ISSN 1063-7842, Technical Physics, 2015, Vol. 60, No. 11, pp. 1658–1662. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.B. Boledzyuk, Z.D. Kovalyuk, Z.R. Kudrinskii, A.D. Shevchenko, 2015, published in Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 60, No. 11, pp. 86–90.
SOLID STATE
Structural Characteristics and Magnetic Properties of Cobalt-Intercalated A 52B63 Single Crystals V. B. Boledzyuka, Z. D. Kovalyuka, Z. R. Kudrinskiia, and A. D. Shevchenkob a
Institute for Problems in Materials Science, National Academy of Sciences of Ukraine, Chernovtsy Branch, ul. Vil’de 5, Chernovtsy, 58001 Ukraine b Institute for Metal Physics, National Academy of Sciences of Ukraine, bul. Vernadskogo 36, Kyiv, 03680 Ukraine e-mail:
[email protected] Received January 13, 2015
Abstract—The magnetic properties of the Bi2Te3 and Bi2Se3 single crystals intercalated by cobalt in a dc magnetic field are considered. The effect of a dc magnetic field on the introduction and the magnetic properties of the formed intercalates is studied. It is shown that the introduction of cobalt into Bi2Te3 (Bi2Se3) is accompanied by a change in the lattice parameters depending on the presence of a magnetic field during intercalation and that the structure type of the crystals remains unchanged. Co0.15Bi2Te3 and Co0.15Bi2Se3 intercalates exhibit ferromagnetism: the magnetic-field dependence of the magnetic moment has the shape of hysteresis loops typical of hard magnetic ferromagnets. DOI: 10.1134/S1063784215110079
INTRODUCTION Layered compounds A 52B36 and the solid solutions based on them are widely used as materials for thermoelectric transducers [1, 2]. The main structural unit of these compounds is a five-layer unit consisting of alternating monoatomic planes of bismuth and chalcogenide, …Te(Se)–Bi–Te(Se)–Bi–Te(Se)…. Atoms inside the unit are connected by strong covalent bonds and van der Waals forces are operative between units. Interest in studying such materials doped with various metals has quickened, since the joining of metallic and semiconductor layers on a microscopic level can result in the appearance of crystals with new electronic properties. The ferromagnetic properties of semiconductor systems, such as GaAs, Ge [3, 4], and GaSe [5], were investigated. It was experimentally found that p-Bi2Te3 doped with Fe acquires a ferromagnetic order at T = 12 K [6]. A so-called diluted semiconductor forms when a low content of a magnetic impurity is introduced into a semiconductor lattice in the absence of a direct exchange interaction between magnetic atoms [7]. Here, an exchange interaction occurs due to free charge carriers. This finding opens up fresh opportunities for the use of such materials in spintronics [8, 9]. EXPERIMENTAL Bi2Te3 and Bi2Se3 single crystals were grown by the Bridgman technique from components taken in the
stoichiometric ratio. The crystals were solidified into a layered structure the layers of which were perpendicular to a threefold symmetry axis and had a rhombohe5 dral structure with space group D3d (R3m). The lattice parameters were a = 4.3838 Å and c = 30.487 Å for Bi2Te3 and a = 4.147 Å and c = 28.681 Å for Bi2Se3. The crystal structure, the parameters, and the properties of layered A 2V B VI compounds were described in detail in [2, 10]. Samples for an investigation were cleaved from single-crystal ingots along the (0001) layer plane. The introduction of Co2+ ions was performed by electrochemical intercalation using the drawing electric field method [11]. As an electrolyte, we used a saturated aqueous CoSO4 solid solution. To prevent the deposition of an introduced impurity or its salts on the samples or electrochemical cell electrodes, interpolation was carried out under galvanostatic conditions at currents lower than 0.4 mA/cm2. The crystal structures of the initial and cobalt-intercalated Bi2Te3 and Bi2Se3 single-crystal samples were controlled by X-ray diffraction on a DRON-2.0 diffractometer using CuKα radiation. The measurement results were processed by the Rietveld method. Since cobalt belongs to the group of transition ferromagnetic 3d metals, penetration was performed in the presence and absence of a dc magnetic field, which was normal to crystallographic axis c of a crystal. A magnetic field was created by two permanent neodymium magnets between which a sample to be intercalated was placed.
