Influence of the reagent types on the characteristics of barium hexaferrites prepared by mechanochemical method

Influence of the reagent types on the characteristics of barium hexaferrites prepared by mechanochemical method

Materials Today Communications 21 (2019) 100614 Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsev...

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Materials Today Communications 21 (2019) 100614

Contents lists available at ScienceDirect

Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm

Influence of the reagent types on the characteristics of barium hexaferrites prepared by mechanochemical method

T



V.A. Zhuravleva, , A.A. Nevmyvakab, V.I. Itinb, V.A. Svetlichnyia, I.N. Lapina, D.V. Wagnera a b

Tomsk State University, Tomsk, 634050, Russia Tomsk Scientific Center, SB RAS, Tomsk, 634021, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: Mechanochemical synthesis Hexaferrites Specific magnetization Ferromagnetic resonance Magnetocrystalline anisotropy Anisotropy

The phase composition, structure parameters, morphology and the basic magnetic characteristics of BaFe12O19 hexaferrites obtained by the method of soft mechanochemical synthesis with the following annealing in air for one hour at temperatures in the range from 800 to 1100 °C were studied. The influence of the reagent type on the phase composition and structural characteristics of hexaferrites are analyzed. It has been established that for the synthesis of a single-phase barium hexaferrite a reaction when only water-containing reagents are used is advantageous. In addition, the participation of water in the mechanochemical synthesis leads to an increase in the rate of reagent interaction. The saturation magnetization and the anisotropy fields were determined. The specific saturation magnetization of the sample obtained using the initial hydrous reagents (FeCl3·6H2O and BaCl2·2H2O) is higher than that of the sample obtained using the anhydrous barium chloride (BaCl2). Magnetocrystalline anisotropy of the synthesized materials is investigated by the ferromagnetic resonance method. Along with the high-anisotropic ВаM phase the samples contain the traces of a phase with a small field of magnetocrystalline anisotropy. It is caused by the presence of the X-ray amorphous phase in the samples. Another source of this phase is the superparamagnetic Ba-M hexaferrite nanoparticles.

1. Introduction Hexagonal M-type ferrites possess a wide range of useful physical and chemical properties and are one of the most commonly used materials for permanent magnets, magnetic recording media and radarabsorbing materials and coatings for millimeter and sub millimeter wavelength bandwidths [1–5]. Hexaferrites are used both in the massive and the dispersive state (in magnetoplasts), including the nanoscale range particles. Magnetic properties of nanocrystalline hexaferrites are determined by the rotation of magnetization vectors, while the magnetic properties of large-crystalline hexaferrites are also affected by the displacement of domain boundaries. When hexaferrites are in the nanocrystalline state their principle parameters such as the magnetocrystalline anisotropy fields and the magnitude of saturation magnetization change due to the increasing effect of the defect surface layer of nanoparticles. This provides the control of their magnetic properties by changing the particle size. Thus, the effect of the dispersion time in a high-energy ball mill on the magnetocrystalline anisotropy fields and phase composition of Ba-M hexaferrite powders was studied in [6]. It was demonstrated that the mechanical treatment of the initial hexaferrite powder leads to a



significant decrease in the value of the anisotropy field [6]. It is worth noting that the M-type barium hexaferrites are of great interest for medicine and biology, and promising tumor treatment methods have been developed on their basis [7–10]. At present, in order to obtain nanocrystalline and nanodispersed powders, a mixture of reagents is subjected to an intensive mechanical treatment in the high-energy equipment (planetary mills, attritors, and disintegrators). To regulate the rate of chemical reactions, an inert additive is added to the mixture of initial reagents. It affects the structural and temperature factors of the chemical reaction rate and, as a result, prevents local combustion and the agglomeration of final products and provides the synthesis of nanodispersed and nanocrystalline powders [11,12]. A series of articles is devoted to the study of the structure and magnetic properties of M-type barium hexaferrites obtained by the mechanical grinding of a powder mixture of BaCO3+6Fe2O3 in a hardened steel vial together with steel balls using a Spex 8000 mixer/mill, followed by calcination at temperatures exceeding 750 °C [13–17]. The composition of the synthesized product corresponds to barium hexaferrite and consists of monocrystalline particles with a size close to 100 nm. It has a high coercive force and saturation magnetization

Corresponding author at Permanent address: 17/62, A. Belentsa Str., Tomsk, 634050, Russia. E-mail address: [email protected] (V.A. Zhuravlev).

https://doi.org/10.1016/j.mtcomm.2019.100614 Received 21 August 2019; Accepted 21 August 2019 Available online 22 August 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.

