STUDY OF THE FEATURES OF THE MAGNETIC AND CRYSTAL STRUCTURES OF THE BaF12-Х ALXO19 AND BaFe12-Х GaXO19 SUBSTITUTED HEXAGONAL FERRITES

The structural and compositional diversity of hexagonal ferrites makes them a convenient object for a research of magnetic and electric properties [1, 2]. The structure of BaFe12O19 substituted ferrites, commonly used in technology is similar to MeFe12O19 (Me2+–Ba2+, Sr2+, Pb2+, Ca2+) magnetoplumbite mineral, in which the oxygen layers are a set of hexagonal (HCP), and cubic (CCP) packing of hexagonal (R) and spinel (S) blocks containing Fe3+ ions [3, 4]. Interest in research of M-type barium ferrites with a hexagonal structure (BaFe12O19) and solid solutions is due to their high functional properties. Excellent chemical stability and corrosion resistance make them environmentally safe and usable virtually without restrictions in time. The combination of high coercive force (Нс~160–55 kA/m) [5] and rather high residual induction provides permanent magnets with satisfactory specific magnetic energy, and their low conductivity (ρ~108 ohm∙cm) allows applying hexaferrite magnets in the presence of high-frequency magnetic fields. 2. Literature review and problem statement


Introduction
The structural and compositional diversity of hexagonal ferrites makes them a convenient object for a research of magnetic and electric properties [1,2].The structure of BaFe 12 O 19 substituted ferrites, commonly used in technology is similar to MeFe 12 O 19 (Me 2+ -Ba 2+ , Sr 2+ , Pb 2+ , Ca 2+ ) magnetoplumbite mineral, in which the oxygen layers are a set of hexagonal (HCP), and cubic (CCP) packing of hexagonal (R) and spinel (S) blocks containing Fe 3+ ions [3,4].
Interest in research of M-type barium ferrites with a hexagonal structure (BaFe 12 O 19 ) and solid solutions is due to their high functional properties.Excellent chemical stability and corrosion resistance make them environmentally safe and usable virtually without restrictions in time.The combination of high coercive force (Н с ~160-55 kA/m) [5] and rather high residual induction provides permanent magnets with satisfactory specific magnetic energy, and their low conductivity (ρ~10 8 ohm•cm) allows applying hexaferrite magnets in the presence of high-frequency magnetic fields.

Literature review and problem statement
Barium ferrite is isostructural with PbFe 12 O 19 magnetoplumbite mineral [6], the crystal structure type of which has been first determined by Adelskold in 1938.The anisotropy energy constant of hexaferrites is by two orders of magnitude greater than that of garnet-type ferrites, which has paved the way for practical applications: permanent magnets, magnetic recorders and microwave equipment [7,8].Hexagonal ferrites have been successfully used in the decimeter and centimeter electromagnetic radiation absorption field (electromagnetic compatibility of microelectronic devices and radio equipment, "STEALTH" technology and so on).Previous research of hexagonal barium ferrite showed that it is an insulator, in which Kramers-Anderson shortrange indirect exchange between the magnetoactive iron ions through oxygen anion prevails, i. e., the pair exchange integral depends on the valence angles and interatomic distances in Fe-O-Fe bonds [9].Strong sublattice exchange in bonds causes collinear ferrimagnetic alignment with the Curie temperature of ~740 K [10].The functional properties of ferrites can be controlled by changing the number of mag-

V . K o r o v u s h k i n
Doctor of Geological-Mineralogical Scienses, Associate Professor* Е-mail: krvsch@mail.ru

I . I s a e v
Engineer* Е-mail: isa@misis.ru

A . T r u k h a n o v
PhD* Е-mail: truhanov86@mail.ru*Department of Technology for Electronic Materials National University of Science and Technology "MISiS" Leninskiy ave., 4, Moscow, Russia, 119049
Isomorphous substitution in the crystal structure of hexagonal Ba ferrites is essential in the formation of their magnetic and electric properties.Isomorphous elements are often Sr 2+ and Ca 2+ cations, which substitute some Ba 2+ ions, and weak-magnetic or non-magnetic In 3+ , Sc 3+ , Al 3+ , Ga 3+ cations, etc. substitute Fe 3+ ions.Non-magnetic cations lead to breakage of exchange coupling between Fe 3+ ions, thus changing the magnetic characteristics of the ferrite [9].
However, the issues remain about localization of both divalent and trivalent ions in the structure due to the presence of 5 structural positions in hexaferrites, as well as the cation distribution impact on the magnetic and electric properties.The isomorphous substitution in hexagonal and spinel blocks may cause the angular magnetic structure.Thus, in [10], the authors believe that the angular magnetic structure at the Al entry into the hexaferrite lattice occurs at х=~3.However, the issues of how various elements influence the noncollinearity angle, in what positions substitution occurs and at what element contents are not fully understood.The ability of Mössbauer spectroscopy to determine the angle θ between the magnetic moment in the hexaferrite and the g-radiation direction with the known direction of the crystallographic axis C allows solving this problem.

