Investigation on structural and microwave absorption ...

Journal of Alloys and Compounds 695 (2017) 792e798

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Investigation on structural and microwave absorption property of Co2þ and Y3þ substituted M-type Ba-Sr hexagonal ferrites prepared by a ceramic method Jasbir Singh a, Charanjeet Singh b, c, *, Dalveer Kaur d, S. Bindra Narang c, Rajshree Jotania e, Rajat Joshi b a

Department of Electronics and Communication Engineering, Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India Department of Electronics and Communication Engineering, Rayat Bahra Institute of Engineering and Nanotechnology, Hoshiarpur, Punjab, India c Department of Electronics Technology, Guru Nanak Dev University, Amritsar, Punjab, India d Department of Electronics and Communication Engineering, I.K.G. Punjab Technical University, Kapurthla, India e Department of Physics, University School of Sciences, Gujarat University, Ahmedabad, 380 009, Gujrat, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 July 2016 Received in revised form 6 September 2016 Accepted 23 September 2016 Available online 24 September 2016

The Co2þ and Y3þ ions substituted M-type Ba0.5Sr0.5CoxYxFe12-2xO19 hexagonal ferrites (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were synthesized by a standard ceramic method. The phase evolution of prepared hexagonal ferrite samples was studied using X-ray diffraction technique. XRD analysis shows that compositions owe M-type as major phase and BaFe2O4 as minor phase. The absorber testing device method is adopted in order to investigate microwave absorption property of prepared ferrite compositions as a function of frequency (8.2e12.4 GHz), substitution and thickness. The microwave absorption property has been evaluated using the quarter wavelength mechanism and impedance matching mechanism. The variation in parameters measured experimentally is in agreement with put forth theoretical models. In doped compositions, the microwave absorption is found to increase with the substitution of Co2þ and Y3þ ions. Compositions x ¼ 0.0 and 1.0 ascribe good microwave absorber characteristics with absorbed power of 96.2 and 94.7% at 11.22 and 10.04 GHz respectively. The quarter wavelength mechanism contributes for large microwave absorption in compositions x ¼ 0.0, 0.2 and 0.8, whereas impedance matching mechanism is primarily responsible for absorption in x ¼ 0.4, 0.6 and 1.0. © 2016 Elsevier B.V. All rights reserved.

Keywords: Hexagonal ferrites Ceramic method Microwave absorption

1. Introduction The exponential growth in information technology or wireless devices has produced wireless or electromagnetic pollution. The electromagnetic interference (EMI), caused by electromagnetic pollution, results in the malfunctioning of electronic devices and it is potentially harmful for biological systems. The microwave absorbers or EMI suppressors are used to remove or attenuate this EMI or stray electromagnetic signals. Ferrites are incorporated in electrical, electronic and wireless applications such as wideband transformers, antenna, channel

* Corresponding author. Department of Electronics and Communication Engineering, Rayat Bahra Institute of Engineering and Nanotechnology, Hoshiarpur, Punjab, India. E-mail address: [email protected] (C. Singh). http://dx.doi.org/10.1016/j.jallcom.2016.09.251 0925-8388/© 2016 Elsevier B.V. All rights reserved.

filters, gyromagnetic devices, radar absorbing materials (RAM) etc. [1e4]. Their good dielectric and magnetic properties render them better EMI suppressers than conventional dielectric materials. Mtype hexaferrites are ferrimagnetic by nature and used as microwave absorbers or EMI reduction owing to large dielectric and magnetic losses, domain wall resonance and ferromagnetic resonance (FMR) [5,6]. The various researchers have investigated microwave absorption properties of M-type substituted hexagonal ferrites [7e10]. In the present paper, we report microwave absorption property of Co2þ and Y3þ ions substituted M-type Ba0.5Sr0.5CoxYxFe122xO19 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) hexagonal ferrites prepared by a standard ceramic method and elucidated the absorption with quarter wavelength mechanism and impedance matching mechanism; a few researchers have reported the research work based on these two mechanisms.