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barium and iron chlorides and analysis of the influence of the type of reagents on the phase composition, structural parameters and the main magnetic characteristics of the synthesis products after calcinations at various temperatures in the range from 800 to 1100 °C

(65–70 emu/g) and in the authors’ opinion can be used to obtain permanent magnets or recording media [13]. After hot pressing in a magnetic field, barium hexaferrite samples have magnetic anisotropy with the easy direction of magnetization being parallel to the direction of pressing. The residual magnetization is 70% of the saturation magnetization and the coercive force and the maximum energy are approximately 5 kOe and 2 MGs*Oe [14–16]. The magnetic viscosity parameter is practically independent of magnetic field strength and corresponds to the activation volume equal to a sphere with a diameter of 25 nm which is close to the width of the domain wall [15]. Substitution in the barium hexaferrite the iron ions by the aluminum ions (BaFe10Al2O19) substantially increases the coercive force (up to 9.3 kOe), which is caused by an increase in the anisotropy field [17]. One of the methods for the Ba-M hexaferrite synthesis is the mechanical grinding of a mixture of hematite (α−Fe2O3) and barium acetate (Ba(CH3COO)2) in a Denver ball mill with further calcination in air, proposed in [18,19]. The study of the structure, phase composition and magnetic properties of the synthesized product showed that the optimal conditions for the mechanical activation of a 5.5Fe2O3+Ba (CH3COO)2 mixture with further calcination were achieved by grinding for 48 h, a Fe/Ba molar ratio of 11, and calcination at 1100 °C for 2 h. As a result, single-phase barium hexaferrite powder BaFe12O19 with an average particle size of about 200 nm and an average crystallite size of 46 nm was obtained. M-type barium hexaferrite powder synthesized using barium acetate as a reagent has high coercivity values (HC = 334.2 kA/m), residual magnetization (Br = 39.7 Am2/kg), and saturation magnetization (MS = 73.9 Am2/kg). For the mechanochemical synthesis (MCS) of complex oxides, in particular, an ultrafine powder of BaFe12O19 hexagonal barium ferrite, the mixture of anhydrous barium chloride (BaCl2) and ferric chloride (FeCl3) with sodium hydroxide (NaOH) is subjected to intense mechanical treatment. In addition, an excess of the anhydrous barium chloride and an inert additive, sodium chloride (NaCl), are added to the mixture [20]. Mechanical grinding forms a product that in the authors’ opinion [20] is most likely to consist of the mixture of sodium hydroxide or oxide-hydroxide. Annealing at 800 °C in air leads to the formation of nanodispersed barium hexaferrite powder (BaFe12O19) from thin plates with the diameter of 20–100 nm and the thickness of 10–20 nm. The coercive force and the specific saturation magnetization are 5.3 kOe and 60 emu/g respectively [20]. It is worth to note that in the mechanochemical synthesis of oxides, anhydrous salts are usually used as reagents. They are solid, so that it increases the level of mechanical loads, the wear of grinding equipment, and the contamination of final product by wear fragments. There is a variant of synthesis that the authors [21] call the soft mechanochemical synthesis (SMCS) [21–23]. It uses solid acids and bases, hydrous oxides, crystalline hydrates, acid and base salts as reagents. As a result, the final product is formed with the participation of water and the solid-phase mechanism of interaction of reagents changes for the mechanism similar to the hydrothermal one, which increases the reaction rate. A reaction is found to be thermodynamically more favorable when one of the products is water of not any, but a certain amount [21]. Reactions of soft mechanochemical synthesis, as well as reactions of mechanochemical synthesis are conventionally divided into two groups: reactions without further heat treatment and with further heat treatment necessary for the formation of final product. The technological advantages of SMCS for the above listed compounds are that as a rule they are cheaper than the anhydrous reagents, and their hardness is several times less. This reduces the wear of the grinding equipment and accordingly the degree of contamination of the final products. The purpose of this study is the mechanochemical synthesis of Ba-M hexaferrite using the various basic reagents: aqueous and anhydrous

