Impact of GO on Non-stoichiometric Mg0.85 K0.3Fe2O4 Ferrite Nanoparticles

Graphene oxide (GO) nanostructures are systems with many fascinating novel properties that can be used to study new science and have significant promise for applications. In this study, graphene oxide was prepared using the modified Hummer’s method. In addition, potassium ferrite is a good candidate for biomedical application, as iron and potassium are biocompatible and non-toxic materials. Mg0.85K0.3Fe2O4/GO nanocomposites were prepared by the citrate auto-combustion method. The effect of adding GO to Mg0.85K0.3Fe2O4 on structure, morphology, electrical, and magnetic properties was discussed. Samples under investigation were characterized using XRD, infrared spectroscopy (IR), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM). The crystallite size of prepared samples was decreased from 28.098 to18.148 nm by increasing GO content. Scanning electron microscope (SEM) confirms the successful adhesion of Mg0.85K0.3Fe2O4 nanoparticles on graphene oxide sheets, which are dispersed in a metal oxide matrix. EDAX analysis confirms the existence of C, O, K, Mg, and Fe elements present in the samples. Magnetic properties were studied by VSM and Faraday's method. GO has a significant effect on the magnetic properties of nanocomposites. For instance, the saturation magnetization and Curie temperature have diverse values, which will be appropriate for numerous applications.


Introduction
Nowadays, nanocrystalline materials have become a very vital and active research subject owing to their rapidly developing in manufacturing sectors and spreading to almost every field of science from engineering to medical [1]. Spinel ferrites (MFe 2 O 4 ; M = Ni, Co, and Mn) represent a significant class of magnetic materials however their uses for biomedical applications are disadvantaged because of the high inherent poisonousness of these metals [2]. So, the necessity of synthesis biofavorable nanoferrite is essential. Potassium ferrite is a good candidate for this application, as iron and potassium are biocompatible and non-toxic materials [3]. Magnesium ferrite is an n-type semiconductor material with a spinel structure [4,5]. Consequently, it has been applied in many fields as sensors, photocatalysts, and anode materials [6][7][8][9]. However, nanoscale ferrites have some drawbacks such as the agglomeration of nanoparticles affecting its applications [10]. GO is considered as massproduction of graphene-based materials with large specific surface area, and good bio-compatibility [11,12]. In recent, the combination of nanoferrites with graphene oxide (GO) has attracted considerable attention due to their desirable properties comparable with their individual counter parts which have excellent properties for biomedical, condensed matter, energy storage as well as water treatment applications [13]. Graphene oxide nanosheets are exfoliated and decorated with nanoferrites [14]. Researchers have keened their efforts to synthesize and study GO/nanoferrite nanocomposites [15].
Herein, the present study is aimed at synthesizing and characterization of GO-based spinel ferrites (Mg 0.85 K 0.3 Fe 2 O 4 ) nanoparticles by the citrate auto-combustion method. The structural, optical, electrical, and magnetic properties of the prepared samples are studied. Also, one of our goals is to reach the critical concentration at which the physical properties reach their optimum values and the sample becomes more applicable.

Preparation of Graphene Oxide (GO)
Graphene oxide (GO) [16] was developed in a standard procedure using a modified method of hummers from pure graphite powder as shown in Fig. 1 hydrogen peroxide (H 2 O 2 ) was progressively reduced and agitated for 10 min. It cooled down as a result of the exothermic process. The Eppendorf Centrifuge 5430R was used to centrifuge 10 mL of hydrochloric acid (HCl) and 30 mL of deionized water (DIW) at 5000 rpm for 7 min. After decanting the supernatant, the residuals were rewashed three times with HCl and DIW. To make GO powder, the cleaned GO solution was completely dried.  double-distilled water. Then, different wt % of GO (1 wt%, 10 wt%) were added to the solution, followed by ultrasonic treatment for about 1 h until GO was well dispersed in the solution. To improve homogeneous mixing, citric acid (C (OH) (COOH) (CH 2 COOH) 2 H 2 O) was used. The molar ratio of cations/citric acid was maintained at 1:1. The solution was continuously stirred for 1 h. Meanwhile, NaOH solution (0.5 mol/L) was added dropwise into the above solution to regulate the pH value of the mixed solution to 7. The obtained precursor solution was then heated on a hot plate at 80 °C with constant stirring until it began to evolve reddish brown colored gases and transform into a highly viscous gel. Finally, the dried gel was finally burnt-out completely to form loose powder. The procedure of the preparation was illustrated in the following chart (Fig. 2).