The purpose and objective of the research
The research aim was to determine the localization of isomorphic Al and Ga impurities in the hexagonal M-type barium ferrite structure and the effect of these impurities on the magnetic properties of the ferrite.
The research objectives included: -determining the substituted hexaferrite sintering temperature; -determining the structural positions of the Fe ions substitution with impurity ions; -determining the magnetic parameters (specific and residual magnetization, coercive force, hysteresis loop shape) in the source barium hexaferrite compared to the substituted one; -determining the angle θ between the magnetic moment and the g-radiation direction in isotropic and anisotropic hexaferrites by analyzing the intensity ratio of resonance lines in the Mössbauer spectra.

Materials and methods of researching the substituted hexaferrites
The The initial composition was subjected to air synthesizing calcination at 1200 °C (6 h) and then sintered at 1300 °C (6 h).After sintering, the sample was slowly cooled in the furnace (~100 °C/h).
Mössbauer spectroscopy, X-ray radiography, and magnetic measurements were used to examine the features of the crystal structure, composition and properties.Mössbauer studies were performed on the Ms1104-Em spectrometer and the spectra were processed by the Univem Ms program (Russia, Southern Federal University, Rostov-on-Don).The measurements were performed at room temperature (300 K) for powders and foil.The isomer shift was determined relative to a-Fe.X-ray diffraction patterns were obtained on the DRON-3M apparatus in CuK a radiation.Magnetic parameters such as saturation magnetization σ s , coercive force H c , residual magnetization σ r , hysteresis loop shape and area were measured by the VS M 250 vibration magnetometer in the 20 kOe magnetic field at 300 K.

The results of studying the substituted barium hexaferrites
X-ray phase analysis of BaFe 12 O 19 samples after annealing at 1100, 1150 and 1200 and 1300 °С was held in order to determine the phase formation kinetics of substituted hexaferrites.It was found that hexaferrite with magnetoplumbite-type structure, isostructural with BaFe 12 O 19 was present in the samples as the main phase (>91 %) after annealing at 1100 °С.The presence of an impurity phase -BaFe 2 O 4 was observed.In addition to the main phase (BaFe 12 O 19 ), the presence of a second phase -Ba 2 Fe 6 O 11 was noted after annealing at 1150 °С.The BaFe 2 O 4 impurity phase was detected after annealing at 1200 °С (Fig. 1).
BaFe 12 O 19 hexaferrite without intermediate phases was found at the sintering temperature of 1300 °С.  2, a. Table 1 shows the parameters of Mössbauer spectra after decomposition.The Mössbauer spectrum decomposition model was based on the physical sense, visual spectrum resolution, achievement of minimum χ 2 and minimum discrepancy between the experimental spectrum and the model.
To study the magnetic properties of substituted hexaferrites, their saturation magnetization, residual magnetization, coercive force, Curie temperature and hysteresis loops in comparison with the unsubstituted hexaferrite were measured.Fig. 3 shows hysteresis loops of BaFe 12 O 19 (Fig. 3, a) and BaFe 9,9 Al 2,1 O 19 (Fig. 3, b) ferrites, and Table 2 -their magnetic characteristics.The effect of Ga dopants on magnetic properties depending on the localization in the hexagonal barium ferrite structure was examined in the BaFe 11.4 Ga 0,6 O 19 sample.Fig. 4 shows a Mössbauer spectrum of the sample, taken at K 300, and Table 3 -the Mössbauer parameters.Unlike spectra of hexaferrites with Al, the BaFe 11.4 Ga 0,6 O 19 hexaferrite spectrum was decomposed into 6 sextets.The additional sextet was designated as 12k 1 and assigned to the 12k sublattice.
The magnetic characteristics of BaFe 11.4 Ga 0,6 O 19 are also of interest.Fig. 5 shows the magnetic hysteresis loop for this sample, and Table 2 -the magnetic characteristics.

