**2.2 Microwave**

The arrangement of all forms of electromagnetic waves based on their wavelengths and frequencies covers a very low range of energy to very high energy called the electromagnetic wave spectrum, as shown in **Figure 5**.

 Microwaves are electromagnetic waves which have a frequency range of about 0.3–300 GHz with wavelengths of around 1–1 mm. The microwave frequency range consists of several bandwidths, namely, L band to D band [8].

#### **2.3 Absorption mechanism**

Coherent and polarized microwaves obey the optical law; this wave can be reflected, transmitted, and absorbed, depending on the type of material it passes. In general, the use of microwaves is based on the phenomenon of reflection and transmission only (**Figure 6**).

But in the last few decades, the phenomenon of microwave absorption has also become very popular as the core concept in the development of rapidly advancing electronic and telecommunications technology as shown in core (**Figure 7**) [9–15]. The main requirement that is needed as microwave absorbing material is that this material has a value of permeability (magnetic loss properties) and permittivity (dielectric loss properties) material [16].

**Figure 5.** 

 *Spectrum of electromagnetic waves and microwaves.* 

**Figure 6.**  *Rules of optical law in microwaves.* 

**Figure 7.**  *Microwave absorption mechanism for materials.* 

In general the electrical and magnetic properties of a microwave absorbent material are characterized by complex permittivity and complex permeability, as shown by the following equations [17–19]:

$$\mathbf{e}\_r = \mathbf{e}^\prime + \mathbf{e}^{\prime\prime} \tag{1}$$

$$
\mu\_r = \mu^\prime + \mu^\prime \tag{2}
$$

The real part of permittivity (ε′) states the measure of the amount of energy from the external electric field stored in the material, while the imaginary part (ε″) states the measure of energy lost due to the external electric field. If the imaginary part is zero then the material is a lossless material and is called a loss factor. The same for permeability, the real part (μ′) expresses a measure of the amount of energy from the external magnetic field stored in the material, while the imaginary part (μ″) shows the amount of energy dissipated due to the magnetic field.

 Permittivity is present from material dielectric polarization. The quantity ε′ can also be referred to as the dielectric constant of a material. The quantity ε″ is a measure of attenuation from the electric field caused by material. Loss of tangent permittivity of a material is defined as follows:

$$\text{Tam } \delta\_e = \frac{\text{e}^{\cdot \cdot}}{\text{e}^{\cdot}} \tag{3}$$

*Metamaterial: Smart Magnetic Material for Microwave Absorbing Material DOI: http://dx.doi.org/10.5772/intechopen.84471* 

 The greater the loss tangent of a material, the greater the attenuation when the wave moves through the material. The same applies to magnetic fields, namely:

$$\text{Tran } \delta\_{\mu} = \frac{\mu^{\cdots}}{\mu^{\cdots}} \tag{4}$$

 Both components contribute to the compression of wavelengths in the material. Because electromagnetic waves (EM) are a combination of two waves between electric and magnetic waves, loss of both magnetic and electric fields will weaken the energy in waves. In most dampers, both permittivity and permeability are functions of frequency and can vary significantly even in small frequency ranges. If permittivity and complex permeability are known in a certain frequency range, the material effect on the wave will be known.

 It is well known that dielectric and magnetic parameters include electric → → → → field vectors *E*, magnetic fields *H*, induction fields *B*, displacement *D*, polarization *P* → , and magnetization *M* → . The interaction of electric fields in materials follows a pattern similar to magnetic interactions in materials. One of the requirements that must be met for practical application as an absorbent of electromagnetic waves is that this material must have the highest permeability and permittivity values with high magnetic saturation. The SI unit of permittivity and permeability respective are farad per meter and henry per meter. In terms of absorption of EM wave energy, the overall interaction can be represented by the dielectric and magnetic impedance matching of the material (Zin) equal to the air impedance (Zo) as a frequency function.

$$Z\_{\rm in} = \sqrt{\frac{\mu\_r}{\varepsilon\_r}} \tanh\left[j\frac{2\pi fd}{c}\sqrt{\mu\_r \varepsilon\_r}\right] \tag{5}$$

 where Zin is the impedance of material, (*μr*) and (<sup>ε</sup>*r*) are the complex relative permeability and permittivity of the material, d is the absorber thickness, and c and f are the velocity of light and frequency of microwave in free space, respectively.

