INTRODUCTION

Recently, one-dimensional (1D) nanostructures have attracted a great deal of interest due to their unique electronic, magnetic and physical properties[1]. Among various nanoscale materials, spinel ferrites (MFe₂O₄; M = Fe, Co, Ni, Mn, Zn, etc.) have become attractive due to their wide practical applications in different fields of the science and technology such as magnetic refrigeration, ferrofluids making, magnetic resonance imaging and high-density data storage[2-5]. These nanoscale structures also found potential applications in other areas such as biomedical, wastewater treatment, catalyst and electronic device with or without other additives [6-9]. Among these materials, nickel ferrite (NiFe₂O₄) is one of the most important spinel ferrites, and it is especially attractive to researchers because of its high magneto crystalline anisotropy, high saturation magnetization and unique magnetic structure[10]. These structures exhibit various kinds of magnetic properties, including paramagnetic, superparamagnetic or ferrimagnetic behavior, by varying their particle size and shape[11, 12].

The unit cell regarding spinel ferrites comprises 32 oxygen atoms packed closely together in cubic positions, with 16 occupied octahedral sites and eight tetrahedral voids. In a typical spinel, all 16 trivalent elements and 8 divalent elements are organized in octahedral spaces. While the 2-valent elements are also organized in inverse spinel’s octahedral spaces, the three-valent elements are split evenly between tetrahedral spaces and cathedral spaces (Fig. 1) [13-15].

The distribution of the cations can impact the mutation of the electrons, interactions of the magnetic exchange, and modification of magnetic characteristics, and therefore, expand and enhance the ferrite applications[16-18]. Using these materials is wide-ranging as a result of the abovementioned modifications, as well as the fact that they can reach a variety of qualities like the coercive force, magnetic permeability, saturation magnetization (μS), anisotropy constant, physical and chemical stability, and mechanical stiffness. As a result, neuromas studies have been conducted on the production and modification of the magnetic characteristics of ferrites as well as doping effects of various 2/3-valent elements within their structures[18-21]. Cobalt ferrite is one of these structures due to its good saturation magnetization (MS), high coercive force (HC), high magnetic permeability, mechanical hardness, and excellent chemical and physical stability. As a result, it is a perfect choice for radar absorption materials (RAM), Ferro-fluid technology, drug delivery, high-density magnetic recording, magnetic hyperthermia (MH) for cancerous tissues, and magnetic resonance imaging (MRI)[8, 22-24].

Inverse spinel structure is present in cobalt ferrite (CoFe₂O₄). Accordingly, half of the 3-valent iron ions are in tetrahedral spaces, while the second half is in octahedral sites for 2-valent cobalt ions, which are in octahedral locations. However, zinc is a cheap substance and may considered for doping into the cobalt ferrite in order to produce appropriate magnetic characteristics, in particular, for the applications involving hyperthermia[25, 26]. In these structures, as a typical spinel, zinc ferrite contains all of the 3-valent ions in octahedral regions and 2-valent ions in tetrahedral places. This could explain why Zn-doped cobalt ferrite structures may have modified spinel structure (Zn²⁺_xFe³⁺_1-x)_A [Co²⁺_1-xFe³⁺_1+x]_BO₄ [14, 24, 27, 28].

The production of zinc cobalt ferrite is of tremendous interest because of its potential applications in a range of domains. The size, shape, and other physicochemical properties of synthetic ferrites affect their final applications[29]. There are several methods to prepare magnetic nanoparticles (gaseous, solid, and liquid phase synthesis)[30-32]. Pulsed Laser Deposition (PLD) is a thin-film deposition technique that is used frequently for in stiu synthesizing and deposition of different nanoscale materials. This method uses high-energy laser pulses to vaporize the surface of a solid target inside a vacuum chamber and condense it on a substrate to form nanoparticles or thin films up to a few micrometers in thickness[33, 34].

In this study, the PLD technique has been used to synthesize ferrite compounds with the formula of Ni_0.5Zn_0.5Fe₂O₄. The physicochemical, morphological and crystalline properties of the resultant nano Zn-ferrite were investigated by different characterization methods including SEM, AFM, XRD, and FTIR techniques. 