2. Materials preparation The following reactions were used to obtain barium hexaferrite powders BaFe12O19: BaCl2·2H2O + 12FeCl3·6H2O + 38NaOH = BaFe12O19 + 38NaCl + 93H2O (sample No 1) BaCl2 + 12FeCl3·6H2O + 38NaOH = BaFe12O19 + 38NaCl + 91H2O (sample No 2) BaCl2 + 12FeCl3 + 38NaOH = BaFe12O19 + 38NaCl + 19H2O (sample No 3) The initial materials for the synthesis were high grade reagents: iron (III) chloride hexahydrate (FeCl3·6H2O) and barium chloride dehydrate (BaCl2·2H2O) purity 99.7%; iron(III) chloride (FeCl3) and barium chloride (BaCl2) purity 97.0%; sodium hydroxide (NaOH) purity 99.0%. Barium chloride was taken in an excess 20 wt. % [20]; hydrous for sample No 1 and anhydrous for samples Nos 2 and 3. The remaining reagents were taken in the stoichiometric ratio according to the chemical reactions presented above. To prevent the agglomeration and the local combustion of the reaction mixture (r.m.) the inert addition (i) – sodium chloride (NaCl purity 97.0%) was added to the reaction mixture in the ratio equal to mr.m.: mi = 1:4. The prepared mixture was sealed in hardened steel drums with steel balls 5 mm in diameter. Barium hexaferrite synthesis was conducted in a high-energy planetary ball mill (APF-type, acceleration 60 g, ZAO NOIC, Novosibirsk, Russia) with the mass ratio of the reaction mixture (p) to the mass of balls (b) mp: mb = 1:10 for τMA =60 min. The obtained product was subjected to heat treatment at 200 °C for 2 h in air and then it was washed by centrifugation (ROTANTA 430 R) with distilled water until complete removal of all salts. The content of impurity chlorine ions was controlled by a qualitative reaction between chloride ions and silver nitrate. Then the products were dried at room temperature of 25 °C (the initial samples) and the structure of hexaferrite was formed by calcining the obtained powders in a muffle furnace in air successively at the temperatures: 800, 900, 1000 and 1100 °C for 1 h for each temperature. The technological scheme for producing barium hexaferrite powders is presented in Fig. 1. 3. Research methods The X-ray phase and the structure analysis of the samples obtained were carried out using a DRON-3 M powder diffractometer (Cu-Kα radiation, NPP “Burevestnik”, Saint Petersburg, Russia). The phase composition and the structural parameters of the phases revealed are determined using a RIGAKU's full-function powder diffraction analysis package, as well as the Powder Cell 2.4 program. Based on the physical diffraction line broadening analysis, the size of coherent-scattering regions (CSR) was calculated by the Sherrer equation. CSR are usually used for a rough estimation of the size of crystallites. The infrared spectra of the final product after heat treatment were obtained using a FTIR spectrometer (NICOLET 5700, Thermo Electron Corporation, Atkinson, USA). Measurements were carried out in the frequency range of 4000–400 cm−1. The Raman spectra of the final product were recorded with the InVia, Renishaw confocal dispersive Raman spectrometer equipped 2

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Fig. 1. The technological scheme for the production of the barium hexaferrite powders by the mechanochemical method.

100 wt. % (Fig. 2a, Table 1). When anhydrous barium chloride is used as the main reagent, after calcination at 1100 °C the sample No 2 contains hematite along with the Ba-M phase (Fig. 2b, Table 1). Anhydrous barium and iron chlorides (sample No 3) used as initial compounds and subjected to 1 -h mechanochemical activation do not form barium hexaferrite (Fig. 2c, Table 1). After calcining the sample at 800 °C, only hematite phase (α-Fe2O3) is observed. Table 2 presents the X-ray diffraction analysis of samples Nos 1 and 2. It can be seen that the lattice constants a and c of synthesized materials are close to the literature data [1,5]. The CSR of the Ba-M phase for samples Nos 1 and 2 after calcination at 800 °C is 31 nm and 35 nm and increases to 50 nm and 63 nm after calcination at 1100 °C. It can be assumed that the synthesized M-type barium hexaferrite is nanocrystalline at all temperatures of heat treatment. The noticeable difference in the phase composition of the products of mechanochemical synthesis identifies a significant role of the water that is present in the system for the samples prepared with the participation of initial hydrous or anhydrous reagents (samples Nos 1–3). As is known from literature data [21], the use of hydrous compounds for mechanochemical synthesis simplifies the formation of nuclei and accelerates diffusion transport. As a result, the faster growth of the solid phase is observed. In addition, water released during neutralization reaction can be an additional source for the acceleration of reactions as compared to conventional solid-phase synthesis. In this case conditions are created for the development of the hydrothermal process, in which water acts as a solvent. This explains the fact that during the interaction of two anhydrous compounds subjected to mechanochemical activation for an hour, the precursor of the Ba-M product was not synthesized (Fig. 2c, Table 1, sample No 3), since the medium favorable for ion exchange was not formed. It is important to note that the use of hydrous compounds for soft mechanochemical synthesis not only accelerates the reaction, but also allows the final product to be obtained at relatively lower temperatures. Thus, calcination at 800 °C sharply increases the content of the barium hexaferrite phase (Table 1). For calcination above this temperature, a slight increase in the Ba-M content is observed. Table 1 shows that with increasing the calcination temperature, the amount of the Ba-M phase in sample No 1 increases faster than in sample No 2. As a result, after annealing at 1100 °C, the content of the Ba-M phase is 100 wt. %. It can be assumed that this is related to the presence of the large amount of water in the initial reaction medium for sample No 1 as compared to sample No 2, which leads to the acceleration of mass transfer. A wide diffuse maximum on X-ray patterns of the samples Nos 1 and 2 is observed at the low angles (10–20°) range. It is observed both in the initial samples and in the samples after annealing at temperatures of 800–1100 °C. This indicates the presence in the samples of a small amount of the substance in an X-ray amorphous state (see inset on

