Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties
Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted hexaferrites BaTixCoxFe₁₂–₂xO₁₉ and SrxTixCoxFe₁₂–₂xO₁₉ (0.8 ≤ x ≤ 2). The dynamics of the growth process is studied and results of the analysis of impurity phases appearing in the initial stag...
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| Cite this: | Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties / A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Y. Ivanov, A.A. Mukhin // Физика низких температур. — 2017. — Т. 43, № 8. — С. 1207-1213. — Бібліогр.: 23 назв. — англ. |
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Balbashov, A.M. Voronchikhina, M.E. Iskhakova, L.D. Ivanov, V.Y. Mukhin, A.A. 2021-01-30T18:04:22Z 2021-01-30T18:04:22Z 2017 Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties / A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Y. Ivanov, A.A. Mukhin // Физика низких температур. — 2017. — Т. 43, № 8. — С. 1207-1213. — Бібліогр.: 23 назв. — англ. 0132-6414 PACS: 75.47.Np, 75.50.–y, 75.85.+t https://nasplib.isofts.kiev.ua/handle/123456789/175127 Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted hexaferrites BaTixCoxFe₁₂–₂xO₁₉ and SrxTixCoxFe₁₂–₂xO₁₉ (0.8 ≤ x ≤ 2). The dynamics of the growth process is studied and results of the analysis of impurity phases appearing in the initial stages of the crystal growth are presented. Compositions and unit-cell parameters of crystals are determined. Electrical, magnetic and magnetoelectric properties of grown crystals are investigated at temperatures 2–365 K and magnetic fields up to 50 kOe. It is shown that the resistivity of annealed in oxygen crystals at room temperature is ∼10⁶ Ohm·cm while at helium temperatures the crystals become good insulators. Magnetic measurements reveal conical spin structures in the crystals at some concentrations and temperatures. Magnetic field induced electric polarization of the low value (∼0.3 μC/m²) is detected at liquid helium temperatures for compositions with Ti and Co concentrations x = 0.8–0.9. The work is supported by the Russian Scientific Foundation (Project No. 16-12-10531). en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low dimensionality and inhomogeneity effects in quantum matter Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties Article published earlier |
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Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
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Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties Balbashov, A.M. Voronchikhina, M.E. Iskhakova, L.D. Ivanov, V.Y. Mukhin, A.A. Low dimensionality and inhomogeneity effects in quantum matter |
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Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
| title_full |
Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
| title_fullStr |
Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
| title_full_unstemmed |
Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
| title_sort |
single crystals growth of hexaferrits m-type mtixcoxfe₁₂–₂xo₁₉ (m = ba, sr) by floating zone and investigation of their magnetic and magnetoelectric properties |
| author |
Balbashov, A.M. Voronchikhina, M.E. Iskhakova, L.D. Ivanov, V.Y. Mukhin, A.A. |
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Balbashov, A.M. Voronchikhina, M.E. Iskhakova, L.D. Ivanov, V.Y. Mukhin, A.A. |
| topic |
Low dimensionality and inhomogeneity effects in quantum matter |
| topic_facet |
Low dimensionality and inhomogeneity effects in quantum matter |
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2017 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Article |
| description |
Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted hexaferrites BaTixCoxFe₁₂–₂xO₁₉ and SrxTixCoxFe₁₂–₂xO₁₉ (0.8 ≤ x ≤ 2). The dynamics of the growth process is studied and results of the analysis of impurity phases appearing in the initial stages of the crystal growth are presented. Compositions and unit-cell parameters of crystals are determined. Electrical, magnetic and magnetoelectric properties of grown crystals are investigated at temperatures 2–365 K and magnetic fields up to 50 kOe. It is shown that the resistivity of annealed in oxygen crystals at room temperature is ∼10⁶ Ohm·cm while at helium temperatures the crystals become good insulators. Magnetic measurements reveal conical spin structures in the crystals at some concentrations and temperatures. Magnetic field induced electric polarization of the low value (∼0.3 μC/m²) is detected at liquid helium temperatures for compositions with Ti and Co concentrations x = 0.8–0.9.
