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Microwave properties of the Ga-substituted BaFe12O19 hexaferrites

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 Mater. Res. Express 4 076106 (http://iopscience.iop.org/2053-1591/4/7/076106) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 22/07/2017 at 07:31 Please note that terms and conditions apply.

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Mater. Res. Express 4 (2017) 076106

https://doi.org/10.1088/2053-1591/aa50b9

PAPER

RECEIVED

9 July 2016

Microwave properties of the Ga-substituted BaFe12O19 hexaferrites

ACCEPTED FOR PUBLICATION

A V Trukhanov1,2, S V Trukhanov1,2, V G Kostishyn1, L V Panina1, I S Kazakevich2, An V Trukhanov1,2, V O Natarov2, D N Chitanov1, V A Turchenko3,4, V V Oleynik5, E S Yakovenko5, L Yu Macuy5 and E L Trukhanova2

30 November 2016

1

PUBLISHED

2

21 July 2017

3

REVISED

16 November 2016

4

5

National University of Science and Technology ‘MISiS ’, Leninskii av., 4, Moscow, 119049 Russia Scientific Practical Materials Research Centre, National Academy of Sciences of Belarus, P. Brovki str., 19, Minsk, 220072 Belarus Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Moscow region, 141980 Russia Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, R. Luxembourg str., 72, Donetsk, 83114 Ukraine Kiev National University named after Taras Shevchenko, Vladimirskaya str., 64/13, Kiev, 01601 Ukraine

E-mail: [email protected] Keywords: substituted hexaferrites, microwave absorbing, crystal structure, magnetic properties, natural ferromagnetic resonance

Abstract The crystal structure features and the unit cell parameters were refined using the powder x-ray method for the solid solutions BaFe12−xGaxO19 (x = 0.1–1.2) barium hexagonal ferrites of M-type at 300 K. With increase of substitution level the unit cell parameters monotonically decrease. The temperature and field dependences of the specific magnetization were investigated by the vibration magnetometry method. The concentration dependence of the TC Curie temperature as well as the MS spontaneous specific magnetization and the HC coercive force at 300 K is constructed. With increase of substitution level the magnetic parameters monotonically decrease. The microwave properties of the considered solid solutions in the external magnetic bias field are also investigated at 300 K. With increase of Ga3+ concentration from x = 0.1 to x = 0.6 the frequency value of the natural ferromagnetic resonance (NFR) decreases in the beginning, and at further increase in concentration up to x = 1.2 it increases again. With increase in Ga3+ concentration the line width of the NFR increases that indicates the increase of frequency range where there is an intensive absorption of electromagnetic radiation (EMR). At the same time the peak amplitude of the resonant curve changes slightly. The frequency shift of the NFR in the external magnetic bias field takes place more intensively for the samples with small Ga3+ concentration. It is shown the prospects of use of the Ga-substituted barium hexagonal ferrite as the material effectively absorbing the high-frequency EMR.

1. Introduction The interest in research of the M-type BaFe12O19 barium and SrFe12O19 strontium ferrites with hexagonal structure and their solid solutions substituted by different diamagnetic cations (La3+, Al3+, Sc3+, Sn3+, Zr2+ etc) [1–10] is caused by their high functional properties. Excellent chemical stability and corrosion resistance [11] do them ecologically safe and suitable for application practically without restrictions in time. The combination of high coercive force (HC ∼ 160 − 55 kA m−1 with rather high residual induction allows receiving permanent magnets with satisfactory specific magnetic energy [12]. Their low conductivity (ρ ∼ 108 Ohm cm) allows to apply hexaferrite magnets in the presence of high-frequency magnetic fields. For the first time, barium hexaferrite, isomorphic to PbO*6Fe2O3, has been received in Philips firm [13] still in the 1950th. The main magnetic, electric and structural properties of hexaferrites were discussed in the recent review [14]. Until recently barium hexaferrites were widely used only as permanent magnets [15] and in magnetic storage devices (devices with high density information) with perpendicular-type of magnetization [16]. Recently there are a lot of publications concerns with substituted M-type barium hexaferrites. It caused by coexistence in these compounds ferrimagnetic and ferroelectric ordering at the room temperature [17–24]. Magnetic properties of the M-type barium hexaferrites can be improved by chemical substitution by diamagnetic ions and by varying by © 2017 IOP Publishing Ltd