with the Leica microscope. The excitation was conducted by continuous-wave semiconductor laser with the wave length of 785 nm and the maximum power up to 100 mWt. The diffraction lattice of 1200 line/mm provided the spectral resolution of 1 cm−1 and the microscope with the 50x lens provided the excitation locality to 2 μm 2. The particle size and morphology of calcined powders were characterized by SЕM (TESCAN, VEGA3 SBH). The particle size distribution was estimated using sample group of not less than 600 particles. The specific surface (S) of the samples was measured by the thermal desorption of nitrogen using the 4-point BET method on a TriStar 3020 analyzer (Micromeretics, USA). Before measurements, the samples were degassed in the He stream for 2 h at 200 °C. The values of the specific saturation magnetization are determined from the study of the magnetization curves in pulsed magnetic fields up to 30 kOe. The measurements were carried out according to the method described in [24]. Investigations of the ferromagnetic resonance (FMR) spectra of powder ferrimagnets with a hexagonal crystal structure are the unique way to determine the magnitude of magnetic anisotropy fields [25]. The standard transmission technique with a rectangular waveguide channel in the frequency range of 37–53 GHz was used to investigate FMR. The powder samples were placed in the thin-walled quartz tubes with an inner diameter of 0.7 mm and a length of ˜ 10 mm. The densities of the samples were identical. The tubes were located parallel to the wide wall of the waveguide. The alternating magnetic field was directed along the sample axis. The magnetizing field was directed perpendicularly to the wide wall of the waveguide. Method of treatment of experimental data to determine anisotropy fields from FMR measurements is described in details in [25]. All magnetic measurements were carried out at room temperature.

4. Results and discussion 4.1. The results of X-ray research Fig. 2 and Tables 1 and 2 present the X-ray phase and diffraction analysis of the products obtained by mechanochemical activation, followed by calcination at different temperatures. The prepared with the participation of the hydrous reagents initial samples Nos 1 and 2, along with the phase of M-type barium hexaferrite BaFe12O19 contain the large amount of hematite (α-Fe2O3) and magnetite (Fe3O4) (or possibly maghemite (γ-Fe2O3)). After calcination at 800 °C, magnetite reflexes are not observed on the X-ray diffraction patterns of samples Nos 1 and 2. Further calcination at 900, 1000, and 1100 °C increases the content of the BaM phase and decreases the amount of hematite in these samples. Using hydrous reagents (sample No. 1), mechanochemical activation and calcination at 1100 °C, the content of the phase BaFe12O19 is 3

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Fig. 2. XRD patterns of the samples after mechanochemical synthesis and successive heat treatments in air for an hour: No 1 – a, No 2 – b, No 3 – c. X-ray reflections from Ba-M, Fe3O4 and α-Fe2O3 phases are labeled with different symbols. Vertical dashed lines denote the position of reflexes of the Ba-M phase. Table 1 XRD analysis of the samples after mechanochemical synthesis and heat treatment in air for an hour. Phase content, wt. % Phase

Heat treatment, °C initial

Sample No 1 BaCl2·2H2O + FeCl3·6H2O BaFe12O19 27.5 α-Fe2O3 45.9 Fe3O4 26.6 Sample No 2 BaCl2 + FeCl3·6H2O BaFe12O19 19.7 α-Fe2O3 59.4 Fe3O4 20.9 Sample No 3 BaCl2 + 12FeCl3 BaFe12O19 – α-Fe2O3 60.4 Fe3O4 39.6

800

900

1000

1100

89.9 10.1 –

96.9 3.1 –

97.8 2.2 –

100 – –

79.8 20.2 –

93.4 6.6 –

93.9 6.1 –

94.6 5.4 –







– 100 –

Fig. 3. Raman spectra of the samples Nos 1, 2. Table 3 Assignment of the main Raman-active modes in Ba-M powders obtained by mechanochemical synthesis and heat treatment in air at 1100 °C for an hour.

