ISSN 0020-1685, Inorganic Materials, 2018, Vol. 54, No. 2, pp. 140–146. © Pleiades Publishing, Ltd., 2018. Original Russian Text © E.P. Domashevskaya, N.S. Builov, A.N. Lukin, A.V. Sitnikov, 2018, published in Neorganicheskie Materialy, 2018, Vol. 54, No. 2, pp. 153–159.
IR Spectroscopic Study of Interatomic Interaction in [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 Multilayer Nanostructures with Metal-Containing Composite Layers E. P. Domashevskayaa, *, N. S. Builova, A. N. Lukina, and A. V. Sitnikovb aVoronezh
State University, Universitetskaya pl. 1, Voronezh, 394018 Russia Voronezh State Technical University, Moskovskii pr. 14, Voronezh, 394018 Russia *e-mail:
[email protected]
b
Received April 17, 2017; in final form, September 6, 2017
Abstract⎯This paper presents an IR spectroscopic study of chemical bonds between components of amorphous [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 multilayer nanostructures (MNS’s) made up of metal-containing composite layers and different interlayers, which influence their electromagnetic properties. Our results demonstrate that, even though the MNS’s have identical elemental compositions, their IR spectra differ significantly. The reason for this is that the main contribution to the IR spectrum of the [(CoFeB)60C40/SiO2]200 MNS is made by its SiO2 interlayers. The formation of other bonds with oxygen and silicon is blocked by the carbon present in the (CoFeB)60C40 composite layers, as evidenced by the presence of the strongest mode in the IR spectra of this structure, which corresponds to boron carbide, BC. The considerable intensity redistribution to the low-frequency region in the IR spectrum of the [(CoFeB)34(SiO2)66/C]46 MNS, containing carbon interlayers, is due to the incorporation of nominal SiO2 into the metal-containing composite layers and the partial redistribution of oxygen bonds from the SiO2 to the 3d transition metals, resulting in the formation of metal oxides and a silicon suboxide. The interaction of the carbon present in the interlayers between the composite layers with elements of the composite layers, in particular with boron, is considerably weaker in comparison with the other MNS, [(CoFeB)60C40/SiO2]200, which has oxide interlayers. Keywords: IR spectroscopy, identification of IR spectral modes, [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 multilayer nanostructures, (CoFeB)60C40 and (CoFeB)34(SiO2)66 metal-containing composite layers, silicon oxide and carbon nonmetallic interlayers DOI: 10.1134/S002016851802005X
INTRODUCTION Research interest in nanostructures is stimulated by the possibility of significant modification and radical changes in the properties of known materials upon a transition to a nanocrystalline state. Low-dimensional magnetic materials exhibit unusual physical phenomena, such as a colossal magnetoresistance, magneto-optical response, planar Hall effect, and other effects, which are not only of scientific interest per se but also of practical importance for applications in solid-state electronics [1–4]. The magnetic properties of heterophase systems depend on many parameters of the atomic and electronic structures of their components. In particular, multilayer films consisting of superparamagnetic composite layers and nonmagnetic interlayers, including a [(Co40Fe40B20)34(SiO2)66/C]46 multilayer nanostructure (MNS) to be studied, have interesting features. As shown by Dunets et al. [5], if the thickness of
a ferromagnet–insulator composite layer is a few nanometers, its composition is below the percolation threshold, and there is a continuous layer of a nonmagnetic interlayer, the multilayer system undergoes a transition from a superparamagnetic state of the layers to ferromagnetic order in the entire system at room temperature. In previous work [6, 7], the electronic structure and phase composition of the nonmetallic interlayers and interfaces in [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 MNS’s were analyzed by small-angle X-ray diffraction (SAXRD) and ultrasoft X-ray emission spectroscopy (USXES). The structures were shown to be amorphous, but the former MNS, containing SiO2 interlayers, retained planar interfaces. As a result, the MNS had a superstructure of bilayers, which gave sharp diffraction peaks for four orders of reflection in SAXRD studies [6]. At the same time, in the case of the latter MNS, containing C inter-
