Structural, dielectric, and thermal properties of Zn and Cr doped Mg- Co spinel nanoferrites

Nanoferrites play a pivotal role in resolving worldwide electronic and microwave devices. Spinel ferrites have exceptional structural, morphological, and dielectric properties. The composition Zn 0.5–xMg0.25+x Co 0.25Cr1–x Fe 1+x O 4 (ZMCCF) where x varies from 0–0.5 with the difference of 0.25 was synthesized via auto combustion (sol-gel) route. The structural, thermal, and dielectric characterizations were done to observe the responses of variation of x in designed nanoferrites. The designed nanoferrites with a variation of x experienced a promising change in structural, thermal, and dielectric responses. Based on Koop’s theory, the dielectric constant decreases with the increase in frequency, which is the favorable trend of spinel ferrites. The different cationic distributions in the spinel structure endorse this behavior. The maximum value of the tangent loss at low frequencies reflects the application of these materials in medium-frequency devices. Therefore, planned spinel nanoferrites may benefit advanced electronics and microwave devices.


Introduction
The study of materials at the atomic and molecular scale and their manipulation is known as nanoscience. Materials behave strikingly response at the nanoscale with enhanced physical and chemical properties. This patently characteristic is not nature' s new law but physics explains it significantly earlier about the reduction in size difference of properties [1]. Therefore, nanoscience is a renowned structure with a minimum of one dimension in the range of 1-100nanometer. Nanotechnology discusses these types of materials, which are devoted to novel applications because of nanosized materials [2]. This achievement increased the efficacy of power consumption that follow-up to reduce the cost of products. In the last few decades, nanotechnology gained pivotal attention in biology, physics, chemistry, geology, and many more owing to augmented features of reduced size especially below 100 nm. In nanoscience, nanotechnology deals with the study of nanoparticles, nanorods, nanotubes, nanofibers, nanospheres, etc, with varying sizes, but one dimension is less than 100 nm. The nanomaterials pull towards versatile physical and chemical properties due to the size and shape variation, leading to multipurpose applications and optical, magnetic, or electrical properties [3][4][5]. Nanoparticles have distinguished characteristics from bulk materials only by the reduction in size, namely, chemical reactivity, energy absorption, and biological mobility in physics, chemistry, and biology, which lead to combination in materials science [6].
In recent years, the development in nanotechnology has focused on synthesizing nanocrystalline materials at the nano and sub-nano scale. In a myriad of fields of technology, nanocrystalline ceramic materials have efficient electric, dielectric, and magnetic properties, which are very beneficial for numerous varieties of electronic devices [7,8]. This class of nanomaterials is a transition metal and lanthanides base elements. These are known as metal oxide ceramic ferrites and are hard, brittle, non-conducting, iron-containing gray or black, and polycrystalline [9]. By virtue of proficient application, ferrites have a plethora for this field of interest. These are the chemical combination of iron oxide with one or more other metals like magnesium, aluminum, barium, manganese, copper, nickel, cobalt, or iron. Conventional electrical, electronic, and magnetic devices are based Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. on ferrites. Ferrite nanoparticles are used in many equipment to suppress and dissipate high frequency noise levels caused by electromagnetic devices [10,11]. Ferrites have immensely changed magnetic, electrical, optical, and structural tunability and applications in different electronic technology [12], biomedical fields [13], energy storage [14], environmental protection [15], etc. Spinel ferrite nanomaterials have become more attractive due to their promising physicochemical properties, including their good electro-optical properties, ease of functionalization, and superparamagnetic properties. A ferrite is usually described by the formula M (Fe x O y ), where M represents any metal that forms divalent bonds, such as any of the elements mentioned earlier. Ferrites are soft and hard and are also classified into spinel, garnet, and hexagonal ferrites.
Spinel nanoferrites have a ceramic material structure consisting of iron oxide and other metallic elements [2]. Spinel nanoferrites have a significant variety of usages, including switching and high-frequency devices [11], removal of organic industrial contaminants [16], hyperthermia treatments [17], high-performance energystorage equipment [10], drug delivery for cancer treatment [18], and waste-water management [19]. The spinel structure ferrites also have been used in lithium-ion batteries [20] in antimicrobial [21,22], and microwave applications [23,24]. The crystal structure of spinel soft ferrites has the formula AB 2 O 4 and is an FCC lattice structure with 64 A sites and 32 B sites for one unit cell that contains eight formula units [2]. The combination of a trivalent cation (Fe 3+ ) and another divalent metallic cation, such as either a transition or post-transition metallic cation (A = Mn, Mg, Co, Ni, Zn). Their chemical compositions and synthesis techniques strongly influence the physical properties that determine their applications [25].
For the synthesis of spinel structure materials, different synthesized techniques were used, including the solgel auto combustion technique [26,27], solvothermal process [28], and coprecipitation route [29][30][31]. The auto combustion (sol-gel) process is commonly used because of its easy synthesis and economical fabrication [25]. The sol-gel methods have also been assessed as an important way to prepare nanoferrites with high purity, homogeneity, and large porosity. Properties are changed/modified owing to the distribution of cations at A and B sites in the spinel soft ferrite nanostructure.
Omelyanchik et al [32] reported To the best of our knowledge, no research has been reported for synthesizing Mg and Cr doping on Zn nanoferrites with the dopant concentration selected for the present research work. The effect of doping on the physical properties of nanoferrites is investigated by x-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermo-gravimetric Analysis (TGA), and Impedance analysis. The designed nanoferrites may benefit a medium range of frequency device applications.

