Spin-Coating Technique for Fabricating Nickel Zinc Nanoferrite (Ni0.3Zn0.7Fe2O4) Thin Films

Functional nanoferrite thin films are used in various fields of our life. There are many different methods used to fabricate thin films including sputter deposition, flash laser evaporation pulsed laser deposition (PLD), chemical vapor deposition (PVD) and spin-coating process. In each of these methods, it produces an amorphous phase of the deposited film. To produce a crystalline film, an additional high-temperature processing is required. The high-temperature process can lead to considerable constraints in combining the desirable characteristics of a crystalline nanoferrite thin film with those of thermally unstable substrates and other device components. High-temperature thin-film processing is also a considerable cost to manufacturing. This chapter will report a simple procedure of the sol-gel precursor method for fabrication of NiZn nanoferrite (Ni0.3Zn0.7Fe2O4) thin films and spin-coating method in coating a chemical solution. This method generally provides for both low-temperature deposition and crystallization of NiZn nanoferrite thin films.


Introduction of NiZn ferrite thin films
NiZn nanocrystalline ferrite thin films have a spinel crystal structure which have been a subject of extensive attempt because of their potential applications in high-density magneto-optic recording devices, magnetic refrigeration and microwave materials due to its high electrical resistivity, low magnetic coercivity and low eddy current losses. NiZn nanoferrite thin films are structure sensitive, and it is not easy to produce a stoichiometric and point-defectfree NiZn ferrite, for high-resistivity applications. In NiZn ferrite thin-film fabrication, the accurate composition control and the uniformity throughout the film deposition are important. It is well known that properties of ferrite materials strongly depend on the preparation conditions. This issue is important in NiZn ferrite thin films because the effect of temperatures will lead to the change in the chemical composition of the ferrite films. These also will result in non-uniformity of film composition and the magnetic hysteresis parameter of ferrites. Hightemperature synthesis of NiZn thin-film ferrite results in the evaporation of some constituents that lead to the nonstoichiometry, and zinc volatilization at higher temperature can result in the formation of Fe 2+ ions that lead to increase the electron hopping and reducing the electrical resistivity [1]. Therefore, a low-temperature synthesis is required for the synthesis of NiZn ferrite film. Properties of ferrite film depend on the preparation route, due to its strong influence on type of the film (polycrystalline and epitaxial), particle size, chemical homogeneity, microstructure and cationic distribution between tetrahedral and octahedral sublattice sites [2,3]. Synthesis of ferrite thin film is of great interest among researchers in this field of study. Widely used techniques are utilized to produce desirable final product of nickel zinc ferrite. These techniques can be classified into two major techniques which are the conventional technique and the nonconventional technique. The starting materials are conventionally oxides or precursors of oxide of the cations. This process involves the interdiffusion of the various metal ions of preselected compositions to form a mixed crystal. The nonconventional powder processing in a liquid medium may produce intermediate, finely divided mixed hydroxides or mixed organic salts to assist the subsequent diffusion process [4]. Most of ferrite films have been prepared using sputtering and pulsed laser deposition. Somehow, sol-gel method is a kind of potential film preparation process, which possesses advantages of chemical homogeneity, easy component adjustment, low calcination temperature and low cost. Spin coating gives an advantage to liquid film that leads to uniformity in thickness during spin-off [5]. Once uniform, it tends to remain provided the viscosity is not shear-dependent and does not vary over the substrate. Other than that, sol-gel and hydrothermal routes of ferrite synthesis have shown increasing importance. Recent years are marked by growing interest in sol-gel processed films in new areas, particularly in microelectronics. This is mainly due to intensively developing applications of silicate or siloxane sol-gel films in the VLSI multilevel interconnection process, the preparation of ferroelectric films for nonvolatile memory [6].

