Structure parameters and magnetic properties of nanosized strontium hexaferrite prepared by the sol-gel combustion method

The phase composition, structure parameters, and main magnetic characteristics of SrFe12O19 hexaferrite prepared by the sol-gel combustion method in combination with subsequent annealing at the temperature of 850°C for 6 hours are investigated. From the study of the magnetization curves in the pulsed magnetic fields the value of saturation magnetization of synthesized material was determined. The value of the magnetocrystalline anisotropy field was determined from the survey of the ferromagnetic resonance.


Introduction
Hexagonal ferrite of M-type SrFe 12 O 19 is characterized by a large coercive force, rather large value of the saturation magnetization, high Curie temperature, large magnetocrystalline anisotropy, high electric resistance, high chemical stability and corrosion resistance. Due to the qualities listed and its low cost the ferrite is used in different spheres of science and technology as permanent magnets and bonded magnets, in various microwave devices (circulators, isolator, phase shifters, filters) and in magnetic and magneto-optic recorders of information with high density [1]. For the reasons listed above the materials which possess the controllable size and form of particles, narrow distribution of particles according to the size and with given magnetic characteristics are needed.
The traditional ceramic method of strontium hexaferrite synthesis is burning the mixture of SrCО 3 and Fe 2 O 3 powders at a high temperature (> 1200 °C). The disadvantages of this method are high power consumption, uncontrolled morphology of particles, chemical heterogeneity and big size of particles and their agglomerates. Grinding of synthesized strontium hexaferrite powder leads to admixtures, elastic stresses and worsening of magnetic properties [2,3].
To control the size and form of the particles as well as the magnetic properties of materials new energy-saving methods have been suggested. Such as sol-gel method and its modifications [4,5], pyrolysis of aerosol, coprecipitation, hydrothermal method, SHS and etc. The full description of these methods can be found in the review [1]. As a rule, using these methods the nanosized and nanostructured particles are obtained which possess the properties different from the properties of massive polycrystalline materials due to the significant part of atoms present in the defective surface layer. By means of additional annealing one can change the size of particles, their morphology and the magnetic properties. In the present work, the sol-gel combustion method is used to prepare nanosized strontium hexaferrite powders of M-type with the SrFe 12 O 19 composition. The method is based on the preparation of initial colloid nanosystem that can react in the combustion mode. This method is the most suitable for the preparation of powder materials with good stoichiometric control, narrow particle size distribution and it requires relatively short processing time at lower temperatures [6]. Results of investigations the phase composition, structural parameters, and magnetic properties of the synthesized materials are presented.
Research of the magnetocrystalline anisotropy of the samples was carried out by the method of ferromagnetic resonance (FMR). FMR is the only method for the study of the magnetocrystalline anisotropy of the multi-phase powder samples consisting of a phases with significantly different values of the anisotropy fields [7]. The concentrated solution of NH 4 OH ammonium hydroxide was added dropwise to the prepared mixture with constant stirring until pH of the solution became equal to 7. An HI98103 checker portable digital tester (HANNA Instruments) was used to measure pH of the solution mixture. The prepared sol was heated in an ES-6120 magnetic stirrer (EKROS) at temperatures from 80 to 90°С for 3-5 h. As a result of heating, the sol was transformed into a viscous gel of brown color which foamed. Further heating of the gel foam to temperatures 150-160°С caused its ignition and combustion that lasted for several minutes. After combustion, a friable powder was obtained which was easily grinded in a mortar converting into a fine powder. The SrFe 12 O 19 hexaferrite nanoparticles were finally formed by annealing of the powder at the temperature of 850°С for 6 h.