Experimental part 2.1. Materials
All analytical grade nitrates salts of respective metal were used for the synthesis of ferrites. Hydrated salts of magnesium nitrate, zinc nitrate, chromium nitrate, cobalt nitrate, iron (III) nitrate, and citric acid were purchased from Sigma Aldrich.

Methodology
The sol-gel auto-combustion method was used to prepare ZMCCF1, ZMCCF2, and ZMCCF3 specimens. The nitrates and citric acid solution were prepared with a stoichiometric amount of corresponding metal nitrates that act as an oxidizing agent and fuel citric acid as a reducing agent for the combustion reaction. Placed the beaker on a magnetic stirrer to form a homogenous solution. After it, the ammonia solution was added dropwise to maintain the solution pH 7, and when the pH reached 7, the magnetic stirrer switched on. Evaporation turned the solution into a solid-liquid phase called gel, and then auto-combustion gel was converted into fine powder. The powder was placed in the furnace to perform calcination at 600°C, and after being grounded, the prepared ferrites powder was used for different characterizations. The step-by-step procedure of ZMCCF1, ZMCCF2, and ZMCCF3 ferrites preparation is depicted in figure 1.

Characterizations
For the thermal behavior of ZMCCF1, ZMCCF2, and ZMCCF3 nanoferrites, the Perkin Elmer Diamond Thermogravimetric analyzer, Japan, is used for the thermogravimetric analysis and differential thermal analysis. The recorded thermograms were observed from room temperature to 570°C in the air atmosphere. The structural variations were observed using x-ray diffraction STOE with a scan angle 20-60°with a scan rate 2°min −1 . by Cu Kα (λ = 1.5406 Å) at room temperature. The morphological analysis was studied using a Scanning electron microscope, Jeol Japan. The RF impedance/Material analyzer (Agilent E4991A) in the frequency range of 1MHz-1GHz is used to study the dielectric properties of the prepared sample and measure the complex permittivity (ε′ and ε′′) and dielectric tangent loss (tan δ).