Sample Preparation of GO
X-ray diffractometer (XRD) and Fourier-transform infrared (FTIR) spectra were used to ratify the crystallinity and structure of the samples. Morphology of the prepared samples was studied by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Magnetic properties of samples were discussed through the vibration sample magnetometer (model 9600-1 LDJ, USA) (VSM) as well as Faraday's method. Electrical conductivity was studied as a function of temperature and frequencies by the LCR bridge (Hioki 3532, Japan). However, the DPs of GO are not clearly identified for the prepared samples. This can be attributed to the addition of a small amount of graphene during the synthesis process as well as the low-diffraction intensity of GO sheets [17]. The crystallite size "L" of the prepared samples is calculated using the Scherrer equation as mentioned in the previous work [18].

Structural Analyses
An additional tool for the characterization of GO/ Mg 0.85 K 0. 3Fe 2 O 4 is FTIR spectroscopy. Figure [19]. The peak at 1550 cm −1 can be attributed to the formation of COO_ after decorating with Mg 0.85 K 0.3 Fe 2 O 4 . The specific peak at 590 is related to the SV of the Fe-O band. From the mentioned results, one can demonstrate that GO is bonded to spinel nanoparticles through a covalent bond [20].  The IR spectra of GO indicate a broad and strong peak around 3400 cm −1 matching the SV of hydroxyl "OH" groups. The additional peaks at 1040 cm −1 , 1224 cm −1 , 1494, and 1785 cm −1 correspond to C-O stretching, C-OH stretching, C-O-H bending, and C-O stretching vibrations of carboxyl (COOH) groups. The peak at 1619 cm −1 is related to aromatic C-C indicating the hybrid sp 2 structure of graphene.
Finally, the results of XRD and FT-IR spectra verify that the nanocomposites are formed in the synthesis. Figure 5 shows AFM of Mg 0.85 K 0.3 Fe 2 O 4 . Quantitative information such as surface area, particle size, and surface  Table 1.
Atomic force microscopy images of the single-layer flakes show high-quality graphene oxide with well-defined edges and a relatively low amount of surface defects in the form of cracks and wrinkles.
Figure 6a-f shows the scanning electron micrographs (SEM) and energy dispersive spectra (EDAX) for the investigated samples. The agglomeration of ferrite nanoparticles can be well detected. This agglomeration is attributed to the high surface energy and magnetic interaction between the nanocrystallites.
The SEM images of the prepared Mg 0.85 K 0.3 Fe 2 O 4 display a narrow-size distribution at a large-scale. In addition, it is noticed that the auto combustion synthesis technique produces highly monodispersed nanospheres of As shown from Fig. 6e, carbon and oxygen are present in the pure GO sample, which approves the purity of the investigated material. The carbon peaks that are not noticeable in the XRD diagram (Fig. 3) Fig. 7a-f. The surface of the specimens is smooth with a cubic shape. Figure 7b depicts the HRTEM image of the GO sheets, which have multilayered structures and rough texture as seen in the figure. Along with some wrinkles and folds, the GO sheet possesses a large aspect ratio, which may be the chief parameter behind the strengthening of the contact area with ferrite. The configuration of the selected region (SAED) is illustrated in Fig. 7c. The image clearly demonstrates a bright dot layout that confirms the nano-structure characteristic of the samples. The corresponding SAED pattern displays diffraction rings, corresponding to the planes  figure (Fig. 7f). The HRTEM data are consistent with the previously mentioned XRD results.