|
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0132-6414 |
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https://nasplib.isofts.kiev.ua/handle/123456789/175127 |
| citation_txt |
Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties / A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Y. Ivanov, A.A. Mukhin // Физика низких температур. — 2017. — Т. 43, № 8. — С. 1207-1213. — Бібліогр.: 23 назв. — англ. |
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2025-11-25T13:10:02Z |
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2025-11-25T13:10:02Z |
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| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8, pp. 1207–1213
Single crystals growth of hexaferrits M-type
MTixCoxFe12–2xO19 (M = Ba, Sr) by floating zone
and investigation of their magnetic and magnetoelectric
properties
A.M. Balbashov1, M.E. Voronchikhina1, L.D. Iskhakova2, V.Yu. Ivanov3, and A.A. Mukhin3
1Moscow Power Engineering Institute, Moscow, Russia
E-mail: BalbashovAM@mpei.ru
2Fiber Optics Research Center of RAS, Moscow, Russia
3Prokhorov General Physics Institute of RAS, Moscow, Russia
Received January 2, 2017, published online June 26, 2017
Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted
hexaferrites BaTixCoxFe12–2xO19 and SrxTixCoxFe12–2xO19 (0.8 ≤ x ≤ 2). The dynamics of the growth process is
studied and results of the analysis of impurity phases appearing in the initial stages of the crystal growth are pre-
sented. Compositions and unit-cell parameters of crystals are determined. Electrical, magnetic and
magnetoelectric properties of grown crystals are investigated at temperatures 2–365 K and magnetic fields up to
50 kOe. It is shown that the resistivity of annealed in oxygen crystals at room temperature is ~ 106 Ohm·cm
while at helium temperatures the crystals become good insulators. Magnetic measurements reveal conical spin
structures in the crystals at some concentrations and temperatures. Magnetic field induced electric polarization of
the low value (~ 0.3 µC/m2) is detected at liquid helium temperatures for compositions with Ti and Co concen-
trations x = 0.8–0.9.
PACS: 75.47.Np Metals and alloys ;
75.50.–y Studies of specific magnetic materials;
75.85.+t Magnetoelectric effects, multiferroics.
Keywords: magnetic and magnetoelectric properties, hexaferrits, crystals growth, floating zone melting.
1. Introduction
Hexaferrites are ferrimagnetic materials with hexagonal
structure which are actively studied as a promising materi-
al with wide applications: permanent magnets, devices for
recording and storage of information, components of vari-
ous electronic devices [1]. Among them a significant atten-
tion is attracted to barium and strontium hexaferrites of M-
type as well as their substituted compositions, for example,
BaxTixCoxFe12–2xO19 (x = 0–6) which possess a high mag-
netization, and controllable magnetic anisotropy at room
temperature. A new impetus to their study aroused in re-
cent years with the discovery of multiferroelectricity in
number of Y, Z and M-type hexaferrites including relatively
high (up to room) temperatures [2–4]. Usually polycrystal-
line samples have been used for practical applications and
physical studies; these compounds were obtained by hydro-
thermal synthesis, sol-gel technology (see, e.g., [1,5–7]).
Recently, an interest to the hexaferrite producing in the
form of nanomaterials or oriented films has grown [8,9].
Only a small number of papers are devoted to the study of
single crystalline multiferroic hexaferrites. Obviously, re-
searches on single crystals allow to perform more detailed
study of physical mechanisms of magnetoelectric phenom-
ena and to obtain the maximum of magnetoelectric cou-
pling as well as to clarify the effects of magneto-crystalline
anisotropy in these processes.
