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Synthesis and properties of nanoscale films of the Y2O3-Fe2O3 system on silicon
I. A. Milyaeva1, N. S. Perov2, V. V. Bessalova2, M. V. Berezhnaya1, V. O. Mittova3, Anh Tien Nguyen4, I. Ya. Mittova1
1 Voronezh State University, Voronezh, 394018 Russia 2Lomonosov Moscow State University, Moscow, 119991 Russia 3Burdenko Voronezh State Medical University, Voronezh, 394036 Russia 4Ho Chi Min City University of Education, Ho Chi Min City, Viet Nam
cnurova2010@yandex.ru
PACS 75.50.Cc, 81.07.Wx DOI 10.17586/2220-8054-2018-9-3-417-423
Thin films of the Y2O3-Fe2O3/Si system with nanometer-scale thicknesses were synthesized by centrifugation with application of sol and gel of YFeO3. It was found that the samples formed from the gel are characterized by the following composition: Fe2O3, Fe3O4, and Y2O3, YFeO3, which is not consistent with the data, obtained for powder products, and can indicate that the thermal annealing conditions were insufficient for the formation of a single-phase product. For the first time, the magnetic properties of thin ferrite films of yttrium on the surface of the silicon formed from the gel were measure. The nature of the hysteresis loop of nanofilms of yttrium ferrite with different parameters of time and speed of centrifugation suggests that the samples exhibit soft magnetic properties of a ferromagnet. The correlation between the value of specific magnetization and thickness of nanofilms was established.
Keywords: sol-gel synthesis, nanofilms, centrifugation, yttrium orthoferrite, magnetic properties.
Received: 9 February 2018 Revised: 20 March 2018
1. Introduction
The wide application of multiferroics is due to the fact that these materials have two kinds of orderings -ferromagnetic and ferroelectric - which have utility in microelectronics, recording devices, and the storage and reading of information [1-7]. According to the study [8], yttrium orthoferrite also shows multiferroic properties.
There are various methods of producing films of semiconducting metal oxides. The sol-gel method is most promising for the formation of nanofilms of ferrites involving the deposition of metal hydroxides, their conversion into a colloidal state, the colloidal solution applied on the substrate and crystallization of oxide phases during thermal dehydration [9].
The popularity of this technology is associated, primarily, with high chemical homogeneity of the products, which in turn reduces the temperature and duration of final thermal treatment. Despite the large number of studies devoted to the sol-gel technology, the synthesis of specific samples requires an individual approach, providing the precipitation of highly dispersed, easily translatable in a colloidal state [10-12].
The goal of this study was the formation and investigation of magnetic properties of nanodimensional Y2O3-Fe2O3 films, synthesized by the sol-gel method on a surface of Si.
2. Materials and methods
2.1. Initial substances
For the formation of thin films, the sol-gel synthesis of YFeO3 technique was used [13]. The following reagents were used as original reactants: nitrate iron (III) 9-water, Fe(NO3)3-9H2O (h), yttrium nitrate 6-aqueous Y(NO3)3-6H2O (H. h), distilled water.
2.2. The synthesis of Y2O3-Fe2O3 films
The substances were taken in stoichiometric ratio, concentration of the solutions was 0.008 M, the total volume of solution was 100 ml. Solutions of nitrates of iron (III) and yttrium in distilled water was boiled prior to gel formation, and then applied on the silicon substrate by centrifugation, at a speed of 2000 - 5000 rpm within 1 -15 min. The films were annealed in a muffle furnace for one hour at 750 °C. Such thermal annealing parameters were chosen based on the results of [13] for the synthesis of YFeO3 nanopowders.
2.3. Methods of investigation
The thickness of the synthesized films was determined by spectroscopic ellipsometry (Ellipse 1891). Determination of sample compositions was conducted by x-ray diffraction (XRD, x-ray diffractometer Thermo ARL X'tra) and infrared spectroscopy (X-ray, ft-IR spectrometer Vertex 70).
The surface morphology of the formed films was investigated by atomic force microscopy (AFM, Solver P47 Pro Corporation NT-MDT).
The study of the magnetic characteristics (magnetization J, the coercive force HC) of films on silicon synthesized from the gel was performed at room temperature using vibrating magnetometer sample (VSM, "Lakeshore", model 7404).
3. Results and discussion
The results of determination of film thickness, confirming the nanometer range, are presented in Table 1.
