An Inexpensive Route to Synthesize High-Purity CrO 2 for EMI Shielding in X-Band Frequencies

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, 68emu/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 e ﬀ ectiveness 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.


Introduction
The ferromagnetic chromium oxide (CrO 2 ) is considered to be one of the most promising candidates for new generation of spintronics devices [1][2][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][9][10][11][12]. This study is inspired by the recent advances in the development of materials with magnetically controlled attenuation.

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][14][15][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.

Results and Discussion
The XRD pattern of the calcined powder of CrO 2 determined by Rigaku Miniflex II, step size = 0.02, with Cu Kα radiation of wave length λ = 1.5406Å, is shown in Figure 1   CrO 2 (P4 2 /mnm) were calculated, a = 4.421Å and c = 2.913Å. 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 S 11 (S 22 ), S 21 (S 12 ) 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 S 11 and S 21 measurements, the reflectivity (R), transmissivity (T), and absorptivity (A) 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 SE (dB) = −10 log(P t /P 0 ), where P t and P 0 are the transmitted and incident electromagnetic powers, Here, A 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 1 − R. Hence, the effective absorbance [17] (A eff ) can be described as  Figure 4 shows the variation of EMI shielding effectiveness of SE A and SE R versus frequency for CrO 2 sample. It explicitly confirms that the pure CrO 2 possesses much more effective electromagnetic absorbing effect compared to γiron and ferrites [18][19][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.

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 SE A (∼20 dB) is quite interesting for strategic technology applications.