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crystalline size. It makes these materials perspective for practical application as absorption materials. It could be used for absorption of the electromagnetic radiation (EMR) in the microwave range. There are many works on effective absorption of EMR by these materials at high frequencies [25–28] that provides electromagnetic compatibility of microelectronics and radio equipment devices. The high values of the magnetocrystalline anisotropy and the ferrimagnet-paramagnet phase transition temperature (TC ∼ 740 K) are main advantages of the M-type barium hexaferrites. The collinear ferrimagnetic ordering is formed strong interlattice exchange interactions between Fe3+ cations. The change of the magnetic bonds number for Fe3+ cations in magnetic oxides can be produced by diamagnetic substitution in B-lattice [29] or by creation of anion-deficient compounds [30]. It allows operating its functional properties. The analysis of the M-type barium hexaferrite electromagnetic properties shows that the effective EMR absorption range is in the centimetric spectral range [31]. The application of the substituted M-type barium hexaferrites with large magnetocrystalline anisotropy as fillers in composite materials allows carrying out frequency selective EMR absorption using the controlled resonances of domain boundaries and rotation of magnetization (a natural ferromagnetic resonance, NFR) [32–34]. It is known that the absorption peak of the BaFe12O19 M-type barium hexaferrite is in the area of ∼47–50 GHz [35] whereas the substitution of the Fe3+ by Mn2+ + Co2+ + Ti4+ reduces an absorption maximum to 13.4 GHz [7]. It is considered that losses at high frequencies are generally caused by NFR. By controlling the diamagnetic substitution level it is possible to displace the absorption peak. Production of the solid solutions of the BaFe12O19 M-type barium hexaferrite with different substitution leads to expand an absorption strip in the microwave range. In this paper we present the results of the structural, magnetic and microwave parameters investigations for the solid solutions BaFe12−xGaxO19 (x = 0.1–1.2). The results of this work can find application for ensuring of the electromagnetic compatibility.

2. Methods The BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) polycrystalline samples were obtained from the Fe2O3, Ga2O3 oxides and the BaCO3 carbonate (all powders manufacturer—Xiamen Ditai Chemicals Co., Ltd) taken in the corresponding proportions. Chemically purity of all components is 99.99%. The powders mixture was milled on a vibratory mill during 1 h at 300 rpm (wet milling with the addition of ethyl alcohol) for better homogenization. After that powders mixture was pressed in tablets (in diameter – 7 mm, the height of the tablet – 5 mm). The initial compositions in tablets were exposed to the synthesizing firing in air at 1200 °C (6 h), and then sintered at 1300 °C (6 h). After agglomeration the samples were slowly cooled (∼100 °C h–1). The crystal structure of obtained samples was investigated by x-ray diffraction which is carried out on the powder D8 Advance diffractometer (Bruker) with the following parameters: 40 kV, 40 mA, Cu-Kα radiation (λ = 1.5406 Å). The refinement of x-ray diffraction data was carried out by the Ritveld full-profile analysis [36] by means of the FullProf [37] program. The specific magnetization was investigated by means of universal cryogenic high-field measuring system (Liquid Helium Free High Field Measuring System by Cryogenic Ltd, London, UK) at a temperature of 300 K in external magnetic fields up to 2 T (field magnetization curve) and in the field of 0.86 T in the temperature range of 300–750 K (temperature magnetization curve) [38]. The magnetic measurements were taken on polycrystalline samples with average sizes of 2 × 3 × 5 mm3. The spontaneous magnetization was determined from field magnetization curve by linear extrapolation to the zero field. The ferrimagnet-paramegnet phase transition temperature—the Curie temperature—was defined as an inflection point on temperature magnetization curve. This point is equivalent to a point of a minimum of specific magnetization derivative on temperature (min{dMFC/dT}). At the minimum point of the derivatives the behavior of the temperature magnetization curve changes from ‘curved up’ to ‘curved down’ that corresponds to transition from fast decrease to slow [39]. The measurements of the absorbing properties were taken in the range of 36–56 GHz. The sample was located in a metal wave guide with a cross-section of 5.2 × 2.6 mm2. The measurements were taken by means of the P2-68 chains scalar analyzer. The external magnetic bias field was put parallelly to the vector of electric field wave in a wave guide. The ktr transmission and kref reflection coefficients are defined as: ⎛P ⎞ k tr = 10* lg ⎜ tr ⎟ ; ⎝ P inc ⎠

