Microwave Absorbing Properties of W-Type Hexaferrite Ba ( MnZn ) xCo 2 ( 1 − x ) Fe 16 O 27

MnZn-doped W-type barium cobalt ferrite powder composites of Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2, 0.3, 0.4, and 0.5) were prepared in a sol-gel process. The microwave absorbing properties of the composites in the range of 2 18 GHz and their electromagnetic loss mechanisms were studied. The results demonstrated that the synthesized Ba(MnZn)xCo2(1−x)Fe16O27 samples possess a W-type phase of the crystal structure with a hexagonal flaky shape in micro-morphology, and the samples exhibited a soft magnetic trait that enables improving their microwave absorption properties through suitable MnZn doping. For Ba(MnZn)0.4Co1.2Fe16O27 with a thickness of 2.8 mm, the reflection loss peak was −40.7 dB at a frequency of 7.3 GHz, with a bandwidth of 6.6 GHz at a loss of less than −10 dB. The microwave absorption primarily resulted from magnetic losses caused by magnetization relaxation, domain wall resonance, and natural resonance.


Introduction
Ferrite is one type of the important electromagnetic-wave absorbing materials, of which spinel and magnetoplumbite hexagonal ferrites are the most widely used in applications.However, because of Snoek's limitation, the working frequency of those spinel ferrites is generally confined to the range below 3 GHz [1][2][3][4][5].Hexagonal ferrites, which exhibit a planar anisotropy and ferromagnetic resonance in the GHz range caused by a high magnetocrystalline anisotropy field, are applicable for use as electromagnetic-wave absorption materials working in the GHz wavelength range [5][6][7][8][9].Hexagonal ferrites can be classified into many types, including M-, Y-, W-, X-, Z-, and U-type phase, according to their chemical formulas and crystal structures.The M-type is the most studied among those hexagonal ferrites.It has been confirmed that different ions can be substituted for Ba 2+ and Fe 3+ in M-type phase ferrite, such as La for Ba 2+ and Co-Ti, Mn-Ti, Mg-Ti, Co-Mn-Ti, Mn-Cu-Ti, Mn-Cu-Zr and Zn-Zr-Sn for Fe 3+ .Consequently, the electromagnetic parameters of the ferrites can be adjusted or controlled, and their microwave absorption performance can be improved [2][3][4][5][6][7][10][11][12][13][14][15][16][17].
W-type phase ferrite can be viewed as a complex ferrite consisting of M-type phase and spinel phase block, which is M + 2S (M: BaFe 12 O 19 , S: Fe 2+ Fe 2 O 4 ).In BaFe 12 O 19 , chemical doping is commonly utilized by replacing Fe 2+ with Co 2+ to improve its thermal stability and other properties, which results in a W-type bariumcobalt ferrite of BaCo 2 Fe 16 O 27 .Because of the existing Fe 2+ and Fe 3+ cation sites, Fe cations are easily substituted by other divalent and trivalent cations; the magnetic parameters, including saturation magnetization, Curie temperature, coercivity, magnetic anisotropy and ferromagnetic resonance frequency, are thus more adjustable.However, there are fewer studies on microwave electromagnetic characteristics of W-type phase ferrites than on the characteristics of M-type ferrites.The substitution of Co 2+ or Ba 2+ by other metal ions was determined to help improve the microwave absorption properties of BaCo 2 Fe 16 O 27 .Materials with improved microwave absorption include Ba(Zn x Co 1−x ) 2 Fe 16 O 27 [10], BaZn 1.1 Co 0.9 Fe 16 O 27 [13], Ba 1−x Er x (Zn 0.3 Co 0.7 ) 2 Fe 16 O 27 [18] and Ba(Zn 0.5 Co 0.5 ) 2 Fe 16 O 27 [19].The microwave electromagnetic effects are expected to be richer when Co 2+  , and Ba 2+ nitrate and manganese acetate were dissolved in deionized water and stirred to form a uniform brown transparent solution.Citric acid was added to the above solution according to the molar ratio of 1:1.5 between the metal ions and the citric acid, and the ammonia water was dripped to ensure the pH value that was in the range of 6 -7.Next, the solution was strung for 4 h in a water bath at 80˚C to form a sol.
The solution was then dried in an oven (100˚C) to obtain the dry gels.The dry gels were slightly grinded after ashing at 450˚C.Finally, the black Ba(MnZn) x Co 2(1−x) Fe 16 O 27 ferrites were obtained by cooling the powder, which was sintered in an electric furnace at 1235˚C for 4 h under the atmosphere circumstances.

Measurements
The crystal structure of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) was investigated through X-ray diffraction (XRD) using a system from X'pert PRO of Holland PANalytical that utilized a Cu target at a voltage/current of 35-kV/25-mA.Scanning electron microscopy (SEM) examination was performed using QUANTA 200 to obtain the morphology and size of the powder particles of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 .The hysteresis loops of the Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 particles were measured using a vibrating sample magnetometer (VSM).
The prepared crystalline powders were mixed with paraffin wax at a ratio of 8:3 in weight by heating and pressed into annular samples.The complex permittivity and permeability of the samples were measured using an AV3629 microwave vector network analyzer in the frequency range of 2 -18 GHz.The relationship between microwave reflectivity (R) and frequency (f) can be obtained according to a previous formula [20,21] that used the measured data.