$$RL = -2\mathbf{O} \log \left| \frac{Z\_{\text{in}} - Z\_o}{Z\_{\text{in}} + Z\_o} \right| \tag{6}$$

This impedance adjustment is important in the microwave frequency range. A transmission line that is given the same load as the characteristic impedance has a standing wave ratio (SWR) equal to one and transmits a certain amount of power without wave reflection. Also the absorption efficiency is optimum if there is no reflected power. Matching means giving the same impedance as the characteristic impedance of electromagnetic waves. Measured parameters are reflection loss (RL), if there is a matching impedance Zin = Zo, meaning that RL will be infinite or all waves have been absorbed perfectly.

### **3. Microwave absorbing material**

 This chapter focuses specifically on microwave absorbent materials from smart magnet materials which have been thoroughly studied by the authors, which include perovskite, ferrite, hexagonal ferrite, and composite systems. The results have all been reported and published in several globally indexed journals.

#### **3.1 Perovskite system**

 Perovskite systems have an empirical formula ABO3. In this research, the authors have focused on the LaMnO3-based system. An LaMnO3 is a magnetic material that has high permittivity but low permeability because it is paramagnetic at room temperature [20]. In a previous study [21], after LaMnO3 was substituted with barium atoms forming the compound La0.8Ba0.2MnO3, this material was ferromagnetic where the permeability of the material increased. However, the results of testing microwave absorption are still relatively low, only in the range of ~6.5 to ~3 dB at a frequency of 14.2 GHz. In this chapter book, we will also present the results of advanced material engineering based on the results obtained previously, namely, manganite-based materials with a composition of La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 (x = 0.1–0.8) [22]. A designed La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 composition was prepared using a conventional milling technique. Stoichiometric quantities of analytical grade BaCO3, Fe2O3, MnCO3, TiO2, and La2O3 precursors with a purity of greater than 99% were mixed and milled using a planetary ball mill to powder weight ratio of 10:1 for up to 10 h. The quasicrystalline powders were then compacted into pellets and sintered in the electric chamber furnace at 1000°C for 5 h to obtain crystalline materials and confirmed using an X-ray diffractometer (XRD). The results of XRD analysis show that the highest fraction of the LaMnO3 phase was found in the sample with composition x < 0.3 to 99%, while the LaMnO3 phase mass fraction decreased for composition x > 0.3 as illustrated in **Figure 8**.

The results of magnetic properties analysis were measured using vibrating sample magnetometer (VSM). The results of the VSM analysis show that in all samples, La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 (x = 0.1–0.8) contains a BaFe12O19-based hard magnetic phase. This magnetic phase increases with increasing composition of x as shown in **Figure 9**.

The characteristic of microwave absorption is measured using vector network analyzer (VNA) in the frequency range 9–15 GHz as illustrated in **Figure 10**. The results of the VNA analysis show that the highest reflection loss is found to be three absorption peaks of ~9, ~8, and ~23.5 dB which is located at 9.9, 12.0, and 14.1 GHz frequency, respectively. Based on the calculation of the reflection value obtained, microwave absorption reaches 95% with a sample thickness of 1.5 mm.

Investigation on this perovskite system has also been carried out by previous researchers. Zhang and Cao [23] succeeded in synthesizing transition metal (TM)-doped La0.7Sr0.3Mn1−xTMxO3±δ (TM: Fe, Co, or Ni) for microwave absorbing materials. La0.7Sr0.3Mn1−xTMxO3±δ has shown good properties for

**Figure 8.**  *X-ray diffraction pattern of La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 (x = 0.1–0.8) [22].* 

*Metamaterial: Smart Magnetic Material for Microwave Absorbing Material DOI: http://dx.doi.org/10.5772/intechopen.84471* 

#### **Figure 9.**

*The hysteresis curve of La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 (x = 0.1–0.8) [22].* 

microwave absorption. The maximum reflection loss was 27.67 dB at a 10.97 GHz frequency, which was obtained from a sample thickness of 2 mm. Zhou et al. [24] reported the successful synthesis of a modified of manganite-based compound La0.8Sr0.2Mn1-yFeyO3 (0 < y < 0.2). They showed that the absorption bandwidth reached 8.5 GHz above 8 dB and 6.2 GHz above 10 dB; the highest absorption peak reached 34 dB.

