Microwave absorption in 0.1–18 GHz, magnetic and structural properties of SrFe12-xRuxO19 and BaFe12-xRuxO19

Journal Pre-proof Microwave absorption in 0.1–18�GHz, magnetic and structural properties of SrFe12-xRuxO19 and BaFe12-xRuxO19 Yidi Chang, Yanan Zhang, Lu Li, Shunquan Liu, Zhi Liu, Hong Chang, Xin'an Wang PII:

S0925-8388(19)34176-3

DOI:

https://doi.org/10.1016/j.jallcom.2019.152930

Reference:

JALCOM 152930

To appear in:

Journal of Alloys and Compounds

Received Date: 29 July 2019 Revised Date:

4 November 2019

Accepted Date: 5 November 2019

Please cite this article as: Y. Chang, Y. Zhang, L. Li, S. Liu, Z. Liu, H. Chang, Xin'. Wang, Microwave absorption in 0.1–18�GHz, magnetic and structural properties of SrFe 12-xRuxO19 and BaFe12-xRuxO19, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152930. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Microwave Absorption in 0.1-18 GHz, magnetic and structural properties of SrFe12-xRuxO19 and BaFe12-xRuxO19 Yidi Chang1,2, Yanan Zhang1, Lu Li1, Shunquan Liu*3, Zhi Liu2, Hong Chang**1, Xin’an Wang4 1 2

School of Physics, Inner Mongolia University, Hohhot, 010021, China

School of Electronics and Information Engineering, Changchun University of Science and Technology, Changchun, 130022, China 3

4

School of Physics, Peking University, Beijing, 100871, China

Ningxia Magvalley Novel Materials Technology Co. Ltd. New energy sub-park, Ningdong Energy & Chemical Industry Base, Ningxia, 751411, China Abstract The hexagonal SrFe12O19 and BaFe12O19 ferrites have a high magnetocrystalline

anisotropy due to the ions at the 2b site. The axial component at the 2b site is weakened by the Ru doping, as it is reflected in the xrd refinements and the FT-IR spectra. Accompanied with this, the coercivity of SrFe12-xRuxO19 decreases from 4.38 kOe in x = 0 to 0.68 kOe in x = 0.5 and 0.42 kOe in x = 1.5. That of BaFe12-xRuxO19 decreases from 2.47 kOe in x = 0 to 1.10 kOe in x = 1.0. With high magnetocrystalline anisotropies, the ferromagnetic resonance (FMR), above 40 GHz, of both BaFe12O19 and SrFe12O19 falls out of 0.1-18 GHz. In BaFe12-xRuxO19 with x ≥ 0.3, FMR is observed at 14.5 GHz in 0.1-18 GHz, but the reflection loss (RL) is very low with RL ≥ -10 dB. In SrFe12-xRuxO19, strong microwave absorptions with RL ≤ -10 dB in 0.1-18 GHz range are observed with 0.5≤x≤1.5. The optimum RL is -32 dB observed in x = 1.0 and 1.3, and the widest bandwidth is 6.55 GHz in x = 1.0 with the thickness as 2.3 mm. In x = 0.5, RL ≤ -10 dB occurs in 14.2 -18 GHz. As the Ru content varies, the microwave absorption of SrFe12-xRuxO19 fully covers a large frequency range of 4.0-18 GHz. The Ru doping drives SrFe12-xRuxO19 (0.5≤x≤1.5) become good microwave absorbing materials in 0.1 -18 GHz.

