Microwave absorption studies of magnetic sublattices in microwave sintered Cr3+ doped SrFe12O19

Journal of Magnetism and Magnetic Materials xx (xxxx) xxxx–xxxx

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Microwave absorption studies of magnetic sublattices in microwave sintered Cr3+ doped SrFe12O19 ⁎

⁎

K. Praveenaa, , K. Sadhanab, , Hsiang-Lin Liua, M. Bououdinac,d a

Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan Department of Physics, University College of Science, Osmania University, Saifabad, Hyderabad 500 004, India Nanotechnology Centre, College of Science, University of Bahrain, Bahrain d Department of Physics, College of Science, University of Bahrain, Bahrain b c

A R T I C L E I N F O

A BS T RAC T

Keywords: Microwave-Hydrothermal synthesis Complex permittivity X-band Reflection coefficient Line width

The partial substitution of Fe3+ by Cr3+ in strontium hexaferrite has shown to be an effective method to tailor anisotropy for many novel microwave applications. Some basic studies have revealed that this substitution leads to unusual interactions among the magnetic sublattices of the hexaferrite. In order to investigate these interactions, Cr3+ doped SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) (m-type) hexaferrites were prepared by microwave-hydrothermal (m-H) method and subsequently sintered at 950 °C/90 min using microwave furnace. The magnetic hysteresis (m-H) loops revealed the ferromagnetic nature of nanoparticles (NPs). The coercive field was increasing from 3291 Oe to 7335 Oe with increasing chromium content. This resulting compacts exhibited high squareness ratio (Mr/Ms–80%). The intrinsic coercivity (Hci) above 1,20,000 Oe and high values of magnetocrystalline anisotropy revealed that all samples are magnetically hard materials. A material with high loss as well as high dielectric constant may be desired in applications such as electromagnetic (EM) wave absorbing coatings. The room temperature complex dielectric and magnetic properties (ε′, ε′′, µ′ and µ′′) of Cr3+ doped SrFe12O19 were measured in X-band region. The frequency dependent dielectric and magnetic losses were increasing to large extent. The reflection coefficient varied from −16 to −33 dB at 10.1 GHz as Cr3+ concentration increased from x=0.0 to x=0.9. Ferromagnetic resonance spectra (FMR) were measured in the X-band (9.4 GHz), linewidth decreases with chromium concentration from 1368 to 752 Oe from x=0.0 to x=0.9, which is quite low compared to commercial samples. We also have detailed origins of the FMR linewidth broadenings in terms of some important theoretical models. These results show that chromium doped strontium hexaferrites are useful for microwave absorption in the X-band frequency and also have potential for use in low frequency self-biased microwave/millimeter devices such as circulators and isolators.

1. Introduction Since their discovery, hexagonal ferrites have continued to be very attention-grabbing group of materials credited to their significant physical and chemical properties. Their captivating applications in microwave devices, microstrip antennas, high frequency transformers, memory core, radar devices [1,2] and high density recording media [3,4] have enforced several researchers to explore new such materials. Interest in hexaferrites has been rekindled by the discovery of intrinsic magnetoelectrics with strong coupling of magnetic and electric order as well as by the emergence of various low-dimensional hexaferrite systems, e.g., nanoparticles, fibres, thin layers, or composites [5]. The increase in electromagnetic interference (EMI), originating from the rapid development of gigahertz (GHz) electronic systems and

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wireless telecommunications, has resulted in an intensive growing interest in electromagnetic absorber technology [6,7]. Recent developments in microwave absorber technology have been focused on the materials with high wave absorption coefficient, good physical performance and lower production cost [8,9]. There are variety of absorber materials that can be used to suppress EMI depending on their suitability for low or high frequency applications [10–12]. According to the thickness and working frequency bandwidth, the magnetic composites have obvious advantages. The magnetic fillers often used in such composites are ferrite materials, such as spinel ferrites and hexaferrites [13,14]. M-type hexaferrites have been widely used as microwave attenuation materials for the GHz frequencies and have recently received considerable attention from the researchers [15,16], because of their low cost, low density, high stability, large electrical

Corresponding authors. E-mail addresses: [email protected] (K. Praveena), [email protected] (K. Sadhana).