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layers, we observed a considerable decrease in the number and intensity and considerable broadening of SAXRD reflections, which suggests broadening of the interfaces between the metal-containing composite layers and carbon interlayers as a result of interatomic interactions at the interfaces in the [(Co40Fe40B20)34(SiO2)66/C]46 MNS [7]. Moreover, no 3d transition metal silicides were detected on the composite/nonmetallic interlayer interfaces in USXES depth profiling studies [6, 7], unlike in the case of a (Co45Fe45Zr10/SiO2)32 MNS and, especially, (Co45Fe45Zr10/a-Si)40, containing amorphous silicon interlayers [8]. The lack of silicide formation on the interfaces in the MNS’s with C under investigation is attributable to a “shielding” effect of the carbon, which forms carbide phases. In addition, USXES depth profiling results for the carbon-containing MNS’s indicate that the composition of the dielectric component of the MNS’s deviates from the stoichiometry of the sputtered quartz toward lower oxygen content, resulting in the formation of the SiO1.3 suboxide in the case of [(CoFeB)60C40/SiO2]200 and SiO1.7 in the case of [(CoFeB)34(SiO2)66/C]46. The purpose of this work is to gain insight into interatomic interactions both within the metal-containing (M) composite layers and between the M layers and interlayers in the [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 MNS’s using IR spectroscopy, which is one of the most powerful modern physical tools for probing interatomic interactions and chemical bonding by measuring vibrational spectra of chemical compounds. EXPERIMENTAL [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 MNS’s were produced by ion-beam sputtering of two targets onto a rotating glass-ceramic substrate [9, 10]. The metallic component of the composites in both MNS’s was Co40Fe40B20, an easily amorphizable alloy. For each MNS, we used two targets, one of which was composite. In the case of the [(CoFeB)60C40/SiO2]200 samples, the composite target was made up of the Co40Fe40B20 alloy and graphite inserts, and the other target was a SiO2 (quartz) plate. In the case of the [(CoFeB)34(SiO2)66/C]46 samples, the composite target was made up of the Co40Fe40B20 alloy and SiO2 (quartz) inserts, whereas the other target contained graphite. During the sputter deposition process, a Vshaped shield was placed between the target and substrate, which allowed the thickness of the composite layers and interlayers to be varied in a wide range, depending on the mutual arrangement of the target and substrate. This allowed a composite layer of continuously varying thickness to be grown in a single processing cycle. Immediately before multilayer film growth, the substrate surface was cleaned by ion millINORGANIC MATERIALS
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ing in order to improve the surface adhesion between the growing structure and substrate. Next, using the process parameters chosen (the voltage applied to the ion-beam sputtering sources, plasma current, argon pressure over each source, total pressure in the sputter chamber, substrate rotation rate, and others), we simultaneously sputtered the two targets. When the substrate holder passed through the position corresponding to the sputtering of each target, a particular layer of the structure was grown. The thickness of the bilayers in the MNS’s, consisting of metal-containing composite layers and silicon dioxide or carbon nonmetallic interlayers, was determined by SAXRD and varied from ~4 to 8 nm. A total of four samples were characterized by IR spectroscopy—two samples differing in thickness for each MNS: samples 1462-1 and 1462-2 for the [(CoFeB)60C40/SiO2]200 MNS and samples 1112-1 and 1112-5 for the [(CoFeB)34(SiO2)66/C]46 MNS. The four-digit series number, 1462 or 1112, specifies the number of the processing step in the fabrication of MNS samples with a given composition on a glassceramic substrate with a gradually varying bilayer thickness. An increase in the one-digit number after the series number corresponds to an increase in the overall MNS thickness on account of an increase in bilayer thickness in the range indicated above. The IR spectra of the above-mentioned MNS samples were measured on a Bruker VERTEX 70 Fourier transform infrared spectrometer (Shared Research Facilities Center, Voronezh State University), which is intended for measuring optical transmission and reflection spectra in the mid-IR spectral region: 400 to 4000 cm–1. IR reflection spectra were obtained by frustrated total internal reflection (FTIR) measurements. The spectra were decomposed into Gaussian components using MagicPlot Student software. RESULTS AND DISCUSSION The [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 MNS’s under investigation differ fundamentally in that the C and SiO2 in them are located either in the composite layers and interlayers, respectively, or conversely. Whereas in the former MNS the carbon enters into the composition of the (CoFeB)60C40 metal-containing composite layers and the SiO2 forms dielectric interlayers, in the latter MNS, conversely, the SiO2 enters into the composition of the composite layers, whereas the carbon forms interlayers. Given that, as shown earlier [6, 7], there is no silicide formation on the interfaces in the MNS’s under consideration and taking into account the position of the C and SiO2 in the composition or interlayer, we expect the formation of carbides in [(CoFeB)60C40/SiO2]200
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0.14 0.12
Fe—O 1168—
1231—
B—C
Si—O—Si 1108—
0.08
Co—Si—O
454— Si—C Si—O—Si 827
0.02
B—C Co—Si—O 790—
968—
Si—O
337—
0.04
O—Si—O
397—
0.06
496—
Fe—O (Co—Fe) 1038—
Absorbance
0.10
0 1400
1200
1000
800 600 Wavenumber, cm—1
400
200
Fig. 1. IR spectrum of sample 1462-1 of the [(CoFeB)60C40/SiO2]200 MNS in the range 100–1400 cm–1.