MATERIALS AND METHODS

Materials and reagents

Iron nitrate [(Fe(NO₃)₃.9H₂O)], nickel nitrate [(Ni(NO₃)₂.6H₂O)], Zinc nitrate [Zn(NO₃)₂.6H₂O] and citric acid (99%) were purchased from Sigma–Aldrich chemical reagent Co. (USA). All the reagents were of analytical grade with a high purity of 99.99% and used as received without further purification.

Preparation of NiZnFe₂O₄ Nanoparticles by PLD

The nitrate compounds of nickel, zinc and iron with a molar ratio of (0.5:0.5:2) were combined with citric acid (1:2) as the host to create nano-powder of cations. The powder that was thus produced was hydraulically pressed into pellet shape. The pellet was sintered for three hours at 1200°C. The substrate used for film deposition was Si (100) of size 10 × 10 mm. A Lambda Physik KrF excimer laser (model COMPex 102 and PLD chamber) of wavelength 248 nm has been used to deposit Ni/Zn ferrite films on Si substrate. Pulse energy was adjusted to 220 mJ, pulse duration 20 ns, and repetition rate was kept 10 Hz. The laser beam was directed at 45° from the target normal and focused on a 3.5 × 0.5 mm² area of the ablated target. The deposition of the thin films was done in a high-vacuum chamber, which was evacuated to a base pressure lower of 1 × 10^-6 Torr by means of a SD-200 Turbo V200 pump. The gas pressure inside the PLD chamber was measured using a cold cathode gauge and adjusted via an electronic mass flow controller. The substrate was mounted at a distance of 4 cm from the target. The substrate temperature was kept 600 °C. Flowing O₂ gas pressure at 100 mTorr was used during deposition process in order to preserve the stoichiometry in the films. The deposition was carried out under constant laser energy to reduce the occurrence of ferrite droplets caused by splashing from the target. Fig. 2, Schematically represent the sample preparation and PLD process used in this study for deposition of nanoparticles on Si substrate.

Characterization

The morphological characteristic of the nanoparticles-deposited thin films was studied by SEM analysis using Scanning Electron Microscopy (Philips XL 30 and VEGA\\TESCAN) instrument at 30 kV. The particle size distribution of the prepared nanoparticles-deposited thin films was analyzed by determining the size of about 100 randomly selected particles using ImageJ 1.52v software (National Institutes of Health, USA, http://imagej.nih.gov/ij/).  To examine the surface morphology and topography of the samples, atomic force microscopy (AFM) was applied using a silicon nitride cantilever in contact mode on the Nano scope E Digital equipment. The cantilever’s force constant was 0.56 N/m, and the tip radius was 20 nm. The chemical structure and composition of the resultant thin films were investigated using an FTIR instrument (FTIR, Bruker Model Vertex 70 FTIR, Germany) in the range 500-1000 cm^-1. The crystalline structure of films was examined by X-ray diffraction method using the SCINTAG DMS 2000 XRD equipment in a θ-2θ configuration with Cu-K radiation (λ = 1.93604 Å).

RESULTS AND DISCUSSION SEM and AFM analysis

Fig. 3 shows the SEM images of the nanoparticle deposited on Si thin films. As we can see from the background of Fig. 3A, the prepared thin films exhibited a smooth blacked surface with several droplet contamination in the nanoscale sizes, which is often associated with PLD[31, 35].  The occurrence of these droplets is the result of incomplete elimination of target splashing during laser ablation, in spite of the lower energies of the laser beam. The high repetition rate of the laser pulses, which is required to ensure a high deposition rate, may cause droplets and cluster aggregates to occur during ablation. The repetition and deposition rate of 5 Hz and 2 nm/min were considered an optimum values for growing good quality Ni/Zn ferrite on thin films[36]. In order to further investigate the surface morphology features of the prepared Ni/Zn ferrite thin films, the size distribution of the droplet contamination was measured by ImageJ software and the results are depicted in Fig. 4. As we can see, the size distribution of the droplet contamination is in the nanoscale size with an average size of 147.40 ± 119.25 nm. These results confirm the successful synthesizing and deposition of Ni/Zn ferrite nanoparticles on Si thin films. Moreover, regarding the nanoscale size of these droplets, we can conclude that the majority of the nanoparticles were less than these sizes, resulting in highly uniform and smooth-surfaced thin films.