Literature review on synthesis techniques of NiZn ferrite thin films
Spin coating is widely used in modern optical and microelectronic industries [7]. The understanding of its underlying physics remains limited, a fact attributed to the lack of experimental data for the evolution of various parameters during the process [8,9], leading to the need for new evolution tools. A relative study of nickel zinc ferrite by sol-gel route and conventional solid-state reaction was carried out [10]. It was claimed that the homogeneity and high purity in the sol-gel samples and small grains confirmed the finer particles. Ni 0.36 Zn 0.64 Fe 2 O 4 (NZF) thick films have been synthesized using sol-gel dip-coating method [11]. Combination with dispersion of ceramic NZF particles in starting sols has been proved to be useful for producing thick nickel zinc ferrite films. The best NZF powders are formed from dispersing at 300°C by hydrothermal grow. Nickel zinc ferrite thin films are successfully prepared using spin-deposited citrate-precursor route [12]. The formation of crystalline film at low temperature even though films were found to be X-ray amorphous revealed the formation of uniform grains in nanometer size range. Besides that, NiZn ferrite film was successfully fabricated by using photosensitive sol-gel method. The photosensitive gel film can be the photoresist of itself during the preparation process. The fine pattern of Ni 0.5 Zn 0.5 Fe 2 O 4 film is obtained through UV radiation, rising and heat treatment [13]. The great potential of combining the microwave technique with nonaqueous sol-gel chemistry was successful [14]. Many transition metal ferrite nanoparticles with high crystallinity are uniformly morphological besides homogenous metal ferrite thin films on flat and curved substrates. The thickness of film can easily be adjusted in the range of 20-80 nm using precursor concentration. Other synthesis techniques of nickel zinc ferrite thin films are chemical vapor deposition, spray pyrolysis, sputtering, pulsed laser deposition and spin spray. Of these methods the earliest used was vapor deposition of metals followed by oxidation [15]. The films were porous and polycrystalline and approximately 1000 Å thick. Many common ferrites produced by this method were single-phase spinel in crystal structure. Spray pyrolysis is complicated and expensive and required special equipment and sometimes high processing temperature above 500°C. By using spray pyrolysis technique [3,16], very homogenous ferrite thin films were obtained with good reproducibility. Nevertheless, for spinel ferrite thin-film growth, this method is used rarely. Also only a few works can be found on NiZn ferrites, obtained by spray pyrolysis where a focus on the investigation of microstructural, optical and magnetic properties was held [17]. Apart from that, the effect of oxygen plasma treatment on magnetic and NiZn ferrite films using the spin-spray plating method has been employed [18]. The oxygen plasma treatment increased the number of nucleation sites of ferrite and enhanced adhesion of the films to the substrates. It has been reported that spinel Zn ferrite can be synthesized without substrate heating by pulsed laser deposition [19]. However, this technique needs post-deposition and requires sputtered film at a high temperature to grow the spinel ferrite structure. The results were optimized and obtained 4000 Å NiZn ferrite films with low in-plane coercivity of H c = 15.2 Oe and relatively high saturation magnetization M s = 318 emu/cm 3 . Some other research works was working on preparing NiZn ferrite films by magnetron sputtering method. Most of the sputtered ferrite films must be deposited at a high substrate temperature and need high heat treatment to obtain ordered spinel structure. Sputtering method is prepared at room temperature without any post-annealing treatment [20]. By controlling the relative oxygen flow, grain size in a range 10-20 nm was developed. It revealed a maximum saturation magnetization of about 151 emu/cm 3 . The static magnetic measurement results are affected by the crystallinity, grain dimension and cation distribution.

Preparation of NiZn ferrite thin films
NiZn ferrite thin film is prepared by a sol-gel process and spin-coating technique. The starting materials nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O) (Sigma Aldrich, 99.999%), iron nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) (Alfa Aesar, 99.999%) and zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) (Alfa Aesar, 99.999%) with high purity were used as a precursor for the starting sol preparation. The materials are in metal nitrate hydrates which are soluble in alcohol solvents. Acetone and deionized water were used as a medium for sol-gel reaction. Acetic acid (C 6 H 8 O 7 H 2 O) (Alfa Aesar, 99.99%) acts as the chelating agent. The precursors were dissolved in deionized water and stirred for 15 min with a molar ratio of Ni:Zn:Fe = 1:1:2 using hot plate. The former salt solution was dissolved into acetic acid solution with a molar ratio of 1:1 and stirred for 3 h at 80°C. A sol-gel formed was left 24 h for age.

Thin film deposition and spin-coating technique
The thin film was deposited on indium tin oxide (ITO) glass. ITO has higher melting point around 1926°C. There is no phase change of substrate during deposition of the film. The typical properties of ITO glass substrate are listed in Table 1. The film deposition consists of substrate wash and spin coating. The steps are to wash the substrates firstly with distilled water in ultrasonic bath for 15 min. The substrate was then washed in ultrasonic bath using acetone liquid for 15 min. Coating was carried out in a clean room by using a spin coater. The setting parameters were listed in Table 2. The aged sol of 1.0 ml (Section 3.1) was dropped on ITO glass substrate and spin coated for 25 s at 3500 rpm (revolutions per min). The deposition was repeated several times to obtain the required thickness (300 nm). The film thickness can be controlled by the number of coating. The film with desired thickness can be obtained by repeating the deposition cycle. Then drying films were performed in a room temperature for a few minutes and annealed in air at temperature 400, 500, 600 and 700°C, respectively, with an increment of 100°C for 1 h. Annealing process was performed in a box furnace with rate of 5°C/min.