MNS samples and the formation of oxides and silicides in [(CoFeB)34(SiO2)66/C]46 MNS samples. The solid lines in Figs. 1–4 represent measured IR spectra of the samples of the two types of MNS and the dashed lines represent model spectra obtained via decomposition into Gaussian components. Table 1 lists the peak positions of the modes obtained by decomposing the IR spectra of the MNS’s under investigation and identified using previously reported data [11–22]. Figures 1 and 2 show the IR spectra of the two [(CoFeB)60C40/SiO2]200 MNS samples: 1462-1 (thinner) and 1462-2 (thicker). It is seen from the results obtained that, in these MNS’s, absorption is stronger in the high-frequency region of their spectra, which contains modes corresponding to bonds of the 3d transition metals Co and Fe and Si with oxygen. In addition, this region contains the strongest mode, B–C, which results from the interaction between the nonmetallic elements—boron and carbon—in the (CoFeB)60C40 composite layers. This attests to the formation of boron carbide on the interface between the metallic granules and carbon in the composite layers. The Si–C mode (827 and 829 cm–1) of the silicon carbide forming on the composite layer/SiO2 interlayer interface is considerably weaker in this MNS.
Figures 3 and 4 show the IR spectra of the two samples of the other MNS, [(CoFeB)34(SiO2)66/C]46: 1112-1 (thinner) and 1112-5 (thicker). In the IR spectra of this MNS, which has carbon interlayers, the intensity of the IR spectrum redistributes to a lower frequency region, where bonds of cobalt and silicon oxides prevail. The B–C modes in the high-frequency region have considerably lower relative intensity, suggesting that there is a lower probability of the formation of boron carbides through a SiO2 layer, which shields metallic granules in the (CoFeB)34(SiO2)66 composite. Note also that this MNS has few or no modes of silicon carbide. For comparison, Fig. 5 shows IR spectra of thermal SiO2 reported by Lucovsky et al. [23]. The relative mode intensities in these spectra correlate well with those in the IR spectra obtained in this study for the [(CoFeB)60C40/SiO2]200 MNS samples. This suggests that the SiO2 in the interlayers mainly retains the silicon–oxygen bonds and does not destroy interfaces, which is consistent with the SAXRD results reported previously for these samples [6]. Comparison of the IR spectra of the two types of MNS demonstrates a considerable intensity redistribution from the high-frequency region to the low-frequency one as the SiO2 moves from the interlayers in the MNS’s to the composite of the metal-containing composite layers. INORGANIC MATERIALS
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0.14 0.12
Fe—O 1162—
1226—
B—C
1103—
Si—O—Si
0.08
Fe—O (Co—Fe) 495—
Co—Si—O
O—Si—O 456—
0.04
966—
Si—O
Co—Si—O
Si—C Si—O—Si
B—C 389—
792—
829
0.02
333—
0.06
1034—
Absorbance
0.10
0 1400
1200
1000
800 600 Wavenumber, cm—1
400
200
Fig. 2. IR spectrum of sample 1462-2 of the [(CoFeB)60C40/SiO2]200 MNS in the range 100–1400 cm–1.
