Study on Crystallization and Magnetic Property Deviation of Ni-Zn-Cu Ferrite Depending on the State of the Starting Material in the Annealing Process

0e spinel structure of the nanosize powder (Ni0.3Zn0.3Cu0.4Fe2O4) substituted by Ni, Zn, and Cu was fabricated by the sol-gel process. Changes in weight, crystal formation, and magnetic properties were observed by XRD, TG-DTA, VSM in the annealing process of the sol and gel. 0e saturation magnetization of the sol showed 54.8–58.6 emu/g at 500–800°C, and the gel showed 52.3–56.8 emu/g at 600–800°C. 0e coercive force of the sol decreased in the range −136Oe to −11.4Oe at 500–800°C, and the gel decreased in the range −95Oe to −44Oe at 600–800°C. 0erefore, the deviation of the annealing temperature of the nanopowder fabricated in the sol process and the gel process was about 100°C.


Introduction
Data usage has increased rapidly in the development of wireless communication and electronic industry.To transmit a large amount of data, gigahertz frequency is used.
erefore, wave interference of the gigahertz frequency occurs frequently [1].us, the study of the wave absorber is continuously carried out by a lot of researchers [2,3].Ferrite is the most widely used wave absorber material.Ferrites form various structures such as iron oxides, the spinel, the garnet, and the hexagonal [4].
e hexagonal ferrite could be classi ed into M-type, U-type, X-type, and Y-type by an additional atom and by stacking method [4].It can be con rmed from the previous studies that the hexagonal ferrite can absorb in the frequency range of several MHz to tens of GHz depending on the type [5].
e spinel structure of the nanosize powder substituted by Ni, Zn, and Cu was fabricated by the sol-gel process.Elements such as Ni, Zn, Cu, and Mn can be substituted at the tetrahedral site and octahedral site in the Fe 2 O 4 ferrite [6].And the magnetic properties of the spinel ferrite could be changed by substituted elements [6].e other factors are crystal size and uniformity of spinel ferrite.Also, according to the previous studies, the ferrite is con rmed by wave absorption in the frequency range of the tens of MHz to several hundred MHz ranges [7].However, the other study con rmed that the nanometer size crystal of the spinel ferrite could absorb waves in the gigahertz frequency region [3].In this study, the nanopowder of spinel ferrite substituted by Ni, Zn, and Cu was synthesized by the sol-gel process.Zn and Cu are mainly substituted at the A-site in the tetrahedral, and Ni is mainly substituted at the B-site in the octahedral.
e substituted Ni, Zn, and Cu change the saturation magnetization value of ferrite.It is because that Ni 2+ , Zn 2+ , and Cu 2+ ions reveal the di erent net moments from Fe 2+ ion.Di erences in properties were mentioned in our previous paper in detail [8].
e sol-gel process has advantages.It is simple and can fabricate a high-purity powder.In a typical sol-gel process, the sol was dried to form the gel, and then the nanopowder was obtained after the annealing process of the gel.In this study, we con rmed the in uence on the spinel crystal formation by annealing the sol and the gel, respectively.And, the deviation in the process of crystal formation and magnetic properties of spinel ferrite was analyzed.

