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Розглянуто мехатзм формування гек-сафериту барю з замiщенням юнами А1 i Ga. Замiщення вгдбуваються в тдгратщ 12^ Це призводить до виникнення неекв^ валентних положень ютв Ге?+, i в месба-уерiвських спектрах з алюм^ем видшя-ються 7 секстетов, а з галiем 6. Замiщення алюмШем тдвищуе коерцитивну силу, а входження галю знижуе коерцитивну силу. Визначено кути мiж магттним моментом i напрямком у-випромтювання в дослиджувальних составах
Ключовi слова: замещенный гексафе-рит барю, мезбауерiвська спектроскотя, намагтчетсть, коерцитивна сила, температура Кюрi
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Рассмотрен механизм формирования гексаферритов бария с замещением ионами А1 и Ga. Замещения происходят в подрешетке 12k. Это приводит к возникновению неэквивалентных положений ионов Ре3+ и в мессбауэровских спектрах с алюминием выделяются 7 секстетов, а с галлием 6. Замещение алюминием повышает коэрцитивную силу, а для галлия отмечено понижение коэрцитивной силы. Определены углы между магнитным моментом и направлением у-излучения в исследованных составах
Ключевые слова: замещенные гекса-ферриты бария, мессбауэровская спектроскопия, намагниченность, коэрцитивная сила, температура Кюри -□ □-
UDC 535.421; 537.622; 538.91
|DOI: 10.15587/1729-4061.2017.91409|
STUDY OF THE FEATURES OF THE MAGNETIC AND CRYSTAL STRUCTURES OF THE BaFE12-x ALxO19 AND BaFe^ GaxO19 SUBSTITUTED HEXAGONAL FERRITES
V. Kostishyn
Doctor of Physical and Mathematical Sciences, Professor, Head of Department* Е-mail: drvgkostishyn@mail.ru V. Korovushkin Doctor of Geological-Mineralogical Scienses, Associate Professor* Е-mail: krvsch@mail.ru I. I saev Engineer* Е-mail: isa@misis.ru A. Trukhanov PhD*
Е-mail: truhanov86@mail.ru *Department of Technology for Electronic Materials National University of Science and Technology "MISiS" Leninskiy ave., 4, Moscow, Russia, 1 19049
1. 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 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 (Hc~160-55 kA/m) [5] and rather high residual induction provides permanent magnets with satisfactory specific magnetic energy, and their low conductivity (p~108 ohm-cm) allows applying hexaferrite magnets in the presence of high-frequency magnetic fields.
2. Literature review and problem statement
Barium ferrite is isostructural with PbFe12O19 magnetoplumbite mineral [6], the crystal structure type of which has been first determined by Adelskold in 1938. The an-isotropy 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 short-range 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-
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netic couples of iron ions, for example, introducing diamag-netic Al, Sc, Ga and In ions. Depending on the doping degree and temperature, the solid solutions formed undergo phase transitions accompanied by different types of alignment.
Hexaferrites contain only Fe3+ions as magnetoactive, which are localized at the nodes having octahedral (4f2, 2a, 12k), tetrahedral (4fi) and bipyramidal (2b) oxygen environment [6].
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 Sr2+ and Ca2+ cations, which substitute some Ba2+ ions, and weak-magnetic or non-magnetic In3+, Sc3+, Al3+, Ga3+ cations, etc. substitute Fe3+ ions. Non-magnetic cations lead to breakage of exchange coupling between Fe3+ 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 x=~3. However, the issues of how various elements influence the noncol-linearity angle, in what positions substitution occurs and at what element contents are not fully understood. The ability of Mössbauer spectroscopy to determine the angle 0 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.
3. 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 0 between the magnetic moment and the g-radiation direction in isotropic and anisotropic hexaferrites by analyzing the intensity ratio of resonance lines in the Mossbauer spectra.
BaC03+5.7Fe2O3+xMe2O3=BaFe12-xMexO19+CO2.
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).
Mossbauer spectroscopy, X-ray radiography, and magnetic measurements were used to examine the features of the crystal structure, composition and properties. Mossbauer 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 CuKa radiation. Magnetic parameters such as saturation magnetization as, coercive force Hc, residual magnetization ar, hysteresis loop shape and area were measured by the VS M 250 vibration magnetometer in the 20 kOe magnetic field at 300 K.