http://dx.doi.org/10.1016/j.jmmm.2016.11.013 Received 20 July 2016; Received in revised form 11 October 2016; Accepted 2 November 2016 Available online xxxx 0304-8853/ © 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Kuruva, P., Journal of Magnetism and Magnetic Materials (2016), http://dx.doi.org/10.1016/j.jmmm.2016.11.013

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transport events on the atomic scale [23]. The electromagnetic energy was absorbed by the material as whole (also known as volumetric heating) due to microwave-matter coupling and deep penetration and then converted in to heat through dielectric (in case of ceramics), magnetic permeability/eddy currents (metals) loss mechanisms. Since there was an energy conversion and no thermal conductivity mechanism involved, the heating is very rapid, uniform and highly energy efficient. Due to the internal heating in the microwave processing, it is possible to sinter many materials at a much lower temperature and shorter time than that are required in conventional methods reported by other researchers [24,25]. Commercially available kitchen microwave oven has been modified by us and used for sintering many ferrite samples [26]. Therefore, we have selected microwave sintering method in the present investigation. Fig. 2. gives the line diagram with names of the different parts of the kitchen microwave oven. The main control unit magnetron was converted in the oven cavity with a wave guide. The specimen was placed in a casket made of insulation material and SiC plate. A thermocouple enter the casket from the top of the microwave oven and is in contact with the specimen. The microwave oven used for sintering studies was of power level 1 kW and has a cavity of 35×21×35 cm3. The control system is a microprocessor based control with a timer and has a provision to vary average power in discrete steps of 30%, 50%, 70% and 100% using simple on-off control. The microwave power was controlled using variac interfaced to main control. The variac is introduced at the input to the primary of the magnetron transformer so that the input voltages can be regulated to the magnetron by varying the input voltage to the transformer upto the full value. The heat input to specimens is regulated and can be varied continuously. The variac unit along with relay timer was connected to a thermocouple (TC) and a temperature indicator. The heating filament supply of the magnetron therefore gets its continuous rated supply voltage of 230 V and keeps the microwave source efficient. Depending on the nature of load to be heated and the rate at which heating needs to be given, one can make a simple selection of ‘on’ and ‘off’ times and in this manner control the heat cycles. The casket used has dimensions of 130 mm ODx70 ID and 140 mm long tube formed by Alumina fiber mat. End mats were also cut into minimum thickness of 3 cm. The tubular shape was maintained by wrapping with glass cloth ribbons. SiC plates of 35×35×3 mm were used to form a rectangular container with porous Zirconia plates as a

Fig. 1. Unit cell of hexaferrite SrFe12O19. The five nonequivalent Fe sites are octahedral 2a (site symmetry), bipyramidal 2b (m2), tetrahedral 4fIV, octahedral 4fVI (both 3m), and 12k (site symmetry m). The three- and six fold local axes are parallel to the hexagonal axis c of the crystal (vertical direction in the picture), which is also the easy axis of magnetization.

resistivity, large anisotropy field and moderate saturation magnetization. Many studies have been carried out to investigate the influence of partial Fe3+ ion substitution [17–20] on improving microwave performance of various hexaferrites. Finally, there is still effort devoted to classical hexaferrite systems aimed on improving their performance in applications and unveiling related physics. One of the important properties of strontium M-type hexaferrites SrFe12O19 (SrM) is the magnetocrystalline anisotropy. The anisotropy arises mainly from contributions of ferric cations. In the hexaferrite structure (space group P63/mmc), Fe atoms occupy sites 2a, 2b, 4fIV, 4fVI and 12k, which form five magnetic sublattices (see Fig. 1). In our previous work [21,22], the synthesis and improved magnetic properties of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) were synthesized by microwave hydrothermal method and sintered using microwave furnace. In this work, the effect of particle size and size distribution on the structure and microwave properties of Cr-substituted M-type strontium hexaferrites were investigated. 2. Experimental A series of Cr3+(0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) doped SrCrxFe12O x 19 powders were prepared by microwave-hydrothermal method. The complete details of the synthesis route were given elsewhere [21]. The obtained powders were mixed with an appropriate amount of 2 wt% polyvinyl alcohol as a binder. Then the powders were uniaxially pressed at a pressure of 800 kg/cm2 to form green pellet specimens. The compacts were sintered at 950 °C/90 min using microwave sintering method. Sintering was defined as a thermal treatment for bonding particles into a coherent, predominantly solid structure via mass

Fig. 2. Internal view of Microwave Oven.