и

⎛P ⎞ kref = 10* lg ⎜ ref ⎟ ; ⎝ P inc ⎠

(1)

where Pinc—the incident EMR power, Ptr—the transmission EMR power, Pref—the reflection EMR power. The ktr transmission and kref reflection coefficients are negative that indicates the reduction of the transmitted and reflected EMR power in relation to the incident EMR power and they are measured by dB. 2


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Figure 1. Powder x-ray diffraction patterns at T = 300 K for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

3. Results and discussions 3.1. Crystal structure The x-ray diffraction patterns of BaFe12−xGaxO19 (x = 0.1, 0.3, 0.6, 0.9 and 1.2) solid solutions at the room temperature are presented in figure 1. X-ray diffraction data correspond to the single-phase samples with a hexagonal unit cell of crystal structure and P63/mmc space group that will well be agreed with the results received earlier on solid solutions with substitution by Al3+ and In3+ cations [40–42]. The (a and c) unit cell parameters almost linearly and slightly decrease. In figure 2 the results of calculations of dependences concentration of the unit cell parameters for the BaFe12−xGaxO19 solid solutions are presented (x = 0.1, 0.3, 0.6, 0.9 and 1.2). The insignificant reduction of the unit cell parameters and, as a result, the unit cell volume is caused by the insignificant divergence of values of ionic radiuses of Ga3+ (0.62 Å) and Fe3+ (0.64 Å) cations [43]. The isotropic and almost linear reduction of the unit cell parameters is caused by statistical distribution of diamagnetic substitutional Ga3+ cations on different nonequivalent crystallographic positions with octahedral, tetrahedral and bipiramidal anion environment that is consequence of proximity of ionic radiuses of iron and gallium cations. 3.2. Magnetic properties The model of magnetic structure of barium hexaferrite offered by Gorter [44] assumes that the Fe3+ magnetic cations are located in nonequivalent crystallographic positions which have octahedral (Fe1 – 2a, Fe4 – 4fVI and Fe5 – 12 k), tetrahedral (Fe3 – 4fIV) and bipiramidal (Fe2 – 2b) oxygen environment. The substitution of the Fe3 + cations by the diamagnetic Ga3+ cations depending on preference of the positions taken by them can lead to some reduction of the values of the magnetic moments in the corresponding nonequivalent crystallographic positions. Furthermore the magnetic moments of the above called positions initially have the little different values due to different oxygen coordination [45]. According to temperature dependence of specific magnetization of BaFe12−xGaxO19 (x = 0.1, 0.3, 0.6, 0.9 and 1.2) polycrystalline samples given on figure 3 the ferrimagnet—paramagnet phase transition is the phase transition of the second order. At increase of Ga3+ TC 3


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Figure 2. Concentration dependence of the a (+) и c (,) unit cell parameters at the T = 300 K for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

Figure 3. Temperature dependences of the specific magnetization in external magnetic field of H = 8.6 kOe for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions. Insert demonstrates the derivative of the specific magnetization on temperature and Curie temperature of the corresponding samples.

decreases from 646 K for x = 0.1 to 577 K for x = 1.2. These values are lower in comparison for non-substituted BaFe12O19 compound – 740 K. It confirms that substitution of the Fe3+ cations by the diamagnetic Ga3+ cations leads to reduction of the number of magnetic neighbors of the iron cations, and, as a result, to earlier temperature destruction of the long-range magnetic order when heating. At the diamagnetic substitution there is a frustration of the magnetic subsystem of the solid solution due to change of the bond lengths between the magnetic Fe3+ cations and O2− anions with the partial distortion of the valence bond angles that reduces the exchange interaction energy. The behavior of the specific magnetization for the BaFe12−xGaxO19 (x = 0.1, 0.3, 0.6, 0.9 and 1.2) samples from field dependence (figure 4) indicates the decrease of the maximum magnetic energy at the concentration increase of the Ga3+ cations. It is noted the monotonic decrease in residual magnetization and coercive force (figure 5). The lack of anomalies, i.e. the deviations from linear dependence of the magnetic energy decrease at increase of the substitution cations concentration on the temperature and field dependences of specific magnetization evidences in favor of the version of statistical distribution of Ga3+ cations in different nonequivalent crystallographic positions in structure of the M-type barium hexaferrite. 4


