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ISRN Materials Science

Volume 2012 (2012), Article ID 948219, 4 pages

http://dx.doi.org/10.5402/2012/948219

## An Inexpensive Route to Synthesize High-Purity for EMI Shielding in X-Band Frequencies

^{1}Magnetic Standards, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India

^{2}Department of Physics, Hindu College, University of Delhi, Delhi 110007, India

Received 19 December 2011; Accepted 12 January 2012

Academic Editors: N. Bahlawane and N. Martin

Copyright © 2012 Vivek Verma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Rod-shaped high-purity samples of CrO_{2} have been synthesized by an inexpensive and simplified procedure. Here, we have prepared pure CrO_{2} without applying any external pressure or control it during synthesis. The sample prepared exhibited an improvement in saturation magnetization values, 68 emu/g at 300 K, 136 emu/g at 80 K, and uniform grained microstructure. The complex permittivity, permeability, and microwave absorption properties of high-purity CrO_{2} sample were investigated in the 8.2–12.2 GHz (X-band) microwave frequency range. Microwave measurements have shown the high shielding effectiveness due to absorption (SE_{A}) of 20.3 dB. The high value of SE_{A} suggests that CrO_{2} can be used as a promising electromagnetic shielding, EMI, material in 8.2–12.2 GHz (X-band) microwave frequency range.

#### 1. Introduction

The ferromagnetic chromium oxide (CrO_{2}) is considered to be one of the most promising candidates for new generation of spintronics devices [1–3] and is a well-established magnetoresistive material. CrO_{2} is a peculiar compound, which behaves as a half-metallic ferromagnet, with a Curie temperature *T _{c}
* ~ 114°C [4, 5]. It shows giant magnetoresistance (GMR) and optical properties for spintronics and optoelectronic devices [6]. There is an increased interest in shielding against electromagnetic radiation [7] in commercial, military equipments, scientific electronic devices, and communication instruments that are widely being used. The ferromagnetic CrO

_{2}with high permeability has been utilized in many microwave applications [8–12]. This study is inspired by the recent advances in the development of materials with magnetically controlled attenuation.

#### 2. Experimental

The most widely accepted synthesis route for CrO_{2} is initiated by using a highly hygroscopic compound CrO_{3}, which is treated at elevated pressures (270 GPa) in raw form or by mixing with other oxides of Cr and some catalysts that are often needed to bring down the working pressure [13–16]. In this paper, we report microwave absorption properties of high-purity sample of CrO_{2} synthesized by a simplified procedure, where the pressure is not a control parameter, thus drastically reducing the cost of production. This synthesis technique involves a simple two-step process. The first step involves the preparation of intermediate precursors oxide, which can be prepared under ambient pressure by heating CrO_{3} for 3 hours at a temperature of 250°C in air. In a second step, the sample is sealed in an evacuated quartz tube at ambient pressure followed by palletization and subsequently treated in furnace at a temperature of 400°C for 3 hours.

#### 3. Results and Discussion

The XRD pattern of the calcined powder of CrO_{2} determined by Rigaku Miniflex II, step size = 0.02, with Cu K*α* radiation of wave length *λ* = 1.5406 Å, is shown in Figure 1(a). It is observed that the sample consists of the pure CrO_{2} phase confirmed by PCPDFWIN card no. 76-1232. The lattice parameters corresponding to the tetragonal structure of CrO_{2} (P4_{2}/mnm) were calculated, *a* = 4.421 Å and *c* = 2.913 Å. Figure 1(b) shows the SEM micrograph of pure CrO_{2} sample which depicts uniform grain size distribution.

The magnetic properties of a representative CrO_{2} sample have been determined by the M-H hysteresis loop at 300 K and 80 K as shown in Figure 2. The saturation magnetization (Ms) value of the sample has been measured as 68 emu/g and 136 emu/g at 300 K and 80 K, respectively, by vibrating sample magnetometer (Lakeshore, 7304). The saturation magnetization value obtained in our samples is higher as reported by A. Bajpai and A. K. Nigam [15]. The coercive force is found nearly the same at 300 K and 80 K as shown in the inset of Figure 2. Curie temperature *T _{c}
* = 391 K is depicted from M-T curve at 3 k Oe for CrO

_{2}sample as shown in the inset of Figure 2. Comparisons of relevant physical parameters as exhibited by our sample to those reported in literatures are given in Table 1.