Growth of high quality single crystals of hexaferrites is
a rather difficult task. There were enough successful at-
tempts of the growth of the BaFe12–x–0.05ScxMg0.05O19
crystals by floating zone melting [10]. The Ref. 11 describes
the growth of samarium-activated single crystals of the
SrFe12O19 in the form of hexagonal plates with dimensions
of 10 mm in length and 3–4 mm width by spontaneous
crystallization from the melt. Single crystals of Ti-substi-
tuted barium hexaferrite BaFe12–xTixO19 with x up to 1.3
© A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Yu. Ivanov, and A.A. Mukhin, 2017
A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Yu. Ivanov, and A.A. Mukhin
and sizes 2–8 mm were grown by spontaneous crystalliza-
tion from molten sodium carbonate flux [12]. The difficul-
ty of single crystal growth of substituted hexaferrites M-type
is due to an incongruent nature of melting. A phase dia-
gram of the BaO–Fe2O3 system was studied only partially,
but nevertheless it allows to make a conclusion on a nature
of the incongruent melting of BaM–ferrite [1]. According
to Ref. 13, where an iron oxide-rich region of the SrFe12O19
crystallization is studied, the hexaferrite remains stable up to
1410 °С and decomposes at higher temperatures with a for-
mation of W- and X-hexaferrites. The Ref. 14 on the exam-
ples of growth of the МFe12O19 (M = Sr, Ba) crystals shows
that the degree of incongruent melting is affected by in-
creasing of partial pressure above the melt, and a congruent
melting can be achieved at a pressure above 100 atm. Fur-
thermore, at low partial oxygen pressure the Fe3+ ions
transform partly to Fe2+ ones resulting thus in a significant
increase of electrical conductivity.
The aim of this work was to grow high quality bulk sin-
gle crystals of substituted hexaferrites SrTixCoxFe12–2xO19
and BaTixCoxFe12–2xO19 and study their electrical and
magnetic properties. Our interest in the study of hexa-
ferrites is determined by the search for new compounds
possessing multiferroelectric properties. The conventional
mechanism of electric polarization in hexaferrite M- [10],
Y- [2,15] and Z-types [3,4] is the mechanism of the spin
current (or inverse Dzyaloshinskii–Moriya interaction)
[16,17], which is manifested in transverse conical spin
structures including magnetic field induced ones. Such
structures and associated multiferroelectric properties were
observed in the substituted system BaScxFe12–xO19
[10,21]. The conical magnetic structure obtained by neu-
tron diffraction was discovered in another M-type substi-
tuted system: BaTixCoxFe12–2xO19 [18,19]. Some magnetic
and electric properties of one single crystal from this series
were investigated recently [22,23]
2. Experimental
The growth of the SrTixCoxFe12–2xO19 and
BaTixCoxFe12–2xO19 crystals was carried out by floating-
zone melting method with radiation heating using the zone
melting apparatus URN-2-ZM, equipped with the crystalli-
zation chamber of the high pressure, allowing to carry out
the processes of growth under oxygen pressure up to
100 atm, and possessing a high-temperature annealing of
the grown crystals [14].
Polycrystalline feed rods with a diameter of 7 mm, were
made by usual ceramic technology using high purity Fe2O3,
CoO, BaCO3, SrCO3, TiO2. Crystal growth was performed
with a rate of 5–10 mm/h; the oxygen pressure in the
chamber was 60–70 atm; the rotation of the crystal and the
seed were 40 rpm and 1 rpm, respectively. The grown
crystals were obtained as cylinders with a diameter ~ 6 mm
and a length ~ 60 mm.
The characterization of the samples was performed by
diffraction methods (x-ray phase analysis and Laue method).
Microstructure of the samples was investigated using JSM-
5910LV (JEOL) scanning electron microscope in Z-contrast
back-scattered electrons mode. Determination of the chemi-
cal composition was carried out by means of x-ray energy
dispersive analysis using AZtecENERGY (Oxford Instru-
ments) analytical system. Single crystals of TiO2, CoGa2O4,
SrFe12O19 and BaFe12O19 were used as standards/referen-
ces for the calculation of the chemical element composition.
The x-ray analysis (XRD) of the samples was performed
using Bruker D2 Phaser powder x-ray diffractometer with
Cu-Kα radiation. Processing of the results and the phase
analysis of samples was accomplished by software packag-
es DIFFRACplus (EVA and TOPAS 4.2.0.2). The orienta-
tion of the samples with respect to the crystallographic
axes was performed by the x-ray Laue using digital appa-
ratus of photonic science.