corresponding author: *[email protected], **[email protected]. 1

Introduction Accompanied with the rapid development of electronic equipment and wireless communication, microwave absorption materials have been quickly developed in order to avoid electromagnetic interferences. Magnetic materials with high magnetic permeability, such as ferrites and metal or alloy particles, are used as intrinsic microwave absorption materials [1-3]. Even though some interesting works on the metal-organic frameworks with magnetic porous carbon nanorods have been reported [4-6], intrinsic magnetic microwave absorbing materials are still catching researchers’ eyes. Ferrites are divided into spinel-type, garnet-type and hexagonal-type by their different crystal structures [7]. Most spinel ferrites possess high static permeability, and are applied as wave absorbers in electric and electronic technologies at high frequency (3–30 MHz), very high frequency (30–300 MHz), and ultrahigh frequency (300 MHz – 3 GHz) [8]. Garnet ferrites are very useful in reciprocal and non-reciprocal microwave devices, because of their high gyromagnetic properties and very low magnetic and dielectric loss tangents [8]. Hexagonal ferrites are good microwave absorbers in the GHz frequency with their high magnetization, high stability, low cost and easy fabrication. Hexagonal ferrites are composed of S blocks (spinel block) without alkaline earth metals, R blocks (hexagonal block) with alkaline earth metals and 2 oxygen layers, and T blocks (hexagonal block) with alkaline earth metals and 4 oxygen layers. With different numbers and arrangements of the blocks, hexagonal ferrites are classified into M-, W-, Y-, Z-, X-, and U-types [7]. With large c-axis anisotropies, the resonance frequencies of the W-type ferrites of BaM2Fe16O27 with M = Fe, Ni and Zn are as high as 36 GHz [7, 9]. The Co substitution turns the anisotropy to be in the c-plane. The resonance frequency is between 2.5 – 12 GHz in BaCoxZn2-xFe16O27 [10]. The microwave absorption of nano Ba0.8Al0.2Co0.9Zn1.1Fe19O27 ferrites moves to 9.62-13.29 GHz [11]. Most of the Y-type hexagonal ferrites have c-plane anisotropy, except for Ba2Cu2Fe12O22. The resonance frequency of Ba2Co2-xZnxFe12O22 decreases from 1.5 GHz to 0.5 GHz with the Zn doping [12]. The Z-type ferrites are important soft magnetic hexagonal ferrites. Ba3Co2Fe24O41 has a relatively large permeability, 2

and is one of the most important microwave ferrite materials in the range of 1-3.5 GHz [13]. The X-type ferrites of BaZnxCo2-xFe28O46 have a resonance frequency around 1 GHz. Ba2(Zr0.5Mn0.5)xFe28-xO44+0.25x displays a narrow microwave absorption in the range of 15-18 GHz [14]. The U-type ferrites of Ba4Ni2-xCoxFe36O60 are microwave absorbers at high frequencies. The resonant frequency of (Ba0.7Bi0.2)4(Co1-xNix)2Fe36O60 is at 11.3 GHz [15]. The M-type BaFe12O19 and SrFe12O19 are important permanent materials [16,17]. The resonant frequencies fR of BaFe12O19 and SrFe12O19 with high anisotropic fields are above 40 GHz [18]. Nowadays, microwave absorption materials in the frequency range of 1 - 18 GHz have been extensively used in commerce, industry and defense applications [1-3]. fR is a function of the anisotropic field Ha with 2πfR = γΗa for the c-axis anisotropy or the out-of plane anisotropy H and in-plane anisotropy H with 2πf = γ H H

for the c-plane anisotropy with γ as the gyromagnetic factor.

Decreasing Ha or H and H

is expected to be effective at decreasing fR. With M =

CoTi, the microwave absorption of BaFe12-2xMxO19 moves to 26-40 GHz [19]. The microwave absorption of M-type Ba-ferrites is in the 18.0-26.5 GHz frequency range in Ba(1-2x)LaxNaxFe10Co0.5TiMn0.5O19 [20]. The Cu2+ and Zr4+ codoping drives Ba/Sr hexaferrites have a minimum reflection loss at 11.1 GHz [21]. However, not all dopings are effective at decreasing the anisotropic field and increasing the microwave absorption properties. The La, Co, Al and Mn dopings are known to result in an increasing anisotropic field [22-24]. It was reported that the Ir4+ and Zn2+ codoping turns the EMD from the c-axis to ferroxplana with a strong c-plane anisotropy [25]. Just as Ir4+, Ru4+ is a rare metal, but Ru4+ has a weaker spin-orbital coupling due to its lower element number than its peer Ir4+. It is expected that Ru4+ doping is able to change the magnetocrystalline anisotropy. We investigate the Ru doping on the microwave absorption of SrFe12-xRuxO19, together with the analysis based on the magnetic properties and the structure.