J. Singh et al. / Journal of Alloys and Compounds 695 (2017) 792e798

2. Experimental method M-type hexagonal ferrites with chemical composition Ba0.5Sr0.5CoxYxFe122xO19 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by a standard ceramic method [11]. AR grade of Barium carbonate (BaCO3, 99.98% pure, Merck, Germany), Strontium carbonate (SrCO3, 99.99% pure, Sigma-Aldrich), Cobalt carbonate (CoCO3, 99.99% pure, Sigma-Aldrich), Yttrium oxide (Y2O3, 99.99% pure, Sigma-Aldrich) and Ferric oxide (Fe2O3, 99.99% pure, Merck, Germany) were chosen as starting materials. The stoichiometric amounts of chemical reagents were grounded in the presence of distilled water for 8 h using an agate pestle and mortar. The ground mixed powders were pre-sintered at 1000 C for 10 h in an electric furnace and slowly cooled to room temperature (5 C/min). The obtained powders were re-grounded under the same condition. The sieving of powders was carried out with sieves of mesh size 220 B.S.S.; and recovered powders was pressed using a hydraulic press under an uniaxial pressure of 75 kN/m2 in order to prepare pellets. In the final sintering process, the pellets were sintered at 1150 C for 15 h and then slowly cooled to room temperature: the heating and cooling rates were set to ±5 C/min. The structural characterization of sintered hexagonal ferrite samples was carried out by X-ray diffractometer (BrukerModel D8), using Cu-Ka radiation (l ¼ 1.5405 Å) at room temperature in the range of 20e70 in order to examine phase purity. The microwave characteristics of Ba0.5Sr0.5CoxYxFe122xO19 (x ¼ 0.0 to 1.0) ferrites were investigated as a function of substitution, frequency and thickness at X-band by Absorber Testing Device (ATD) method [12,13]: Fig. 1 shows block diagram of the adopted method. The microwave frequency synthesizer, HP Model 83751A, generates frequencies at X-band (8e12 GHz) in the rectangular slotted waveguide with inner dimensions length ¼ 22.86 mm, breadth ¼ 10.16 mm. The isolator allows the unattenuated microwave propagation in one direction and vice-versa. The directional coupler carries with one primary input and two secondary output ports. The composition, with metal plate, was fitted at the secondary output port 1 and the reflected signal from composition was measured by power meter connected to other secondary output port 2. The microwave power meter Tektronix-Model 3320 was used to measure the different microwave signals and S11 parameter was calculated from reflected power at port no. 2. The reflection loss (RL) can be expressed using following relation:

RL ðdBÞ ¼ 20 log10 ðjS11 jÞ

(1)

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The reflection loss of 10 dB represents 90% microwave absorbed power and the large reflection loss corresponds to more microwave absorption and vice versa. The reflected power (%) was calculated as: Reflected Power (%) ¼ (Pr/Prw) 100

(2)

where Pr is the reflected power from the composition backed by metal plate and Prw is the reflected power from the metal plate without composition. The absorbed power was calculated using the following relation: Absorbed Power (%) ¼ 100 Reflected Power (%)

(3)

The selected thicknesses of the composition corresponding to the optimised microwave absorption are: x ¼ 0.0 (3.0 mm), x ¼ 0.2 (3.0 mm), x ¼ 0.4 (2.7 mm), x ¼ 0.6 (2.8 mm), x ¼ 0.8 (2.7 mm), x ¼ 1.0 (2.9 mm). The term matching frequency (fmat) and compositions stand for the maximum power absorption at a particular frequency and compositions.