5. The results of studying the substituted barium hexaferrites
X-ray phase analysis of BaFei2Oi9 samples after annealing at 1100, 1150 and 1200 and 1300 °C was held in order to determine the phase formation kinetics of substituted hexaferrites. It was found that hexaferrite with magne-toplumbite-type structure, isostructural with BaFe12O19 was present in the samples as the main phase (>91 %) after annealing at 1100 °C. The presence of an impurity phase -BaFe2O4 was observed. In addition to the main phase (BaFe12O19), the presence of a second phase - Ba2FegOu was noted after annealing at 1150 °C. The BaFe2O4 impurity phase was detected after annealing at 1200 °C (Fig. 1).
BaFe12O19 hexaferrite without intermediate phases was found at the sintering temperature of 1300 °C.
Fig. 1. BaO-5.6Fe2O3 line radiograph at Tf=1200 °C
4. Materials and methods of researching the substituted hexaferrites
The research objects were samples of polycrystalline M-type BaFe12O19 hexaferrite in the powder form, poly-crystalline BaFe12-xAlxO19 in the d=0.1 mm foil form and polycrystalline BaFe114Ga0,gO19. The samples have been prepared of Fe2O3, Me2O3 oxides, and BaCO3 carbonate according to the known ceramic technique in appropriate proportions:
The high-temperature X-ray radiography revealed that BaFe12O19 is formed in two stages:
at t=700-900 °C BaCO3+6 Fe2O3^
^BaFe2O4+5 Fe2O3+CO2|; at t=900-1200 °C BaFe2O4+5 Fe2O3=BaFe12O19. At the same time, other intermediate phases may be formed during the second stage [11]. The formation mechanism and the sintering temperature of polycrystalline aluminum- and gallium-substituted barium hexaferrite are similar.
Mossbauer spectra of BaFe12O19 and BaFe12-xAlxO12 substituted ferrite samples in powder and foil forms are shown in Fig. 2, a. Table 1 shows the parameters of Mossbau-er spectra after decomposition.
and BaFe99Al21O19 (Fig. 3, b) ferrites, and Table 2 magnetic characteristics.
their
Fig. 2. Mossbauer spectra: a — BaFe12 O19 powder, b — BaFe12-x AlxOi9 powder, c — BaFe12-x AlxOi9 foil
The Mossbauer spectrum decomposition model was based on the physical sense, visual spectrum resolution, achievement of minimum x2 and minimum discrepancy between the experimental spectrum and the model.
To study the magnetic properties of substituted hexafer-rites, 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 BaFe12O19 (Fig. 3, a)
Table 1
Mossbauer parameters of the samples: BaFe12O19 powder, BaFe12-xAlxO19 powder, BaFe12-xAlxO19 foil
Sample, material Sublattice, spectral component Isomer shift 5, mm/s Quad-rupole splitting A, mm/s Magnetic fields at Fe57 nuclei H, kOe Component areas S, %
Isotropic BaFe12O19, powder 12k-C1(Fe3+)VI 0.35 0.42 414 49.9
2a-4(Fe3+)vi 0.33 0.00 507 8.8
4fi-C3(Fe3+)iv 0.26 0.22 491 18.6
4f2-2(Fe3+)vi 0.39 0.20 516 17.6
2b-C5(Fe3+)V 0.29 2.21 400 5.1
Isotropic BaFe12-x AlxO19, powder 12k1-C1(Fe3+)VI 0.31 0.38 370 18.0
4f2-C2(Fe3+)vi 0.37 0.13 474 22.9
12k-C3(Fe3+)IV 0.31 0.43 397 23.2
2a-C4(Fe3+)VI 0.37 0.52 311 8.3
4fi-C5(Fe3+)iv 0.28 0.24 428 16.6
4fi1-C6(Fe3+)iv 0.22 0.37 345 2.6
2b-C7(Fe3+)V 0.29 2.1 383 8.4
Anisotropic BaFe12-x AlxO19, foil 12k-C1(Fe3+)VI 0.29 0.39 396 25.8
4f2-C2(Fe3+)vi 0.37 0.13 485 17.4
4fi-C3(Fe3+)iv 0.25 0.32 423 14.8
2a-C4(Fe3+)VI 0.37 0.52 312 6.8
4fi1-C5(Fe3+)iv 0.27 0.02 449 12.3
12k1-C6(Fe3+)IV 0.26 0.32 366 13.8
2b-C7(Fe3+)V 0.29 2.21 400 8.9
b
Fig. 3. The hexagonal ferrites hysteresis loops: a — BaFe12O19, b — BaFe99Al21O19
a
b
c
Table 2
Magnetic properties of ВаFe12O19, BaFe^Al^O-^ and BaFe-i-uGao^O-ig hexagonal ferrites
Ferrite Specific saturation magnetization gs, Am2/kg Specific residual magnetization, Gr, Am2/kg Coercive force Hc, kA/m Curie temperature, K
BaFe12O19 64.61 40.13 273 720
BaFe9,gAl2,iOi9 21.68 12.13 532 543
BaFeiUGao,6Oi9 37.10 23.17 246 620
60
40
20
-20
-40
-60
: BaFe,, ,Ga„aO,r
;T=300 K
ff
7........... .... 1 ... . ....................