Table 1. The thickness of the films synthesized from the gel on the Si surface after annealing at 750 °C, defined by spectral ellipsometry
No. t rotation speed, min. u rotation speed, rpm thickness, nm
1 5 2000 17
2 15 2000 19
3 5 4000 20
4 5 5000 15
5 15 5000 15
6 1 5000 49
7 15 5000 38
The qualitative picture of the concentration of components in the film was established by x-ray diffraction analysis conducted by x-ray diffractometer Thermo ARL X'tra in a small angle range. This was necessary to ensure that the high-intensity peaks of the substrate do not overlap the peaks of the film. The data obtained (Figs. 1, 2 and Tables 2, 3) demonstrated that samples no. 6 and no. 7 showed in Table 1 contain following phases: Fe2O3, Fe3O4, Y2O3, YFeOs.
Table 2. The results of digital processing of diffraction pattern of sample No. 6
The angle of incidence of the x-ray beam Dhki (A) Phase
25.47 3.32 Fe2O3
49.38 1.84 Fe2O3
37.47 2.40 Fe3O4
57.98 1.59 Fe3O4
40.80 2.21 Y2O3
49.38 1.84 Y2O3
51.91 1.76 Y2O3
23.37 3.80 YFeO3
51.91 1.76 YFeO3
It should be noted that the most intensive peaks were peaks corresponding to the substrate at all diffractograms. This fact suggests that the thickness of the film grown on the surface did not exceed a few hundred nanometers and it was confirmed by the studies of thickness using spectroscopic ellipsometry.
^ Fe:03
■ Fe;0, • YzOs ♦ YFe03
*
* ./kjjL^bVviJVA \ /wA .A k / A Wk Aa A i/yi Auu ■ Wl u tl • J Ji.,-J\ flAtawi.A „-.fi^mn—ntih
40,00 45,00
1 theta, erad
Fig. 1. X-ray powder diffraction patterns of sample No. 6 (t = 1 min,u = 5000 rev./min.), synthesized from the gel on the surface of Si
Table 3. The results of digital processing of diffraction pattern of sample No. 7
The angle of incidence of the x-ray beam Dhki (A) Phase
40.23 2.24 Fe2O3
60.18 1.53 Fe2O3
59.20 1.56 Fe3O4
33.15 2.70 Y2O3
34.91 2.57 Y2O3
56.92 1.61 Y2O3
26.13 3.41 YFeO3
53.22 1.72 YFeO3
56.92 1.61 YFeO3
IR absorption spectra of the samples were recorded for establishing the correlation with XRD data.
According to IR spectroscopy data (Fig. 3), the composition of the films included: Y2O3 (v = 480 cm-1), Fe2O3 (v = 620 cm-1), Fe3O4 (v = 1160 cm-1), YFeO3 (v = 1300 cm-1) and these data correlated with the results of XRD.
The surfaces of the synthesized samples had a developed structure with a pronounced wavy nature (the AFM data). Comparison of the images detected the decrease of the grain surface with increasing size of the individual nanocrystallites.
The observed result indicates the significant effect of centrifugation parameters on the nature and structure of the resulting films. Increasing the speed and time of rotation contributed to the smoothing of film topography. For
Fig. 2. X-ray diffraction pattern of sample No. 7 (t =15 min., y = 5000 ob./min). Synthesized from the gel on the surface of Si
example, by increasing the centrifugation time (Figs. 4 and 5) during film formation from the gel decreased the difference in height of the terrain from 8 to 6 nm.
Fig. 3. The IR absorption spectrum of the sample No. 6, synthesized from the gel on the surface of Si
Increasing the centrifugation speed from 4000 rpm to 5000 rpm also caused surface smoothing, the height of the terrain decreased from 12 nm to 10 nm.
A
'i A
1 y.
S \ \ / \ 1
H i 1 i;
(a)
(b)
Fig. 4. AFMimage (a) and surface profile (b) of sample No. 1 (t synthesized from the gel on the surface of Si
5 min, u = 2000 rpm),
(a)
(b)
Fig. 5. AFMimage (a) and surface profile (b) of sample No. 2 (t synthesized from the gel on the surface of Si
15 min, u = 2000 rpm),
Thus, by varying the process parameters, it is possible to achieve the formation of films with a given structure and morphology.