Ni 0.3 Zn 0.7 Fe 2 O 4 thin-film characterizations
The X-ray diffraction (XRD) pattern of Ni 0.3 Zn 0.7 Fe 2 O 4 thin films was obtained by using a Philips X'pert diffractometer model 7602 EA Almelo operating at 40 kV/30 mA in the 2θ range (20-80°) with CuKα radiation, λ = 1.5418 Å. The microstructural properties were   (311) is most intense in each annealing temperature, whereas others are at relatively low intensity [22]. These plane formed nickel zinc ferrite phases. The small peak intensities in XRD pattern revealed the existence of fine grain nanocrystalline with the most part as amorphous. The height of the highest XRD intensity is more intense at high temperature and improves the crystallinity of the films. As the annealing temperature increases, the grain size also increases, as indicated in the narrowing of the XRD spectrum lines. Increasing annealing temperature will enhance the crystallinity besides releasing the internal strains within the samples which results in better optical and magnetic properties. The intensity of the (311) peak increases as a function of the substrate temperature showing an improvement of the film crystallinity. Moreover the peak intensity increases, while its full width at half maximum (FWHM) decreases. Further increasing of substrate temperature leads to a slight decrease of the peak intensity for films prepared at 700°C. It indicates a saturation of film crystallinity. The crystallite sizes of all ferrite thin films are found to be between 16 and 18 nm. The average crystallite size, D ( Table 4), was determined using the Scherrer's formula [23] as given by Eq. (1): Reproduced with permission from [21]. where D is the crystallite size, β is full width at half maximum of the diffraction peak, λ is the wavelength of 1.54 Å, and θ is scanning angle.
The lattice constants of these films were calculated using indexing method [24] given by Eq. (2): where d is interplanar spacing, λ is X-ray wavelength, and θ is reflection angle. The λ/4a 2 is a constant and d = h 2 + k 2 + l 2 , which is determined by sin 2 θ value.
These lattice constants are tabulated in Table 3. The lattice parameters of all the ferrite films do not match precisely with the standard JCPDS bulk values which could be attributed to the strains present on the surface of the films during the synthesis [25,26]. Annealing temperature has a pronounced effect on grain size. The lattice parameter calculated for nickel ferrite thin film is 8.338 Å [27]. This is in accordance with the variation in lattice parameter with Zn content reported for the bulk ferrites.

Microstructure analysis
The FESEM images revealed that the Ni 0.3 Zn 0.7 Fe 2 O 4 films have dense and homogenous grains with an average grain size. Film annealed at 400°C was homogenous with dense microstructure, and they have high adhesion to the substrate. Film annealed at 500°C shows a welldeveloped grain. The grains slowly appeared with increasing annealed temperature. This is because the grain tends to combine with closer grain to form larger grain size. The structure formed in the thin films is a normal characteristic of film derived from sol-gel. 28.12 nm (600°C) and 41.32 nm (700°C). The grains of the films are spherical and uniform, and cohesion of grains is due to the magnetic attraction. The average grain size of the films is presented in Figure 2. The histogram of grain size distribution shifted to the larger grain size as the annealing temperature increased. However, based on the coercivity, H c results (Section 4.3), the transition from 600 to 700°C of annealing temperature exhibits a fall of the H c value. Figure 3 illustrates the cross section of the samples annealed at 400, 500, 600 and 700°C, respectively. The deposited films were uniform with two cycles of number deposition cycle. It was found that the thin films have thickness in the range of 145.7-285.6 nm which was confirmed by cross-sectional FESEM images. The grain size over 26 nm was further increased with a higher annealing temperature. Accordingly, the number of grain sizes beyond the single domain to multidomain critical size also increased. Therefore, the number of domain wall increased as the movement of domain wall contribution to make ease of magnetization increased [28]. Figure 4 presented the thickness of the Ni 0.7 Zn 0.3 Fe 2 O ferrite thin films and shows its relationship with the grain size. Annealing is a process related to secondary grain growth in the film. Thompson discussed the secondary grain growth mechanism and came to the conclusion that the secondary grain growth is driven by the reduction of the total grain boundary energy. Since the grain boundary energy is film-thickness-dependent, the secondary grain growth rate increases when the film thickness is reduced [29].

