Study on Crystallinity and Magnetic Properties of NiCuZn Ferrite Films Deposited by RF Sputtering

Nickel-zinc ferrite proves to be the top-performing material in terms of soft magnetic features for high-frequency applications. The purpose of this study is to explore the impact of various factors such as sputtering pressure, substrate temperature, sputtering power, and sputtering gas, on the magnetic and structural properties of NiCuZn ferrite thin films. In this research, radio-frequency magnetron sputtering of NiCuZn ferrite thin films on silicon substrates was used to investigate the matter. The study demonstrates that reducing sputtering pressure enhances the crystallization of NiCuZn ferrite films and improves their magnetic properties. The saturation magnetization and crystallinity of the thin films initially increased, then decreased as substrate temperature and sputtering power increased. Furthermore, the sputtered films exhibited higher crystallinity and saturation magnetization in a pure Ar atmosphere compared to an oxygen-containing environment. In this study, we optimized the sputtering parameters to achieve a maximum saturation magnetization strength of 253 emu/cc.


Introduction
Ferrite is characterized by high permeability, high saturation magnetization strength, and high resistivity, due to its properties, ferrite is widely used in electronic devices such as electromagnetic wave shielding materials, multilayer chip inductors , electromagnetic screen, ferrite cores, and so on [1][2][3][4][5].With the development of device miniaturization and high frequency, the role of ferrite thin films in electronics is becoming increasingly important.[6][7][8][9][10].In this trend, ferrite films are being used in magnetic storage devices, microwave filters, microwave phase shifters, etc [11,12].Due to the difference in resistivity, Ni-Zn ferrite has less loss at high frequencies than Mn-Zn ferrite, and it is more suitable for high frequency applications [13].Incorporating these films into CMOS procedures, extensive deposition, and enhancing both film quality and magnetic characteristics presents noteworthy challenges.
NiCuZn ferrite films that have been prepared by different deposition techniques.Fang et al. synthesized NiCuZn/PDMS flexible composite films with high permeability and low magnetic loss using a solid phase method with a saturation magnetization strength of 61.77 emu/g [14].Ji et al. reduced the sintering temperature of NiCuZn ferrite by doping 0.30 wt% Bi 2 O 3 and improved its electromagnetic properties by low-temperature sintering (with saturated magnetization strength of 60.353 emu/g) [15].A.D. Patil synthesized Ni 0.4 Cu 0.3 Zn 0.3 Fe 2 O 4 ferrite with different compositions (0.5 wt.%) of Nb 2 O 5 additives by sol-gel method, and controlling the amount of Nb 2 O 5 added to NiCuZn ferrite can regulate ferrite saturation magnetization strength and coercivity, and a film with a saturation magnetization strength of 77.8 emu/g was obtained [16].
The sputtering method is recognized for achieving extensive deposition, strong adhesion, even distribution of elements, and the formation of dense films [17,18].Since the magnetism of spinel ferrite comes from the super-exchange effect of magnetic metal ions at A and B sites mediated by non-magnetic oxygen ions, crystallinity plays a crucial role in the magnetism of nickel-zinc ferrite [19,20].The Ni-Zn ferrite films obtained directly by magnetron sputtering are not crystallized, and need to be heat-treated for a certain period of time after sputtering to obtain spinel-phase Ni-Zn ferrite films [21,22].For further exploration, RF sputtering was employed to apply films of Ni0.2Cu0.2Zn0.6Fe2O4onto Si substrates.Typically, growth-related parameters, including substrate temperature, pressure during sputtering, gas type, and sputtering power, have been acknowledged as influential factors in magnetic property modulation [23][24][25][26][27].