Discussion of the results of studying ВаFe 12-х Al х O 19 and BaFe 12-х Ga х O 19 hexaferrites
The analysis of the BaFe 12 O 19 sample spectrum (Fig. 2, a) shows that it is a superposition of five sextets corresponding to 12k, 4f 1 , 4f 2 ), 2а, 2b sublattices.The occupancies of sublattices correspond to their theoretical values for the hexagonal structure M, 12:4:4:2:2, respectively.The Mossbauer spectra parameters for iron ions, localized in these sublattices coincide with the data obtained in [11].
Satisfactorily decomposition of Mössbauer spectra of BaFe 12-x Al x O 12 substituted hexaferrite into 5 sextets proved impossible.The best option was the spectrum decomposition into 7 sextets, corresponding to 12k, 12k 1 , 4f 1 , 4f 2 , 4f 1 1 2а and 2b sublattices.The emergence of 12k 1 and 4f 1 1 sublattices is associated with the Al 3+ localization features and 12k and 4f 1 division into two.Comparison of the sextets areas (Table 1) allows concluding that Al 3+ ions are localized mainly in 12k positions, resulting in allocation of 7 sextets in the spectrum.If, according to the BaFe 12 O 19 ferrite formula, 50 % rel., falling to 6 Fe 3+ ions should theoretically be in the 12k sublattice, then 41.2 % accounts for iron ions of the 12k sublattice in ВаFe 12-x Al x O 19 isotropic ferrite, while the occupancy change in other sublattices is within the error.Proceeding from the 12k sublattice occupancy, the crystal-chemical formula for ВаFe 12-x Al x O 19 powder will be ВаFe 9,9 Al 2,1 O 19 .
When analyzing the Mössbauer spectra parameters (Table 1), we can note a decrease in the values of magnetic fields at Fe 57 nuclei in almost all sublattices, and not only for the 12k sublattice.Indeed, indirect exchange interactions between Fe 3+ ions of different Fe(12k)-O-Fe(4f 2 ), Fe(12k)-O--Fe(2b), Fe(4f 2 )-O-Fe(2b) Fe(12k)-O-Fe(4f 1 ) and Fe(12k)-O-Fe(12k) sublattices occur in the hexaferrite structure.Therefore, the Fe 3+ -Al 3+ substitution in the 12k sublattice leads to the breakage of magnetic coupling with other sublattices and, consequently, to a decrease in magnetic fields at Fe 57 nuclei.If the Н eff dependence is performed as 4f 2 >2a>4f 1 >12k>2b in the unsubstituted hexaferrite, then in the substituted isotropic hexaferrite due to the different number of exchange couples of 12k with other sublattices -4f 2 >4f 1 >12k>2b>2a.In addition, the isomer chemical shift in the substituted hexaferrite is markedly reduced for Fe 3+ ions of all sublattices as compared with the unsubstituted hexaferrite.This is probably due to the covalence increase of Fe 3+ ions bond, with a decrease in the unit cell parameter, since the ionic radius 3 VI rAl + (0.051 nm) is less than 3 VI rFe + (0.064 nm).
The integral intensity ratios, characteristic of polycrystals are not satisfied in the spectrum of hexaferrite foil with isomorphic aluminum.So, the most appropriate foil sample decomposition was carried out by setting 7 sextets at the intensity ratio of 3:1.39:1.13.Such intensity ratio is characteristic of the samples with a certain degree of texture.According to the obtained ratio, the deflection angle of the magnetic moments from the g-radiation wave vector θ is 44.6º.Based on the results, we can say that hexagonal Ba ferrites in the foil form have an evident texture.The foil sample Mössbauer spectrum, taken at 87 K showed the worst peak resolution from Fe 3+ ions of different sublattices than at 300 K, which is explained by opposite directions of spins of Fe 3+ ions of structural sublattices.The angle θ remained almost unchanged, being 44.2º.
Breakage of Fe-O-Fe magnetoactive couples between hexaferrite sublattices leads both to a reduction in local magnetic fields at Fe 57 nuclei, the emergence of non-equivalent 12k and 4f 1 nodes, and a change in the magnetic characteristics.Fig. 3 shows the BaFe 12 O 19 and BaFe 9,9 Al 2,1 O 19 ferrite hysteresis loops, and Table 2 -their magnetic characteristics.
Table 2 shows that the exchange coupling weakening decreases the Curie temperature by 177 degrees, the specific saturation magnetization by 43 Am 2 /kg compared with the BaFe 12 O 19 ferrite.The hysteresis loops show a significant squareness decrease and area increase compared to the unsubstituted Ba hexaferrite.At the same time, the hexaferrite coercive force increased almost 2-fold.This result can be essential when choosing hard-magnetic materials for non-metallic permanent magnets.