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heating. Sutton has described the characteristics of microwave absorption in great detail [27]. Microwave irradiation is becoming an increasingly popular method of heating samples in the laboratory. It offers a clean, cheap and convenient method of heating often resulting in high density and shorter reaction times, it can be used for any ceramic oxide/composite material [31–36]. Phase formation and morphology of the sintered samples were studied using X-ray diffraction (XRD) using Bruker D8 advanced equipped with Cu-Kα (λ=1.5406 Å) and field emission scanning electron microscopy (FESEM) FEI model. The magnetic properties such as saturation magnetization (Ms), remnant (Mr) and coercive field (Hc) were obtained by measuring magnetization versus magnetic field (mH) curves at room temperature using vibrating sample magnetometer (VSM) Lakeshore 7500, USA equipped with a magnet of 15 kOe. The reflection curve of an electromagnetic absorber is a function of a complex interplay between the permittivity, the permeability and the thickness of the absorber. From material point of view, both permeability and permittivity can be varied, whereas an additional degree of freedom of design is obtained (thickness). Absorptive layer can be made from either dielectric or soft magnetic materials with appropriate loss tangent. Usually the absorber layers are made from composite materials, mixtures of dielectric (carbon black, aluminum flakes, etc.,) and/or soft-magnetic (ferrites, carbonyl iron) particles in some matrix; however, excellent absorbers can be made from ferrite ceramics for use at lower frequencies. Since every material has advantages and disadvantages, the selection of materials is difficult and depends on actual application [37]. Microwave magnetic properties were characterized by the ferromagnetic resonance (FMR) measurements using a shorted waveguide at 22.5–48 GHz. In the FMR measurements, the external magnetic field was applied perpendicular to the plane and swept. The frequency was fixed during the field sweep. Because the magnetic easy axis was perpendicular to the film plane, we applied the external magnetic field H0 parallel to the easy direction, i.e. perpendicular to the film plane. Thus, the microwave magnetic field ~h was applied in the plane of the sample. At f =9.65 GHz (X-band), the FMR linewidth of the sample was measured using a field sweep FMR/electron paramagnetic resonance, FMR/EPR, facility with both dc magnetic field and microwave excitation field in the plane of the samples.

separator on either side. This in turn provides a sample volume of 30×30×30 mm inside the casket. The sample is placed in the casket and is located at the centre of the cavity. The Platinum-Rhodium thermocouple has been introduced from the top of the oven to measure the temperature. With platinum sheathing tube we can monitor temperature continuously without microwave signal pickup. Very inexpensive neon lamps of 5 mm long are used at various joints and openings to detect any microwave leakage as these lamps start glowing even if they are exposed to low power microwave radiation. Internally disconnecting the input leads of the magnetron and routing it through the timer interface results in the continuous microwave power control scheme. The thermocouple leads are also routed through contactor of the scheme so as to prevent microwave power reaching the measurement system when microwave is on. The on time of the timer is adjusted based on the heating rate required and also reduced in proportion to off time set for measurement during holding of sample temperature. The most notable effect of microwave radiation is the heating effect. In microwave heating, unlike conventional heating, heat is generated internally within the material instead of originating from external sources. As a result of internal and volumetric heating, thermal gradients and direction of heat flow in microwave heated materials cab be just the opposite of those in materials heated by conventional methods [27–29]. Microwave heating is fundamentally different from conventional heating in which electrical resistance furnaces are typically used (Fig. 3). In microwave heating, heat is generated internally by interaction of the microwaves with the atoms, ions, and molecules of the material. Heating rates in excess of 1000 °C can be achieved in a short time and significantly enhances densification. The degree of interaction between the microwave electric and magnetic field components with the dielectric or magnetic material determines the rate at which energy is dissipated in the material by various mechanisms. The most important properties for the interaction are the permittivity (ε) for a dielectric material and the permeability (μ) for a magnetic material [30]. The samples of any shape can be heated rapidly and uniformly under microwave exposure. Thermal stresses are reduced which decrease cracking while processing. Also, microwave absorption varies with composition, which introduces a new possibility of selective

