Application of Calcium Hexaferrite as Microwave Absorbing Material: Review

Many studies have been conducted on Spinel, Garnet, and Hexa-ferrites in single and multi-doped concentrations. This article is an attempt to review the researcher's work on Ca, Ba, and Sr hexaferrite by substituting a variety of various ions such as Al, La, Sn, Zr, Co, Cr, and Ir. This review paper investigates M-type (Ca, Sr, Ba) Hexa-ferrites with a space group of P63/ mmc that were synthesized using various techniques and characterized by XRD for crystallographic information, SEM and TEM for surface morphology, VSM for magnetic behaviour, and Vector Network Analyzer (VNA) for microwave absorption properties. Changes in a material's chemical composition affect features such as coercivity, saturation magnetization, and Curie temperature, as well as managing these properties and utilizing these compounds in the field of microwave absorption properties and magnetic field industry.


Introduction
Numerous studies have been conducted since the invention of radar to create radar-absorbing materials. This material is beneficial for antenna pattern shaping, radar cross reduction, and EMI reduction. The radar absorbing materials work by absorbing radar electromagnetic radiation. 1 The magnetic and dielectric properties of the materials influence the absorption of the E-M wave. The absorbing material must have the lowest reflectance throughout the largest bandwidth, the weight and size of the absorbing material are also critical for radar. Radar absorption materials are divided into two types, conductive materials and magnetic materials. 2 Hexa-ferrites and carbonyl iron are used in magnetic radar absorbers. Hexaferrite is a type of hard ferrite that is a permanent magnet that retains its magnetism after being magnetized and has a high coercivity and remanence after magnetization. 3 Certain ferrites are further split into the following groups: M, W, Y, Z, and X compounds. Each of them possesses a unique crystalline phase via which absorption materials have been absorbed in the MHz and GHz band ranges. 4 The performance of a material is determined by its structure and physical qualities. Changing the chemical makeup of an absorber can result in significant changes in chemical and physical properties across a wide range of frequency bands. 5 There have been no publications published on M-type barium ferrites or strontium ferrites, although there have been a few studies on the microwave absorption magnetic characteristics of calcium hexaferrite. As a result, we took up the review paper for calcium hexaferrite. 6 Octahedral R-S six ↑

Experimental Technique
The sol-gel technique, coprecipitation method, hydrothermal method, and solid-state method are easy chemical ways for economically producing hexagonal ferrites. These techniques are effective for regulating the grain size and morphology of ferrite particles. 7 When creating a ferrite powder, we pay close attention to the following factors: Heat treatment, chemicals used in beginning powders, powder stoichiometry Using these crucial components, the Ca hexaferrite material was substituted with Mg-Ti, and several hexaferrite samples were generated using the citrate sol-gel method. 8 Between the frequencies of 12 and 20 GHz, the microwave absorption characteristics of calcium hexaferrite replaced by Mg-Ti were investigated. Similarly, the Ca substituted Sr hexaferrite was produced using a standard solidstate reaction approach [9], before being applied in magnetic media and absorbing microwaves.
For the field of high-density recording media, the Sn-Zr substituted Ca hexaferrite was synthesized using the sol-gel auto combustion process with urea as a fuel. 10 The Co-Sn and Sn-Zr substituted Ca hexaferrite with the chemical formulas Ca(Co-Sn) xFe 12-2x O19 and Ca(Co-Zr) xFe 12-x O19 for x= 0 to 5 were synthesized by sol-gel technique. 11 The M Type Ba hexaferrite substituted by Ca generated by microemulsion and a stearic acid sol-gel method is useful in the storage media industry. Previously, Ir-Co substituted Ca hexaferrite with the chemical formula CaFe 12-2x IrxCoxO 19 , which was manufactured using the chemical-coprecipitation process. 12 The microwave absorption capabilities of calcium and strontium hexaferrite substituted with Co and Ti by boll milling technique and solid-state reaction were investigated at frequencies ranging from 8.4 GHz to 12.4 GHz. A ceramic approach was used to create the Cu-Ti and Al-Co substituted M Type Ca hexaferrite for x=0 to 5 and x=2 to 5. 13,14

Results and Discussions XRD Report
The sol-gel method, solid-state reaction, ceramic process, and coprecipitation method were all used to successfully synthesis the M-type Calcium hexaferrite nanoparticle. X-ray diffraction studies reveal the creation of a hexagonal structure with a space group of P63/mmc and planes (107), (114), (203), and (204 (108). 15 The lattice parameters were discovered to be in the range of a = 5-6 and c = 22-24. The Debye-Scherer formula was used to determine the particle's averaged crystallite size. 16 The TEM pictures revealed that the produced ferrites nanoparticles were spherical with a size distribution between 20 and 60 nm, which corresponded to the particle size determined by XRD. Due to unit cell elongation, bigger La, Mg-Ti, Sn-Zr, Cu-Ti, Co-Ti, Co-Sn, and Co-Al ions are replaced by smaller Fe ions. 17,18