#### **3.2 Ferrite system**

 For the ferrite system, we have conducted research on nickel ferrite-based microwave absorbers [25]. A research to study the microwave absorption properties of nickel ferrite in the X-band range has been conducted by using high energy milling technique. The synthesis of nickel ferrite (NixFe3-xO4) was performed using solid-state reaction

**Figure 10.**  *Reflection loss (RL) of La0.8Ba0.2FexMn½(1-x)Ti½(1-x)O3 [22].* 

method with the material composition (2x)NiO: (3-x)Fe2O3 (x = 0.5, 1.0, 1.5 and 2.0) according to the molar ratio. This powder mixture was being milled for 10 hours then sintered at 1000°C temperature for 3 hours. Diffraction patterns of all varied NixFe3 xO4 (x = 0.5, 1.0, 1.5 and 2.0) which have been synthesized by using milling technique are shown in **Figure 11**. It can be noticed that a single phase of all varied NixFe3-xO4 (x = 0.5, 1.0, 1.5 and 2.0), which had spinel structure with lattice parameters a = b = c (space group Fd3m), has successfully formed.

The magnetic properties were measured by using vibrating sample magnetometer (VSM) as shown in **Figure 12**. To study the effects of Ni2+ doping on saturation magnetization (Ms), coercivity (Hc), and remanent magnetization (Mr) of NixFe3 xO4 of (x = 0.5–2.0), M-H hysteresis loops were recorded using VSM under the applied magnetic field in the range of −10 up to 10 kOe at room temperature. The VSM result shows that all the samples exhibited a ferromagnetic behavior and fine hysteresis loops with a decrease in magnetization (Ms and Mr) but coercivity (Hc) with increase in Ni2+ concentration. Its coercivity value is in the range of 164–217 Oe, and the maximum value is found at x =1.5 composition.

The microwave absorption measurement was carried out by Vector Network Analyzer (VNA). The VNA characterization shows the ability of microwave absorption with a parameter of RL (reflection loss) value. **Figure 13** shows that the highest RL peak reached −28 dB at frequency of 10.98 GHz. It means that the Ni1.5Fe1.5O4 sample can absorb microwave about ~96% at 10.98 GHz.

Other ferrite materials, such as barium mono-ferrite-based microwave absorbers, have also been studied by Ade Mulyawan et al. [26]. Barium mono-ferrite (BaFe2O4) has a more complex structure that exhibits orthorhombic structure. In this study, barium strontium mono-ferrite has been successfully synthesized using mechanical milling technique. BaCO3, SrCO3, and Fe2O3 powders are each weighed in accordance with the mole ratio of a total weight of 10 g. The chemical composition for the Ba(1-x)SrxFe2O4 samples was in the range of 0 < x < 0.5. X-ray diffraction patterns of all varied Ba(1-x)SrxFe2O4 (0 < x < 0.5) show a single phase of all composition.

 **Figure 14** shows the RL result of Ba(1-x)SrxFe2O4 (0 < x < 0.5). The results of VNA analysis show that the highest reflection loss of the Ba(1-x)SrxFe2O4 (0 < x < 0.5) was

**Figure 11.**  *X-ray diffraction patterns of NixFe3-xO4 of (x = 0.5–2.0) [25].* 

*Metamaterial: Smart Magnetic Material for Microwave Absorbing Material DOI: http://dx.doi.org/10.5772/intechopen.84471* 

#### **Figure 12.**

*Hysteresis curve of NixFe3-xO4 (x = 0.5–2.0) [25].* 

**Figure 13.**  *Reflection loss (RL) curve of NixFe3-xO4 (x = 0.5–2.0) [25].* 

**Figure 14.**  *Reflection loss (RL) curve of Ba(1-x)SrxFe2O4 (0 < x < 0.5) [26].* 

 found in the sample with composition x = 0.1, while the reflection loss of the Ba(1-x) SrxFe2O4 (0 < x < 0.5) decreased for composition x > 0.1. A significant property of microwave absorption has also been displayed for the composition of x = 0.1, in which the value of −38.25 dB (~99.9%) for the reflection loss in the frequency range of 11.2 GHz was achieved.