Magnetic properties
The plots of magnetization, M, against magnetic field strength, H (M−H hysteresis loop), for Ni 0.3 Zn 0.7 Fe 2 O 4 films annealed at 400, 500, 600 and 700°C were shown in Figure 5. The hysteresis shape is narrow and has linear loops which have a low saturation magnetization, M s . The saturation magnetization, M s , and coercivity, H c , values have been directly extracted from these curves and have been listed for various annealing temperatures in  (Figure 6(a)) could be caused by several reasons. A large grain boundary volume presented in thin films would result in the increase of the M s [30].
Other reasons for the increase M s are due to complex spinel structure; it was difficult to gain    Reproduced with permission from [21]. , which will result in a partially disordered cation distribution in the crystal lattice [31]. The saturation magnetization (M s ) increases with the grain size, and the observations on larger decrease are interpreted mostly by oxygen absorption, characteristic to the preparation technique. The coercivity, H c , was decreased as the annealing temperature and average grain size increased (Figure 6(b)). The maximum value of H c was 16.54 Oe for the grain size 26.25 nm. The H c observed were closed to the reported value of H c which is  within the range of 20-210 Oe [20]. The decreases of H c were contributed from the transition of the single domain to the multidomain [32]. The coercivity (H c ) has a maximum grain size of about 26 nm and a steep decrease at larger grain sizes (41.3 nm). The smaller grain sizes and the decrease of H c are due to the randomizing effects of thermal energy. Thermal energy has an important role in magnetic instability of single-domain magnetic particles. Due to the smaller grain sizes, the thermal agitation becomes small and will not be able to cause fluctuations in the magnetic spin orientations of the nanoparticles where they freeze in random orientations. The latter is probably due to the decreased anisotropy constant, which leads to a sharp decrease in coercivity according to the random anisotropy model. The relation between the decrease H c and increase grain size shows the linear inverse proportionality between coercivity, H c , and grain size, D, by H c α 1/D [22,33]. Figure 7 demonstrates the curves of absorbance and transmittance, respectively. The absorption spectrum exhibits that NiZn ferrite thin films have low absorbance in visible region and it is close to infrared region (Figure 7(a)). However, absorbance in UV region is high. This result of the optical behaviour is analogous to those claimed by [34] or cobalt ferrite thin film using microwave-assisted nonaqueous sol-gel process. Optical transmittance is plotted in a wavelength range of 200-800 nm as shown in Figure 7(b). The films are highly transparent in the visible range below 90%. The average transmittance is calculated and tabulated in Table 5.

Optical properties
The optical transmittance spectra of annealed thin films show a good transmission in the visible region and a sharp fall in the UV region which corresponds to the band gap. The decrease of the transmittance is due to the interaction of the incident long-wavelength radiation with the free electron in the films [35].  Table 5 do change with thickness. A dependence of band-gap energy shift on the grain size is attributed to electron confinement effect related with the grain size in the films. As a result, the observed decrease in E g with increasing grain size is due to the decrease of resistivity and the increase of film thickness. It has been studied that the band gap does not change significantly with the thickness after the film grows completely [37]. The band gap becomes saturated for a particular value of thickness [38]. The presented values of optical band-gap energy are larger than reported value for NiZn ferrite film 1.66 eV [39] and bulk NiZn ferrite, 1.55-1.66 eV [40]. The direct and indirect band energy increases could be the effect of strain present in the films during heat treatment [41].

Summary
NiZn ferrite thin film with composition Ni 0.3 Zn 0.7 Fe 2 O 4 was successfully prepared using solgel spin-coating technique. The structure formed in the thin films is a normal characteristic of film derived from sol-gel. Sol-gel spin-coating method was able to produce the similar trend and behaviour, among others, of ferrite thin film. The results are summarized as follows: • The phase analysis of films produced the complete phase with the formation of spinel structures of Ni 0.3 Zn 0.7 Fe 2 O 4 ferrite which were observable at annealed 400°C and upwards. Further annealing temperature demonstrated the improvement in the degree of crystallinity of the annealed films.
• The saturation magnetization, M s , of the synthesized Ni 0.3 Zn 0.7 Fe 2 O 4 ferrite thin films has been obtained at room temperature from the hysteresis loops which increases with annealing temperatures. The hysteresis shape shown is narrow and has linear loops which have a low M s . The coercivity, H c , decreases as the average grain size increases since the alignment of the magnetic moments inside the domain is fully controlled by thermal energy.
• The micrograph revealed the increasing average grain size with the annealing temperature. The grains of the films are spherical and uniform, and cohesion of grains is due to the magnetic attraction.
• The absorption spectrum exhibits that NiZn ferrite thin films have low absorbance in visible region and close to infrared region. The films are highly transparent in the visible range below 90%. The optical transmittance spectra of annealed thin films show a good transmission in the visible region and a sharp fall in the UV region which corresponds to the band gap.