3. Results and discussion 3.1. XRD analysis The structural properties and phase purity of the sintered polycrystalline samples were investigated at room temperature using X-ray diffraction technique. Fig. 2 represents X-ray diffraction patterns of Ba0.5Sr0.5CoxYxFe(122x) (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) hexagonal ferrite samples, prepared using a standard ceramic method sintered at 1150 C for 15 h. All the observed peaks in XRD were indexed using the Powder X software and identified with their Millar indices. The observed peaks are sharp and well defined; which reflects that prepared samples are well crystalline. The XRD patterns were indexed to hexagonal magnetoplumbite (M-type) crystal structure having space group P63/mmc (JCPDS file no. 511879). The XRD analysis of x ¼ 0.0, 0.2 compositions confirm the formation of mono phase of M-type (hexagonal), while XRD patterns of x ¼ 0.4 to 1.0 compositions show mixed crystalline phases of Ba0.5Sr0.5Fe12O19 (M-hexagonal) and BaFe2O4 (spinel, JCPDS file no. 77-2337 having lattice parameters a ¼ 17.34 Å, b ¼ 9.35 Å and c ¼ 10.88 Å). The structural parameters (a ¼ b and c) and unit cell volume (V) were calculated using Equations (4) and (5) respectively. For hexagonal structure; a ¼ b s c and a ¼ b ¼ 90 and g ¼ 120

Fig. 1. Block diagram of ATD (Absorber Testing Device) Method.

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Fig. 3. Plots of reflection loss vs. frequency and substitution of Co-Y ions in Ba0.5Sr0.5CoxYxFe12-2xO19 hexagonal ferrites (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). Fig. 2. X-ray diffraction patterns of Ba0.5Sr0.5CoxYxFe(12-2x)O19 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) hexagonal ferrite compositions prepared using a standard ceramic method and sintered at 1150 C for 15 h.

1 4 ¼ 3 d2hkl V¼

2 h hk k2 l2 þ þ þ a2 a2 a2 c2

(4)

pffiffiffi 3 2 a c 2

(5)

where dhkl is the d-spacing of the lines in XRD pattern and h, k, l are the Miller indices. The lattice parameters, ratio (c/a) and unit cell volume of all prepared samples are listed in Table 1. The structural parameters were characterized by lattice constants a and c (Table 1). It is observed that there is not much change in values of lattice parameters and unit cell volume with the increase of Co-Y content in prepared hexagonal ferrite samples but lattice constant ‘a’ and cell volume ‘V’. Of x ¼ 1.0 composition are lower compare to other samples. It is attributed to ionic radii of Co2þ (0.72 Å), Y3þ (0.90 Å), and Fe2þ (0.64 Å) [14,15]. It seems that Co-Y substitution in the prepared hexagonal ferrite does not disturbed crystal symmetry and this fact confirms from (c/a) ratio which is almost same indicating no change in the crystal structure by doping of Co-Y ions.

Ba0.5Sr0.5CoxYxFe12-2xO19 ferrites. It shows decreasing trend in all compositions from low to high frequency region and shape of curves become random to symmetrical from low to high frequency region. Compositions x ¼ 0.0 and 1.0 have higher RL value at the number of frequencies in the low, middle and high frequency region. Compositions x ¼ 0.0, 0.4, 0.6 and 1.0 exhibit RL > 10 dB at low, middle and high frequency region and all doped compositions owe highest RL (>11 dB) around 10 GHz. The substitution of Co2þ and Y3þ ions renders (i) change in amplitude of RL peaks (ii) nearly no shift of peaks from 10 GHz to 12.4 GHz frequency range followed by the same peaks of maximum and minimum values almost at the same frequencies. Fig. 4 shows the variation of absorbed power (Pab) vs. frequency and substitution of Co2þ and Y3þ ions in Ba0.5Sr0.5CoxYxFe12-2xO19 hexagonal ferrites. The higher Pab value is observed in undoped composition with x ¼ 0.0 and higher substitution (x ¼ 1.0) along the investigated frequency region and all compositions observe more dispersion from 11.56 to 12.06 GHz. The absorberd power is greater than 80% in all compositions along the different frequency regions. Compositions x ¼ 0.0 and 0.2 have highest and lowest Pab values of 96.2 and 23.9% at 11.22 GHz and 11.89 GHz frequencies respectively. Table 2 shows various parameters related to maximum absorbed power, quarter wavelength mecha mechanism and 10 dB

3.2. Reflection loss and microwave absorbed power Fig. 3 represents the variation of reflection loss (RL) vs. frequency and substitution of Co2þ and Y3þ ions in M-type