Experiments 3

The SrFe12-xRuxO19 (x = 0, 0.3, 0.5, 0.7, 1 and 1.5) and BaFe12-xRuxO19 (x = 0, 0.3, 0.5, 0.7 and 1) compounds were synthesized with the solid state reaction. The starting materials SrO, Fe2O3 and RuO2 with proper weights were ground in an agate mortar, and heated at 850℃ in air for 12 hours. Then the obtained powder was reground and fired at 1050℃ for 24 hours and this process was repeated for 3 times. Powder x-ray diffraction (XRD) measurements were carried out using Rigaku D/max2500 powder X-ray diffraction with Cu Kα radiation. The diffraction data were collected for structure analysis, and the scan range 2θ was from 5˚ to 80˚, with a scanning step of 2θ = 0.01˚ and a sampling time 1 s. The magnetically aligned samples are made by mixing the sample with the epoxy, and being solidified with the applied field about 10 kOe perpendicular to the surface. The scanning electron microscope (SEM) was used to characterize the particles’ morphology. Fourier transform infrared spectroscopy (FT-IR) were obtained with the wave numbers ranging from 4000 cm-1 to 400 cm-1 via a NICOLET AVATAR 330 spectro-photo meter manufactured by Thermo Electron Corporation. The resolution was set at 2 cm-1 during the measurement. Thermal Gravimetric Analyzer (TG-DTA) measurement was conducted on a Meittler TGA/DSC 1 SF/1382 calorimeter in the temperature range of 20 oC to 1000 oC with the heating rate of 10 oC / min equipped with a magnet below the sample. The magnetization was measured with a commercial Quantum Design SQUID magnetometer in the temperature range of 5 - 400 K. The microwave absorption spectra were measured on Agilent N5234A vector network analyzers. The scattering coefficients S21 and S11 are directly obtained from the setup. By defining V1 = S21 + S11, V2 = S21 – S11 and X = (1-V1V2)/(V1-V2), the parameter Γ is set as Γ = X +- (X2 -1)1/2 with | Γ | ≤ 1. The value Γ is related to the permittivity (ε) and permeability (µ) with µ / ε = (1+ Γ)2 / (1- Γ)2, and µ ε = [c/ωd ln(1/z)]2 with d as the sample thickness and z = (V1- Γ) / (1 - V1 Γ). Therefore, µ and ε can be obtained [26].

Results and discussion SrFe12-xRuxO19 (x = 0, 0.3, 0.5, 0.7, 1.0, and 1.5) and BaFe12-xRuxO19 (x = 0, 0.3, 0.5, 0.7, and 1.0) are characterized as the M-type ferrites. All the xrd patterns are 4

refined by the FullProf program with the Rietveld method. The space group is P63/mmc. No impurity is observed in all the samples. Fig. 1 (a) shows the xrd pattern of SrFe11.3Ru0.7O19, as a representative. The doping Ru4+ increases the lattice parameters a and c of BaFe12-xRuxO19 with the ratio

∆ /

=

∆ /

= 0.05%, as shown

in Fig. 1(b). On the other hand, the doping Ru4+ of SrFe12-xRuxO19 slightly increases the lattice parameter a with

∆ /

= 0.01%, while decreases c with

∆ /

= −0.02%,

as shown in Fig. 1(b). The refinement results show that the Sr2+ (or Ba2+) ions are at the 2d site, which is coordinated by 6 O2-, and the Fe3+ are at the octahedral 2a, 12k, 4f2 sites, the tetragonal 4f1 site, and bipyramidal 2b site, and most of the Ru4+ ions are at octahedral 2a and the tetragonal 4f1 site with some at the 4f2 and 12k sites. On the other hand, most of the Ru4+ ions in BaFe12-xRuxO19 are at the 2a, 4f2 and 12k sites, with a small amount of them enter the 4f1 site. The SEM images of SrFe12-xRuxO19 do not have significant change with the different Ru content. As shown in Fig. 2, crystals form in the hexagonal shape with particle sizes being about 1 µm.