Crystal Structure, Morphology and Hysteresis Loop of the Powders
Figure 1 shows the XRD pattern of the Figure 3 shows the hysteresis loop of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 powders calcined at 1235˚C for 4 h.The coercitity (Hc), magnetization (Ms) and retentivity (Mr) are 28.702Oe, 67.320 emu/g, 3.7964 emu/g, respectively.The hysteresis loop indicates that Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 is a soft magnetic material.

The Microwave Absorption Properties
Figure 4 shows the variation of reflection loss versus frequency for Ba(MnZn) x Co 2(1−x) Fe 16 O 27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) at a thickness of 2.8 mm.For x = 0.1, the reflection loss exhibits a peak at approximately −12 dB at a frequency of 8.1 GHz, with a bandwidth of 5 GHz for losses less than −10 dB.For x = 0.2, the reflection loss exhibits a peak at approximately −16 dB at 5.0 GHz, with a bandwidth of 6 GHz for losses less than −10 dB.For x = 0.3, the reflection loss exhibits a peak at ap-  proximately −23 dB at 8.0 GHz, with a bandwidth of 6.5 GHz for losses less than −10 dB.For x = 0.4, the reflection loss exhibits a peak at approximately −40.7 dB at 7.3 GHz, with a bandwidth of 6.6 GHz for losses less than −10 dB.For x = 0.5, the reflection loss exhibits a peak at approximately −12 dB at 5.5 GHz, with a bandwidth of 3 GHz for losses less than −10 dB.It was shown that the compositions Ba(MnZn) 0.3 Co 1.4 Fe 16 O 27 and Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 have obviously higher microwave absorptions than the other compositions.This would explained by doped MnZn elements can change the electric and magnetic structures of the system, result in the change of the magnetocrystalline anisotropy field, the increase of the domain rotation resistance, pinning of domain wall displacement, thereby the magnetic loss is increased and the resonance frequency is changed.However, overdope will increase the lattice distortion, result in the increase of electric resistivity and the decrease of Ohmic loss.Thus, the appropriate doping can enhance the impedance matching and microwave attenuation and change the position of the absorption peak.As a result, W-type barium-cobalt ferrite doped by certain amounts of MnZn can act as a wide-band microwave absorber.As shown in Figure 4, the frequency position of the absorption peak changes with the amount of doped MnZn.This absorption peak shifts with doping because the amount of doped material will affect the magnetocrystalline anisotropic field and thus cause a change in the natural resonance frequency.
Figure 5 shows the variation of the reflection loss versus frequency for Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 at different thicknesses.For d = 2.0 mm, the maximum reflection loss is approximately −17 dB at a frequency of 4.8 GHz, with a bandwidth of 3 GHz for losses less than −10 dB.For d = 2.4 mm, the maximum reflection loss is approximately −22.5 dB at a frequency of 5.5 GHz, with a bandwidth of 5 GHz for losses less than −10 dB.For d = 2.8 mm, the maximum reflection loss is approximately   −40.7 dB at a frequency of 7.3 GHz, with a bandwidth of 6.6 GHz for losses less than −10 dB.For d = 3.2 mm, the maximum reflection loss is approximately −32.5 dB at a frequency of 9.0 GHz, with a bandwidth 6.5 GHz for losses less than −10 dB.The absorption peak obviously exhibits a shift towards higher frequencies (blue shift) with an increase in the thickness of the samples.Thus, the matching thickness of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 as a microwave absorber is 2.8 mm.