Hysteresis Loop and Magnetic Parameters
Generally, there are no nonmagnetic materials, and even if the effect is very small, diamagnetism is inherent to all materials. Numerous types of carbon allotropes display diamagnetic susceptibility χ at room temperature RT [21]. Figure 8 shows the hysteresis loops of the investigated samples at 300 K and the fitted Langevin plot. It is clear that the magnetization for all investigated samples increases with the applied magnetic field until reaching saturation behavior. A weak ferromagnetic behavior with narrow loops is detected for the pure and doped samples. However, the incorporation of GO into the Mg 0.85 K 0.3 Fe 2 O 4 nanoparticles increases the cation redistribution, changes the morphology, increases surface roughness, and decreases the crystallite sizes of the investigated samples [22]. It has been shown theoretically [22] and experimentally [23] that defects lead to an increase in the magnetic moment of graphene. These changes are related to the increase of unpaired spins.  Another source of magnetic moments and magnetic ordering of the GO can be derived from the localized edge states of graphene. First principles and mean-field theory calculations have shown that carbon structures that consist of zigzag edges can be magnetic [24]. Theoretical calculations show that localized electron spins in zigzag ribbons are polarized, resulting in ferromagnetism [25].
By increasing the amount of GO, saturation magnetization (Ms) decreases from 24.672 to 13.514 emu/g. This may be related to the decrease in particle size when Mg 0.85 K 0.3 Fe 2 O 4 is distributed on the GO surface, aside from the diamagnetic behavior of the graphene oxide. The small values obtained of remnant magnetization (Mr) and coercivity (Hc) for Mg 0.85 K 0.3 Fe 2 O 4 /GO are desirable for many practical applications that require strong magnetic signals at small applied magnetic fields.
The Ms values that correspond to 20 kOe is 24.672 emu/g. The M s of Mg 0.85 K 0.3 Fe 2 O 4 nanoparticle can also be noticed using the Stoner-Wohlfarth theory by extrapolating the plot of magnetization versus 1/H 2 to approach 0 [26,27]. This theory is applied to non-interacting particles as approved by the M r /M s values in Table 2. In this way, the M s value is equal to 25 emu/g. The estimated and investigational values are comparable with each other, signifying that the field of ± 20 kOe is fitting to saturate the testified samples.
All magnetic parameters as M S , H C , Mr, squareness (Mr/ Ms), and magnetic moment (n B ) are tabulated in Table 3. The types of inter grain group exchanges can be detected from Mr/Ms values [28].
Herein, the ratio between (M r /M s ) is listed in Table 3, which means that the samples have magneto static interactions. Anisotropy constant K is obtained from the following equation [7]: The addition of GO causes a noticeable shift of the magnetic loop that is ascribed to exchange bias phenomena.
The exchange bias field is estimated from Eq. (3) and tabulated in Table 3.  The exchange bias values confirm the presence of various spin configurations. In the nanostructure sample, the magnetic structure of the core is numerous from that of the surface where spin frustration predominates [29,30].
The size of the NPs has a crucial role in the magnetization of the samples. From the previous discussion in structure analysis, the decrease of the particle size D with increasing content of GO leads to a decrease in Ms as illustrated in the Table 3. This can be attributed to the existence of the GO layers that reduce the magneto crystalline and surface anisotropy and introduce shape and stress anisotropy due to the wrinkle layered nature of GO [31,32]. The overall competition between the above-mentioned anisotropies reduces the anisotropy constant K as well as Hc in Eq. (1).
On the other hand, the smaller particle size of GO/ Mg 0.85 K 0.3 Fe 2 O 4 increases the inert layer, causing "Ms" to decrease. Furthermore, the surface enhanced property of GO, intensifying the surface energy of the carriers. As a result, the magneto-static energy of the ferrite decreases [33]. Accordingly, the exchange in the cationic preference, the increase in the anti-site defects, and the decrease in the "Ms" are expected. This property of the materials enables the magnetization of the GO-doped samples to take place at a faster rate with a lower applied field [34]. This feature makes the examined samples suitable for high-frequency devices, data storage devices, and erasable memories.
The switching field distribution (SFD) for the samples is one of the important standards for high-density recording performance [35]. Figure 9 shows the SFD value for the 1% GO/Mg 0.85 K 0.3 Fe 2 O 4 and10% GO/Mg 0.85 K 0.3 Fe 2 O 4 as a function of coercivity. The SFD value is estimated from Eq. (3) as the ratio of the ½ width of the peak of ∂M/∂H and the magnetic field plot.
The present plot displays a double peak (DP) typical of a two-step reversal. Commonly, the DP reveals the competition between exchange coupling and strong dipolar interactions [36][37][38]. In the case study, the dipolar interactions are more effective, for the testified samples, which implies a strong reduction of the inter bump exchange coupling in our bump array.