1.2
1.0
433—
905—
274—
Fe—O Co—Fe—O 136
862—
949
Co—O
Fe—O (Co—Si—O) 206—
1337—
B—C
1188—
0.2
B—C
1139—
Si—O B—C
997—
0.4
1043—
Co—Si—O Co—O Si—O
Si—B (B—C)
Co—Si—O
111—
O—Si—O (B—C)
Co—Fe—O
321—
0.6
381—
Fe—O (Co—Si—O)
1090—
Absorbance
0.8
0 1400
1200
1000
800 600 Wavenumber, cm—1
400
200
Fig. 3. IR spectrum of sample 1112-1 of the [(CoFeB)34(SiO2)66/C]46 MNS in the range 100–1400 cm–1.
In the composite layers of the [(CoFeB)34(SiO2)66/C]46 MNS, the components of the SiO2 form stronger bonds with the metals, Co–Si–O and Co– Fe–O, in comparison with which the modes of the silINORGANIC MATERIALS
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icon dioxide show up more weakly. This is accompanied by changes in the stoichiometric composition of the SiO2: the formation of the SiO1.7 suboxide along with the dioxide in the composite layers [7].
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0.6
0.5
438—
0.3
Co—Fe—O B—C 897
944—
Co—O Si—O
856—
B—C
1077—
1323—
B—C
1167—
0.1
1119—
Si—B (B—C) Si—O
988—
0.2
Co—O 217—
Co—Si—O
118— Fe—O (O—Si—O)
O—Si—O (B—C)
1032
Absorbance
379—
Co—Si—O
146
0.4
Co—O Co—O (Co—Si—O) (O—Si—O) 273—
327—
Co—Si—O
0 1400
1200
1000
800 600 Wavenumber, cm—1
400
200
Fig. 4. IR spectrum of sample 1112-5 of the [(CoFeB)34(SiO2)66/C]46 MNS in the range 100–1400 cm–1.
Table 1. Modes in the IR spectra of two [(CoFeB)60C40/SiO2]200 MNS samples of series 1462 and two [(CoFeB)34(SiO2)66/C]46 MNS samples of series 1112 in the range 100–1400 cm–1 Mode, cm–1 [(CoFeB)60C40/SiO2]200 1462-1
1462-2
1231 1168
1226 1162
1108
1103
1038 968
1034 966
827 790 496 454
829 792 495 456
397
389
337
333
[(CoFeB)34(SiO2) 66/C]46 1112-1
1112-5
1337 1188
1323 1167
1139
1119
1090 1043 997 949 905 862
1077 1032 988 944 897 856
433
452
381 323 274 206
399 327 273 217 146
136 118 111
Previous results B–C: 1340 [22] B–C: ~1200 [11] Fe–O: 1160 [12] Si–O: 1120, 1150–1170 [22] Si–O–Si: 1080 [13] 1100 [14], [15] Si–B: 1080 [22] B–C: 1110 [21] Co–Si–O: 1030 [22] Si–O: 980 [17] 960 [22] 1000 [20] Co–O: 936–940 [16] Co–Fe–O: 900 [22] B–C: 850 [21] Si–C: 817 [18] Si–O–Si: 800 [13] Fe–O: 496 [20] Co–Fe: 489,516–540 [19] O–Si–O: 454–456 [20] B–C: 420 [21] O–Si–O: 454–456 [20] B–C: 400 [22] Co–Si–O: 375 Fe–O: 375 [22] Co–Si–O: 320–330 [22] Co–O: 275 Co–Si–O: 260 [22] Co–Si–O: 205 Co–O: 200, 220 Fe–O: 200 [22] O–Si–O: 150 Fe–O: 135 [22] Fe–O: 135 [22] O–Si–O: 120 Co–O: 120 [22] Co–Fe–O: 105 [22]
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1077 cm–1 1070 cm–1
Stretching
Absorbance
1066 cm–1
Rocking Bending
1000°C 800°C 700°C 1400
1200 1000 800 Wavenumber, cm–1
600
400
Fig. 5. IR spectra of silicon oxide (SiO2) layers grown on annealed Si in dry oxygen at temperatures of 700, 800, and 1000°C; borrowed from Lucovsky et al. [23].