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Figure 4. Field dependences of the specific magnetization at the T = 300 K for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

Figure 5. Concentration dependences of the TC (a) Curie temperature, the MS (b) spontaneous specific magnetization and the HC (c) coercive force at the T = 300 K for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

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Figure 6. Frequency dependences of the ktr transmission coefficient at the T = 300 K in zero H = 0 kOe external magnetic bias field for the BaFe12−xGaxO19 (x = 0.1; 0.9 and 1.2) solid solutions (a), in H = 0.7 kOe, 2 kOe and 2.5 kOe external magnetic bias fields for the BaFe12−xGaxO19 (x = 0.1) solid solution (b) and in H = 0.7 kOe, 1.5 kOe and 3.5 kOe external magnetic bias fields for the BaFe12−xGaxO19 (x = 1.2) solid solution (c).

3.3. Microwave properties In work [46] the expression determining the energy value of the EMR absorbed in material is: x abs = 2pf *(e//*E 2 + m //*H 2) ,

(3)

where ε//, μ//—imaginary parts of dielectric and magnetic permeability of material; f—frequency of electromagnetic oscillations. This implies that at interaction of the real materials with the electromagnetic field there are losses both due to the μ// magnetic losses and due to the ε// dielectric losses. The last include the conduction currents and the dielectric hysteresis phenomenon that takes into account by imaginary part of dielectric permeability. In the same work it is shown that absorption of electromagnetic energy in the ferromagnetic materials is generally defined by the magnetic losses—as a result of a NFR and a resonance of domain borders [47]. In figure 6 the EMR transmission spectrums through the samples with different concentration of Ga3+ cations including in different external magnetic bias fields are presented. In the 46–50 GHz frequency range the most intensively absorption of an electromagnetic wave caused by NFR phenomenon is observed. The abscissa value of the global minimum of the EMR transmission spectrum determines the fres resonant frequency of the transmission. The ordinate value of the global minimum of the EMR transmission spectrum determines the Ares resonant amplitude of the transmission. The global minimum width value measured at the Ares/2 half of the resonant amplitude determines the width of the Wres absorption band–band width. From figure 6 it is shown that all these three quantities are sensitive to the substitution level. The power of the transmission radiation decreases almost by 100 times. The external magnetic bias field also changes the amplitude-frequency characteristics of NFR. The samples with the low substitution level are more sensitive to the external magnetic bias field. At the small values of the magnetic bias field there is a shift of the resonant frequency towards major frequencies due to the increase of an effective internal magnetic field of the anisotropy. At that the amplitude of the EMR transmission curve changes a little. However at the BaFe10.8Ga1.2O19 sample magnetic biasing for which the Ga3+ cations concentration is the greatest together with the NFR frequency increase the amplitude of the EMR transmission curve also 6


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Figure 7. Concentration dependences of the amplitude-frequency characteristic of the EMR transmission process: the fres NFR frequency (a), the Ares = ktr (max) NFR amplitude (b) and the Wres NFR band width (c) at the T = 300 K in zero H = 0 kOe external magnetic bias field for the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