5.894 BaFe Ru O 12-x x 19 5.892

23.20

20

40 o 60 2θ ( )

80

(b)

5.890 5.885

23.044

5.884 5.883

0

23.22

23.040

SrFe12-xRuxO19

0.0

0.5 1.0 Ru content x

c (angstrom)

exp. cal. difference Bragg position

a (angstrom)

Intensity (a.u.)

(a)

1.5

Fig. 1 (a) The refined xrd pattern of SrFe11.3Ru0.7O19; (b) the lattice constants, a and c, versus the Ru content x of BaFe12-xRuxO19 and SrFe12-xRuxO19.

5

Fig. 2 SEM micrograph of SrFe12-xRuxO19 (x = 0, 0.5, 1.0 and 1.5).

3

10

2

x = 0.3 0.7 1.0

u'

1

5 ε''

u''

0 0

6 12 f (GHz)

(b)

ε'

ε ' and ε''

u' and u''

(a)

18

0

0

x = 0.3 0.7 1.0

6 12 f (GHz)

18

Fig. 3 (a) The real u’ and imaginary permeability u’’, and (b) The real ε’ and imaginary permmitivity ε’’ of BaFe12-xRuxO19.

6

RL (dB)

0

x = 0.3 x = 0.7 x = 1.0

-5 (b)

-10

0

6 12 f (GHz)

18

Fig. 4 RL of BaFe12-xRuxO19.

Microwave absorbing materials are able to convert electromagnetic waves into thermal energies with dielectric or magnetic losses. In the GHz frequency region, the microwave absorbing properties strongly depend on the performance of the ferromagnetic resonance (FMR) of the magnetic material body [27]. Fig. 3 (a) and (b) shows the complex permeability and permittivity spectra of BaFe12-xRuxO19. The imaginary permeability µ'' has the maximum at the FMR frequency of 14.4 GHz in x = 1.0, at which the real permeability µ' drops to half its maximum. Similarly, the FMR frequency of x = 0.7 is at 15.0 GHz, and that of x = 0.3 is at 14.4 GHz. Both the real ε' and the imaginary permittivity ε'' increase with the increasing Ru content in BaFe12-xRuxO19. ε' is higher than 6.7 in x = 1.0, and 5.9 in x = 0.3. ε'' is higher than 0.5 in x = 1.0, and 0.1 in x = 0.3. Furthermore, both the real ε' and the imaginary permittivity ε'' decrease with the increasing frequency. It is consistent with the Debye theory [28]. In x = 1.0, ε' decreases from 9.7 at 0.1 GHz to 6.7 at 18 GHz, and ε'' decreases from 2.3 at 0.1 GHz to 0.5 at 18 GHz. Generally, in the frequency range of 0.1 GHz to 18 GHz, the dielectric loss is largely contributed by the polarization of the defects [28,29]. The higher ε' and ε'' imply that the electric dipoles accumulate at the defects. The higher ε' in x = 1.0 indicates that it has a higher capacity to store electromagnetic energy, and a higher ε'' in x = 1.0 leads to a higher energy dissipation. Based on the relative complex permeability and permittivity, the reflection loss (RL) with a given absorber thickness is calculated by the following equations, 7

= 20 =

|(

− 1)/(

#$ /%$ &'(ℎ [+(

,-./ 0

+ 1)|, )√#$ %$ ].