3. Results and discussion 3.1. Structure and morphology The XRD profiles of microwave sintered Cr3+ doped SrCrxFe12−xO19 showed the patterns of hexaferrites. All the peaks were indexed using JCPDS card No.27–1029 within space group P63/ mmc (No. 194). The results were presented elsewhere [21,22]. From FESEM micrographs of SrCrxFe12−xO19, we observed that for x > 0.3, in addition to the hexagonal shaped grains, a small amount of impurities were observed and the micrographs are given in ref [21,22]. According to literature [38], barium and strontium hexaferrite nanoparticles with hexagonal pyramidal and hexagonal plate like morphology were the best materials for the electromagnetic wave absorption applications, effective radar absorbing materials (RAM). It is found to be in the range of 700–280 nm. The average grain size decreases with Cr3+ doping since it inhibited the grain growth of the hexaferrite. The grain size distribution is obtained (Fig. 4) by analysing FE-SEM micrographs (six) for each sample. We notice that the particle size distribution followed a log-normal distribution. 3.2. VSM analysis Magnetic hysteresis of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) were given in elsewhere [21]. The squareness ratio (Mr/Ms) is ~80%, which clearly shows the permanent magnetic behaviour of

Fig. 3. Principle of microwave and conventional sintering methods.

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Fig. 4. Grain size distribution using FESEM micrographs of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9).

Brown et al. also gave an expression to estimate the magnetocrystalline anisotropy and intrinsic coercive field [43]

chromium doped strontium hexaferrites. Here we have adopted the Stoner-Wohlfarth (SW) model to extract the saturation magnetization (Ms) and the fitting results are given in Fig. 5. these values are not the Ms values. The Ms for each sample is estimated by SW model extrapolating Ms to 1/H2 approaches to zero [39]. The S-W model applies for non-interacting, single domain hexaferrites. The magnetocrystalline anisotropy, shape anisotropy and grain size are the major factors affecting coercivity (Hc) of polycrystalline hexaferrites [40]. In our present report, Hc increases with Cr3+ concentration. In contrast to our results, Auwal reported [41,42] that by doping Bi, La and Y in Fe site Hc decreases with increasing dopant concentration.

⎛ b ⎞ σ = Ms⎜1 − 2 ⎟ , ⎝ H ⎠

(1)

where b is a parameter related to magnetocrystalline anisotropy. In Fig. 5. a linear relationship is observed between magnetization and 1/ H2. The Figure also includes linear fit lines for experimental data. The slope of these linearly fitting lines is the product of Msb and hence the parameter b is calculated. When b value is substituted at an approximate relation, Eq. (2), an effective anisotropy constant (Keff) is 4

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Fig. 5. Plots of magnetization as a function of 1/H2 for SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9).

uniaxial anisotropic property for SrFe12O19. Strontium hexaferrites owed its magnetic hardness to its crystal anisotropy. We calculated the intrinsic coercivity (Hci) of hexaferrites by using Eq. (3) [46] and given in Table 1

obtained for a uniaxial magnetic nanocrystal [44,45].

⎛ 15b ⎞1/2 Keff = Ms⎜ ⎟ ⎝ 4 ⎠

(2)

The values of b and Keff are listed in Table 1. Cullity assigns the hexagonal c axis as the easy axis and crystal anisotropy constant Keff is little bit larger than 3.3×106 ergs/cm3 (or 6.22×105 ergs/g) for strontium hexaferrites [44]. We found that the Keff value as 5.27×105 ergs/g. This value might be attributed to crucial

Hci =

2Keff Ms

.