## **3.3 Hexagonal ferrite system**

 The M-type hexagonal ferrite system (BaFe12O19) is generally an oxide permanent magnet. The hexagonal ferrite is one of the hard magnetic materials that is widely used in many applications. Apart from being applied to electric motors, they can be used for electronic devices because they have good phase stability at high temperatures and very high frequency responses and as switching with narrow field distribution. Material engineering for this application requires material that has magnetic and electrical specifications, and to obtain it, system modification is needed through a substitution process where trivalent iron ions Fe3+ will be replaced in part by M2+ divalent and M4+ tetravalent metal ions; using a material processing route can vary. We have succeeded in modifying this material into a microwave absorbent material. Ba0.6Sr0.4Fe12-zMnzO19 (z = 0, 1, 2, and 3) was successfully synthesized by solid-state reaction through a mechanical milling method [27]. The raw materials of MnCO3, BaCO3, Fe2O3, and SrCO3 pro-analytic with purity > 99% were mixed according to stoichiometry composition of Ba0.6Sr0.4Fe12-zMnzO19 (z = 0, 1, 2, and 3). Based on the results of quantitative analysis using XRD, it shows that the best phase composition was found in the composition z = 1, namely, Ba0.6Sr0.4Fe11MnO19 as shown in **Figure 15**. Refinement of X-ray diffraction patterns reveals that Ba0.6Sr0.4Fe11MnO19 is a single phase and has a hexagonal structure with space group P63/mmc.

Powder Ba0.6Sr0.4Fe11MnO19 has an average particle size of 850 nm. Magnetic properties of Ba0.6Sr0.4Fe11MnO19 have a relatively low coercivity field and high remanent magnetization as shown in **Figure 16**.

 The results of the microwave absorption test in the sample Ba0.6Sr0.4Fe11MnO19 in the frequency range 8–14 GHz show that the absorption peak values were −8 and −10 dB at 8.5 and 12.5 GHz, respectively (**Figure 17**). When compared with the results of the study by Azwar Manaf et al. [28], they have conducted research on the Ti2+-Mn4+ ions which substituted BaFe12-2xTixMnxO19 samples with x = 0.0–0.8 through a mechanical alloying process and have studied the effect of ion substitution on

**Figure 15.**  *X-ray diffraction patterns of Ba0.6Sr0.4Fe12-zMnzO19 (z = 0, 1, 2, and 3) [27].* 

*Metamaterial: Smart Magnetic Material for Microwave Absorbing Material DOI: http://dx.doi.org/10.5772/intechopen.84471* 

**Figure 16.**  *Hysteresis curve of Ba0.6Sr0.4Fe12-zMnzO19 (z = 0, 1, 2, and 3) [27].* 

microstructure, magnetic, and microwave absorption characteristics. They obtained the results of reflection loss (RL) on series of Ti2+-Mn4+ ions substituted BaFe12- 2xTixMnxO19 samples with x = 0.0–0.8 samples which could be increased from 2.5 dB in composition x = 0 to −22 dB in composition x = 0.6 in the 8–12 GHz frequency range.

The results of this study can be concluded that the modified hexagonal ferrite system is also a good candidate for microwave absorbing material.

#### **3.4 Composite system**

At present there have been many materials developed from other types of polymer-based composite materials, because application requires that these