Magnetic Susceptibility
The molar magnetic susceptibility (χ) as a function of absolute temperature at three different field intensities is illustrated in Fig. 10a-c. For all studied samples, χ decreases with increasing temperature until it reaches to 0 at the Curie temperature Tc. This can be attributed to the increase in the thermal energy of the system, increasing the disturbance of the dipoles. At Tc, a complete disorder is taking place and samples are transformed from ferromagnetic into paramagnetic phases [39].
The addition of GO has a great effect on the Curie temperature of the investigated samples, as shown from Table 4. The distribution of Mg 0.85 K 0.3 Fe 2 O 4 nanoparticles on the GO surface decreases the spin disorder. Consequently, the domains become well-oriented [40] and the ferromagnetic region becomes wider compared to the paramagnetic one.
The maximum magnetic entropy change ΔS(T, H) of Mg 0.85 K 0.3 Fe 2 O 4 and Mg 0.85 K 0.3 Fe 2 O 4 /GO is determined from the magnetization data in the temperature range from 300 to 800 K and by the use of the Maxwell equations as follows [41]:  The change of the magnetic entropy ΔS M with temperature as a function of magnetic field (H = 1010, 1340, 1660 Oe) is revealed in Fig. 10. The figure displays a distinctive shape with a maximum change near the Curie temperature, Tc. As shown from the figure, the increase of applied magnetic field (H) increases the ΔS M of the system. On the other hand, the increase of the GO decreases the disorder and the ΔS M . This can be clarified as the increase in the H and the content of GO affect the magnetic order of the prepared sample. The calculated Tc from the Curie-Weiss law agrees well with Tc obtained from the maximum ΔS M as tabulated in Table 4.

Conclusion
Mg 0.85 K 0.3 Fe 2 O 4 /GO nanoparticles have been successfully synthesized by the citrate combustion technique. Adding GO displays the varying structures and magnetic properties. The XRD pattern confirms the single spinel structure of the prepared samples.
The Curie temperature increases from 473 to 776 K with increasing GO content while "Ms" and "Hc" decrease.
The micrographs revealed successful decoration of GO with Mg 0.85 K 0.3 Fe 2 O 4 nanoparticles, with a reduction in particle size.
The massive transition temperature enhancement is demonstrated for the GO-doped samples compared to the pure ones.

Future Work
The numerous uses of Mg 0.85 K 0.3 Fe 2 O 4 /GO spinel nanoferrite particles will be the main issue for future work. The main advantages of the synthesized sample are ease of separation, high adsorption, low cost, and recyclable with notable efficiency. Thus, it is suggested as a promising candidate for waste water treatment [42].
The cheap and non-toxic Mg 0.85 K 0.3 Fe 2 O 4 /GO spinel nanoferrite particles can be employed as an anode for lithium-ion batteries (LIBs). It can demonstrate superior electrochemical performance in terms of specific capacity, cycle performance, and rate capability [43].
In addition, potassium ferrite is a good candidate for biomedical application, as iron and potassium are biocompatible and non-toxic materials [44].  Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.