The AFM images of the prepared thin films in Fig. 5 also depicted typical thin film surfaces with visibly displaying smooth surfaces and good optical quality and definition. In agreement with SEM images, the deposited nanoparticles on the surface of thin films, along with droplet contamination from target splashing, resulted in a grainy appearance of the resultant thin film surfaces. However, as we can see, all AFM images from the prepared thin film surfaces represented a highly smooth and uniform deposition of nanoparticles. This resulted in considerably even surface topography for resultant thin films, which is a desired feature for obtaining unique and specific properties from these samples.   

FT-IR analysis

The typical spectra of the FT-IR regarding the prepared Ni/Zn ferrite nanoparticle-deposited thin films have been depicted in Fig. 6. FT-IR spectra were used for identifying the ferrite phase’s development. The peaks observed at 426.27 cm⁻¹ and 462.92 cm⁻¹ are the peaks of the metal-oxygen vibration extended towards the tetrahedral and octahedral site (at sites A and B, discussed above) with a pure cubic spinel ferrite structure, which confirms the formation of Ni/Zn-doped ferrite nanoparticles. The comparatively lower wavenumbers (which are generally detected at 544–569 cm⁻¹) are due to the octahedral metal–oxygen vibrational band at 400–500 cm⁻¹. At 426.27 cm⁻¹, the lower wavenumber absorption band was extra sharp for the octahedral sites, while the band at 462.92 cm⁻¹ signifies the tetrahedral sites in the inverse spinel structure of Ni_0.5Zn_0.5Fe₂O₄[7, 37]. The broad peak at 3400 cm⁻¹ is due to the chemisorbed water molecule and having an HO-H bond[38]. The remaining peaks could be attributed to the Si (100) and citric acid vibrations used as a substrate or host material for the fabrication and deposition of nanoparticles. In this regard, the 1789.94 cm⁻¹ peak was attributed to carbonyl groups of the citric acid, while the two peaks at 2852.72 cm⁻¹ and 2763.99 cm⁻¹ were attributed to its asymmetric and symmetric -CH₂ groups vibration stretching. The broad peak at 1382.96 is also related to the vibration of C-O bonds of citric acid[39, 40].

X-ray Diffractometer (XRD)

Fig. 7 illustrates XRD patterns of synthesized Ni_0.5Zn_0.5Fe₂O₄-deposited thin film nanocomposites. From this figure, all the observed diffraction peaks can be indexed to a cubic spinel phase (in agreement with the FTIR spectra section) with FCC structure magnetite according to JCPDS card No 82-1042. Nine characteristic XRD peaks marked by their Miller indices (220), (002), (311), (111), (400), (422), (511), (440) and (311) at 2θ values of 30.28°, 33.28°, 35.68°, 37.28°, 43.36°, 53.56°, 57.28°, 62.76°, and 74.32°. All of the XRD peaks were matched with JCPDS cards 8–1935 (MgFe₂O₄) and 89-1012 (ZnFe₂O₄)[37, 41]. Moreover, the (002), (111) and (311) peaks at 33.28, 37.28, and 74.32 were attributed to the Ni₂O₃, Ni and NiO crystalline structure the resultant thin films[41, 42]. The most intense peak detected for the (311) plane clearly proves the formation of the cubic spinel ferrite structure, with other major diffraction planes being observed at different 2θ angles.

CONCLUSION

Pulsed laser deposition has been successfully used as the target to create Ni_0.5Zn_0.5Fe₂O₄ nanoparticles with an optimal structure on the Si substrate thin films.

A uniform and evenly distributed nanoparticles on the Si surface were observed in SEM and AFM analysis. The morphological studies also revealed that the grains are clear with well-defined grain boundaries. Moreover, some droplet and cluster aggregates also observed in SEM and AFM images, which is occurred due to the incomplete elimination of target splashing during ablation. The FTIR spectra showed the characteristic chemical bands of the materials used for the fabrication of thin films. The single-phase cubic spinel structure of samples has been confirmed from X-ray diffraction analysis. Furthermore, it was noted that the PLD-synthesized and deposited nanoparticles had extremely small and nanoscale particle sizes. Therefore, it can be concluded that PLD is a superior synthesizing and nanoparticle deposition method for producing desired nanoparticle-deposited thin films.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.