For measurements of electric and magnetoelectric prop-
erties of crystals plane-parallel samples were cut in a form
of disks or rectangular plates with a thickness 0.8–1.5 mm
and an area 10–30 mm2. Usually the measurements were
carried out of samples with a hexagonal axis lying in the
plane of the plate, however, in some cases the axis was
oriented perpendicular to the sample plane. Contacts made
of conductive silver paste were applied to flat surfaces of a
sample. Electrical properties were measured using a
Keithley 6517A electrometer.
The magnetization was measured by a vibrating sample
magnetometer in fields up to 14 kOe, and a SQUID magne-
tometer of Quantum Design (MPMS) in fields up to 50 kOe
in the temperature range from 2 to 365 K. Measurements
were carried out on samples with ~ 2 mm size and a shape
closed to cubic. No correction for the demagnetization fac-
tor was performed.
3. Experimental results and discussion
3.1. Crystal growth details
When growing single crystals in the system
SrTixCoxFe12–2xO19 (x = 0.9–2.0) using both mono- or
polycrystalline seed no single crystal growth occurs imme-
diately after crystallization starting. In accordance with the
phase diagram of the system SrO–Fe2O3 [13] iron oxide is
crystallized from the melt of stoichiometric composition,
and a composition of melt is displaced, reaching the
hexaferrite crystallization region. This process promotes by
the growth performed under high oxygen pressure. Further,
the solidification front formed embryos hexaferrite phases,
which progress in size, forming a blocky crystal with trac-
es of impurity phases. With further crystallization the
growth of the main hexaferrite phase occurs while the
phases of other compositions disappear. Usually a single-
phase growth of the crystal starts after ~ 15 mm of the
grown boule at the crystallization rate of 5 mm/h. Further
1208 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8
Single crystals growth of hexaferrits M-type MTixCoxFe12–2xO19 (M=Ba, Sr) by floating zone
single crystal grows according a mechanism of the com-
peting growth. This process is confirmed by x-ray Laue
analysis of grown crystals along the growth axis. Optimal
composition of the seed was clarified to provide the
growth of single crystals of the substituted M-hexaferrites
MTixCoxFe12–2xO19. The phase compositions of the two
cross sections of the boule of the SrCo2Ti2Fe8O19 are
mapped in Fig. 1 as an example, where shown are the ini-
tial nonequilibrium part of the crystal containing three
crystalline phases (a) and single phase-to-phase of the
nominal composition SrCo2Ti2Fe8O19 (b). As can be seen
from Fig. 1(a) it was found in the initial (not single phase)
part of the crystal the main M-hexaferrite SrCo2Ti2Fe8O19
phase, SrTiO3 phase and SrCo2Ti6Fe8O27 unknown phase.
The later Sr–Ti–Co enriched phases were not identified by
x ray due to its low content in the single crystal and con-
tained mainly in floating zone melt. Thus, for the imple-
mentation of the process of single crystal growth using a
single crystalline seed it is necessary to modify the initial
part of the supplying feed rod end by non-stoichiometric
composition with low content of iron oxide. It is well
known process of “moved solution crystal growth”
(MSCG), where as a solvent components of composition
are used. At MSCG the crystal growth speed should be
decreased essentially.
During growth of the BaCoxTixFe12–2xO19 (х = 0.8–1.1)
single crystals the crystallization occurs by a similar manner
as for the Sr-hexaferrite. But unlike to the Sr compounds in
the Ba ones the formation of only single impurity phase with
the BaFe2O4 structure has been observed (in the single crys-
talline sample with x = 0.9). According to the review [1], the
hexagonal BaFe2O4 were often observed as an impurity phase
for various methods of synthesis of M-type hexaferrites, par-
ticularly at temperatures below 1000 °C.
The results of the composition analysis of the Co and Ti
substituted various hexaferrites found in their single-phase
part of crystals as well as the refined unit-cell parameters
of compounds (space group P63/mmc) are presented in
Table. 1. As can be seen from Table 1 the found composi-
tions are closed to the as grown nominal compositions.
Some their differences could be attributable to a small con-
tent of impurity phases due to large crystallization speed in
this MSCG processing.
