning the magnetostatic interactions, while FORC diagrams of Co 0.75 (Fe 2 O 4 ) 0.25 and Co 0.5 (Fe 2 O 4 ) 0.5 NPs show similar distributions along H u axis, the interaction eld distribution of Co 0.25 (Fe 2 O 4 ) 0.75 NPs (Fig. 7(c)) is reduced, likely due to the separation of the magnetic phases.

Figure 4 Size
Figure 4

Figure 7 Room
Figure 7 Basically, magnetic properties of CoFe 2 O 4 nanoparticles (NPs) strongly depend on annealing temperature, heating rate, and NP size and shape[16].The morphological properties of NPs are directly related to the method of preparation.There are a number of reports on the chemical synthesis of CoFe 2 O 4

Table 1
The molarity and weight of the precursors used are presented in Table1.The molarity and weight of precursors used in the co-precipitation synthesis of Co 1 − x (Fe 2 O 4 ) x NPs.
In order to prepare the NPs, FeCl 3 and CoCl 2 .6H 2 O precursors were weighed (according to stoichiometric proportions with molarity listed in Table study crystal structure of CoFe 2 O 4 NPs synthesized with different cobalt ion concentrations, X-ray ).Thereby, sets of FORCs were acquired.To obtain the distribution of FORC, (H, H r ), the Room-temperature magnetic properties of CoFe 2 O 4 NPs were investigated using hysteresis loop analysis via a vibrating sample magnetometer (VSM, MDK, Iran) equipped with FORC software.To carry out FORC diagram technique, a strong external magnetic eld (H) was applied to magnetically saturate the NPs.Afterwards, H decreased to a reversal eld (H r ), while also performing the measurement of magnetization M (H, H r

Table 2
. As observed, the crystallite size considerably increases from 7.077 to 23.559 nm with decreasing the cobalt ion concentration from Co 0.75 (Fe 2 O 4 ) 0.25 to Co 0.25 (Fe 2 O 4 ) 0.75 NPs.Meanwhile, the lattice parameter slightly changes from 8.247 to 8.349 Å.The crystallite size and lattice parameter of CoFe 2 O 4 NPs synthesized with different cobalt ion concentrations.

Table 3
Magnetic properties of as-synthesized and annealed Co 1 − x (Fe 2 O 4 ) x NPs with x = 0.25, 0.5 and 0.75 extracted from hysteresis loop measurements.