Experimental
In this study, Ni 0.3 Zn 0.3 Cu 0.4 Fe 2 O 4 (NZCF04) was synthesized by sol-gel process.Iron nitrate, nickel acetate, zinc acetate, copper acetate, citric acid, ethylene glycol, and DI water were used as starting materials.e starting materials were mixed by stoichiometry, and citric acid and ethylene glycol were added as the total mole of the starting materials.
e mixture was dissolved with DI water.At this time, DI water was added to the minimum amount so that the NZCF04 mixture could be dissolved.
e hydrogen ion concentration of the NZCF04 mixture was adjusted to pH 6 by adding ammonia solution.A NZCF04 sol was prepared by heating the NZCF04 mixture at 85 °C for 8 hours.e generated vapor was cooled and re uxed to keep the concentration of the mixture constant.And the NZCF04 gel was prepared by heating the NZCF04 sol on a hotplate at 100 °C for 4 hours.To convert the fabricated NZCF04 sol and gel into nanopowder, annealing process was performed at 400-800 °C for 3 hours.TG-DTA was measured to con rm the phase change of NZCF04 sol and gel with temperature.
e heating rate of TG-DTA was set as 5 °C/min, which was the annealing heating rate.Lattice parameter and crystal size of nanopowder fabricated with NZCF04 sol and gel were measured by XRD (Rigaku, SmartLab, Cu Kα). e size and shape of the nanocrystals were analyzed by SEM (JEOL, JSM-7800F).e saturation magnetization, coercive force, and initial permeability of nanopowders fabricated with NZCF04 sol and gel were analyzed by VSM (Quantum Design, VersaLab VSM, ±3,000 Oe).

Results and Discussion
TG-DTA analysis was performed to con rm the phase change of the sol and the gel with temperature.e results of TG-DTA analysis are displayed in Figure 1.In the sol and the gel of NZCF04, the weight was reduced by evaporation of water at less than 200 °C.e weight of the NZCF04 sol was reduced rapidly under 200 °C, and the weight of NZCF04 gel was reduced rapidly under 100 °C to 200 °C.e sol and the gel showed endothermal reaction simultaneously at 200 °C. is con rmed that the residue of the sol-gel process vaporized up to 200 °C and endothermic reaction by crystallization occurred.It was con rmed that the gel showed rapid endothermic reaction at 200 °C, and the sol showed it at 200-350 °C range.
e sol and the gel showed weight reduction at 320 °C to 550 °C.
e results of the TG-DTA analysis showed that the in ection point of heat ow of the sol and the gel was at 320 °C and 350 °C, respectively.ese in ection points of heat ow indicate that NZCF04 initial crystals were formed at these temperature regions.For the gel, in ection points were found in the weight ratio curve and the heat ow curve in the region of 550 °C to 650 °C, and this phenomenon was estimated to be caused by the phase change of the spinel crystal.e phase change of the sol is estimated at 330 °C to 550 °C range.e temperature range from 700 °C to 900 °C was the grain growth sector, in which the sol and the gel showed similar weight ratio change.However, the sol showed more rapid change in heat ow than the gel. is means that the sol requires more energy for crystal growth.
e XRD pattern of the sol and the gel is displayed in Figure 2. e pattern of the sol and the gel showed broad peaks at 400 °C.It means the crystal size of 400 °C annealing process was below a few nanometer.And It could be conrmed that this annealing process was the spinel crystal generating stage, compared with TG-DTA result.
e sol and the gel showed sharp peaks at over 500 °C, and thus it was con rmed that spinel crystals were formed.
e in ection point of the gel at 550-600 °C in Figure 1 could be con rmed in the XRD pattern at 600 °C.It can be seen that the intensity of the XRD main peak (311) at 600 °C is reduced, and the FWHM is increased above 500 °C.e FWHM of the gel was 0.267 and 0.299, respectively, at each temperature.It means that the crystal structure at 600 °C was more irregular than at 500 °C.at is, it could be estimated that a change has occurred in the spinel crystal structure.
e change in the crystal size with annealing temperature is displayed in Figure 3.It could con rm that the crystal size of the sol and gel increases with increasing annealing temperature.It can be seen that the initial crystal size of the sol and the gel was 19 nm and 36 nm at 400 °C, respectively.e crystal growth has progressed at 800 °C, and the crystal size of the sol and the gel was 102 nm and 94 nm, respectively.e crystal size was calculated by the following Debye-Scherrer equation: where D p average crystallite size, β line broadening in radians (FWHM), θ Bragg angle, and λ X-ray wavelength (1.54 Å). e main factor of the crystal size in (1) is β (FWHM).Looking at FWHM of the sol and the gel, it can be seen that 2 Advances in Materials Science and Engineering the FWHM of the sol decreased from 1.08 to 0.19 and that of the gel decreased from 0.59 to 0.2.It con rms that the decrease in FWHM is the main factor for crystal growth.e SEM image of the sol and the gel is displayed in Figure 4. Calculated initial crystal sizes of the sol and the gel annealed at 400 °C were 19 nm and 36 nm, respectively.It could also be con rmed in Figures 4(a) and 4(b).In Figure 4(c), the sol annealed at 800 °C, and the crystal size was seen over 100 nm.us, it con rmed that the crystal has grown rapidly at this temperature.
en, it can be con rmed that the sol showed more endothermic reactions than the gel for crystal growth.
e initial permeability is displayed in Figure 5.It shows that the initial permeability of the sol and the gel increased with increasing annealing temperature.e initial permeability of the sol increased from 0.033 to 0.096 and the gel from 0.042 to 0.090.e sol showed a continuous increase in proportion to the annealing temperature, and the gel showed rapid increase from 0.055 to 0.088 after 700 °C.
e reason was estimated to be due to the rapid growth of the crystal. is could be con rmed from the 700 °C XRD pattern and the result of TG-DTA of the gel.
It could be con rmed that the initial permeability was related to the crystal size and uniformity.
e coercive force and the saturation magnetization of the sol and the gel are displayed in Figure 6. e saturation magnetization of the sol showed 54.8-58.6 emu/g at 500-800 °C, and the gel showed 52.3-56.8emu/g at 600-800 °C.It can be seen that the range where the saturation magnetization of the sol is kept constant was 500 °C to 800 °C and of the gel was from 600 °C to 800 °C.e coercive force of the sol decreased in the range −136 Oe to −11.4 Oe at 500-800 °C, and the gel decreased in the range −95 Oe to −44 Oe at 600-800 °C.It was because the crystal growth and stabilization of the spinel crystal have progressed in these ranges.By analyzing the magnetic properties, it was con rmed that the crystal growth and the crystal stabilization of the sol were lower than the gel by 100 °C.