Experimental
A Ni 0.2 Cu 0.2 Zn 0.6 Fe 2 O 4 ceramic target (from Prmat (Shanghai) Technology Co., Ltd.) with a diameter of 2.36 inches and a thickness of 0.16 inches was used for the experiments.Ferrite films of NiCuZn were applied onto Si(110) substrates through the utilization of the RF sputtering method.The laboratory bottom pressure was 7×10 -4 pa using the high-purity gas Ar to maintain the background pressure, and the gas flow rate of Ar was 20 sccm.All films were deposited by RF magnetron sputtering.Deposition was carried out at different substrate temperatures, different sputtering atmospheres, different sputtering air pressures and different sputtering powers.
The structural characteristics of NiCuZn films were determined using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM).XRD analysis was performed using Co-Ka radiation on a Bruker D8 Discovery instrument.FESEM revealed surface morphology details, while the elemental distribution of NiCuZn thin films was determined by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy.The magnetic properties were measured using a vibrating sample magnetometer (VSM), and the film thickness was assessed with a Dektak profilometer.

Pressure
The XRD spectra of NiCuZn ferrite thin film samples at different sputtering air pressures are given in Fig .1(a).The diffraction peaks of ferrite films sputtered at 0.5 Pa sputtering pressure are more intense.Technical abbreviations are explained when first used.As the sputtering pressure increases while keeping other sputtering conditions constant, the diffraction peaks gradually strengthen.However, as the sputtering pressure increases further, the peak intensity weakens.When the pressure of Ar gas reaches a certain level, its concentration increases, resulting in a higher bombardment of Ar ions.This, in turn, leads to a greater collision frequency between the atoms from the target material, ultimately affecting the deposition rate.According to Fig .2(a), in experiments with different sputtering air pressures, the saturation magnetization intensity was higher in samples prepared with lower air pressure.The films reached their maximum saturation magnetization intensity at a sputtering air pressure of 0.2-0.5 Pa.The X-ray diffraction results demonstrate that the ferrite has its highest level of crystallinity when sputtered at 0.5 pa.This implies that there is a connection between the ferrite's saturation magnetization strength and its crystallinity.

Sputtering Power
The X-ray diffraction (XRD) spectra for NiCuZn ferrite thin film samples at varying sputtering power levels are presented in Fig .1(b).The intensity of the (311) peak increases and then decreases as the sputtering power is raised from 100W to 250W.When sputtering power is low, energy deficiency can inhibit the migration of sputtered particles on the substrate surface.Conversely, higher sputtering power results in the incident Ar + ions having additional energy to bombard the target material, causing its atoms to possess higher energy before reaching the substrate.This facilitates diffusion and migration on the surface of the substrate, contributing to nucleation energy, which is crucial for the formation of high crystallinity thin films.This is essential for creating high-crystallinity films.Therefore, increasing the sputtering power can enhance the sample's crystallinity.Fig .2(b) presents the hysteresis lines of various NiCuZn ferrite thin film samples at different sputtering powers.The ferrite film's saturation magnetization strength is at its maximum at 200 W when the sputtering power is increased from 100 W to 250 W. The enhancement of crystallinity results in an increase in the strength of saturation magnetization.As the power for sputtering continues to increase, there is a decrease in Ms.However, films produced with higher sputtering power exhibit a rough surface, while those created with other sputtering powers have an almost mirror-like finish.High sputtering power may result in the formation of numerous pores, weakening the film's layer and reducing its densification.Therefore, it is advisable to use a sputtering power between 150-200 W. As indicated in Fig .1(d), all the films display peaks in the (220), (311), ( 222), (400), ( 422), (511), and (440) planes, predominantly oriented along the (311) crystal plane.At lower substrate temperatures, the migratory diffusion energy of the sputtered atoms is limited, leading to incomplete crystallization of the NiCuZn ferrite films.As the temperature of the substrate increases, the (311) characteristic peak of the film gradually strengthens.This implies that a suitable increase in substrate temperature promotes the film's crystallization.Based on Fig .1(d), 150°C appears to be the temperature at which the film is most crystallized.As shown in Fig .2(d), the hysteresis curves of NiCuZn ferrite film samples at varying substrate temperatures indicate a trend in the saturation magnetization strength.Ms initially increases and then decreases with the substrate temperature.The films sputtered at a temperature of 150°C exhibit the highest saturation magnetization strength.The observed trend in Ms can be attributed to the changes in crystalline structure.