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The magnetic field at the site of sample location was 4 kOe. The magnetic properties of Co0.15Bi2Te3 and Co0.15Bi2Se3 intercalates were studied by magnetometry on a Vibrating Magnetometer 7404 VSM vibratingsample magnetometer. The magnetometer sensitivity was ~10–7 emu, which allowed us to investigate a magnetic moment in samples several milligrams in mass. The magnetic moment was measured at room temperature along and across the crystal layer plane. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction patterns of cobalt-intercalated Bi2Te3 single crystals formed (a) without and (b) with a magnetic field. A comparison of these X-ray diffraction patterns with those of pure Bi2Te3 samples indicate that the structure type and the space group of the Co0.15Bi2Te3 intercalates remain 00-15
3136 Bi2Te3 Co0.15Bi2Te3
(a)
00-21
00-12
00-3
00-9
00-6
00-18
1568
0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 2θ, deg 00-15
3134
00-24
00-12
00-9
00-6
00-3
00-18
1567
(b)
00-21
Bi2Te3 Co0.15Bi2Te3
0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 2θ, deg Fig. 1. X-ray diffraction patterns of initial Bi2Te3 samples and Co0.15Bi2Te3 intercalates formed by intercalation (a) without and (b) with a dc magnetic field. TECHNICAL PHYSICS
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unchanged irrespective of the presence of a magnetic field during intercalation. The penetration of Co2+ ions in the crystal structure of Bi2Te3 is accompanied by an insignificant broadening of the diffraction peaks of the intercalated samples. Moreover, the Co0.15Bi2Te3 intercalates formed in a magnetic field have weak additional peaks, which are likely to be caused by cobalt localized in the interlayer space (Fig. 1b). The lattice parameters of the Co0.15Bi2Te3 formed without and with a magnetic field are a = 4.3439 Å, c = 30.3954 Å and a = 4.3586 Å, c = 30.4384 Å, respectively. When comparing these data with the lattice parameters of Bi2Te3 [2], we can state that the intercalation of Bi2Te3 by cobalt leads to a decrease in the lattice parameters of the samples. It should be noted that a similar change in the lattice parameters is also observed in the Co0.15Bi2Se3 intercalates. This decrease in the lattice parameters is characteristic of many layered semiconductors intercalated by the irongroup 3d transition metals [5, 12–15] and is caused by the hybridization of the 3d orbitals of metal atoms (in our case, Co) with the p states of a chalcogen (Te, Se). This hybridization favors the appearance of covalentlike bonds between an intercalant and the crystalline matrix, and these bonds contract the crystalline layers, which results in a decrease in the lattice parameters. During the electrochemical intercalation of bismuth telluride by cobalt, the intercalant is localized in van der Waals gaps Te(1)–Te(1) formed by neighboring layer atoms—telluride quintets [5, 13]. Foreign atoms or molecules behave as an independent system and retain the properties that are inherent in them before intercalation. The authors of [16, 17] showed that the intercalation of Bi2Te3 by metal (Cu, Ag, Ni) atoms along the plane of (0001) layers is accompanied by the accumulation, redistribution, and formation of intercalant nanofragments in the interlayer space; that is, an additional layer of nanofragments of an intercalated metal appears in the crystal lattice. Bi2Te3 and Bi2Se3 are diamagnets. When introducing transition metal atoms into the structure of such compounds, one can form magnetic composite nanostructures, which consists of a semiconductor layered matrix and metallic magnetic layers located along the plane of crystal layers [18]. Moreover, the magnetic properties of intercalated layered semiconductors can be controlled in intercalation and is performed in a dc magnetic field [5, 13], since this field affects the ordering and self-organization of impurities in a layered matrix [19]. Figure 2 shows the magnetic moment of a cobaltintercalated Bi2Te3 single crystal versus the magnetic field. Figure 3 shows m = f(H) dependences for Co0.15Bi2Se3 intercalates. As is seen from these curves, both intercalated compounds have a hysteresis loop that is characteristic of ferromagnets [20]. These compounds exhibit a ferromagnetic hysteresis loop at T =