where Zin is the normalized input impedance at the absorber surface, f is the microwave frequency and C is the velocity of the light. Fig. 4 shows the RL of BaFe12-xRuxO19 (x = 0.3, 0.7, and 1.0) in the frequency range of 1 - 18 GHz with the optimal thickness as 1.9 mm, at which thickness RL has the lowest value. The microwave absorption of BaFe12-xRuxO19 at 14.4 GHz is due to the FMR. However, RL ≥ -10 dB is too low to be useful as a microwave absorption material, but it clarifies that the microwave absorption of BaFe12-xRuxO19 is due to FMR. Fig. 5 (a) and (b) show the complex permeability and permittivity spectra of SrFe12-xRuxO19. The real permeability µ' has 2 peaks. Generally, the permeability is contributed by the hysteresis loss, domain-wall resonance, eddy effect, and natural resonance [30]. The hysteresis loss is negligible at zero applied field. The domain-wall resonance is generally about 1~ 100 MHz, which is out of the present frequency range. Considering that SrFe12-xRuxO19 has very high resistance, the eddy effect is excluded. Therefore, the µ' peaks are contributed to natural resonances. As shown in Fig. 5 (b), compared to that of BaFe12-xRuxO19, both the real ε’ and the imaginary permittivity ε’’ increases. It indicates that SrFe12-xRuxO19 have better dielectric absorption and loss than the peer BaFe12-xRuxO19. The wave-like permittivity ε’ and ε’’ suggest that there are several relaxation processes with the Ru doping. In Fig. 6, the variation of RL with the frequency and the thickness are shown on the left of SrFe12-2xNixRuxO19 with x ≥ 0.5. RL of SrFe12O19 is close to 0 in the range of 0.1-18 GHz, as the resonant frequency is very high at 42.5 GHz [18]. In the sample of x = 0.5, RL ≤ -10 dB is observed above 14.8 GHz. The sample of x = 1.0 has the highest bandwidth as 7.3 GHz in 8.4-15.7 GHz with RL as -27.8 dB at the optimal frequency 9.8 GHz. In x = 1.3, the bandwidth is 5.87 GHz from 7.8 to 13.67 GHz, and RL is -32 dB at optimal frequency 12.0 GHz. In x = 1.5, the widest microwave absorption at a single layer further moves to the low frequency range of 8

5.5-9.95 GHz with the bandwidth as 4.45 GHz and the lowest RL is -28.68 dB at 8.0 GHz. The optimum values at the highest RL are listed in table I, including the optimum frequency, the optimum thickness, and the absorption bandwidth and the corresponding coercivity Hc of SrFe12-xRuxO19. The microwave absorption with RL ≤ -10 dB fully covers the frequency range from 4.0 GHz to 18 GHz with a varying x in 0.5 ≤ x ≤ 1.5 of SrFe12-xRuxO19. The RL peak moves to the low frequency with the increasing thickness of the absorption layer, as shown in the left part of Fig. 6. As the quarter wave cancellation contributes to the RL, the relation of the optimal frequency with the corresponding thickness follows the equation of dm = n λm / 4 = n c / [4fm |µr εr|1/2 ], where dm, fm and

λm are the thickness, optimal frequency and the quarter-wavelength of the microwave, respectively [31]. In the right part of Fig. 6, the calculated and the experimental optimum thickness dm versus the frequency are shown. The experimental data of x > 0.5 fall on the calculated curves fairly well. The quarter-wavelength criteria explains the relationship between the thickness and the optimum frequency in x > 0.5. The deviation from the calculated one in x = 0.5 is probably due to poor RL ~ f contour, which makes the optimum RL frequency inaccurate.

3

(a)

u'

1.0

0.5

1.3

0.7

1.5

12 (b) ε ' and ε''

u' and u''

2

x=0

1 0 -1

ε'

6 ε''

u''

x=0

1.0

0.5

1.3

0.7

1.5

0

0

6 12 f (GHz)

18

0

6 12 f (GHz)

18

9

Fig. 5 (a) The real u’ and imaginary permeability u’’, and (b) The real ε’ and imaginary permmitivity ε’’ of SrFe12-xRuxO19.