Table 2 XRD results of the samples Nos 1, 2 after mechanochemical synthesis and heat treatment in air at 800 and 1100 °C for an hour. Temperature, °C

Sample No 1 800 1100 Sample No 2 800 1100

Phase content BaFe12O19, mass. %

Lattice constant, Ă

Wave number, [cm−1]

CSR, nm

a

c

89.9 100

5.899 5.894

23.313 23.313

31 50

79.8 94.6

5.896 5.899

23.313 23.313

35 63

4

[26,27]

[28]

Samples Nos 1, 2

Symmetry

Assignment

173 184 212 250 285 340 385 409 467 529 614 684 713

174 186 – – – 339 – 412 470 – 618 686 721

170 179 208 244 282 332 378 410 464 521 612 682 713

E1g E1g E1g E1g E1g A1g E2g A1g A1g E2g A1g A1g A1g

Whole spinel block Whole spinel block

Octahedron (mixed) Fe(5)O6 octahedra dominated Octahedra Fe(1)O6 and Fe(5)O6 Octahedra Fe(4)O6 Bipyramid Fe(2)O5 Tetrahedra Fe(3)O4

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Fig. 2a).

Table 4 Characteristic FTIR vibrations of BaFe12O19 powders obtained by mechanochemical synthesis and heat treatment in air at 1100 °C for an hour.

4.2. Raman spectra investigation

IR bands (cm−1)

Raman spectra of the final product after heat treatment in air at 1100 °C are given in Fig. 3 and in Table 3. The articles by Kreisel et all. [26,27] were used for their interpretation. These articles report that 42 active Raman modes (11А1g+14E1g+17E2g) and 30 active infrared modes (13A2u+17E1u) can be expected for the Ba-M structure with a D6h symmetry at room temperature. In addition, the position of Raman modes of thin monocrystalline films and powders is almost similar [28]. If to compare the Raman spectra of the products synthesized in the present investigation with those one in the articles [26–28], it becomes clear that they are almost the same and the final product is hexaferrite BaFe12O19. The mode at 713 cm−1 belongs to the tetrahedron positions of (3) Fe O4. The mode at 682 cm−1 is related to bipyramid Fe(2)O5 and the mode at 612 cm−1 belongs to the octahedron positions of Fe(4)O6. All these modes have the А1g symmetry. The mode at 464 cm−1 belongs to the mixed octahedron positions of Fe(1)O6 and Fe(5)O6 with the А1g symmetry, as well as the modes at 410 cm−1 и 332 cm−1. The bands at 170 cm−1 and 179 cm−1 are the modes of a whole spinel block with the Е1g symmetry. If to compare the Raman spectra of products obtained with the use of different reagents and heat treatment, it becomes clear that the position and intensity of active modes are close in value (Fig. 3, Table 3). The assessment of the Raman modes width on their half-height shows that this value is almost the same for all final products. Thus, the sizes of the particles obtained for different samples do not differ significantly. This fact is proven by the CSR values from the XRD diffraction analysis (see Table 2).

Samples No 1 3421 1644 1450 1017 890 817 593 548 448

Characteristic vibrations No 2 3432 1629 1437 1022 889 842 602 546 436

ν (O–H) δ (FeO–OH) δ (FeO–OH) ν (CO32−) δ (–OH) δ (–OH) ν (Fe–O), ν (Ba–O) ν (Fe–O), ν (Ba–O) ν (Fe–O), ν (Ba–O)