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Specific magnetic moment mS and coercive force Hc for interstitial Co0.15Bi2Te3 and Co0.15Bi2Se3 compounds Co0.15Bi2Te3 Magnetic moment measured
introduction without magnetic field
Co0.15Bi2Se3 introduction with magnetic field
mS, emu/g
Hc, G
mS, emu/g
Hc, G
mS, emu/g
Hc, G
Along layer plane
2.168 × 10–3
162.63
20.236 × 10–3
201.89
0.1762
99.63
Across layer plane
0.731 × 10–3
265.42
11.257 × 10–3
262.14
0.1023
487.53
300 K, in contrast to the Bi1.92Fe0.08Te3 compound, which exhibits a hysteresis loop at low temperatures (T = 2 K) [21]. Layered semiconductors are known to be strongly anisotropic due to their crystal structure [22]. The hysteresis loop width of the Co0.15Bi2Te3 samples formed in a magnetic field (Fig. 2b) is larger than that of the Co0.15Bi2Te3 intercalates formed without a magnetic field (Fig. 2a). In both cases, this width depends on crystallographic orientation. The mag-
0.20
1
(a)
0.15
0.001
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0
Moment/mass, emu/g
Moment/mass, emu/g
0.002
netic moment measurements along and across the plane of intercalate layers in Co0.15Bi2Te3 and Co0.15Bi2Se3 showed that the m = f(H) dependence in the former case is typical of a ferromagnetic interaction between cobalt atoms. In the case of measurements performed normal to the layers, the m = f(H) dependence corresponds to a ferromagnetic interaction between the layers. The table gives the specific
0.10 0.05
2
0
−0.05
−0.001
−0.10 −0.15
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0 1000 2000 3000 Field, Oe
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0.01 0 −0.01 −0.02 −3000 −2000 −1000
Moment/mass, emu/g
(b) Moment/mass, emu/g
1
(a)
1
(b)
0.05 2
0
−0.05 −0.10
0 1000 2000 3000 Field, Oe
Fig. 2. Specific magnetic moment vs. the magnetic field (1) along and (2) across the layer plane for the Co0.15Bi2Te3 intercalates formed by intercalation (a) without and (b) with a dc magnetic field.
−8000
−4000
0 Field, Oe
4000
8000
Fig. 3. Specific magnetic moment vs. the magnetic field (a) along and (b) across the layer plane for (1) Co0.15Bi2Se3 and (2) Co0.15Bi2Te3 intercalates formed by intercalation in a dc magnetic field. TECHNICAL PHYSICS
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saturation magnetic moment (mS) and the coercive force (Hc) for both types of samples. The obtained coercive forces are characteristic of hard magnetic ferromagnets [20]. As noted above, the magnetic properties of layered semiconductors intercalated by transition metals, which have unfilled 3d electron shells, are determined by the exchange interaction between the magnetic ions of an intercalant localized in the octahedral and tetrahedral interstices of the interlayer space [12, 23]. Layered crystals are characterized by point defects (chalcogen vacancies) in the basal planes of anions. During the introduction of cobalt into A 52B36 compounds, intercalant atoms can accumulate and occupy the sites around dislocation pits and chalcogen vacancies on the (0001) plane in Bi2Te3 or Bi2Se3. Intercalant atoms move in a drawing electric field in the interlayer space of a layered crystal, touch a primary nucleus at the site of a chalcogen vacancy, adhere to it, and form impurity nanoinclusions characterized by a domain structure. The application of a magnetic field in combination with an applied electric field during the cobalt intercalation of bismuth chalcogenides influences intercalant diffusion, the kinetic processes of nucleation and growth of Co nanoinclusions, and their ferromagnetic ordering. It should also be noted that the presence of a magnetic field during intercalation and the formation of nanoinclusions of 3d transition metal (in particular, Co) atoms in layered crystals can lead to a magnetic exchange