The complex permittivity, permeability, and (), () measurements were carried out on an Agilent E8362B vector network analyzer in the microwave frequency range of 8.2–12.2 GHz (X-band). The rectangular pellet of 2 mm thickness with a dimension to fit the waveguide dimensions has been prepared for microwave measurements. From and measurements, the reflectivity (), transmissivity (), and absorptivity () were calculated. The real and imaginary parts of complex permittivity (*ε*′ and *ε*′′) and permeability (*μ*′ and *μ*′′) versus frequency are shown in Figures 3(a) and 3(b). Compared with conventional ferrite materials, the CrO_{2} exhibits larger saturation magnetization and complex permeability value in X-band frequency range. In CrO_{2} sample, permittivity and permeability are found to be decreasing with an increase in frequency because the dipole present in the system cannot reorient themselves along with the applied electric field. The CrO_{2} sample shows highest real permittivity (*ε*′ = 33), imaginary part (*ε*′′ = 1.4), and permeability (*μ*′ = 6) which ultimately enhances the shielding effect. Magnetic materials with high permeability have been utilized in many microwave applications. The EMI shielding effectiveness (SE) of a material is defined as the ratio of transmitted power to incident power and is given by , where and are the transmitted and incident electromagnetic powers, respectively. For a shielding material, total , where is due to reflection, is due to absorption, and is due to multiple reflections. In two-port network, parameters (), () represent the reflection and the transmission coefficients, and absorption coefficient . Here, is given with respect to the power of the incident EM wave. If the effect of multiple reflections between both interfaces of the material is negligible, the relative intensity of the effectively incident EM wave inside the material after reflection is treated equally to . Hence, the effective absorbance [17] () can be described as with respect to the power of the effectively incident EM wave inside the shielding material. It is convenient to express the reflectance and effective absorbance in the form of and in decibel (dB), respectively, which results in and as and
.

Figure 4 shows the variation of EMI shielding effectiveness of and versus frequency for CrO_{2} sample. It explicitly confirms that the pure CrO_{2} possesses much more effective electromagnetic absorbing effect compared to *γ*-iron and ferrites [18–20]. CrO_{2} has shielding effectiveness mainly due to absorption, which is found in maximum ~20 dB. The shielding effectiveness due to reflection was nominal and of value ~10 dB.

#### 4. Conclusions

In conclusion, the pure phase CrO_{2} can be prepared by an inexpensive and simplified procedure adopted. Structural and magnetic properties are compatible as reported earlier [6, 15]. These CrO_{2} have a relatively uniform dimension and well-defined rod-shaped structure. The microwave absorption properties strongly depend on the intrinsic properties of CrO_{2}. The value of EMI shielding effectiveness due to absorption (~20 dB) is quite interesting for strategic technology applications.