(3)

According to Table 1, we observed that coercivity and intrinsic coercivity of hexaferrites increases with increasing Cr3+ concentration 5

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ity values at 8.1 and 10.2 GHz frequency are tabulated in Table 1. The observed magnetic spectra's (Fig. 6(c) and (d)) are in agreement with the mechanism of natural magnetic resonance involving domain-wall displacement and domain rotation. These motions lag behind the applied magnetic field and cause magnetic loss in material. The relations expressing the resonance-relaxation phenomena near the characteristic frequency of spin rotation or domain-wall displacement are available in literature [55,56]. For most EM wave absorption materials, eddy currents can be caused by an alternating electric field, due to finite conductivity or poor insulation between particles. Although the currents can cause significant loss by resistive heating of the materials when the incident EM wave contacts the absorber, relatively high intensity eddy currents would cause the permeability to decrease [57]. Cr3+ doping has a slightly effected on resistivity [53], resulting in decrease of real part of permeability and an increase in imaginary part through both eddy current loss and ferromagnetic resonance. At low frequency, domain wall resonance plays the major role and at high frequency, ferromagnetic resonance is the dominant phenomenon in the ferrite materials [58]. The high value of µ″ (Fig. 6(d)) at 7.9 GHz and 12.85 GHz may be due to the coalescence of the two adjacent resonances. It is worth noting that the dielectric loss (Fig. 6(b)) and magnetic loss (Fig. 6(d)) increases with frequency which can be attributed to the fact that the substitution of Cr3+ ions for Fe3+ ions favours the transformation of the Fe3+ ions into Fe2+ ions and electric hopping between these two sites enhances the dielectric and magnetic loss factors.

Table 1 Data of Ms, b, Keff and Hci values with Cr3+ concentration for SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9). Composition x

Ms (emu/g)

Msb (emu. Oe2/g)

b (Oe2)

Keff (J/ m3)

Hci (Oe)

0.1 0.3 0.5 0.7 0.9

44 38 36 31 30

4.44×107 3.17×108 3.37×108 2.57×108 2.46×108

1.00×106 0.88×107 0.93×107 0.82×107 0.82×107

5.7×106 6.1×106 7.2×106 7.4×106 8.7×106

129545 160526 200000 238709 290000

which is due to increasing mangetocrystalline anisotropy in the samples. The critical size of a single domain particle is determined from the formula [47,48]

Dc =

9εw 2πMs2

,

(4) 1/2

where εw = (2kb Keff Tc / a ) is the wall density energy, kb is the Boltzmann constant, Tc is the Curie temperature and a is lattice constant. For SrFe12O19, kb is 1.38×10–16 erg/K, a is 6×10−8 cm [48,49] and Tc is 737 K. By substituting the Keff =5.27×105 erg/m3 and Ms =64.72 emu/gm, Dc is determined as 378 nm. The particles sized below Dc were single domain particles while D > Dc were multidomain structures. The FE-SEM micrographs grain size distribution (Fig. 4.) show that grains have size between 300–400 nm. The grains exhibit single domain structure and grain size does not play significant role for lower values of coercive fields.

3.4. Microwave absorption properties Characterization of absorbers can be made directly with standardized measurements of reflection in free space or indirectly through measurements of constituent materials' electromagnetic properties. The latter method is greatly used in the designing phase, since in general the reflection from the absorber can be analytically calculated from known materials' characteristics and geometry. Fig. 7. shows how a simple one-layer absorber works. The reflection coefficient (RC) is calculated according to the EM parameters derived from the coaxial measurements [59], using the following equations:

