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Shanigaram Mallesh,Ki Hyeon Kim 한국자기학회 2019 Journal of Magnetics Vol.24 No.4
We report the phase stability, microstructure and magnetic properties of MgFe2O4 nanoparticles under different annealing conditions. Magnetic properties of MgFe2O4 are strongly influenced by the crystal structure and morphology, which in turn, depend on annealing conditions. The as-prepared and samples annealed at 1200 °C show a pure spinel phase. Whereas the samples annealed and quenched at 600 °C-1000 °C exhibit the spinel phase along with a small fraction of the secondary phase of α-Fe₂O₃, which causes deterioration of magnetic properties. On the other hand, samples annealed at 600 °C-1000 °C under Argon atmosphere display superior magnetic properties (M = 44-56 emu/g at room temperature) due to the presence of pure spinel phase. Interestingly, the sample quenched at 1200 °C exhibits large saturation magnetization, MS = 63 emu/g at 5 K which is arising from the optimum cationic distribution (δ = 0.77) grown at elevated temperature is retained in the rapid cooling process.
Microwave Absorption Properties of Carbonyl Iron Particles Filled in Polymer Composites
장원규,Shanigaram Mallesh,김기현 한국물리학회 2020 새물리 Vol.70 No.4
We report the electromagnetic wave absorption performance of carbonyl iron-particles (CIP) magnetic fillers in a polydimethylsiloxane (PDMS) matrix prepared by using a doctor blade method. The X-ray diffraction and the field-emission scanning electron microscopy studies of the CIPs revealed a BCC crystal structure and an average diameter of 3 m. For the CIPs, the saturation magnetization, MS, was 205 emu/g, and the coercivity, HC was around 12 Oe. The microwave absorption properties were measured using a vector network analyzer in the frequency range from 0.1 to 18 GHz. We observed a systematic increase in the minimum reflection loss (RL) with increasing weight fraction of CIP fillers (46 wt% to 72 wt%) in the PDMS matrix. The minimum RL was -27.5 dB at 14.6 GHz with a thickness of only 1.5 mm and an effective absorption bandwidth of up to 6.8 GHz (RL @ -10 dB). Therefore, the present study suggests that CIP magnetic fillers in a PDMS matrix are a good candidate for thin, broadband absorbing fillers.
Microwave absorption properties of core-shell structured FeCoNi@PMMA filled in composites
장원규,Shanigaram Mallesh,이상복,김기현 한국물리학회 2020 Current Applied Physics Vol.20 No.4
FeCoNi-coated PMMA (FeCoNi@PMMA) core-shell structures prepared by an electroless plating process were investigated in this study. Further, we fabricated FeCoNi@PMMA in polydimethylsiloxane sheets with volume fractions of fillers from 10 vol% to 25 vol% to evaluate electromagnetic wave absorption performance. Subsequently, results were compared with those of carbonyl iron particles (CIP). The maximum reflection loss (RL) of FeCoNi@PMMA composite (25 vol%) reached a value of −37.2 dB at 12.2 GHz with a thickness of 1.5 mm and a broad absorption bandwidth (RL ≤ −10 dB) of 4.5 GHz, attributed to strong dielectric and magnetic losses. Though the microwave absorption performance of the core-shell structured FeCoNi@PMMA composites is similar to that of CIP composites, the weight fraction of FeCoNi@PMMA is reduced about 38% compared with CIP with the same volume fraction (25 vol%). These results indicate that FeCoNi@PMMA is a lightweight material and a good candidate for high-performance microwave-absorbing devices.
Minji Gu,Shanigaram Mallesh,Ki Hyeon Kim 한국자기학회 2021 Journal of Magnetics Vol.26 No.3
The 1D chain-like α-Fe₂O₃ nanostructures were synthesized using a facile solvothermal method followed by subsequent annealing in air at 450 °C for two hours. We have investigated the microstructure and magnetic properties of the samples in detail. Field emission scanning electron microscopy revealed the formation of 1D chain-like morphology of α-Fe₂O₃ nanostructures. The presence of the pure hematite phase in all samples was confirmed by X-ray diffraction (XRD). Further, results obtained from Raman and FT-IR spectra are consistent with XRD outcomes. The magnetic studies (M-H curves) of samples exhibited a wide range of properties. The S₁ and S5 samples exhibit large coercivity (HC) values of 878 Oe and 1092 Oe, respectively. Interestingly, large magnetization 15 emu/g and lowest HC (= 56 Oe) were observed for sample S₂ compared to all other samples. The unusual magnetic properties of the samples are due to morphology (shape, size, and self-assembled 1D orientation of particles), surface spin disorder, and (surface/interface) anisotropy of the nanoparticles.