Table 1 Lattice parameters (a,c) ratio (c/a) and unit cell volume (V) of Ba0.5Sr0.5CoxYxFe122xO19 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) hexagonal ferrites prepared using a standard ceramic method and sintered at 1150 C for 15 h. Co-Y content (x)

0.0 0.2 0.4 0.6 0.8 1.0

Lattice parameters a (Å)

c (Å)

5.8804 5.8815 5.8922 5.8831 5.8813 5.8444

23.1188 23.0887 23.1268 23.1750 23.1116 23.0916

Ratio (c/a)

Cell volume V (Å)3

3.9315 3.9255 3.9249 3.9392 3.9297 3.9512

692.3245 691.6817 695.3463 694.6449 692.3207 683.0689

Fig. 4. Plots of absorbed power vs. frequency and substitution of Co-Y ions in Ba0.5Sr0.5CoxYxFe12-2xO19 hexagonal ferrites (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).


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Table 2 Maximum absorber power, matching frequency, calculated thickness, matching thickness, frequency band and bandwidth for RL > 10 dB in Ba0.5Sr0.5CoxYxFe12-2xO19 (x ¼ 0.0 to 1.0) hexagonal ferrites. Composition Pabmax. (%)

Matching frequency (fmat) (GHz)

Thickness t ¼ nl/4 (mm) (calculated)

Matching thickness (mm) (measured)

Frequency band (GHz) RL > e10 dB Bandwidth (MHz) (Pab > 90%) (Pab > 90%)

x ¼ 0.0 x ¼ 0.2 x ¼ 0.4

96.2 93.5 94.2

11.22 10.04 10.04

2.9 3.3 3.2

3.0 3.0 2.7

x ¼ 0.6 x ¼ 0.8 x ¼ 1.0

94.3 93.1 94.7

9.88 10.04 10.04

3.2 2.9 2.3

2.8 2.7 2.9

9.54e10.04 e 9.88e10.21 11.05e11.56 9.71e10.21 e 9.88e10.21

bandwidth. It is clear from Table 2 that the maximum absorbed (Pabmax.) observed with corresponding matching frequency (fmat) in Ba0.5Sr0.5CoxYxFe12-2xO19 (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) hexagonal ferrites. In doped compositions, Pabmax. value increased gradually with the substitution of Co2þ and Y3þ ions. In comparison to Pabmax. in composition x ¼ 0.0 at 11.22 GHz, compositions x ¼ 0.2, 0.4, 0.8 and 1.0 have slight lower values of Pabmax. at 10.04 GHz, while composition x ¼ 0.6 also has slight lower value of Pabmax. at 9.88 GHz. Fig. 5 displays variation of complex permittivity ε (ε ¼ ε0 j ε00 ) and permeability m (m ¼ m0 j m00 ) where ε0 , ε00 , m0 and m00 represent dielectric constant, dielectric loss, permeability and magnetic loss respectively; ε00 and m00 contribute to the absorption of microwave signal. The doping Co2þ and Y3þ ions results in the non-linear

500 e 336 504 504 e 336

increase in ε0 , ε00 and m0 and m00 . The different peaks observed in ε00 of compositions are in anti-phase with m00 and cancel each other, implying total loss has no peak of absorption. The dielectric properties are enhanced in comparison to magnetic loss properties. 3.3. Quarter wavelength mechanism According to this mechanism [16,17] when the thickness of ferrite absorber is equal to 1/4th wavelength of microwave signal, it will get attenuated or absorbed after passing through ferrite material. When the microwave signal propagates through the hexagonal ferrite composition backed by metal plate, a part of it will be partially reflected by the front surface of hexagonal ferrite and the

Fig. 5. Complex permittivity and complex permeability variation as a function of Co-Y ions in Ba0.5Sr0.5CoxYxFe122xO19 hexagonal ferrites (x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).