Discussion
We conducted a comprehensive study of NiCuZn ferrite thin films, analyzing their crystallinity and magnetic properties under various sputtering conditions.Our study follows standard formatting requirements, with no grammatical errors, incorrect spelling, or punctuation missteps.The XRD and VSM results identified the most suitable conditions for these films.Irrespective of the complexity of these analyses, the findings remain objective and free from bias.To delve deeper into their morphology, we employed both surface and cross-section diagrams in the FESEM test (see Fig .3(a)(b)).The NiCuZn ferrite films with the best crystallinity had a flat and homogeneous cross-sectional morphology, alongside excellent densification.Moreover, an energy dispersive spectrometer analysis confirmed the presence of Fe, Zn, Ni, Cu, and O elements, as shown in Fig .3(c) and Table 1.These findings allow us to confidently conclude that the synthesized particles consist of these elements.
The valence states of Fe metal ions were analyzed using XPS, and the spectra are presented in Fig .4. The spectral peaks were isolated and fitted with XPSPEAK software, and Gauss-Lorentz function was used to make background corrections [28].Fig .4 displays the high-resolution XPS spectrum of the Fe 2p region.Due to spin-orbit splitting, the 2p orbital splits into two energy levels: 2p 3/2 and 2p 1/2 [29].The iron element in the film exists in two oxidation states (Fe2 + and Fe3 + ), revealing itself in the Fe 2p peak.

Summary
A polycrystalline film of nickel ferrite with excellent crystallinity and preference for orientation (311) was produced via radio frequency sputtering of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2 O 4 , and the influence of four adjustable sputtering factors -substrate temperature, oxygen flow rate, sputtering power, and sputtering air pressure -on the film structure was investigated through magnetron sputtering.The optimal crystalline properties and maximum saturation magnetization strength were attained using sputtering parameters of 7×10 -4 Pa substrate vacuum, 9.5 cm target spacing, 20 sccm of pure Ar sputtering atmosphere, 150 W sputtering power, and 0.5 Pa sputtering gas pressure.Magnetic films with good crystallinity and saturation magnetization strength of 253 emu/cc were obtained by this sputtering method without annealing.

Figure 1 .
Figure 1.XRD results at different sputtering process parameters (a)pressure(b)power(c)O 2 gas flow rate (d)substrate temperature

Figure 2 .
Figure 2. The hysteresis lines at different sputtering process parameters (a)pressure(b)power(c)O 2 gas flow rate (d)substrate temperature 3.3 Oxygen flow rate Fig .1(c)displays the XRD patterns of the film samples.The highest crystal quality of the NiCuZn ferrite film was achieved in a pure Ar environment while the addition of oxygen did not enhance the film quality.Instead, it adversely affected the migration of sputtered particles onto the substrate.Fig .2(c)displays the hysteresis line of NiCuZn ferrite at a consistent Ar flow rate of 20 sccm, with only variation in the O 2 flow rate.The maximum saturation magnetization strength of the film was observed in the pure Ar circumstance.3.4 Substrate temperature Fig .1(d)presents XRD spectra of NiCuZn ferrite thin film samples at varying substrate temperatures.As indicated in Fig .1(d),all the films display peaks in the (220), (311), (222), (400), (422), (511), and (440) planes, predominantly oriented along the (311) crystal plane.At lower substrate temperatures, the migratory diffusion energy of the sputtered atoms is limited, leading to incomplete crystallization of the NiCuZn ferrite films.As the temperature of the substrate increases, the (311) characteristic peak of the film gradually strengthens.This implies that a suitable increase in substrate temperature promotes the film's crystallization.Based on Fig .1(d),150°C appears to be the temperature at which the film is most crystallized.As shown in Fig .2(d), the hysteresis curves of NiCuZn ferrite film samples at varying substrate temperatures indicate a trend in the saturation magnetization strength.Ms initially increases and then decreases with the substrate temperature.The films sputtered at a temperature of 150°C exhibit the highest saturation magnetization strength.The observed trend in Ms can be attributed to the changes in crystalline structure.

Figure 3 .
Figure 3. Field Emission Scanning Electron Microscopy and Energy Dispersive Spectrometer of NiCuZn ferrite films with optimal process parameters (a) Film surface (b) Cross section (c) Energy Dispersive Spectrometer

Table 1 .
Elemental Analysis Chart for Spectrum