Fig. 1. Microphotography (Z-contrast) of some parts of the
cross section of the SrCo2Ti2Fe8O19 sample: (a) region corre-
sponding to the initial stages of growth including phases of
the SrTiO3 (light enable), SrCo2Ti2Fe8O19 (gray region) and
SrCo2Ti6Fe8O27 (black areas), (b) subsequent grown practically
pure SrCo2Ti2Fe8O19 phase.
Table 1. The cationic composition of the substituted single crystals and their unit-cell parameters
Nominal composition Found cationic composition, at.% Unit-cell parameters
Fe Co Ti а, Å c, Å V, Å3
BaCo0.9Ti0.9Fe10.2O19 10.17 1.00 0.90 5.8946(6) 23.237(2) 699.21
BaCo1.1Ti1.1Fe9.8O19 10.01 0,96 1.03 5.8943(7) 23.254 (3) 699.64
SrCo1.25Ti1.25Fe9.5O19 9.9 0.99 1.1 5.8835(4) 23.085 692.02
SrCo1.5Ti1.5Fe9O19 9.03 1.47 1.45 5.8839(3) 23.086(3) 692.14
SrCo1.75Ti1.75Fe8.5O19 8.36 1.84 1.81 5.884(1) 23.097(3) 692.50
SrCo2Ti2Fe8O19 8.0 2.02 1.99 5.886(3) 23.112(1) 693.41
The compositions were determined with an accuracy of ±0.06.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8 1209
A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Yu. Ivanov, and A.A. Mukhin
3.2. Electrical properties
The room temperature resistivity of grown crystals in
the surface layers exceeds 107 Ohm·cm, but at cleaves it is
equal ~ 104–106 Ohm·cm. After annealing of the single
crystalline plates of 5×5×1 mm size at temperatures of
600–800 °C the resistance at the surface was increased above
109 Ohm·cm, but at cleaves it was not exceed 106 Ohm·cm,
i.e., the resistance is increased only in a fairly thin (few
micrometers) surface layer, indicating a small oxygen dif-
fusion into the crystal. Such resistance is not enough for
the magnetoelectric measurements at room temperature.
Besides, it turned out that the current-voltage characteris-
tics are highly nonlinear, namely, the resistance signifi-
cantly decreases with the increase of applied voltage. As
temperature decreases, the resistance increases approxi-
mately exponentially (Fig. 2). At liquid nitrogen tempera-
tures the resistance becomes ~ 1010–1013 Ohm·cm, whose
value already suitable for pyroelectric measurements in the
absence of applied voltage. However, at voltage of ~ 100 V
a relatively noticeable current will leak in addition to pos-
sible pyroelectric currents. Finally, at helium temperatures,
the samples are almost perfect insulators, and voltage of
several hundred Volts is not cause the ohmic currents ca-
pable for measurements.
3.3. Magnetic properties
The results of measurements of magnetization in bar-
ium hexaferrites BaTixCoxFe12–2xO19 (x = 0.8, 0.9, and
1.1) are in a reasonable agreement with known previ-
ously data cited in the literature, for example, for single
crystals at room temperature [19] or polycrystals at liq-
uid helium [20]. Some quantitative differences could be
due to different technologies of their obtaining and,
consequently, some differences between the real and
nominal compositions (see Fig. 3). In particular, crystals
with x = 0.8 have a clear uniaxial anisotropic character
with easy direction along c axis. When x = 0.9 the ani-
sotropy is slightly reduced, and at low temperatures af-
ter reaching the saturation magnetization perpendicular
to the c axis becomes higher than along it. Crystals with
x = 1.1 have the intermediate anisotropy from the easy
axis to the easy plane. At room temperature and in weak
fields at low temperatures, they are still easier magnet-
ized along c axis, but at the field ~ 7 kOe, the magneti-
zation perpendicular to the c axis increases sharply at
low temperatures and exceeds the magnetization along
the c axis. According to Ref. 19 the hexaferrite with x =
= 1.1 behaves already like easy plane one at the room
temperature. It should also be noted that during the ini-
tial magnetization in the basal plane saturation is
achieved in stronger fields than in subsequent cycles.