Electromagnetic Loss Mechanisms
Figure 6 shows the spectra of the complex permittivity and permeability of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 .The imaginary part of the permittivity is small, and the real part is in the range of 5.5 -6.5 over the frequency range of 2 -18 GHz.The real part of the permeability decreases with an increase of frequency, but it does not change when the frequency exceeds 10 GHz.Two peaks of the imaginary part ε'' ε' μ′′ are located at 4.5 GHz and 8 GHz, but μ′′ decreases when the frequency exceeds 4.5 GHz.
Using the dielectric loss tangent tanδ e = r r and the magnetic loss tangent tanδ m = ε '' ε' r r '' ' μ μ , the variation of tanδ e and tanδ m over the frequency range of 2 -18 GHz can be determined.As shown in Figure 7, tanδ e is less than 0.1 and gradually changes with frequency over the entire frequency range, which indicates the presence of weak dielectric loss.This low loss may occur because doping will produce intrinsic electric dipole moments, thus forming defected dipoles.In the presence of a microwave electromagnetic field, polarization relaxation in the dipoles can lead to dielectric losses.Relative to the dielectric loss tanδ e , the magnetic loss tanδ m is much greater over the frequency range of 2 -18 GHz.The value of tanδ m is greater than 0.3 over the frequency range of 2 -15 GHz.There are two peaks on the curve of tanδ m , with the first peak exhibiting a value of 0.78 at approximately 4.5 GHz and the second peak exhibiting a value of 0.82 at approximately 8 GHz; these values are consistent with the peaks of μ′′ shown in Figure 6.The peak position of the microwave reflectivity is predicted to be between the two magnetic loss tangent peaks, though closer to the second peak, which is consistent with the data shown in Figure 4.
The magnetic loss primarily results from domain wall displacement, domain turning, domain wall resonance and natural resonance.As shown in Figure 7, the first loss tangent peak originates from domain wall resonance, and the second peak originates from natural resonance, while magnetization relaxation is present over the entire frequency range.Doping with MnZn can result in the effect of domain wall pinning and the change of magnetic anisotropy, so the damping of domain turning and domain wall displacement increases, thus leading to resonance enhancement and resonance frequency-position shift.However, an excessive amount of doping may result in weak microwave absorption, due to the increase in electric resistivity from the enhancement of electron scattering and the increase of the lattice defects.Consequently, Ohmic loss is increased, and the microwave absorption is not conducive to impedance matching.As a result, a suitable amount of MnZn-doping is required to improve the microwave absorption of W-type bariumcobalt ferrites (as in Figure 4).In addition, because the powder morphology was hexagonal flaky, this shape anisotropy can result in magnetocrystalline anisotropy, which increases the microwave scattering in the medium, thereby enhancing the magnetic loss.

Conclusion
The crystal structure of MnZn-doped barium-cobalt ferrite Ba(MnZn) x Co 2(1−x) Fe 16 O 27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) prepared using a sol-gel process was hexagonal W-type phase, and the samples exhibited a soft magnetism.Incorporating MnZn into the structure resulted in nearly no change in the structure with different doping amounts, and the morphology of the powders was hexagonal flaky with an average particle size of 5 μm.As a result, Ba(MnZn) x Co 2(1−x) Fe 16 O 27 ferrite exhibits the excellent microwave absorption properties of wide bandwidth and strong loss over the frequency range of 2 -18 GHz.The amount of MnZn doping will greatly influence the absorption properties of Ba(MnZn) x Co 2(1−x) Fe 16 O 27.The reflection loss peak exhibited the characteristics of a maximum loss of −40.7 dB at a frequency of 7.3 GHz, with an effective bandwidth of 6.6 GHz for losses less than −10 dB when the thickness of the sample was 2.8 mm and x = 0.4.The microwave absorption of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 ferrite originated from weak  dielectric loss and strong magnetic loss.The dielectric loss was caused by the polarization relaxation of dipoles, and the magnetic loss was caused by magnetization relaxation, domain wall and natural resonances.Both the suitable amount of doping of MnZn and the hexagonal flaky morphology of the powder particles are helpful for enhancing the electromagnetic loss of the ferrites.

˚Figure 1 .
Figure 1.XRD pattern of Ba(MnZn) x Co 2(1−x) Fe 16 O 27. Ba(MnZn) x Co 2(1−x) Fe 16 O 27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) ferrites calcined at 1235˚C for 4 h.Compared with the characteristic diffraction peaks of the standard XRD card (PDF#19-0098) for W-type phase ferrite, the crystal structures of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 samples were measured to be a single W-type phase structure.Nearly no variations of the crystal structure were observed for all MnZn-doped Ba(MnZn) x Co 2(1−x) Fe 16 O 27 samples when x was equal to 0.1, 0.2, 0.3 0.4 and 0.5.Figure 2 show SEM images of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 and Ba(MnZn) 0.3 Co 1.4 Fe 16 O 27 powders after high-temperature annealing at 1235˚C for 4 h.The SEM images indicate that both the morphology of the Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 and Ba(MnZn) 0.3 Co 1.4 Fe 16 O 27 are hexagonal flaky with a clear outline.The average size was 5 μm with a narrow distribution.As a result, doping with a small amount of MnZn does not significantly affect the grain size and morphology.Figure3shows the hysteresis loop of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 powders calcined at 1235˚C for 4 h.The coercitity (Hc), magnetization (Ms) and retentivity (Mr) are 28.702Oe, 67.320 emu/g, 3.7964 emu/g, respectively.The hysteresis loop indicates that Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 is a soft magnetic material.

Figure 4 .
Figure 4. Variation of the reflection loss with frequency of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 ferrites at a thickness of 2.8 mm.

Figure 5 .
Figure 5. Variation of the reflection loss with frequency of Ba(MnZn) 0.4 Co 1.2 Fe 16 O 27 ferrites at different thicknesses.

2. Experiments 2.1. Preparation of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 Powders
in BaCo 2 Fe 16 O 27 is substituted by two or more metal ions.In this paper, MnZn-doped W-type barium cobalt fer-rite composites of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 were prepared using a sol-gel process.The microwave absorption properties of Ba(MnZn) x Co 2(1−x) Fe 16 O 27 in the frequency range of 2 -18 GHz are investigated, and the electromagnetic loss mechanism is discussed.