increases (see figure 6(c)). The increase of the Ares resonant amplitude occurs up to value of the 1.5 kOe external magnetic bias field, and then the amplitude begins to decrease that corresponds to behavior of imaginary part of magnetic permeability of the hexaferrite in a magnetic field at a ferromagnetic resonance. From figure 7 it is shown that at the increasing of the Ga3+ cations concentration the value of the NFR frequency in the beginning reduces up to x = 0.6 and then its value increases, and at x = 1.2 its value reaches maximum fres = 50.5 GHz (figure 7(a)). The Ares resonant amplitude changes not monotonously. At x = 0.6 the minimum value of the resonant amplitude Ares = −18.5 dB (figure 7(b)) is observed. The width of the Wres absorption band monotonously increases with the increase of substituent cation concentration and at x = 1.2 reaches maximum Wres = 5 GHz (figure 7(c)). In figure 8 the frequency dependence of reflection loss—reflection coefficient—is presented in decibels for the BaFe10.8Ga1.2O19 sample including in different external magnetic bias fields measured in the matched load mode. This dependence has difficult behavior for an explanation. It is possible to note that at the increase of the magnetic bias field the losses increase and at the 5 kOe field the anomalous value of a minimum kref = −33 dB is reached. The reflection decreases more than 3 orders of magnitude. At further increase of the magnetic bias field leads to decrease of resonant curve intensity. At that the frequency of the resonant curve of reflection loss almost does not change. In figure 9 the dependence of the NFR frequency value of the measured samples on the external magnetic bias field value is presented. The increase of the resonant frequency value at the increase of the magnetic bias field is observed since the internal anisotropy field increases. These dependences have almost linear behavior for all the samples. The resonant frequency is most sensitive to external magnetic bias field for the samples with small Ga3+ cations concentration. While for almost all the samples the resonant frequency increases approximately on 1.5 GHz at increase of the external magnetic bias field on 1 kOe for the sample with x = 0.1 the resonant frequency increase on 2.2 GHz. 7


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Figure 8. Frequency dependences of the kref reflection coefficient (reflection loss) at the T = 300 K in H = 0 kOe, 2 kOe and 5 kOe external magnetic bias fields for the BaFe12−xGaxO19 (x = 1.2) solid solution.

Figure 9. Field dependences of the fres NFR frequency at the T = 300 K for the BaFe12−xGaxO19 (x = 0; 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions.

Materials which are effectively absorbing the microwave radiation attract of the greatest interest [48]. The ferromagnetic resonance leads to the energy losses of the electromagnetic field which are result of a number of processes at a cations spin precession for the ferro- or ferrimagnets connected with the additional fluctuations of the crystal lattice sites [49]. The spin precession at the NFR occurs under the influence of local internal magnetic fields thanks to own magnetic anisotropy. The M magnetization vector precession around the easy direction axis so as if he is affected by a magnetic field—the anisotropy field. The physical principles are responsible for the NFR losses the same that are at the induced ferromagnetic resonance. These are the additional fluctuations of the crystal lattice sites of the hexaferrite under the influence of spin waves. That is the interaction of spin waves with a crystal lattice leads to the fact that the part of an external alternating field energy exciting a thermal spin precession, so and a spin waves, turns into thermal fluctuations of a lattice. At that the NFR frequency is defined as the rotation rigidity of the magnetization vector in the plane of easy direction and the rotation rigidity out of this plane [50]. The production of the samples with the certain physical properties necessary for the effective absorption is usually reached by substitution of the Fe3+ cations by the diamagnetic and (or) paramagnetic ions. At the increase of the substitution concentration of the hexaferrite by Sc3+, Ti4++Co2+ and Ti4++Zn2+ cations [51] it has been established the decrease of the saturation magnetization, Curie temperature and magnetocrystalline anisotropy that is connected with reduction of number of magnetic cations. The decrease of the resonant frequency is a consequence of the anisotropy field decrease. However, the insertion of the Al3+ 8


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cations along with reduction of the saturation magnetization and Curie temperature leads to increase of the resonant frequency that is connected with increase of the magnetocrystalline anisotropy field [52]. The increase of the resonant frequency is also noted at the increase of the substitution level of the barium and strontium hexaferrites by Cr3+ cations [53]. The analysis [54] of amplitude-frequency characteristics of absorption losses in the substituted hexaferrites shows a control possibility of the NFR frequency and the frequency shift by concentration change of the x substitution cations. It is proposed [55] to approximate the resonant frequency change versus the Ti3++Co3+ cations concentration at x = 0–3.5 by polynom of the second order in next form: fres (x) = fres (0) + A*x 2 + B*x ,

(4)

where x—the substitution level, fres(x) and fres(0)—resonant frequency for the samples with the x and 0 substitution level, A = −1.9 and B = −4.2. In our case the concentration dependence of the resonant frequency is not linear and also characterized by a minimum at x = 0.6. This dependence can be well approximated by a polynomial of the second order in next form: fres (x ) = 50.04 + 3.37*x 2 - 3.73*x .