The Fig. 3 also shows decrease of saturation of magnet-
ization with increasing x.
As already mentioned, neutron diffraction data indicate
the existence of the conical spin structures at low tempera-
tures for the compositions with x = 0.8 and 1.1 [19]. Our
magnetic data also suggest the presence of such structures,
at least for the compositions with x = 0.9 and 1.1. This
conclusion is supported by the presence of maximum in the
temperature dependences of the magnetization measured at
fixed fields (~ 280 K for x = 1.1 and ~ 170 K for x = 0.9).
However, temperatures at which the maxima occur depend
on the values of the field, and very slightly manifested in
measurements along the c axis. The existence of conical
structures is also confirmed by different slopes of the mag-
netization curves at liquid helium and room temperatures
(Fig. 3). In the case of x = 0.8 the slope of the curves at 5
and 295 K are approximately equal, and the maximum in
the temperature dependence of the magnetization almost is
not seen. Therefore, if the conical structure occurs for this
composition, its cone angle is quite small, so the transition
to the conical structure is not manifested significantly in
the magnetic properties.
Let us consider the magnetic properties of an isostructural
system SrTixCoxFe12–2xO19, whose systematic researches are
still not carried out so far, especially for single crystals. Mag-
netization curves at room temperature measured along and
perpendicular to hexagonal c axis for crystals with x = 1.25,
1.5, 1.75 and 2.0 are shown in Figs. 4(a)–(d). As can be seen,
Fig. 2. (Color online) The temperature dependences of electri-
cal resistivity of some SrCoxTixFe12–2xO19 crystals measured at
applied voltage of 1 or 10 V. All curves were obtained for the
annealed samples, while the green curve obtained for not an-
nealed ones.
1210 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8
Single crystals growth of hexaferrits M-type MTixCoxFe12–2xO19 (M=Ba, Sr) by floating zone
the character of the magnetic anisotropy is changed from the
easy axis (x = 1.25) to the easy plane (x ≥ 1.75) with in-
creasing of concentration x. The concentration x = 1.5 ex-
hibits an intermediate character: in weak magnetic fields
the susceptibility along the c axis is large, however, the
magnetization in the basis plane exceeds the magnetization
along the c axis in sufficiently high fields, i.e., the change
of the anisotropy in the Sr hexaferrite is similar to that ob-
served in the BaTixCoxFe12–2xO19 system near x = 1.1.
Figure 4 clearly shows the reduction of the magnetization
saturation upon the substitution of Fe to Co and Ti.
At low temperatures (Fig. 5) the magnetization curves
are more complicated: there is a noticeable hysteresis, and
the curve at the first cycle of magnetization in the basal
plane differs noticeably from the curves for subsequent
cycles, especially in the easy plane crystals. The latter could
be explained by the fact that in the demagnetized state of
easy plane crystals there are ferromagnetic domains oriented
at various directions in the basis plane and their alignment
along the magnetic field requires more efforts than a change
of resulting magnetization for 180° of the already magnet-
ized crystal. We note also the possible formation of conical
structures at low temperatures for the compositions with
x < 1.5 similar to substituted BaCoxTixFe12–2xO19 [19] and
BaScxFe12xO19 [10] systems, that is evidenced by the max-
ima in the temperature dependence of the susceptibility
along and perpendicular to the c axis as well as decreasing
in the magnetization curves slope when the temperature
goes up from helium to room ones. However, as in the case
of the BaCoxTixFe12–2xO19, system these effects are too
weak compare with the BaScxFe12xO19 [21], and only
magnetic measurements are not enough to conclude unam-
biguously about the existence of the conical structure in the
SrCoxTixFe12–2xO19 system.
Fig. 3. (Color online) The magnetization curves of the BaCoxTixFe12–2xO19 crystals of different compositions at liquid helium and room
temperatures.