-10

-20

3 1 mm 1.3 mm 1.7 mm 1.9 mm 2 mm 2.1 mm 2.3 mm 3 mm

0

6

d (mm)

RL (dB)

0

x= 0.5

12 f (GHz)

1 12

18

-30

1 mm 1.3 mm 1.7 mm 1.9 mm 2 mm 2.1 mm 2.3 mm 3 mm

0

6

x= 1.0

-30 0

2 x = 1.0

0

12 f (GHz)

18

6

12 f (GHz)

18

4 1 mm 1.3 mm 1.7 mm 1.9 mm 2 mm 2.1 mm 2.3 mm 3 mm

d (mm)

RL (dB)

-20

18

(b)

0 -10

14 16 f (GHz)

4

d (mm)

RL (dB)

-20

x = 0.5

2

(a)

0 -10

Cal. Exp.

x= 1.3

2 x = 1.3

(c)

6

12 f (GHz)

18

0

6

12 f (GHz)

18

10

0

6

1.6 mm 1.8 mm 2 mm 2.2 mm 2.4 mm 2.6 mm

-20 -30

x = 1.5

4

x= 1.5

0

2.8 mm 3 mm 3.5 mm 4 mm 3.3 mm

6

12 f (GHz)

d (mm)

RL (dB)

-10

2

(d)

18

0

6

12 f (GHz)

18

Fig. 6 The relations of RL with the frequency and the thickness on the left, and the right is the experimental and calculated optimum thickness of SrFe12-2xNixRuxO19.

Table I The absorption frequency, bandwidth, minimum RL, optimal thickness and Hc of SrFe12-xRuxO19.

x

0

0.5

1.0

1.3

1.5

Absorption

--

14.2-18 7.8-14.35 7.8-13.67 4.15-7.76

Bandwidth (GHz)

--

3.8

6.55

5.87

3.61

Minimum RL (dB)

--

-17.6

-31.16

-32.0

-33.3

Optimal thickness

--

1.7

2.3

2.3

3.5

0.552

0.495

0.438

frequency (GHz)

(mm) 4.43 2.66

20

x=0 1.5

114

Counts

006

107

008

Hc (kOe)

40 o 2θ ( )

60

Fig. 7 The xrd patterns of the magnetically aligned SrFe12-xRuxO19 with x = 0 and 1.5. 11

As the FMR frequency, fR, is related to the anisotropic field Ha with 2πfR = γΗa for the c-axis anisotropy and γ as the gyromagnetic factor, it is necessary to study the magnetic properties of both BaFe12-xRuxO19 and SrFe12-xRuxO19. In Fig. 7, the xrd patterns of the magnetically aligned SrFe12-xRuxO19 with x = 0 and 1.5, as representatives, are shown. Both of the patterns are similar. The (00l) peaks are enhanced, while the (hk0) peaks are suppressed. It indicates that the easy magnetization directions of the x = 0 and 1.5 samples are along the c-axis. The hysteresis loops of BaFe12-xRuxO19 and SrFe12-xRuxO19 are shown in Fig. 8 (a) and (b). The coercivity of BaFe12-xRuxO19 decreases from 2.47 kOe in x = 0 down to 1.10 kOe in x = 1.0. It decreases much faster in SrFe12-xRuxO19 from Hc = 4.43 kOe in x = 0 to 0.68 kOe in x = 0.5, as shown in Fig. 8 (c), and it keeps almost the same in x ≥ 0.5. Considering that all the samples are synthesized with the solid state reaction under similar conditions, the variation of the coercivity reflects the change of the anisotropic field. Because the FMR frequency is proportional to the anisotropic field, the

5

x=0 0.3 0.5 0.7 1.0

(a)

BaFe12-xRuxO19

-15 -10 -5 0 5 H (kOe)

10

15

15 10 5 0 -5 -10 -15

x=0 0.3 0.5 0.7 1.0 1.5

-15 -10

SrFe12-xRuxO19

4 Hc (kOe)

15 10 5 0 -5 -10 -15

M (µ B / f.u.)