The research techniques
The X-ray diffraction analysis was performed with a SHIMADZU XRD-6000 polycrystalline diffractometer in the Bragg-Brentano geometry with a focusing pyrographite crystal-monochromator inserted into the secondary gamma-quantum beam. For qualitative analysis of the phase composition, the РDF4+ computer database of x-ray powder diffractometry of the International Center of Diffraction Data (ICDD, Denver, USA) was used. The quantitative analysis of the phase composition and the refinement of the structural parameters of the detected phases were performed using the Powder Cell 2.4 program of full profile analysis. The infrared spectrum was registered with a Nicolet 5700 infrared Fourier spectrometer (France) equipped with an attachment of diffuse reflection.
Magnetic measurements involved investigations of magnetization curves in pulsed magnetic fields up to 30 kOe using a setup described in [8] and measurements of ferromagnetic resonance (FMR) spectra by the standard waveguide passing through technique at frequencies in the range 37-53 GHz with an automated radio spectroscope. To investigate the FMR, powders of the examined samples were put in thin-walled quartz tubes with inner diameter of 0.7 mm and length of 10 mm. The tubes were arranged into a rectangular waveguide parallel to the wide wall of the waveguide in order that the alternating magnetic field was directed along the sample axis. A constant magnetizing field was directed perpendicularly to the wide wall of the waveguide.

The results of x-ray researches
The X-ray diffraction patterns of the synthesis product after ferritization at 850 °С for 6 hours are shown in Fig. 1. It can be seen that it contains basically diffraction peaks corresponding to SrFe 12 O 19 hexaferrite. The traces of accompanying hematite phase (α-Fe 2 O 3 ) are also present.  Results of the x-ray diffraction analysis of the sample are given in Table 1. The content of the Sr-M main phase exceeded 99% and hematite α-Fe 2 O 3 was the additional phase. The average crystallite sizes (~ 100 nm) were estimated based on an analysis of the physical broadening of diffraction lines from sizes of coherent scattering region (CSR). In the last column of the Table 1 shown the relative change in the interplanar distances Δd/d. The estimates of internal elastic microstrain σ = E(Δd/d) can be carried out by that. Here E -Young's modulus.

FTIR spectrum of the sample
The Figure 2 shows the infrared spectrum of transmission coefficient of the obtained hexaferrite powder after annealing at T = 850 °C for 6 hours. At frequencies in the range 3500-3300 cm -1 , a very weak absorption band caused by strain-induced vibrations of O-H groups is observed, wide in comparison with other bands. It corresponds with a weak absorption band at 1625 cm -1 caused by H-O-H vibrations (bending vibrations of water molecules). The bands at 1484 -1448 cm -1 and 860 cm -1 are due to the С-O vibrations due to the presence of lemon acid residues in the water. The band localized at 909 cm -1 is probably attributed to the impurity phase α-Fe 2 O 3 which corresponds with the data of X-ray diffraction analysis ( Table 1). The bands observed at 433, 549 and 591cm -1 can be attributed to valence vibrations metal-oxygen of strontium hexaferrite. Similar results were obtained in [4] for strontium hexaferrite, synthesized by sol-gel combustion method with the usage of egg white.
Thus, the X-ray diffraction and spectrometric data given above prove that the synthesized material is hexaferrite SrFe 12 O 19 .

Research of the magnetization curves
A study of the magnetization curves of SrFe 12 O 19 hexaferrite powder was conducted at the room temperature in the pulsed magnetic field by the method described in [8]. The results are presented in the  The values of the specific residual magnetization (σ r ) and the specific saturation magnetization (σ s ) are presented in the first row of the Table 2. In the last column of the table the saturation magnetization per unit volume calculated according to the formula M S = σ S •ρ are shown. Here ρ = 5.11 g/сm 3 is the density of strontium hexaferrite [1]. In the subsequent rows of the table for comparison are presented the results obtained in other studies. In the paper [4] where the egg white was used as a fuel the value of σ s measured in the field ~10 kOe is close to the one obtained in this study though the sizes of the particles differ significantly due to the different temperature of annealing. In the paper [5] the modified sol-gel synthesis is used and the hexaferrite particles were formed and annealed in the NaCl matrix. So a big value for the specific saturation magnetization was obtained. The data from the review [1] are given in the line 4 of the table. It is clear that the above values of σ s are typical for strontium hexaferrites that synthesized by the sol-gel methods.