The analysis of the BaFe 11.4 Ga 0,6 O 19 ferrite spectrum showed, unlike ВаFe 9,9 Al 2,1 O 19 hexaferrite, minor differences in the isomer shift and quadrupole splitting for all sublattices compared with the unsubstituted hexaferrite, but the magnetic field decrease from 2 to 4 kOe is observed for all sublattices, except 2b.More substantial changes occur in the areas of all spectral components, except 4f 2 .The area reduction from Fe 3+ ions of 12k, 2a and 2b sublattices indicates entry of Ga ions into them.Additional sextet, labeled 12k 1 , was assigned to the 12k sublattice, as their isomer shift and quadrupole splitting values are similar.Reduction of magnetic fields at Fe 57 nuclei is due to the iron ions being surrounded with nonmagnetic Ga ions and breakage of its magnetic couples.
The analysis of magnetic characteristics showed (Fig. 5) that the hysteresis loop does not go into a state of magnetic saturation even in magnetic fields of up to 20 kOe, which can be explained by the intralattice exchange interaction weakening.The maximum specific magnetization in 20 kOe fields was 55.04 Am 2 /kg.The data (Table 2) show that gallium doping (x=0.6) of BaFe 12 O 19 hexaferrite leads to a decrease in almost all magnetic parameters.The spontaneous magnetization decreased by 27.5 Am 2 /kg, residual magnetization by 17 Am 2 /kg, coercive force by 27 kA/m, Curie temperature by 100 K.This confirms that the substitution of iron ions with diamagnetic gallium ions reduces the number of magnetic neighbors of iron ions and, as a consequence, leads to an earlier failure of long-range magnetic order when samples are heated.The comparison of barium hexaferrites doped with various metals shows that gallium doping makes the material magnetically softer in relation to BaFe 12 O 19 , and aluminum doping -magnetically harder.A complete dependence of the magnetic condition of BaFe 12 O 19 hexaferrite on doping with different elements can be observed on their quantitative composition dependences, which is planned in further work.2. It is found that the Fe-Al substitution in BaFe 9,9 Al 2,1 O 19 hexaferrite occurs mainly in the 12k sublattice, whereby the 12k and 4f 1 sublattices are divided into two.The Fe -Ga substitution in BaFe 11,4 Ga 0,6 O 19 hexaferrite occurs in the 12k sublattice, as well as in the 2a and 2b sublattices, and only the 12k sublattice is divided into two.
3. Measurements of magnetic characteristics of BaFe 9,9 Al 2,1 O 19 and BaFe 11,4 Ga 0,6 O 19 hexaferrites showed that substitution of Fe 3+ ions with Al 3+ and Ga 3+ leads to a decrease in specific saturation magnetization, residual magnetization, and Curie temperature.It is shown that the Al (х=2.1)entry into the lattice increases the coercive force from 273 to 532 kA/m, increasing the hexaferrite magnetic hardness, and the Ga entry lowers the coercive force and reduces the magnetic hardness.The rate of changes in the magnetic parameters depends on the degree of substitution of Fe 3+ ions.
4. The angle θ between the magnetic moment and the g-radiation direction in isotropic and anisotropic hexaferrites was determined by analyzing the intensity ratio of resonance lines in the Mössbauer spectra.The deflection angle of the magnetic moments from the g-radiation wave vector θ for the substituted hexaferrite in the foil form is 44.6º, indicating the occurrence of texture.The ВаFe 9,9 Al 2,1 O 19 powder Mössbauer spectrum showed no texture, as it showed the intensity ratio of 3:2.11:1.18 and the angle θ is equal to 53.9º.
The research is performed under the Research Task No. 11.2502.2014/Kon 17.07.2014within the project part of the state task in the field of scientific activity (Subject No. 3219022).

7. Conclusions 1
. It is shown that hexagonal Ba ferrite formation occurs in several stages via intermediate phases.BaFe 2 O 4 intermediate phase with a spinel structure and Ba 2 Fe 6 O 11 intermediate phase with orthorhombic structure are formed in the 1100-1200 °C sintering temperature range.Formation of BaFe 12 O 19 hexaferrite with magnetoplumbite-type structure starts at 1200 °C and ends at 1300 °C.