M (µ B / f.u.)

microwave absorption moves to the 0.1-18 GHz range with the Ru doping.

(b)

0 10

(c)

2 1

SrFe12-xRuxO19

-5 0 5 H (kOe)

BaFe12-xRuxO19

3

15

0.0

0.5 1.0 Ru content x

1.5

Fig. 8 The hysteresis loops of (a) BaFe12-xRuxO19 and (b) SrFe12-xRuxO19, and (c) Hc of BaFe12-xRuxO19 and SrFe12-xRuxO19.

It is generally accepted that the magnetocrystalline anisotropy of BaFe12O19 (SrFe12O19) is induced by the 2b site. The ions at the 2b site are not at the center but are distributed between the two adjacent 4f1 site. The crystal field has a pronounced axial component, which determines the high magnetocrystalline anisotropy along the 12

c-axis [24]. In BaFe12-xRuxO19, the lattice expands at the same ratio along different directions. The bond lengths of Fe-O at the bipyramidal site are calculated based on the structural refinement results, and are listed in table II. The bond length of Fe-3×O in the c-plane and those of 2 kinds of Fe-×1O along the c-axis decrease with the increasing Ru doping. On the other hand, the lattice of SrFe12-xRuxO19 expands along the a axis and compresses along the c axis with the increasing Ru4+ doping, as shown above in the lattice parameters. With the increasing x, the bond length of Fe×3O in the c-plane increases, and those of Fe×1O along the c-axis decreases, as listed in table II. Fig. 9 shows the schematic representation of the bipyramidal 2b site of SrFe12-xRuxO19 with x = 0 and 1.5. The bond length contraction along the c-axis effectively decreases the magnetocrystalline anisotropy. Even though the Ru doping in SrFe12O19 does not change the easy magnetization direction, it greatly decreases the anisotropy at room temperature. It is reflected in the decreased Hc, and has a high impact on the FMR. Similarly, the reduced anisotropy was observed in PbFe12-xRuxO19. It was reported that the Ru doping in PbFe12-xRuxO19 decreases the anisotropic constant K1 with x > 0.1. In x = 0.35, a spin reorientation from c-axis to c-plane is observed at 200 K [32].

Table II The bond length of Fe at the 2b site with its neighboring O of BaFe12-xRuxO19 and SrFe12-xRuxO19. BaFe12-xRuxO19

SrFe12-xRuxO19

x

Fe×1O

Fe×1O

Fe×3O

Fe×1O

Fe×1O

Fe×3O

0

2.1412

2.4985

1.8248

2.3519

2.4617

1.9438

0.3

2.1415

2.4988

1.8252

2.3362

2.4514

1.9487

0.5

2.1419

2.4993

1.8255

2.3200

2.4353

1.9538

0.7

2.1423

2.4997

1.8257

2.2923

2.4075

1.9564

1.0

2.1423

2.4998

1.8257

2.2645

2.3797

1.9610

1.5

--

--

--

2.2553

2.3705

1.9846

13

2.3519 Å

2.2553 Å

1.9846 Å

1.9438 Å

2.4617 Å

2.3705 Å

(a)

(b)

Fig. 9 The schematic structure of the ion at 2b site with its coordinating oxygen ions of SrFe12-xRuxO19 (a) x = 0; (b) x =1.5.