Conclusion
NZCF04 nanopowder was fabricated using the sol and the gel.
is process was analyzed by TG-DTA analysis, and the nanopowder was analyzed by SEM, XRD, and VSM.As a result, it was found that the temperature of the phase change of the nanopowder prepared by the gel was 100 °C higher than that of the sol.e spinel crystals were formed at 500 °C in the sol process and at 600 °C in the gel process.
e size of the crystals was 150 nm and 90 nm at 800 °C, respectively.In the sol process, the saturation magnetization was stabilized as 54.8 to 58.6 emu/g at 500 °C to 800 °C, and the gel process was stabilized as 52.3 to 56.8 emu/g at 600 °C to 800 °C.In this study, the highest Advances in Materials Science and Engineering coercive force of the sol and the gel were −136 Oe and −95 Oe, respectively, also observed at the same annealing temperature as initial temperature of the saturation magnetization stabilization.It was con rmed that the spinel crystal was formed at each temperature in the sol and gel processes.In the sol-gel process, the deviation of the annealing temperature of the nanopowder fabricated in the sol process and the gel process was about 100 °C.e soft   Advances in Materials Science and Engineering magnetic property exploited wave absorber requires high saturation magnetization value and low coercive force value.In this study, the nanopowder fabricated by the sol-gel process showed su cient magnetic properties for the wave absorber.It is also expected that the wave absorption frequency exhibits gigahertz range in nanocrystal.

Figure 1 :
Figure 1: TG-DTA result: weight ratio and heat ow of the sol and gel.

Figure 3 :Figure 2 :
Figure 3: NZCF04 crystal size of the sol and the gel with annealing temperature.

Figure 5 :
Figure 5: Initial permeability of the sol and the gel with annealing temperature.