Microwave Absorption Properties of Cobalt-doped BaFe 12 O 19 Hexaferrites
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Microwave Absorption Properties of Cobalt-doped BaFe12 O19 Hexaferrites Ngo Tran, Min Young Lee and Bo Wha Lee∗ Department of Physics and Oxide Research Center, Hankuk University of Foreign Studies, Yongin 17035, Korea (Received 21 February 2020; accepted 24 March 2020) Cobalt doping at the iron site of barium hexaferrites (BaFe12 – x Cox O19 , x = 0 – 2.0, Δx = 0.5) was done using a co-precipitation method followed by a heat treatment at 900 ◦ C. Careful refinement of the X-ray diffraction patterns by using the Rietveld method revealed a transformation from a hexagonal to a mixed hexagonal and cubic spinel structure. The undoped sample was ∼95% barium M-type hexaferrites (BaFe12 O19 ), with the other 5% being made up barium Y-type hexaferrites (Ba2 Fe14 O22 : 3.50%) and spinel ferrites (Fe3 O4 : 1.62%). The replacement of Fe3+ by Co2+ led to a charge imbalance which promoted the growth of Y-type hexaferrites (Ba2 Cox Fe14 – x O22 ) and spinel ferrites (Cox Fe3 – x O4 ). The impurity phases grew linearly from 5.12% for x = 0 to 67.21% for x = 2.0, leading to a linear decrease in barium M-type hexaferrites (94.88% for x = 0 to 32.79% for x = 2.0). A structural study by using Raman scattering spectroscopy confirmed the growth of Y-type hexaferrites and spinel ferrites by detecting new modes at 319, 325, 519, 569, 640 and 675 cm−1 . As to the electromagnetic properties, three of the four parameters were changed by doping in the frequency range from 2 to 18 GHz for samples with thickness of t = 1.50 mm. Both the real (μ ) and the imaginary (μ ) parts of the complex permeability and the real part (ε ) of the complex permittivity increased with increasing doping. This tuned the reflection loss (RL) from poor (RL = −5.0 dB at f = 12.5 – 13.0 GHz) for the undoped sample to fairly good for doped samples (RL = −10.5 dB and −16.0 dB for x = 2.0 and 1.0, respectively, at f = 17.0 GHz). A study of the thickness (t) dependence of the RL in the range t = 2.75 – 1.50 mm pointed out that our samples also had some features similar to those of metamaterials, especially for thick samples at a frequency close to 18 GHz for the x = 0 sample. Keywords: Hexaferrites, M-type, Y-type, Structural transformation, Electromagnetic, Reflection loss DOI: 10.3938/jkps.77.1125
I. INTRODUCTION Due to the continuous development of information technology and high-power electronic equipment, various electronic products in modern society play an important role in human’s lives. At the same time, these electronic products generate electromagnetic (EM) radiation, thereby causing a serious microwave pollution in the environment and affecting the human health [1–3]. If the environmental pollution problem is to be solved, an EM wave absorber that can absorb EM wave energy and convert it into heat energy and other forms of energy needs to be developed [4–6]. Hexagonal ferrites (hexaferrites) exist in various types: M-, W-, Y-, X-, Z- and U-types. These types correspond to (M O + M eO)/Fe2 O3 ratios of 1:6, 3:8, 4:6, 4:14, 5:12 and 6:18 [7, 8], where M represents
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