Fig. 4. The magnetization curves of the SrCoxTixFe12–2xO19 crys-
tals (x = 1.25, 1.5, 1.75 and 2.0) at the room temperature.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8 1211
A.M. Balbashov, M.E. Voronchikhina, L.D. Iskhakova, V.Yu. Ivanov, and A.A. Mukhin
3.4. Magnetoelectric properties
We have tried to detect the electric polarization induced
by the joint action of electric and magnetic fields. Accord-
ing to the results of previous sections it follows immediate-
ly that at room temperature this is impossible due to high
conductivity. At nitrogen temperature (77.3 K) no change
pyrocurrent was observed with the magnetic field (only a
slow monotonic drift due to a discharge of the effective
capacitance after turning off the voltage was observed).
However, at helium temperatures in crystals with the
smallest x in the classical geometry (P ⊥ H ⊥ c) and after
preliminary cooling down in electric and magnetic fields (pol-
ing) we were able to observe very small changes of electric
polarization (Fig. 6). The curves are obtained as the result
of averaging over multiple cycles, since the small
pyrocurrent signal was not so easy to extract on the back-
ground of the parasitic drift and interference. Therefore
they allow us to estimate the effect only qualitatively and
to get its order of magnitude without claiming quantitative
analysis. Interestingly, that the polarization P(H) does not
change a sign when a magnetic field direction is inverted
thus showing the same behavior as was observed in crys-
tals BaScxFe12–xO19 in the magnetic field at the angle of
45° to the c axis at T = 30 K [10,21].
Thus, in both investigated substituted systems
BaCoxTixFe12–2xO19 and SrCoxTixFe12–2xO19 the electri-
cal polarization is manifested, which is related to the coni-
cal spin structures and their reorientation in magnetic
fields. The reason for the extremely low value of polariza-
tion is likely to be fairly low electrical resistivity of crys-
tals that does not allow preliminary polarization of samples
by cooling down in a sufficiently large electric field from
the temperatures above than the temperature of the transi-
Fig. 5. The magnetization curves of the SrCoxTixFe12–2xO19 crystals (x = 0.9, 1.25 and 1.5) at helium temperatures.
Fig. 6. The field dependences of electric polarization of the
BaCo0.8Ti0.8Fe10.4O19 and SrCo0.9Ti0.9Fe10.2O19 crystals at liq-
uid helium temperatures.
1212 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 8
Single crystals growth of hexaferrits M-type MTixCoxFe12–2xO19 (M=Ba, Sr) by floating zone
tion from the conical state to the collinear ferrimagnetic
one. Our study of Ba(Sr)ScxFe12–xO19 [21] have shown
that the value of the induced polarization is very sensitive
to the magnitude of the poling electric field which decrease
significantly at low values of electric fields. Two opposing
factors are very important: (i) larger polarization is ex-
pected in crystals with large x and large angles of the coni-
cal structure, (ii) but they have a higher transition tempera-
ture (> 200 K for x > 1) that hampering a poling of a sample.
On the other hand, crystals with smaller x have the lower
transition temperature and can be cooled down in stronger
electric fields, but a collinear ferrimagnetic structure is
preserved up to the lowest temperatures and, if the conical
structure still occurs, its angle remains too small resulting
in a low polarization. Therefore, it is very important to
increase of the electrical resistivity of the grown crystals
that requires further technological developments.
4. Conclusion
Thus, as a result of the presented researches it is estab-
lished:
1. High-quality single-phase hexagonal M-type barium
and strontium ferrites substituted by titanium and cobalt
can be obtained by floating zone melting.
2. Their magnetic properties indicate the existence of
the conical spin structures at some temperatures and con-
centrations, which promote of magnetoelectric properties.
3. Field-induced electric polarization is observed for Ti
and Co concentrations of x = 0.8–0.9. Very low value of
the polarization is due to high electrical conductivity of the
grown crystals which does not allow properly implement
the preliminary poling of the samples and requires further
improving of the technology.
The work is supported by the Russian Scientific Foun-
dation (project 16-12-10531).
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1. Introduction
2. Experimental
3. Experimental results and discussion
3.1. Crystal growth details
3.2. Electrical properties
3.3. Magnetic properties
3.4. Magnetoelectric properties
4. Conclusion
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