CONCLUSIONS The present results demonstrate that, even though the MNS’s have identical elemental compositions, their IR spectra differ significantly not only in shape but also in the number of modes. The most striking distinction is the intensity redistribution between the high- and low-frequency regions of the spectra. The main cause of this is that, in the case of the [(CoFeB)60C40/SiO2]200 MNS, the predominant contribution to the IR spectrum is made by the SiO2 interlayers. Formation of other bonds with oxygen and silicon is blocked by the carbon present in the (CoFeB)60C40 composite layers, as evidenced by the presence of the strongest mode in the IR spectra of this structure, which corresponds to boron carbide, BC, and the considerably weaker Si–C mode. In the case of the [(CoFeB)34(SiO2)66/C]46 MNS, the significant intensity redistribution in its IR spectra to the low-frequency region is due to the incorporation of SiO2 into the metal-containing composite layers and the formation of Co–Si–O and Co–Fe–O bonds in these layers. The interaction of the carbon present in the interlayers between the composite layers with elements of the composite layers, in particular with boron atoms, is considerably weaker. Thus, IR spectroscopy offers the possibility of assessing how the composition of chemical bonds in MNS’s is influenced by a particular nonmetallic component in relation to its position in the composite layers or interlayers. INORGANIC MATERIALS
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The distinctions between the IR spectra and the intensity redistribution between the high- and lowfrequency regions in the spectra of the two types of MNS are due to the following: The main contribution to the IR spectrum of the [(CoFeB)60C40/SiO2]200 MNS is made by the SiO2 interlayers in the form of many Si–O–Si bonds. The carbon in the composite layers of the [(CoFeB)60C40/SiO2]200 MNS blocks the formation of bonds of the metals with the silicon and oxygen in the interlayers, forming bonds with the boron within the composite layers. The intensity redistribution to the low-frequency region in the IR spectrum of the MNS containing carbon interlayers, [(CoFeB)34(SiO2)66/C]46, is due to the formation of Co–Si–O and Co–Fe–O bonds in the composite layers with the participation of silicon and oxygen atoms from the dielectric component of the same layers in the MNS. The carbon present in the interlayers between the composite layers forms few or no interatomic bonds with the elements of the composite. ACKNOWLEDGMENTS This work was supported by the Russian Federation Ministry of Education and Science (state research target for higher education institutions in 2017–2019, project no. 3.6263.2017/VU). REFERENCES 1. Buravtsova, V.E., Guschin, V.S., and Kalinin, Yu.E., Kirov, S.A., Lebedeva, E.V., Phonghirun, S., Sitnikov, A.V., Syr’ev, N.E., and Trofimenko, I.T., Magnetooptical properties and FMR in granular nanocomposites (Co84Nb14Ta2)x(SiO2)1001 − x, Open Phys., 2004, vol. 2, no. 4, pp. 566–578. 2. Buravtsova, V.E., Gan’shina, E.A., Gushchin, V.S., Kalinin, Yu.E., Phonghirun, S., Sitnikov, A.V., Stognei, O.V., and Syr’ev, N.E., Colossal magnetoresistance and magneto-optical properties of granular metal–insulator nanocomposites, Izv. Akad. Nauk, Ser. Fiz., 2003, vol. 67, no. 7, pp. 918–920. 3. Gushchin, B.S., Kalinin, Yu.E, Lebedeva, E.V., Phonghirun, S., Sitnikov, A.V., Syr’ev, N.E., Trofimenko, I.T., and Cheol Gi Kim, Effect of percolation processes on the ferromagnetic resonance and magneto-optical properties of granular nanocomposites, Izv. Akad. Nauk, Ser. Fiz., 2004, vol. 68, no. 5, pp. 717–719. 4. Vyzulin, V.A., Buravtsova, V.E., Gushchin, V.S., Gan’shina, E.A, Kalinin, Yu.E., Lebedeva, E.V., Sitnikov, A.V., Syr’ev, N.E., and Phonghirun, S., Magnetic and magneto-optical properties of ferromagnet– ferroelectric nanocomposites, Izv. Akad. Nauk, Ser. Fiz., 2006, vol. 70, no. 7, pp. 949–952. 5. Dunets, O.V., Kalinin, Yu.E., Kashirin, M.A., and Sitnikov, A.V., Electrical and magnetic performance of multilayer structures based on (Co40Fe40B20)33.9(SiO2)66.1
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Translated by O. Tsarev
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