The study of the magnetic anisotropy by the FMR method
The study of ferromagnetic resonance spectrum for powder and polycrystalline hexaferrites gives an opportunity to experimentally determine the values and the sign of the fields of magnetocrystalline anisotropy (H ai ) as well as the value of the gyromagnetic ratio γ = ge/2mc, where g is the effective gfactor of the examined material, e is the electron charge, m is the electron mass, and c is the velocity of light. It has to be noted that in hexaferrites the γ parameter can be anisotropic [9]. The magnitudes and signs of the anisotropy fields H ai define the type of magnetic ordering -the presence of either an easy magnetization plane (EMP) or an easy magnetization axis (EMA). The method described in [9,10] was used to calculate the resonance curves of FMR or components of permeability tensor of the single-domain polycrystalline and powder ferrimagnets with hexagonal crystal structure in the approach of non-interacting grains. The calculations showed:  The shape of FMR curves makes it possible to define what type of anisotropy the material has: an EMA or an EMP. 4 XII International Conference Radiation-thermal Effects and Processes in Inorganic Materials IOP Publishing IOP Conf. Series: Materials Science and Engineering 168 (2017) 012082 doi:10.1088/1757-899X/168/1/012082  The resonance curves or field dependences of the imaginary parts of components the permeability tensor have two features: maximums and (or) steps. The low-field feature is caused by the resonance of crystallites in which the direction of the magnetic field ( H ) is close to the direction of easy magnetization. The high-field feature corresponds to the resonance of the crystallites in which the field is oriented near the hard magnetization directions. The features on the FMR curves are observed close to the values of the magnetizing fields which correspond to the stationary directions on the angular dependence of resonance field: H and  H . The stationary directions are the maxima and minima on an angular dependence of the resonance field of monocrystalline grains. The values of these resonance fields (or frequencies) are determined by the formulas [10]: Here ,  and ,   are resonance frequencies and gyromagnetic ratios for directions along the hexagonal axis с of the crystalline lattice and in the base plane, 1 ' a H ,  H are the magnetocrystalline anisotropy fields for these directions. These fields include contributions from magnetocrystalline anisotropy and anisotropy of the shape of crystallite: In the formulas (2) H ai = 2ik i /M S are the fields of magnetocrystalline anisotropy (i = 1, 2, 3); ,   NN are demagnetizing factors of the particle that has a form of ellipsoid of revolution, 1 2NN    . Figure 4 shows the experimental resonance curves (points) of the sample Sr-M, measured in three frequencies. In the range of magnetizing fields to 23 kOe one maximum is observed in the field close to H , which corresponds with the direction of EMA. With the decrease of frequency the maxima shift to the smaller fields. The increase of losses in zero fields with the decrease of frequency is defined by the approach of the high-frequency magnetic field to the frequency of natural ferromagnetic resonance which is defined by the formula:   The processing of experimental FMR spectra was carried out in two stages. In the first stage, the frequency dependences of the resonance fields were drawn. Then the estimations for magnetomechanical ratios  and anisotropy fields ( curves at different frequencies. In the resonance curves calculations the gyromagnetic ratio was considered to be isotropic  =   = γ, and the value of anisotropy field 1 ' a HH   . The calculated imaginary parts of the diagonal components of the permeability tensor are represented by lines in Figure 4. The calculated curves in the figure are shown in a wider range of magnetizing field than the experimental points. This is done in order to show a presence another peak near the field corresponding to the direction of hard magnetization (  H ). The resonance curves were calculated for values of γ, 1 ' a H , M S and α, shown in the first row of Table 3. Here, α is a decay constant of monocrystalline grains in the Landau-Lifshitz-Gilbert equation of motion. The second row of Table 3 shows the characteristics of barium hexaferrite BaFe 12 O 19 . This sample was synthesized in [11] with the same procedure as in our work. According to the table, the particle size and the magnitude of the gyromagnetic ratio of strontium and barium hexaferrites of M-type differ slightly. The anisotropy field of hexaferrite Sr-M almost two kOe greater than anisotropy field of the BaM. This is consistent with the data for bulk materials [1]. Therefore, the ratio of M r /M S in SrM also higher, indicating the higher value of the coercive force.

Conclusion
Thus, the technology of synthesis of Sr-M hexaferrite by sol-gel combustion produces materials with magnetic properties close to the properties of the materials synthesized by conventional ceramic technology. The advantage of the proposed technology is a significant reduction in synthesis time and energy consuming process.