during the synthesis of barium ferrite proves the participation of crystallization water of initial compounds in the reactions of mild mechanochemical synthesis. Iron monohydrate is formed in the presence of chlorine anions released during the decomposition of chlorides and crystallization water according to the reaction: FeCl3 + 2H2O = FeOOH + 3HCl and is able to participate in the formation of ferrite. The samples calcined at 1100 °C shows three absorption bands in the frequency range of 600 – 420 cm–1 (Fig. 4, Table 5), corresponding to the valence vibrations of Fe – O and Ba – O. This indicates the formation of the phase of hexaferrite BaM. 4.4. SEM-pictures and bar charts of particle size distribution SEM-pictures of the final products obtained by mechanochemical synthesis and final calcination at 1100 °C are shown in Fig. 5. The morphology of particles synthesized with the participation of both hydrous and anhydrous chlorides as main reagents can be characterized as follows. The samples contain particles in the form of hexagonal plates with an aspect ratio equal to about 10, which is typical for ferrites with a hexagonal crystal structure. However, there are also aggregates and small quasi-spherical grains. The number of faceted particles is relatively small. The final product is aggregated and is the thin plates bonded to each other through annealing. Comparison of the particle size distribution bar charts given in Fig. 5 shows that they are similar even changing the basic reagents. The overwhelming majority of particles and aggregates are in the size range from 100 to 800 nm. Thus, the particle size distribution is quite wide, and the difference in the type of main reagents (hydrous or anhydrous barium chloride) does not significantly affect the morphology of the particles, which is mainly due to the high calcination temperature.

4.3. FTIR spectra Comparison of the infrared spectra of the final products synthesized using different initial reaction mixtures (Fig. 4, Table 4) shows that their IR spectra are similar. Initial hydrous reagents form iron monohydrate on the surface of barium ferrite. Thus a broad peak with a small shoulder is observed in both spectra in the frequency range of 3000–3600 cm–1. It can be related to the absorption bands of both free hydroxyl groups and −OH groups involved in different hydrogen bonds, as well as to the valence vibrations of the −OH group in αFeOOH iron monohydrate (goethite) [29]. The narrow absorption peak in the range of 1600–1650 cm−1 and the doublet of lines in the range of 800–900 cm–1 are due to the deformation vibrations of the −OH group in the FeO −OH molecule. The formation of α-FeOOH monohydrate

4.5. BET investigation The specific surface (S) of the samples is given in Table 5. As can be seen, the specific surface area of barium hexaferrite powders after calcination at 1100 °C is 1.50–1.55 m2/g. The total pore volume is 8*10−3 cm3/g and their average sizes are 21–23 nm. Hysteresis is observed on the curve of adsorption-desorption of nitrogen in the relative pressure range of 0.8-0.9. Its presence indicates the mesoporous structure of the sample and the formation of large mesopores. However, the minimum loop area indicates a small amount of such pores in the sample, which is confirmed by the data on the total volume of pores. In addition, the average surface diameter of particles calculated from BET data is 2.5 times larger than the average particle size obtained from electron microscopy data (Fig. 5). This may be due to the fact that the particles partially melted during annealing as can be seen from the SEM data.

Fig. 4. FTIR spectra of the samples Nos 1, 2 and goethite (curve 3). 5

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Table 5 Estimation of the parameters of the porous structure and specific surface of the samples after mechanochemical synthesis and heat treatment in air at 1100 °C for an hour. Sample

S, m2/g

Total volume of pores, cm3/g

Average size of pores, nm

Average surface diameter of particles, nm (for specific density of 5.28 g/cm3)

No 1 No 2

1.55 1.50

0.008 0.009

21 23

734 759

4.6. Magnetization curve investigation Fig. 6 shows the field dependences of the specific magnetization of the samples Nos 1, 2 measured as the pulsed magnetizing field decreases from a maximum value 30 kOe. The measurements were carried out at room temperature. According to Fig. 6, the specific magnetization of sample No 1 is larger than that of sample 2. This can be explained by the presence of the impurity phase of hematite α-Fe2O3 in the sample No 2. Hematite is a weak ferromagnetic above the Morin temperature TM ≈ 250 K [30] and has a significantly lower magnetization value then hexaferrite BaM. The magnetization curves at values of magnetizing fields ˜ 30 kOe almost reach saturation. Therefore, the values of the specific saturation magnetization (σS) of the samples can be estimated by extrapolating the dependences σ (H−1) to the zero value of the argument H−1. The magnetization of the sample No 1 is σS1 = 64.8 Gs·cm3/g, for sample No 2 it is equal σS2 = 59.9 Gs·cm3/g. According to Table 2, the content of the Ba-M phase in the sample No 2 is 94.6%. If we multiply σS1 by this concentration, then we obtain the value 61.3 Gs·cm3/g, which is close to the measured value of σS2. Note that these values of σS of the Ba-M powders are less than the specific magnetization of a massive polycrystalline material 72 Gs·cm3/g [1,5]. However, they are close to the magnitudes of the magnetization synthesized by the sol-gel combusting Ba-M powders [25]. The values of CSR in work [25] are close to the CSR of our samples. Note that the method of studying the magnetization curves in pulsed fields used by us does not make it possible to estimate the magnitude of the residual magnetization (σr) and the coercive force (HC). But, the coercive force value is a characteristic of a specific sample, but not a

Fig. 6. Magnetization curves of the samples Nos 1, 2.

material. A characteristic of the material is the magnitude of the magnetocrystalline anisotropy field, which we measured by the method of ferromagnetic resonance.