Determination of the magnetic characteristics (magnetization J, the coercive force HC) of the synthesized gel film was carried out using a VSM ("Lakeshore", model 7404) at room temperature. The magnetic field was directed in the film plane.
The field dependence of the magnetic moment of the sample with nanofilm on Si (smooth curve obtained by interpolation of the original data of Langevin curve by least square method) deposited by centrifugation of the gel for 1 min at a speed of 5000 rpm./min (sample no. 6 of Table 1) is shown in Fig. 6. The specific magnetization in a field of 1250 kA/m was 47 A-m2/kg. The determination of coercive force of the sample was not possible due to the low magnetic moment and high noise signal. According to estimates, the coercive force of yttrium ferrite nanofilms was not more than 100 Oe, which indicates magnetically soft ferromagnet. On the other hand, the magnitude of the saturation field exceeded 500 kA/m, which shows high magnetic anisotropy. We can assume that the axis of easy magnetization of magnetic anisotropy is oriented perpendicular to the film's plane.
Figure 7 shows the dependence of the magnetic moment of the thin film sample on the Si surface, deposited by centrifugation of the gel for 15 min. at a speed 5000 rev./min (sample no. 7) from the intensity of the applied field. The sample reached magnetic saturation in a field of 450 kA/m, J = 37.5 A-m2/kg. The shape of the hysteresis loop was similar to the previous dependence also indicates the ferromagnetic character of the nanoscale films.
1S00 -1DDD -500 0 5DD 1000 150D
Field intensity, kA/m
Fig. 6. Field dependence of the magnetization of the sample No. 6 YFeO3/Si (t = 5 min, u = 2000 rpm), synthesized from the gel after annealing at 750 °C, 60 min. Smooth curve is obtained by interpolation of the original data curve Langevin least squares
1S00 -1000 -S00 0 SOO 1000 1500
Field intersLty, kA/m
Fig. 7. Field dependence of the magnetization of the sample No. 7 YFeO3/Si (t = 15 min, u = 5000 rpm), synthesized from the gel after annealing at 750 °C, 60 min. Smooth curve was obtained by interpolation of the original data of the Langevin curve by the least square method
Thus, with the decrease in thickness of the YFeO3 films from 49 to 38 nm (Table 1) a decrease of the specific magnetization in a field of 1250 kA/m 47 A-m2/kg to 37.5 A-m2/kg was observed. The values of the saturation field of both samples were close, indicating the magnetocrystalline nature of the magnetic anisotropy. For yttrium ferrite nanopowders, synthesized by the sol-gel method, J = 0.242 A-m2/kg [13]. The relatively high value of specific magnetization of the synthesized samples was due to the presence of impurity Fe2O3, Fe3O4 phases with a pronounced ferromagnetism [14,15].
The results can be used to produce bulk composite materials, as well as for the manufacture of devices requiring a rapid remagnetization using minimal energy, for example, transformer coils [16,17]. The synthesized films, along with other oxides of transition metals [18,19] can act as catalysts for the formation of functional layers by thermal oxidation of binary semiconductor compounds A3B5.
4. Conclusions
Samples of the Y2O3-Fe2O3/Si system with the application of YFeO3 gel were synthesized by centrifugation. These samples were thin films on silicon with nanometer range thickness. The samples formed from the gel were characterized by the following composition: Fe2O3, Fe3O4, Y2O3, YFeO3, which is not consistent with the data obtained for powder products [13], and may indicate that thermal annealing is insufficient for the formation of a single-phase product.
The generated samples had a developed surface morphology with a pronounced wavy structure. The average surface elevation for the samples synthesized from the gel ranged from 8 to 10 nm. Surface morphology of synthesized samples was smoother with increasing time and speed of the centrifugation of the substrate.
Nanofilms of yttrium ferrite are magnetically soft ferromagnets, the specific magnetization increased with increasing film thickness in the range of 38 - 49 nm from 37.5 to 47 A-m2/kg in field 1250 kA/m. The presence of impurity phases of iron oxides in the samples increases the magnetization value and increases the range of application of the formed heterostructures.
Acknowledgements
The study was supported by RFBR grant (no. 18-03-00354a).
This work was performed using the equipment of the Scientific Equipment Sharing Centre, Voronezh State University, Voronezh, Russia, and the Department of Magnetism, Lomonosov Moscow State University, Moscow, Russia; the equipment that was used at the Department of Magnetism was purchased out of the funds of the Moscow State University Development Program.
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