interaction between intercalant nanoinclusions, the so-called collective ferromagnetism of magnetic impurity clusters. The self-organization of nanoinclusions in the van der Waals planes of various layered crystals depends on the electronic structure of these planes and the presence of defects in them [24]. The formation of antisite defects, which are caused by the passage of Bi atoms to the sites of Te, is energetically favorable in Bi2Te3 compounds. Bi2Se3 single crystals are characterized by the existence of donor defects, where Bi atoms are located at interstices. The formation of the antisite defects induced by the passage of Bi atoms to the sites of Se in the lattice is likely to be hindered because of the strong difference between the sizes of these atoms. This feature can also cause the differences in the magnetic properties that were detected in the Bi2Te3 and Bi2Se3 layered crystals intercalated by Co2+ ions in a magnetic field under the same conditions (Fig. 3). CONCLUSIONS During the electrochemical intercalation of layered crystals by cobalt atoms, the structure of complex quintet layers remains unchanged and the structure type of the formed Co0.15Bi2Te3 and Co0.15Bi2Se3 intercalates remains the same. The cobalt intercalation of Bi2Te3 and Bi2Se3 was found to decrease the lat-
A 52B36
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tice parameters of the intercalates as compared to the initial crystals, and the change of the lattice parameters depends on the presence of a magnetic field during intercalation. The detected change in the lattice parameters in the formed intercalates is caused by the hybridization of the 3d orbitals of Co with the p states of chalcogen (Te, Se). The penetration of cobalt into Bi2Te3 (Bi2Se3) induces magnetic properties in Co0.15Bi2Te3 (Co0.15Bi2Se3) intercalates, and they manifest themselves in the dependence of the magnetic moment on the magnetic field, which has the shape of a hysteresis loop. This fact is thought to be caused by the formation of Co nanoinclusions (clusters) at the site of Te (Se) vacancies on the (0001) surface in Bi2Te3 (Bi2Se3). The application of a magnetic field during the cobalt intercalation of A 52B36 compounds influences the nucleation and growth of Co nanoinclusions and causes a magnetic exchange interaction between them (collective ferromagnetism of magnetic impurity clusters). REFERENCES 1. S. A. Aliev and E. I. Zul’fugarov, Thermomagnetic and Thermoelectric Phenomena in Science and Engineering (ELM, Baku, 2009). 2. B. M. Gol’tsman, V. A. Kudinov, and I. A. Smirnov, Semiconductor Thermoelectrical Materials Based on Bi2Te3 (Nauka, Moscow, 1972). 3. Y. D. Park, A. T. Hanbicki, S. C. Erwin, C. S. Hellberg, J. M. Sullivan, J. E. Mattson, T. F. Ambrose, A. Wilson, G. Spanos, and B. T. Jonker, Science 295, 651 (2002). 4. S. Cho, Y. Kim, S. Choi, S. C. Hong, B. J. Kim, J. H. Jung, Y. S. Kim, and J. B. Ketterson, Phys. Rev. B 66, 033303 (2002). 5. Z. D. Kovalyuk, V. B. Boledzyuk, V. V. Shevchik, V. M. Kaminskii, and A. D. Shevchenko, Semiconductors 46, 971 (2012). 6. V. A. Kulbachinskii, A. Yu. Kaminskii, K. Kindo, Y. Marumi, K. Suga, P. Lostak, and P. Svanda, Physica B 311, 292 (2002). 7. R. B. Morgunov and A. I. Dmitriev, Phys. Solid State 51, 1985 (2009). 8. A. Fert, Phys. Usp. 51, 1336 (2008). 9. Yu. G. Kusraev, Phys. Usp. 53, 725 (2010). 10. D. M. Chizhikov and V. P. Schastlivyi, Selenium and Selenides (Nauka, Moscow, 1964). 11. I. I. Grigorchak, Z. D. Kovalyuk, and S. P. Yurtsenyuk, Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 17, 412 (1981). 12. Magnetism of Nanosystems Based on Rare-Earth and 3dTransitional Metals, Ed. by V. O. Vas’kovskii (Ural. Univ., Yekaterinburg, 2007). 13. V. B. Boledzyuk, A. D. Shevchenko, and Z. R. Kudrins’kii, Zh. Nano-Elektron. Fiz. 4, 03017 (2012). 14. A. V. Kuranov, V. G. Pleshchev, A. N. Titov, N. V. Baranov, and L. S. Krasavin, Phys. Solid State 42, 2089 (2000).
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Translated by K. Shakhlevich
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