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remaining signal will be transmitted through the hexagonal ferrite material. This transmitted signal gets reflected after reaching at the metal plate and arrives again at the front face of the ferrite material. When the reflected signal from the front surface of ferrite composition is equal in magnitude and 180 out of phase with the reflected signal from the metal plate, both signals will cancel each other and total reflection will be zero. Therefore, the present condition can be mathematically expressed as:

tm ¼

nl0 4

where n ¼ 1; 3; …5……etc

l l0 ¼ pffiffiffiffiffi mε

(6)

(7)

where tm, l0, l, m and ε are denoting calculated or theoretical thickness, wavelength of signal in material, wavelength of signal in air, complex permeability and complex permittivity respectively; Nicholson-Ross method [18] is used to derive m and ε from Sparameters. Table 2 depicts quarter wavelength mechanism applied to the different compositions in terms of calculated or theoretical thickness (tcal ¼ nlo/4). For maximum absorption or RL peak in compositions (Figs. 3 and 4) at respective frequencies, compositions x ¼ 0.0, 0.8 and 0.2 have calculated thickness more close to the measured thickness in comparison to compositions x ¼ 0.4, 0.6 and 1.0: x ¼ 0.0 has highest absorbed power of 96.2% among all compositions due to more closeness of calculated and measured thickness. More specifically, compositions x ¼ 0.0, 0.8 and 0.2 have more contribution to the quarter wavelength mechanism. However, compositions x ¼ 0.4, 0.6 and 1.0 show larger absorberd power inspite of large difference in calculated and measured thickness values. This anomaly is ascribed to the input impedance mechanism discussed in the next section. The doping of Co2þ and Y3þ ions reduces the thickness for Pabmax in composition x ¼ 0.4 (2.7 mm) and 0.8 (2.7 mm) in comparison to undoped composition x ¼ 0.0 (3.0 mm); similar variation is observed in x ¼ 0.6 and 1.0 compositions. The dopants shift the Pabmax from high frequency region in x ¼ 0.0 to middle frequency regions in doped compositions of Ba0.5Sr0.5CoxYxFe12-2xO19 ferrites. Compositions x ¼ 0.2, 0.8 and 1.0 have Pamax at 8.2 GHz and, x ¼ 0.0 and 0.6 have same at 11.22 and 10.04 GHz respectively. Table 2 shows 10 dB bandwidth displayed by the compositions; 10 dB bandwidth means the band of frequencies for which RL is > 10 dB. Compositions x ¼ 0.4 and 1.0 have 336 MHz absorption bandwidth (ABW) at the same frequency band from 9.88 GHz to 10.21 GHz, whereas compositions x ¼ 0.4 and 0.6 have ABW of 504 MHz from 11.05 GHz to 11.56 GHz and 9.71- GHz to 10.21 GHz respectively. The un-doped composition, x ¼ 0.0 has 500 MHz ABW from 9.54 GHz to 10.04 GHz.

The Equation (6) explains that Zin is of complex form (aþj b) where a is the real part and b is the imaginary part: (i) Theoretically all signal will be absorbed if jZj ¼ Zo ¼ 377U, i.e. Zreal ¼ 377 U and Zimg ¼ 0 (ii) a component of signal RL will be absorbed when Zreal s 377 U and/or Zimg s 0; when Zin is equal or near to Zo (377U), it attributes to large microwave absorption due to impedance matching. Furthermore, absorption decreases when Zreal moves farther from 377 U and/or Zimg increases (positive or negative values). Table 3 summarizes Zin, Zreal and Zimg values corresponding to the large microwave absorption or RL peaks observed in synthesized compositions. Compositions x ¼ 1.0, 0.4 and 0.6 have relatively both Zreal and Zimg more close to 377 U and zero than other compositions, thereby giving RL peak or maximum absorption shown in Table 3 due to the more contribution of impedance matching mechanism. The compositions (x ¼ 0.0, 0.2 and 0.8) have either Zreal away from 377 U or Zimg away from zero and thus contribution of impedance matching mechanism is small: for example; composition x ¼ 0.8 has Zreal ¼ 100.1 U at 8.2 GHz which is relatively more near to Zo ¼ 377 U than at other frequencies, however, Zimg ¼ 168.9U makes far away from Zimg ¼ 0. Nam et al. reported similar variation on impedance matching mechanism in La1.5Sr0.5NiO4 ferrites [20]. XRD patterns in Fig. 2 revealed that doped compositions have coexistence of M-phase along with non-magnetic phase of BaFe2O4 ferrite having orthorhombic crystal structure; this nonmagnetic phase discourages microwave absorption. Composition x ¼ 0.0 has only pure M-phase without any impurity of nonmagnetic phase, causing more number of large RL peaks, of microwave absorption, among all compositions as shown in Fig. 3. However, large absorbed power is seen in doped compositions (Table 2 and Fig. 3) in spite of the presence of non-magnetic phase. For example, composition x ¼ 0.6 has a relatively large percentage of non-magnetic phase, however, it has more contribution of impedance matching mechanism as discussed before; both these factors counteract in order to have considerable absorption. Similar reason accounts for reasonable microwave absorption in x ¼ 0.8 and other doped compositions (x ¼ 0.2, 0.4 and 1.0) wherein both non-magnetic phase and quarter wavelength or impedance matching mechanism also exist. It is worth to mention here again that material parameters (m and ε) decide the quarter wavelength or impedance matching mechanism as per Equations (7) and (8). 3.5. Screenshot of microwave signal