3.3. Complex permittivity and permeability properties The complex permittivity and permeability are generally used to analyse the dielectric and magnetic properties of absorber materials. The real parts (ε′ and μ′) signify the storage capability of electric and magnetic energy, whereas the imaginary parts (ε″ and μ″) stand for the loss of electric and magnetic energy [50]. The complex dielectric permittivity and magnetic permeability of Cr3+ doped SrCrxFe12-xO19 (x=0.0–0.9) were measured via Transmission Reflection (T/R) line method with the waveguide technique at X band. These parameters, shown in Fig. 6(a)–(d), were obtained from an Agilent 85071E Materials Measurement Software. As seen in the figure, the measured permittivity and permeability values are almost constant in the frequency range employed. The real part of permittivity and permeability values are lower as compared with the undoped one. This change in complex permittivity with frequency is mainly due to the intrinsic electric dipole polarization [51]. The substitution of transition metal Cr3+ for Fe3+ may cause the transformation of comparable amount of Fe3+ into Fe2+ for electrical charge neutrality. The electron hopping between the Fe3+ and Fe2+ sites results in the enhancing of the dielectric loss and Fe2+ ions strengthen the interfacial polarization of ferrite material. Furthermore, some extra intrinsic electric moments are formed due to the difference in the radius of Cr3+ among the other cations. Another reason might be that substitution has changed the structure, which results in a non-percolating system [17,52]. The value of electric polarization, ε′, decreases slightly with Cr3+ substitution, which might be due to the diminution of interfacial polarization [53]. In EM wave absorbers, currents can be induced through electric dipole polarization and interfacial polarization through the interaction between incident EM waves, the electric field and mobile electrons. The induced current can be reduced by finite resistivity, which causes heat considered as dielectric loss [53]. With Cr3+ ion substitution in the crystal lattice, more electron hopping will arise, which can increase the intrinsic electric dipole polarization. Therefore, the decrease in ε′ with increased Cr3+ doping could be attributed to the diminution of interfacial polarization [54]. The complex permittivity and permeabil-

RC = 20 log (Zin − 1)/(Zin + 1) , where Zin is given by Zin = μr εr tanh [j (2πfd / c ) μr εr ],

(5)

where εr and µr are the complex permeability and permittivity of the medium, respectively, ‘c’ is the velocity of light in free space, ‘f’ is the frequency and ‘d’ is the thickness of the absorber. As shown in Fig. 8, it could be seen that with increasing Cr3+, the absorbing peaks shift to higher frequencies. Tabatabaie et al. also reported that with substitution of Mn2+ and Ti4+ in barium ferrite, the absorption peak shifted to high frequency regions, which may be related to the resonance frequency [60]. According to ferromagnetic resonance theory [61,62], the resonance frequency is proportional to the anisotropy field (Ha). The substitution of Cr3+ ions for Fe3+ into ferrites changes the direction of the magnetocrystalline anisotropy of the material. The cause of this anisotropy is the effective coupling of the magnetic ion spins and the crystalline electric fields acting upon the ions via spin–orbit coupling. Such electric fields are dependent on the magnitude and the symmetry of the positions of the neighbouring ions [63], so with such substitutions the resonance frequency can be shifted to higher frequencies. Especially for the samples of x=0.7 and x=0.9, they had remarkable absorption. The x=0.9 sample exhibited the largest reflection loss and the widest bandwidth for reflection coefficient compared with other samples. The reflection coefficient varies from 16.5 to 0.71 dB at 8.01 GHz for x=0.0 to x=0.9. As the frequency increases, for x=0.9 sample the maximum reflection loss of −32.7 dB at 9.9 GHz is observed. Above 9 GHz, the EM parameters varied in the high 6

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Fig. 6. a. Frequency variation of real part of permittivity (ε′). b. Frequency variation of imaginary part of permittivity (ε′′). c. Frequency variation of real part of permeability (µ′). d. Frequency variation of imaginary part of permeability (µ′′).

Fig. 7. Scheme of a simple, one-layer absorber.

frequency region and hence the interface impedance, Zin, changes with frequency. As a result, the sample thickness no longer satisfies the quarter wavelength requirement for impedance matching, resulting in impedance mismatch [64]. The RC values are tabulated in Table 2. This shift of minimum value of reflection coefficient towards the

Fig. 8. Reflection coefficient of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9).

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Table 2 Data of complex permittivity and permeability at 8.1 and 10.2 GHz. Composition x