The effect of Ga dopants on magnetic properties depending on the localization in the hexagonal barium ferrite structure was examined in the BaFe^^Gao^O^ sample. Fig. 4 shows a Mossbauer spectrum of the sample, taken at K 300, and Table 3 - the Mossbauer parameters.
o
H kOe
Fig. 5. The BaFeii.4Ga0j6Oi9 sample hysteresis loop
Spontaneous magnetization was determined from the field dependence of the linear extrapolation to zero field.
6. Discussion of the results of studying BaFei2-xAlxOig
and BaFe^vGa^Om hexaferrites
-8.0 -4.0 0.0 4.0 8.0 V, mm/s
Fig. 4. The BaFe-n^Gao^Oig sample Mossbauer spectrum
Table 3
Mössbauer parameters of BaFe-n^Gao^Oig substituted ferrite
Sample, material Sublattice, spectral component Measured shift 5, mm/s Quad-rupole splitting A, mm/s Magnetic field at Fe57 nuclei H, kOe Component areas S, %
Isotropic BaFeiUGao,6Oi9 powder 12k-C1(Fe3+)Vi 0.35 0.41 411 47.6
4f2-C2(Fe3+)vi 0.38 0.20 512 16.3
4fi-C3(Fe3+)iv 0.27 0.21 487 24.2
2a-C4(Fe3+)VI 0.34 -0.02 505 4.3
12k1-C5(Fe3+)IV 0.35 0.44 370 3.0
2b-C7(Fe3+)V 0.29 2.21 400 4.6
Unlike spectra of hexaferrites with Al, the BaFe114 Ga0,gOi9 hexaferrite spectrum was decomposed into 6 sextets. The additional sextet was designated as 12k1 and assigned to the 12k sublattice.
The magnetic characteristics of BaFeu.4Ga0,6Oi9 are also of interest. Fig. 5 shows the magnetic hysteresis loop for this sample, and Table 2 - the magnetic characteristics.
The analysis of the BaFej2Oj9 sample spectrum (Fig. 2, a) shows that it is a superposition of five sextets corresponding to 12k, 4fj, 4f2), 2a, 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 Mossbauer spectra of BaFei2-xAlxOi2 substituted hexaferrite into 5 sextets proved impossible. The best option was the spectrum decomposition into 7 sextets, corresponding to 12k, 12k1, 4fi, 4f2, 4f/ 2a and 2b sublattices. The emergence of 12k1 and 4f/ sublattic-es is associated with the Al3+ localization features and 12k and 4fi division into two. Comparison of the sextets areas (Table 1) allows concluding that Al3+ ions are localized mainly in 12k positions, resulting in allocation of 7 sextets in the spectrum. If, according to the BaFej2Oj9 ferrite formula, 50 % rel., falling to 6 Fe3+ions should theoretically be in the 12k sublattice, then 41.2 % accounts for iron ions of the 12k sublattice in BaFei2-xAlxOi9 isotropic ferrite, while the occupancy change in other sublattices is within the error. Proceeding from the 12k sublattice occupancy, the crystal-chemical formula for BaFej2-xAlxOj9 powder will be BaFe99Al2,iOi9.