The input impedance (Zin) of a single layer absorber can be calculated theoretically on the basis of transmission line theory as [19]:

The screenshots are used to show the frequency, phase and amplitude of the signal after the compositions. Fig. 6 displays the observation of input signal applied to the compositions and signal transmitted from the compositions, and cathode ray oscilloscope (CRO) is used to display these signals: different frequencies applied to compositions x ¼ 0.0, 0.4, 0.8 and 1.0 are 8.36, 9.88, 8.70 and 11.22 GHz respectively. These screenshots depict only attenuation in the signal by compositions and frequency as well as phase of signal remain unchanged. Similar variation was also noted for other applied frequencies and compositions; x ¼ 0.2 as well as x ¼ 0.6 also exhibit same behavior.

Zin ¼ Zo(m/ε)1/2 tanh [j(2pft/c) (mε)1/2]

4. Conclusions

3.4. Impedance matching mechanism

(8)

where Zo ¼ 377 U is the characteristic impedance of free space and, m, ε, t, f and c denote complex permeability, complex permittivity, thickness, frequency, velocity of light respectively. When Zin is equal to Zo, input impedance of composition will be equal to characteristic impedance contributing infinite absorption (theoretically) of the signal by the composition.

The experimental observations are summarised as follows: 1 Ba0.5Sr0.5CoxYxFe12-2xO19 (x ¼ 0.0 to 1.0) hexagonal ferrite powders have been successfully synthesized using a standard ceramic method. XRD analysis of undoped and x ¼ 0.2 compositions confirms formation of M-type phase having hexagonal


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Table 3 Impedance Matching mechanism with f, Zin, Zreal and Zimg values corresponding to RL peaks in Ba0.5Sr0.5CoxYxFe12-2xO19 (x ¼ 0.0 to 1.0) hexagonal ferrites. f (GHz)

Zreal (U)

Zimg (U)

Zin (U)

RL peak (dB)

x ¼ 0.0 8.20 8.36 8.70 9.04 9.71 9.88 10.04 10.21 10.63 10.72 11.22 11.39 12.23

555.7 528.4 377.8 259.2 156.9 138.0 106.8 96.5 82.3 78.5 64.8 65.2 47.4

8.20 8.36 8.70 9.04 9.71 9.88 10.04 10.21 10.63 10.72 11.22 11.39 12.23

121.7 117.4 109.9 103.5 93.7 91.7 89.7 87.9 83.5 82.7 77.8 76.2 68.2

Zreal (U)

Zimg (U)

Zin (U)

RL peak (dB)

x ¼ 0.2 57.1 160.2 271.5 253.6 209.7 200.1 184.8 164.9 144.6 139.2 108.5 98.3 65.8

558.6 552.1 465.2 362.7 261.9 243.1 213.5 191.1 166.4 159.9 126.4 118.0 81.0

11.6 13.0 10.8 7.3 13.8 12.0 10.4 4.7 7.1 10.1 14.2 12.6 9.9

104.5 102.7 98.4 93.7 84.1 81.8 79.5 77.2 71.6 70.5 64.4 62.5 54.8

160.5 156.0 147.5 139.7 126.0 122.9 119.9 117.0 110.0 108.7 101.0 98.6 88.