4.7. FMR spectra investigation Non-oriented hexaferrite powders are a macroscopically isotropic medium. However, their FMR curves differ significantly from the FMR curves of the isotropic media and of the single-crystal samples. Firstly, they have a substantially greater line width and, secondly, there are additional features: the maximums and the steps at the resonance curves [31]. The maximums and the steps on the FMR curves of

Fig. 5. SEM pictures and bar charts of particle size distribution of final product. a – sample No 1, b – sample No 2. 6

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hexaferrites are observed near the magnetizing field’s values H|| and H⊥ corresponding to the maximum or minimum on the angular dependence of the resonant field. The values of these resonance fields (or frequencies) are determined by the formulas [32]:

′ ], ω⊥ = γ⊥ [H⊥ (H⊥ − Hθ )]1/2 . ω∥ = γ∥ [H∥ + (γ⊥ γ∥) Ha1

Table 6 Parameters of the calculated resonance curves. Sample

(1)

No 1

Here ω∥, γ∥ and ω⊥, γ⊥ are the resonant frequencies and magnetomechanical ratio’s for the directions along the hexagonal axis c and in ′ , Hθ , are the fields of magnetic anithe basal plane, respectively; Ha1 sotropy. These fields include the contributions from the magnetocrystalline anisotropy (MCA) and shape anisotropy of the crystallites:

Hθ = Ha′1 + Ha2 + Ha3, Ha′1 = Ha1 + 4πMS [N⊥ −

(γ⊥ γ∥)2N∥].

No 2

Phase content, vol. %

98.3 % BaM 1.7 % LA 98.9 % BaM 1.1 % LA

γ/2π, GHz/ kOe

2.81 2.77 2.81 2.77

H ′a1, kOe

15.4 0.2 15.7 0.2

MS, Gs

342 316

α, dim-less unit 53 GHz

49 GHz

0.08 0.11 0.08 0.15

0.11 0.13 0.11 0.15

corresponds to the direction of easy magnetization. That is why only the ′ and of effective magnetomechanical ratio values of anisotropy field Ha1 γ= γ∥ were evaluated from the experiment. Therefore it was assumed that Hθ = Ha′1 and γ⊥ = γ for the Ba-M phase when calculating theoretical curves. From Fig. 7 it is seen that in the range of magnetizing fields of 16–20 kOe on FMR curves an additional low intensity resonance is observed. It is more noticeable for the sample No 1. According to [6,33,34], it can be interpreted as FMR of the additional phases with a low value of MCA field. Such an additional phase in samples Nos 1, 2 is the X-ray amorphous phase according to the data of X-ray phase analysis (see Fig. 2). In addition to this phase, small nanosized particles of the target phase of BaM can contribute to this resonance if they are in a superparamagnetic state. Table 6 shows the parameters of the samples for which the theoretical curves presented in Fig. 7 were calculated. The second column shows the contents in samples Nos 1 and 2 phase of the Ba-M and the low-anisotropic (LA) phase in the volume percent, obtained from a comparison of the theoretical partial FMR curves with the experimental ones. The values of the saturation magnetization of unit volume MS that is necessary to calculate the components of the permeability tensor was determined in accordance with the formula MS = σS·ρ. Here, ρ =5.28 g/cm3 is the X-ray density of Ba-M hexaferrite [1]. MS was considered the same for high and low-anisotropic phases in our

(2)

In the formula (2) Hai = 2iki/MS is the fields of MCA, N⊥, N∥ are the transverse and the longitudinal demagnetization factors of the grain, and 2N⊥ + N∥ = 1. Here MS is the saturation magnetization per volume unit. Thus, the study of the frequency dependences of the maximums and steps on FMR curves can estimate the values of γ∥, γ⊥ and obtain esti′ and Hθ . Further, by a demates for the values of anisotropy fields Ha1 tailed comparison of the form of calculated and experimental resonance ′ , Hθ . The curves one can clarify the values of the anisotropy fields Ha1 procedure for determining these parameters using the FMR experiments for uniaxial powder hexaferrite is described in detail in the work [25]. This technique was applied to the analysis of FMR in the samples containing along with a highly anisotropic hexagonal phase, a phase with a small magnitude of MCA in papers [6,33,34]. The experimental (dots) FMR curves of the samples Nos 1, 2 and calculated in the independent grains approach imaginary part of the diagonal component of the permeability tensor (line) at frequencies of 49 and 53 GHz are shown in Fig. 7. The experimental curves were normalized to the theoretical ones. In the available for us range of magnetizing fields up to ˜ 23 kOe experimentally only one maximum may be observed in the field close to H|| (see the formula 1) which