(5)

This implies that with the increase of the substitution level up to x ≈ 0.55 the resonant frequency in the beginning decreases, and then it increases. Such concentration behavior is observed at monotonous reduction of magnetic parameters such as the Curie temperature, residual magnetization and coercive force with growth of the Ga3+ cations concentration [56]. Thus it is possible to draw a conclusion that increase of resonant frequency at x 0.6 is caused by increase of the magnetocrystalline anisotropy field which begins to dominate over reduction of magnetic parameters. As the samples have been received at the same time and on identical technology, they have identical morphology of crystallites. It is possible to assume that the domain borders resonance haven’t significant effect on the EMR absorption [57]. The kabs EMR absorption coefficient can be calculated from the experimentally received ktr transmission and kref reflection coefficients taking into account the energy conservation law on a formula: k abs = 10* lg (1 - 100.1 *ktr - 100.1 *k ref ).

(6)

All the coefficients are negative that indicates reduction of EMR energy after interaction with substance. Moreover in practice it is necessary to try to obtain the great negative values of the ktr transmission and kref reflection coefficients to achieve the significant attenuation of the transmitted and the reflected EMR energy. At that the kabs EMR absorption coefficient will be small negative. In our case the absorption coefficient in a zero 0 = -0.09. In the external magnetic bias field of 5 kOe the absorption external magnetic bias field is equal to k abs 5 coefficient increases up to k abs = -0.05. It indicates that almost all the incident EMR energy is absorbed, and in the field of 5 kOe is observed the absorption maximum. Such substances with large negative values of the ktr transmission and kref reflection coefficients and small negative values of the kabs absorption coefficient have a big prospect for creation of a protective and antiradar covering of the air force objects on the 〈〈Stealth〉〉 technology [58].

4. Conclusion The investigations of the crystal structure of the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 and 1.2) solid solutions of the ceramic samples are conducted by x-ray diffraction method in Cu-Kα radiation. All the studied compositions correspond to single-phase samples with hexagonal crystal structure and P63/mmc spatial type of symmetry. Isotropic and almost linear reduction of the unit cell parameters is caused by statistical distribution of diamagnetic substitution cations throughout all the ionic positions (octahedral, tetrahedral and bipyramidal anion surroundings) because of proximity of ionic radiuses of the Fe3+ (0.64 Å) and Ga3+ (0.62 Å). The ferrimagnet-paramagnet phase transition in the BaFe12−xGaxO19 (x = 0.1; 0.3; 0.6; 0.9 и 1.2) samples is the second order phase transition. The temperature of this phase transition at increase of the Ga3+ cations concentration smoothly decreases from 646 K (for x = 0.1) to 577 K (for x = 1.2) that is caused by decreasing of the bonds number of magnetic iron cations with oxygen anions. The magnetization behavior of the samples indicates on decrease of energy of the Fe3+–O2-–Fe3+ exchange interactions at increase in Ga3+ cations concentration. The lack of anomalies on the temperature and field dependences of specific magnetization testifies in favor of statistical distribution of the Ga3+ cations throughout all the sublattices in structure of M-type barium hexagonal ferrite. The realized researches of the absorbing properties have shown noticeable influence of diamagnetic substitution cations on the microwave characteristics of the solid solutions of the barium hexaferrite. At small Ga3+ cations concentrations (x 0.6) the NFR frequency of the samples decreases from 49.6 GHz down to 49.2 GHz and at further increase of the Ga3+ cations concentration (0.6 < x 1.2) the NFR frequency increases up to 50.6 GHz. At that the intensity of resonant curves changes slightly. The 9


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application of the external magnetic bias field considerably shifts the resonant frequency of the samples at small Ga3+ cations concentration. The substitution of the Fe3+ cations by the Ga3+ cations increases the frequency range where there is an intensive absorption of electromagnetic energy that is required for antiradar shields of microwave radiation.

Acknowledgments Present work is realized at joint financing of the Ministry of Education and Science of the Russian Federation on the program of Increase of Competitiveness of NITU ‘MISIS’ among the leading world scientific and educational centers (No. K4-2015-040, No. K3-2016-019), with support and within the state order of the Russian Federation on the organization of scientific work, and with assistance of grants of BRFBR (No. F15D-003 and No. F17D-003) and JINR (No. 04-4-1121-2015/2017). L. Panina acknowledges support under the Russian Federation State contract for organizing a scientific work.

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