ε′ at 8.1 GHz

ε′ at 10.2 GHz

ε′′ at 8.1 GHz

ε′′ at 10.2 GHz

µ′ at 8.1 GHz

µ′ at 10.2 GHz

µ′′ at 8.1 GHz

µ′′ at 10.2 GHz

RC (-dB) at 10.2 GHz

0.0 0.1 0.3 0.5 0.7 0.9

8.4 7.3 6.9 6.6 5.8 5.1

8.8 7.8 7.6 7.3 6.4 5.4

0.5 1.0 1.3 1.6 2.1 2.5

0.5 0.9 1.2 1.5 2.0 2.3

5.2 2.5 2.4 1.9 1.9 1.5

4.5 3.3 2.5 2.4 2.0 1.8

0.6 1.2 1.4 2.2 2.9 3.3

0.6 1.4 1.6 2.4 2.6 3.0

16 20 25 27 30 33

lower and higher frequencies with the substitution of transition element confirmed the claim that the microwave absorption properties of hexagonal ferrites can be tuned with the substitution. This also confirms that the tunable microwave absorber can be synthesized with substitution of transition elements in pure ferrites. These results reflected the applications of presently investigating materials in super high frequency devices (SHF). For achieving the maximum value of the microwave absorption, these conditions must be satisfied. First, the incident microwave can penetrate into the microwave absorbing material at its great extent (impedance matching properties). Second, the incident wave can be attenuated entirely (attenuation characteristic). Among the above mentioned two conditions, the first condition can be justified as; the decreasing trend in µ′ and increasing trend in ε′ makes the ratio approaches to unity and satisfies the first condition [65]. This condition is more important in case of hexagonal ferrites [19]. The attenuation constant can be obtained by relation

α=

4.442f (μ′′ε′′ − μ′ε′) + c

(μ′′ε′′ − μ′ε′)2 + (μ′′ε′ − μ′ε′′)2 ,

(6) Fig. 10. FMR spectra's of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) with applied magnetic field.

The attenuation constant (α) verses frequency graph has been plotted in Fig. 9. It is evident that the maximum values of attenuation constant (Fig. 8) for all the samples are consistent with the reflection coefficient result which is favourable condition for achieving tunable microwave absorber.

and costly to fabricate. It is anticipated that the next generation of magnetic microwave devices will be planar, self-biased and low-loss, and will operate well beyond the performance metrics of today's devices. Self-biasing is an important property that eliminates the need for the permanent magnet and reduces the size, weight, and assembly cost of microwave devices. Fig. 10. depicts the FMR spectra of all the samples, have an intense broad asymmetric peak, The peak-to-peak line width ΔHPP decreases with increasing chromium concentration. Table 3 depicts the FMR parameters for SrCrxFe12-xO19 (x = 0.0, 0.1, 0.3, 0.5, 0.7 and 0.9). The occurrence of the finite line width in FMR spectrum is due to the fact that the electrons interact with external applied magnetic field but they also interact magnetically with the surrounding of the samples. Thus the resultant magnetic field seen by population of electron spins is not quite the same throughout the population even when they are subjected to the same applied field. Consequently, resonance absorption line obtained for a given value of the resultant field will be obtained over a range of values of the applied field. In many magnetic materials, the EPR study has revealed that the variation of the resonance line width ΔHPP is caused by the microscopic magnetic interactions inside the material, mainly the interparticle magnetic dipole interaction and the

3.5. FMR analysis Ferromagnetic resonance (FMR) extends from microwave to millimeter wave frequencies allowing their application in such devices as isolators, filters, phase shifters, and circulators. A key component of radar electronics is the circulator, which usually includes a permanent magnet to provide the biasing magnetic field required for operation. These devices are three-dimensional constructs that are often bulky

Table 3 Data of FMR parameters for SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9).

Fig. 9. Variation of attenuation constant (α) of SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) with frequency.

8

Composition x

HPP (Oe)

g

0.0 0.1 0.3 0.5 0.7 0.9

1368 1221 962 922 875 752

2.08 2.05 2.03 2.01 1.97 1.94

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Fig. 11. frequency dependence of ferromagnetic resonance linewidths for SrCrxFe12−xO19 (x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9) with frequency (22–50 GHz).