Results and discussion
3.1. Thermal investigation Thermogravimetric and Differential Thermal analyses were used to evaluate the phase development of synthesized ZMCCF1, ZMCCF2, and ZMCCF3 nanoferrites powder. Figures 2(a)-(b) shows the TGA and DTA spectra of the as-prepared powder. Each prepared sample demonstrates a single-order decomposition, in which moisture was eliminated from the sample, resulting in weight loss. The temperature versus percentage weight loss of the as-prepared specimen between the temperature range 0°C-570°C is given in figure 2(a). Between the temperatures of 230°and 280°C, the most significant percent weight loss occurred. The percent weight loss between 230°C and 280°C for x = 0.0 and x = 0.50 was nearly the same, while the x = 0.25 nanoferrites exhibited higher stability than the other two at 580°C. The % weight loss was maximum for sample ZMCCF2. It indicates that when the concentration of Fe 3+ and Mg 2+ was enhanced, the thermal stability was enhanced at first, then declined as the value of x increased, reaching a maximum for the ZMCCF3 sample.
As per provided information about % weight loss in ferrites by TGA, DTA analysis helps us calculate the phase transition of the nanoferrites by observing the endo and exothermic reaction when heating the piece of observation with a 10°C rise per minute. The thermogram of temperature versus heat flow is given in figure 2(b). A negative heat flow value means the reaction was endothermic, and the prepared nanoferrites absorbed heat. At a temperature of 500°C heat flow is maximum. It was clear from figure 2(b) that as the value of x increased, the heat absorbed by ferrites decreased. The maximum heat absorbed was observed for the ZMCCF1.

Structural analysis
The XRD spectra of ZMCCF1, ZMCCF2, and ZMCCF3 are given in In equation (6), 'β' indicates full width at half maxima (FWHM), and the values of 'β' are given in table 1. The crystallite size was reduced by increasing the concentration of metal ions from x = 0.0 to 0.5, and the minimum crystallite size was 8.93 nm for the ZMCCF3 sample. The decrease in the crystallite size enhances dislocation density, microstrain, and inter-planar spacing, and also, some lattice distortion will be created in the structure of the host materials. The graphical representation of designed nanoferrites versus crystallite size is depicted in     The micrographs also observed that the agglomeration was increased with the insertion of divalent and trivalent metal ions in the lattice.

Dielectric analysis
The real and imaginary parts of the permittivity of ZMCCF1, ZMCCF2, and ZMCCF3 spinel ferrites are depicted in figures 7(a) and (b). Both plots showed that dielectric constant and dielectric loss were reduced with the frequency. When the frequency was raised from low to high, the dielectric loss and constant drop rapidly, and both permittivity components became independent of the frequency. Koop's model was used to describe this behaviour. According to this model, the spinel ferrites comprised well-conducting grains separated by very resistant boundaries [39]. The grain boundaries were more effective than the grains at low frequencies; hence the dielectric constant reached its highest value at the lowest frequency. Because grains were more effective than grain boundaries at higher frequencies, the dielectric constant was small [40]. The dispersed electrons accommodated on the grain boundaries as a result of the electric field, then these electrons merged, and a space charge polarisation was created. As a result, the dielectric constant and loss were high at low frequencies and decreased as frequency increased. The ratio of dielectric loss to the dielectric constant in figure 7(c) is called   dielectric tangent loss. At low frequencies, greater energy was required for the electron hopping between ferrous and ferric ions. Due to extremely resistive grain boundaries, the dielectric tan losses were highly conducting grains, which reduced electron hopping dielectric tangent losses dramatically as frequency rises, but additional increases in frequency have little effect on tangent loss.

Conclusions
The ZMCCF1, ZMCCF2, and ZMCCF3 designed nanoferrites were successfully synthesized by the sol-gel process and performed different characterizations, including TGA, DTA, XRD, SEM, and LCR analysis. From TGA plots, it was confirmed that the sample possesses single-phase decomposition, mainly due to the removal of moisture in the synthesized material. The minimum % weight loss was observed for ZMCCF3 and the maximum for ZMCCF1. DTA curves also confirmed that the single decomposition process and phase transition occur near 580°C. Moreover, from the XRD technique, it was observed that there is a change in lattice constant, interplanar spacing, and unit cell volume for each parameter and has maximum value for ZMCCF2 and minimum value for ZMCCF3, but on the other hand, the crystallite size reduced with the insertion of divalent and trivalent ions at their respective lattice site. SEM images confirmed the uniform distribution of nanoparticles and increasing agglomeration by adding the dopant ions. LCR plots exhibited that the dielectric constant decreases with increased applied frequency. The minimum tangent loss was observed for sample ZMCCF3. The effect of varying concentrations of 'x' on the designed nanoferrites concluded that dielectric properties could be enhanced and prove a promising candidate for remarkable physical properties.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).