When analyzing the Mossbauer spectra parameters (Table 1), we can note a decrease in the values of magnetic fields at Fe57nuclei in almost all sublattices, and not only for the 12k sublattice. Indeed, indirect exchange interactions between Fe3+ ions of different Fe(12k)-O-Fe(4f2), Fe(12k)-O--Fe(2b), Fe(4f2)-O-Fe(2b) Fe(12k)-O-Fe(4f0 and O-Fe(12k) sublattices occur in the hexaferrite substitution in the 12k
Fe(12k)-
structure. Therefore, the Fe3+-Al3+ sublattice leads to the breakage of magnetic coupling with other sublattices and, consequently, to a decrease in magnetic fields at Fe57 nuclei. If the Heff dependence is performed as
4f2>2a>4f1>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 -4f2>4f1>12k>2b>2a. In addition, the isomer chemical shift in the substituted hexaferrite is markedly reduced for Fe3+ ions of all sublattices as compared with the unsubstituted hexaferrite. This is probably due to the covalence increase of Fe3+ ions bond, with a decrease in the unit cell parameter, since the ionic radius rAl^ (0.051 nm) is less than rFeV (0.064 nm).
The BaFe99Al21O19 powder Mossbauer spectrum showed slight signs of texture because it satisfied the peak intensity ratio 31-6:22-5:13-4 equal to 3:2.11:1.18 and the angle 8, determined from the expression A1-6/A2-5=3(cos28)/4(sin28), is 53.9°. These values are quite close to the ratio of 3:2.2:1.1 in polycrystals at the angle 8=55°.
The integral intensity ratios, characteristic of poly-crystals 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 0 is 44.6°. Based on the results, we can say that hexagonal Ba ferrites in the foil form have an evident texture. The foil sample Mossbauer spectrum, taken at 87 K showed the worst peak resolution from Fe3+ ions of different sublattices than at 300 K, which is explained by opposite directions of spins of Fe3+ ions of structural sublattices. The angle 0 remained almost unchanged, being 44.2°.
Breakage of Fe-O-Fe magnetoactive couples between hexaferrite sublattices leads both to a reduction in local magnetic fields at Fe57nuclei, the emergence of non-equivalent 12k and 4f1 nodes, and a change in the magnetic characteristics. Fig. 3 shows the BaFe12O19 and BaFe99Al21O19 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 Am2/kg compared with the BaFe12O19 ferrite. The hysteresis loops show a significant squareness decrease and area increase compared to the un-substituted 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 BaFe114Ga0 6O19 ferrite spectrum showed, unlike BaFe99Al21O19 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 4f2. The area reduction from Fe3+ ions of 12k, 2a and 2b sublattices indicates entry of Ga ions into them. Additional sextet, labeled 12k1, was assigned to the 12k sublattice, as their isomer shift and quadrupole splitting values are similar. Reduction of magnetic fields at Fe57 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 Am2/kg. The data (Table 2) show that gallium doping (x=0.6) of BaFe12O19 hexaferrite leads to a decrease in almost all magnetic parameters. The spontaneous magnetization decreased by 27.5 Am2/kg, residual magnetization by 17 Am2/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 BaFe12O19, and aluminum doping - magnetically harder. A complete dependence of the magnetic condition of BaFe12O19 hexaferrite on doping with different elements can be observed on their quantitative composition dependences, which is planned in further work.
7. Conclusions
1. It is shown that hexagonal Ba ferrite formation occurs in several stages via intermediate phases. BaFe2O4 intermediate phase with a spinel structure and Ba2Fe6O11 intermediate phase with orthorhombic structure are formed in the 1100-1200 °C sintering temperature range. Formation of BaFe12O19 hexaferrite with magne-toplumbite-type structure starts at 1200 °C and ends at 1300 °C.
2. It is found that the Fe-Al substitution in BaFe99Al21O19 hexaferrite occurs mainly in the 12k sublattice, whereby the 12k and 4f1 sublattices are divided into two. The Fe -Ga substitution in BaFe114Gao,6O19 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 BaFe9,9Al2,1O19 and BaFe11,4Ga0,6O19 hexaferrites showed that substitution of Fe3+ ions with Al3+ and Ga3+ leads to a decrease in specific saturation magnetization, residual magnetization, and Curie temperature. It is shown that the Al (x=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 Fe3+ions.
4. The angle 0 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 Mossbauer spectra. The deflection angle of the magnetic moments from the g-radiation wave vector 0 for the substituted hexaferrite in the foil form is 44.6°, indicating the occurrence of texture. The BaFe99Al21O19 powder Mossbauer spectrum showed no texture, as it showed the intensity ratio of 3:2.11:1.18 and the angle 0 is equal to 53.9°.
The research is performed under the Research Task No. 11.2502.2014/K on 17.07.2014 within the project part of the state task in the field of scientific activity (Subject No. 3219022).
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