#### References

- Y. Ji, G. J. Strijkers, F. Y. Yang et al., “Determination of the spin polarization of half-metallic CrO
_{2}by point contact Andreev reflection,”*Physical Review Letters*, vol. 86, no. 24, pp. 5585–5588, 2001. View at Publisher · View at Google Scholar · View at Scopus - R. J. Soulen Jr., J. M. Byers, M. S. Osofsky et al., “Measuring the spin polarization of a metal with a superconducting point contact,”
*Science*, vol. 282, no. 5386, pp. 85–88, 1998. View at Publisher · View at Google Scholar · View at Scopus - R. A. de Groot, F. M. Mueller, P. G. V. Engen, and K. H. J. Buschow, “New class of materials: half-metallic ferromagnets,”
*Physical Review Letters*, vol. 50, no. 25, pp. 2024–2027, 1983. View at Publisher · View at Google Scholar · View at Scopus - H. Y. Hwang and S. W. Cheong, “Enhanced intergrain tunneling magnetoresistance in half-metallic CrO
_{2}films,”*Science*, vol. 278, no. 5343, pp. 1607–1609, 1997. View at Publisher · View at Google Scholar · View at Scopus - R. M. Costescu, D. G. Cahill, F. H. Fabreguette, Z. A. Sechrist, and S. M. George, “Ultra-low thermal conductivity in W/Al
_{2}O_{3}nanolaminates,”*Science*, vol. 303, no. 5660, pp. 989–990, 2004. View at Publisher · View at Google Scholar · View at Scopus - S. Biswas and S. Ram, “Synthesis of shape-controlled ferromagnetic CrO
_{2}nanoparticles by reaction in micelles of Cr^{6+}–PVA polymer chelates,”*Materials Chemistry and Physics*, vol. 100, no. 1, pp. 6–9, 2006. View at Publisher · View at Google Scholar - C. R. Paul,
*Introduction to Electromagnetic Compatibility*, Wiley, New York, NY, USA, 1992. - Y. Naito and K. Suetaki, “Application of ferrite to electromagnetic wave absorber and its characteristics,”
*IEEE Transactions on Microwave Theory and Techniques*, vol. 19, no. 1, pp. 65–72, 1971. View at Publisher · View at Google Scholar - H. M. Musal and H. T. Hahn, “Thin-layer electromagnetic absorber design,”
*IEEE Transactions on Magnetics*, vol. 25, no. 5, pp. 3851–3853, 1989. View at Publisher · View at Google Scholar · View at Scopus - C. A. Grimes and D. M. Grimes, “A brief discussion of EMI shielding materials,” in
*Proceedings of the IEEE Aerospace Applications Conference*, pp. 217–226, Steamboat Springs, Colo, USA, January 1993. - K. J. Vinoy and R. M. Jha, “Trends in radar absorbing materials technology,”
*Sadhana*, vol. 20, no. 5, pp. 815–850, 1995. View at Publisher · View at Google Scholar · View at Scopus - E. F. Knott, J. F. Shaeffer, and M. T. Tuley,
*Radar Cross Section*, Artech House, Boston, Mass, USA, 1993. - B. L. Chamberland, “Chemical rubber company critical reviews in solid state and materials sciences,”
*Critical Reviews in Solid State and Materials Sciences*, vol. 7, no. 1, 1977. View at Google Scholar - A. Bajpai and A. K. Nigam, Nigam, Indian Patent Appl. No. 783/MUM/2002 and International patent application filed with PCT Appl. No. PCT/IN03/ 00278.
- A. Bajpai and A. K. Nigam, “Synthesis of high-purity samples of CrO
_{2}by a simple route,”*Applied Physics Letters*, vol. 87, no. 22, Article ID 222502, pp. 1–3, 2005. View at Publisher · View at Google Scholar · View at Scopus - P. Norby, A. N. Christensen, H. Fjellvåg, and M. Nielsen, “The crystal structure of Cr
_{8}O_{2}1 determined from powder diffraction data: thermal transformation and magnetic properties of a chromium-chromate-tetrachromate,”*Journal of Solid State Chemistry*, vol. 94, no. 2, pp. 281–293, 1991. View at Google Scholar · View at Scopus - A. Ohlan, K. Singh, A. Chandra, and S. K. Dhawan, “Microwave absorption properties of conducting polymer composite with barium ferrite nanoparticles in 12.4-18 GHz,”
*Applied Physics Letters*, vol. 93, no. 5, Article ID 053114, 2008. View at Publisher · View at Google Scholar · View at Scopus - B.-W. Li, Y. Shen, Z.-X. Yue, and C.-W. Nan, “Enhanced microwave absorption in nickel/hexagonal-ferrite/polymer composites,”
*Applied Physics Letters*, vol. 89, no. 13, Article ID 132504, 3 pages, 2006. View at Publisher · View at Google Scholar - K. Singh, A. Ohlan, R. K. Kotnala, A. K. Bakhshi, and S. K. Dhawan, “Dielectric and magnetic properties of conducting ferromagnetic composite of polyaniline with
*γ*-Fe_{2}O_{3}nanoparticles,”*Materials Chemistry and Physics*, vol. 112, no. 2, pp. 651–658, 2008. View at Publisher · View at Google Scholar · View at Scopus - S. P. Gairola, V. Verma, L. Kumar, M. A Dar, S. Annapoorni, and R. K. Kotnala, “Enhanced microwave absorption properties in polyaniline and nano-ferrite composite in X-band,”
*Synthetic Metals*, vol. 160, no. 21-22, pp. 2315–2318, 2010. View at Publisher · View at Google Scholar