**Figure 17.**  *Reflection loss curve (RL) of Ba0.6Sr0.4Fe12-zMnzO19 (z = 0, 1, 2, and 3) [27].* 

 materials should be easy to form and easy to apply to other media such as electronic devices. For composite systems we have conducted research on microwave absorber sheets that are a composite of silicon rubber—iron oxide [29]. In this study, composites from the raw materials of silicone rubber, toluene, and iron oxide magnet powder have been made. The three raw materials are blended in a beaker and stirred for 60 min. The mixture is then printed on a media at 70°C and left for 15 min. The results of elemental analysis using energy-dispersive spectroscopy (EDS) show that the sample contained carbon, oxygen, sulfur, copper, and iron. While the results of the analysis of X-ray diffraction patterns show that the sample was classified as semi-crystalline with a crystallinity of 46%. So the composite consists of an amorphous matrix and a crystalline filler (namely, the phases of CuFeS2, FeS, FeO2, and Fe). Based on the results of functional group analysis using Fourier transformation infrared (FTIR), it shows that the sulfur addition modifies the polymer by forming a cross bond (bridge) between the individual polymer chain and the bond between the magnetic filler and the rubber matrix. **Figure 18** represents the FTIR spectrum between 4000 and 600 cm<sup>−</sup><sup>1</sup> from the composite sample. The peak transmittance of the FTIR spectrum in the composite sample shows the vibrations of O▬H, C▬H, Si▬C, Si▬O, and Fe▬O bond. Natural rubber consists of suitable polymers of isoprene organic compounds with minor impurities from organic compounds and other water. The transmittance peaks of silicone rubber appear at wave numbers around 3000, 1250, and 1050–750 cm<sup>−</sup>1 which indicate the presence of successive functional groups C▬H, Si▬C, and Si▬O bonds. Peak transmittance oxide iron was also observed at wave numbers around 3700 and 600 cm<sup>−</sup><sup>1</sup> , each of which indicated a vibration of H▬O and Fe▬O bonds. From the results of FTIR analysis, it is suspected that there is a bond between iron oxide as a filler and silicone rubber as a matrix.

The results of magnetic property analysis on absorber sheets were carried out using a vibrating sample magnetometer (VSM) which produced magnetic particle hysteresis curves as shown in **Figure 19**.

 The reflection loss value which is a microwave absorbent indicator from the absorber sheet is shown in **Figure 20** where each profile for sheet-1, sheet-2, and sheet-3 has been compared. **Figure 20** shows the relationship between reflection loss (RL) of the absorber sheet and the X-band microwave frequency in the range 10–15 GHz which was measured at a sample thickness of 1.5 mm. There were two observed absorption peaks and high RL in the high frequency range for all samples.

**Figure 18.** 

*The transmittance spectrum (FTIR) of the composite sample of silicon rubber—iron oxide [29].* 

*Metamaterial: Smart Magnetic Material for Microwave Absorbing Material DOI: http://dx.doi.org/10.5772/intechopen.84471* 

**Figure 19.** 

*Hysteresis curve of composite silicon rubber—iron oxide samples [29].* 

**Figure 20.**  *Reflection loss curve (RL) of composition of silicone rubber—iron oxide [29].* 

 The RL value increases with the addition of filler composition in the matrix. It seems that the RL value increases with the same sample thickness and the absorption frequency slightly shifts. This can be explained by the effects of electromagnetic properties on attenuation characteristics in each sample. Thus according to the results of this study, it was found that the best composite sample was sheet-3 with an absorption peak value of −15 dB at a frequency of 12 GHz.

Based on the results of this study, we conclude that all microwave absorbent materials can be made in the form of silicon rubber-based composite sheets as a composite matrix.

### **4. Conclusions**

 Based on the above explanation, wave absorbing materials are composed of materials that have magnetic and electrical properties which are shown by intrinsic parameters in the form of complex permittivity (ε*r*) and complex permeability (*μr*) and extrinsic parameters in the form of geometry factors (thickness) of a material [14–16]. If a travelling electromagnetic wave is incident upon and

absorbed by a microwave absorbent, spin resonance will occur due to the presence of these material parameters. In smart magnetic materials, the resonance that occurs between electromagnetic waves and material is divided into two mechanisms, namely, the wall resonance domain and spin electron resonance (ferromagnetic resonance). The wall resonance domain is resonance that occurs in magnetic domains caused by induction of electromagnetic waves, while spin electron resonance is resonance that occurs in electrons that are precise in the direction of the internal magnetic field due to the induction of electromagnetic waves. However, it should be noted that smart magnetic material is certainly unable to absorb microwaves on all band frequencies because each smart magnetic material has different resonant characteristics, so the maximum effort that can be done is to find the right composition of smart magnetic material which is expected to have the maximum wave absorption ability.