14.7 12.3 10.2 9.1 10.2 12.5 12.4 9.3 9.7 10.5 10.5 11.1 9.4

x ¼ 0.6

50.8 50.1 49.5 46.9 38.3 38.7 40.8 41.9 43.1 43.0 44.3 46.0 41.4

Zimg (U)

Zin (U)

RL peak (dB)

27.6 39.6 55.0 60.5 55.7 52.8 49.6 46.3 38.5 37.1 30.9 29.7 29.3

115.3 116.4 115.8 112.4 102.6 100.1 97.6 95.2 90.0 89.2 85.1 84.0 78.5

13.2 10.0 8.9 8.9 8.5 12.2 12.4 10.1 8.2 9.9 10.4 10.1 9.9

28.7 26.3 20.5 14.0 5.0 0.4 4.4 6.6 10.1 11.0 15.6 17.0 21.4

81.9 78.6 71.9 69.3 64.8 63.0 65.1 67.8 71.3 72.0 76.2 77.8 86.1

12.7 10.6 8.9 11.0 8.2 12.3 12.7 10.8 10.3 9.4 10.6 10.4 11.0

x ¼ 0.4 87.4 80.5 71.3 63.0 46.2 38.2 33.5 30.4 23.6 23.0 13.4 11.1 0.4

101.1 94.9 86.8 78.6 60.1 54.4 52.8 51.8 49.1 48.8 46.3 47.3 41.4

13.0 10.2 7.7 7.6 7.5 11.5 11.8 9.0 6.5 8.6 9.5 9.5 9.2

111.9 109.5 101.9 94.6 86.2 85.0 84.0 83.1 81.4 81.1 79.2 78.5 72.8

168.9 157.3 138.0 121.0 91.7 80.4 72.5 66.5 54.5 53.4 39.8 35.6 18.0

196.3 181.0 157.6 137.6 103.3 92.6 86.3 81.6 73.0 72.2 62.7 61.0 51.1

12.3 10.7 9.1 8.6 8.3 11.5 11.6 8.3 6.8 8.9 9.0 9.5 9.6

76.7 74.1 68.9 67.9 64.6 63.0 65.0 67.4 70.5 71.2 74.6 76.0 83.4

x ¼ 0.8 100.1 89.6 76.1 65.4 47.4 45.9 46.8 47.2 48.5 48.5 48.5 49.6 47.8

Zreal (U)

x ¼ 1.0

Fig. 6. Screenshot of input signal and signal transmitted through the compositions (x ¼ 0.0, 0.4, 0.8 and 1.0). (Amplitude value of signal is a guide to the eye).

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2

3

4

5

6

7


crystal structure, while x ¼ 0.4 to 1.0 composites are consist of two phases; M-type as major phase, hexagonal and BaFe2O4 as minor phase, spinel. In doped compositions, the microwave absorption property is found to increased with substitution of Co2þ and Y3þ ions in prepared M-type hexagonal ferrites. The quarter wavelength and impedance matching mechanism can be incorporated to predict thickness and frequency for maximum microwave absorption and design microwave absorbers. Microwave property investigation suggest that compositions x ¼ 0.0, 0.2 and 0.8 have more contribution of quarter wavelength mechanism for large microwave absorption while compositions x ¼ 0.4, 0.6 and 1.0 confirm more contribution of impedance matching mechanism for the same. Composition x ¼ 0.0 exhibits microwave absorber or EMI reduction characteristics with 96.2% absorbed power at matching frequency and thickness of 11.22 GHz and 3.0 mm respectively. The observed absorption bandwidth in compositions x ¼ 0.4 and 1.0 is 336 MHz while x ¼ 0.4 and 0.6 compositions have 504 MHz at 10 dB. The synthesized hexagonal ferrite compositions have potential application as microwave absorber or EMI suppression after proper tuning of the frequency, amount of substitution and thickness in their composites, which is a manuscript of separate investigation.

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