able barium ferrites typically have ΔH values of ~2000 Oe [66,67]. This represents ~62% improvement in this critical parameter. The measured FMR linewidths, ~752 Oe, are believed to satisfactorily address most requirements of microwave devices. However, it is assumed that the FMR linewidths can be further narrowed by reducing nonuniformity contributions including porosity, misalignment, roughness, etc. [68]. Broadening of the FMR linewidths from extrinsic sources are indeed inevitable for polycrystalline ferrites. This may present a challenge to realizing low-loss, high-performance devices. In order to find material solutions having narrow linewidths, it is necessary to

superexchange interaction. In the case of ferromagnetic particles, the intrinsic molecular magnetic moments are large; therefore magnetic dipole interaction among these particles is very strong. Magnitude of this interaction is inversely proportional to the cube of average interparticle distance. On other hand, superexchange interaction between magnetic ions through oxygen anions can reduce the value of ΔHPP. More importantly, this ferrites exhibits low FMR linewidths (i.e., ΔH=1320–752 Oe) over a broad frequency range (22.5–48 GHz), as depicted in Fig. 11. These results represent the lowest linewidth observed in polycrystalline Cr-doped hexaferrites. Commercially avail9

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References

estimate contributions to the total linewidth for these materials. In general, the total linewidth is attributed to three major contributions,

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Δ H = ΔHi + ΔHa + ΔHp where ΔHi, ΔHa and ΔHp represent intrinsic, random anisotropy field, and porosity, respectively. Clearly, the latter two terms represent extrinsic contributions present in our samples. Spark conducted a detailed calculation of the scattering linewidth due to pores [69,70]:

ΔHp =

π 2ωPMs (3cos2θu )2 + 1.6 . 2γH0 cos θu

(7)

where P is the porosity, ω is the angular frequency, γ is the gyromagnetic ration, H0 is the internal magnetic field and θu is the spin wave angle between the wave vector and the internal field for the 1 uniform mode. Because 3 < cos2 θu < 1 for non-spherical voids [71], the estimate shows the porosity contribution to line width ranging from 1100 Oe to 800 Oe. The total estimates compare reasonably with the experimental results of ΔHmeas=1328–752 Oe. From this, we conclude that the porosity indeed contributes to the broadening of linewidths for polycrystalline ferrites, and an increase in density is expected to narrow the extrinsic linewidth due to the annihilation of pores [72]. The reduction in pores may be realized by further growth of the grains at higher sintering temperatures. However, such a strategy is not available for the present hexaferrite, which requires grain sizes to be approximately equal to the single domain size needed to obtain high remnant magnetization and hysteresis loop squareness. Therefore, promising solutions in densification include (1) a final sintering carried out under a hot-isotropic pressure [73] and (2) the use of a coldisotropic press [74] to press green bodies before sintering. Because both the techniques feature an isotropic pressure, they will likely not result in significant loss of grain alignment in the highly oriented green body [75]. 4. Conclusions Nanocrystalline Cr-doped SrFe12O19 were synthesized using microwave–hydrothermal method and sintered at 950 °C/90 min using microwave sintering which operates at 2.45 GHz. With increasing Cr concentration, MS decreases while HC increased. This could be due to octahedral and trigonal bipyramidal site occupancy of the nonmagnetic dopants. The larger values of coercive field i.e., 7335 Oe is achieved at low sintering temperature and in shorter time. Hexaferrites exhibited a large uniaxial magnetic anisotropic behaviour. The magnetic hardness of products is attributed to their strong crystal anisotropy. According to FESEM micrographs, the gain sizes were between 660 nm and 280nm. This range is greater for x=0.5 and smaller for above x > 0.5 since the critical dimension exhibited by SrFe12O19 is of 378 nm. Hence by chromium doping samples were exhibiting multi-domain nature. The real part of permittivity changed from 8.4 to 5.2 as Cr3+ varied from x=0.0 to 0.9 at 8.1 GHz, similarly ε′′ increases from 0.57 to 2.5. The real part of permeability varied from 5.2 to 1.5 as Cr3+ varied from x=0.0 to 0.9 at 8.1 GHz, similarly µ′′ increases from 0.6 to 3.3. The shift of maximum value of microwave absorption peak with frequency with the substitution confirmed that the tunable microwave absorber can be achieved. These materials offer low FMR linewidths (1328– 752 Oe) at X-band and Ka-band and high remnant magnetization (Mr/ Ms–80%) and should be suitable for applications in low-frequency microwave devices (1–20 GHz). Acknowledgement Dr K. Praveena acknowledges the Ministry of Science and Technology of Republic of China under Grant nos. MOST 105-2112M-003-013-MY3 and MOST 105-2811-M-003-018 for financial support. 10

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