VSM Report
The magnetic properties of the material, such as saturation magnetization, coercivity (Hc), and Retentivity (Mr), proved its hard ferrite nature. When a material is used for storage, its coercivity is important. 19 As the concentration of replacing ions in the material increases/decreases, if the coercivity is low, it becomes a soft magnet, if the coercivity is high, it becomes a hard magnet. The Fe 3+ ions in the M-type hexaferrite composition are dispersed in five separate crystallographic locations: three up-spin sites (2a, 12k, and 2b) and two down-spin sites (4f1 and 4f2) along the c axis. 20 The saturation magnetization and coercivity were observed to decrease with increasing Mg-Ti concentrations, while increased with increasing Al concentrations using hysteresis loops. 21 The activation energy and dc electrical resistivity of ferrite samples increased with increasing Al concentration at room temperature.
In the presence of surfactants, Ba-Ca hexaferrite exhibited low coercivity. 22 The appearance of non-magnetic anions in the tetrahedral and octahedral positions was frequently responsible for the low Curie temperature. Sites 2a and 2b, 12k, and 4f have been occupied by nonmagnetic ions such as magnesium, titanium, copper, and aluminum. 23

SEM
SEM images of fragmented surfaces of La substituted sintered Ca hexaferrite are shown in the figure.
The micrograph demonstrates that the particle size is reduced as a result of La replacement. Hexaferrite of calcium was used in place of the original substance. The index of crystallinity was derived from the following relationship. 24 A micrograph gives us the average particle size of D, while a Scherer equation tells us the average crystalline size of r. The pinning effect and particle growth inhibition may be caused by a little amount of unsubstituted La2O3 in the sample. 25 Where f is the microwave frequency, d is the absorber layer thickness, and c is the velocity of an electromagnetic wave in a vacuum. and u are the complex relative permittivity and permeability, respectively, while Zin is the absorber's input impedance. 26 The microwave absorption property of material was tested between 3 and 12 GHz, which requires a low matching thickness, low reflection loss, and a wide absorption bandwidth.
When it comes to storing electric and magnetic energy, the real portions of complex permittivity (ε′) and permeability (u′) are used, while the shadowy portions of complex permittivity (ε′′) and permeability (u′′) have been used to express the dissipation of magnetic and electrical energy. 27 It is useful for microwave absorption if the absorbers have large imaginary portions of both complex permittivity and permeability, as reflection loss is impacted by impedance. As a result, to accomplish impedance matching, the real components (ε′ and μ′) must be as identical as possible. 28 The imaginary parts of Ba hexaferrite are very close to one and zero, while the real parts are close to 2-27. For frequencies between 2 and 18 gigahertz, has a very minor change, this indicates that it has a very low dielectric loss. 29

Fig. 4: Reflection loss of M-type hexaferrite
Whenever the RL is below -10 decibels, only 10% of the electromagnetic radiation has mirrored, whereas 90% of the electromagnetic energy consumed. The effective absorption bandwidth is established for a certain frequency range when RL is less than -10 dB. 30 When the thickness of the absorber is changed, the greatest absorption peaks shift to a lower frequency. 31 The electrical test demonstrates the bandgap calculation for the material, which effectively provides a significant choice for microwave absorption. The detailed energy calculation is performed as follows. ... (3) When it comes to matching width, there is a frequency at which the two pieces of information peer. When replacing magnetic Fe 3 with nonmagnetic ions, the matching frequency drops almost exponentially, which would conflict with a drop in the anisotropy field as the substitution level progress. 32 The reflection coefficient of Ca hexaferrite decreased with increasing applied field frequency, as seen in figure 5. The Sr-Ca hexaferrite possesses low conductivity, low dielectric constant (ε′′), and dielectric loss tangent (tan δ). Sr. There is numerous research that must be conducted utilizing various methods of preparation of Ca hexaferrite, with varied ferrite compositions, to determine the effect of preparation procedures on the properties of ferrite. The energy band gap of Lasubstituted powders and sintered pellets is evaluated using the four-probe method, which demonstrates that the sample's resistivity and energy bandgap decrease as the conversion rate for lanthanum rises. The Scherer formula is used to determine particle size. Lanthanum substitution increases with particle size. The temperature-recorded M-H curves revealed that large values of Ms, Mr, and Hc were obtained for all Ca 1-x SrxFe 12 O 19 nanoribbons and that they increased with ribbon-width broadening. Specifically, the best Ms and Hc were around 67.9 emu/g and 7.31 kOe.

Acknowledgment
The author would thankful for the Department of Nanoscience and Nanotechnology Dr. Ambedkar College, Nagpur for providing synthesis and electrical characterizations and VMV college for proving the setup for random measurement of Energy band gap using electronic setup.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.