Fig. 7. Experimental (dots) and the calculated (line) FMR curves of the sample No 1: Fig. 6 a frequency is 49 GHz, Fig. 6 b - frequency is 53 GHz and sample No 2: Fig. 6 c - frequency is 49 GHz, Fig. 6 d - frequency is 53 GHz. Curve 1 is the calculation for the Ba-M phase, 2 is the calculation for the low anisotropic phase, 3 is the total resonance curve.

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References

calculations. The magnetomechanical ratios of all phase are close in the experimental error (± 0.02 GHz/kOe) range of with the magnetomechanical ratio for the spin of a free electron 2.8 GHz/kOe. The similar values of γ/2π were obtained for Ba-M hexaferrite powders obtained by sol-gel combustion in the article [25]. ′ of the sample No 1 has a smaller value than The anisotropy field Ha1 the anisotropy field of the sample No 2. However, taking into account the estimation error ± 0.1 kOe, this difference is not very big. Note that the obtained values of the anisotropy fields are lower than the value Ha1 ≈ 17 kOe for bulk samples of hexaferrite Ba-M [1,5]. There are two possible reasons for this. The first is the influence of the contribution from the anisotropy of the nanoparticles shape (see formula (2) for the ′ and the estimation carried out in the work [35]). The second reason Ha1 is the contribution to the MKA of the grain from the defective surface layer of the nanoparticle [25,35,36]. The last columns of the Table 6 present the damping constant α in the Landau–Lifshitz–Gilbert equation. It should be noted that the constant α in the case under consideration is not the parameter related to the individual monocrystalline grain. It is influenced by the grain shape distribution, the distribution of the values of the MCA fields in different grains and the presence of nanostructured agglomerates consisting of particles with different orientation of the crystallographic axes. From the table one can see that the damping constants of the Ba-M phase for both samples are the same. This suggests that the morphology, structure and composition of the samples Nos 1, 2 after annealing at 1100 °C for one hour is close. The magnitudes of the anisotropy fields and the damping constants of the phases with low anisotropy in Table 6 are approximate, since the intensities of the corresponding FMR curves are small.

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5. Conclusion Thus, the studies have shown: 1 Nanocrystalline M-type barium hexaferrite powders with a particle size in the range of 100–800 nm and a crystallite size of about 50 nm were synthesized. 2 It was found that the use of aqueous chlorides instead of anhydrous ones in the mechanochemical synthesis of the BaFe12O19 hexaferrite leads to an increase in the reaction rate of the reactants and to obtain the target product at lower calcinations temperatures. This is due to the participation in the chemical reactions of water and, possibly, another liquid phase formed during the melting of reagents during mechanical activation. 3 The specific saturation magnetization of sample No 1 obtained using initial hydrous reagents was higher than that of sample No 2 obtained with anhydrous FeCl3, apparently due to the presence of weakly magnetic hematite phase in it. 4 The magnitudes of the magnetocrystalline anisotropy fields of the samples after annealing at 1100 °C were close; the magnetomechanical ratios were the same and coincided with those for spin of a free electron within the experimental accuracy. 5 According to ferromagnetic resonance data, the samples contained the traces of a phase with a small magnetocrystalline anisotropy field. This phase consisted of an X-ray amorphous phase and (or) superparamagnetic nanoparticles of the Ba-M phase.

Acknowledgments This Research is supported by the Tomsk State University Competitiveness Improvement Program and the State task for the Tomsk Scientific Center, Siberian Branch, Russian Academy of Sciences, project No 0365-2019-0004. 8

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xCuxFe12O22 studied by the ferromagnetic resonance method, IOP Conf. Series: Materials Science and Engineering 479 (2019) 012073. [35] E.P. Naiden, V.A. Zhuravlev, R.V. Minin, V.I. Itin, E.Yu. Korovin, Static and dynamic magnetic properties of nanosized barium hexaferrite powders prepared by the Sol-Gel combustion method, Russ. Phys. J. 58 (2015) 125–132.

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