Fig. 10 are the FT-IR spectra of BaFe12-xRuxO19 and SrFe12-xRuxO19. The FT-IR spectra are sensitive to the Fe/Ru-O-Fe/Ru bond. The absorptions ν1 and ν2 at 616 cm-1 and 638 cm-1 are observed in La2CoMnO6 [33]. They are at the same position in all the samples of BaFe12-xRuxO19 and SrFe12-xRuxO19. SrFe12-xRuxO19 (BaFe12-xRuxO19) have 3 kinds of Fe/Ru sites, including tetragonal, octahedral and bipyramidal sites. The absorption peaks at 444 cm-1 (ν3) in SrFe12O19 correspond to stretching vibrations of the Oh sites by Fe–O6 at the octahedral 2a, 12k and 4f2 sites [34]. The ν4 and the ν5 bands are due to the vibrations at the 4f1 site. The ν3 band shifts to the high wavenumber, while ν4 shifts to the low wavenumber with the increasing Ru doping in SrFe12-xRuxO19. In BaFe12-xRuxO19, ν3 and ν4 behave similar but at a smaller ratio than those in SrFe12-xRuxO19. The frequencies of ν3 and ν4 are closely related to the bond length of Fe/Ru–O6 at the octahedral sites. Since the ions at the bipyramidal 2b site are not at the central positions, they would have different impact on the ions at the two adjacent octahedral sites. As a result, the ν3 and ν4 bands are separated. With the increasing Ru4+ content, ν3 and ν4 of both SrFe12-xRuxO19 and BaFe12-xRuxO19 are 14

getting close. They indicate that the 2b ions are getting close to the central position with the Ru doping, and the magnetocrystalline anisotropy decreases in SrFe12-xRuxO19 than in BaFe12-xRuxO19. The ν5 band at 554 cm-1 is due to the vibrations of Fe–O4 at tetrahedral sites [35], and it moves to the high wave number with the increasing Ru4+ doping of SrFe12-xRuxO19, and does not move in

Transmission (%)

(a)

400

ν3ν4

ν5

ν1ν2 x = 1.0 0.7 0.5 0.3 0

500 600 700 -1 wave number (cm )

800

Transmission (%)

BaFe12-xRuxO19 since only a small number of Ru4+ ions are at 4f1 site.

400

(b) ν3 ν 4

ν5

ν1

ν2

x = 1.5 1.0 0.7 0.5 0.3 0

500 600 700-1 wave number (cm )

800

Fig. 10 The FT-IR spectra of (a) BaFe12-xRuxO19 and (b) SrFe12-xRuxO19.

Conclusion

We studied the microwave absorption, the magnetic and structural properties of BaFe12-xRuxO19 and SrFe12-xRuxO19. As BaFe12-xRuxO19 and SrFe12O19 have high anisotropic fields, they are not considered as microwave absorption materials in the frequency range of 0.1 - 18 GHz. The magnetocrystalline anisotropy of the hexagonal ferrite is due to the pronounced axial component of the ions at the 2b site. With the Ru doping, the axial component at the 2b site is weakened as it is confirmed by the xrd refinements and the FT-IR spectra. The weakened anisotropic field is reflected in the decreasing coercivity with the Ru doping in both BaFe12-xRuxO19 and SrFe12-xRuxO19. FMR is observed in BaFe12-xRuxO19, but they are not applicable as microwave absorbing materials with RL≥-10 dB. Interestingly, strong microwave absorbing is observed in SrFe12-xRuxO19 in the a certain range of 0.1 – 18 GHz. In x = 0.5, RL≥ -10 dB occurs in 14.8 - 18GHz. The strongest absorbing occurs in x = 1.0 and 1.3 with 15

RL = -32 dB, and the widest absorbing bandwidth is 6.55 GHz in x = 1.0. With a varying x, the microwave absorbing band of SrFe12-xRuxO19 fully covers a wide frequency range of 5.5 – 18 GHz. The Ru doping drives SrFe12-xRuxO19 become potential candidates for electromagnetic materials with low reflectivity in the microwave band of 0.1 - 18 GHz.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 11864027), Innovation Start Support Program For Overseas Students of Inner Mongolia. The magnetic properties were measured at Zhong-ke-bai-ce company.

16

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1. SrFe12-xRuxO19 and BaFe12-xRuxO19 with high Ru doping crystallize in M-type ferrites. 2. The microwave absorption properties have been greatly improved by the Ru doping. 3. The great microwave absorption improvement is analyzed based on magnetic and structural properties.

The authors declare no conflict interest.