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Article

Fabrication of Multiferroic Co-Substituted BiFeO3 Epitaxial Films on SrTiO3 (100) Substrates by Radio Frequency Magnetron Sputtering

by
Husne Ara Begum
,
Hiroshi Naganuma
*,
Mikihiko Oogane
and
Yasuo Ando
Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05, Aza-aoba, Aoba-ku, Sendai 980-8579, Japan
*
Author to whom correspondence should be addressed.
Materials 2011, 4(6), 1087-1095; https://doi.org/10.3390/ma4061087
Submission received: 6 May 2011 / Revised: 24 May 2011 / Accepted: 3 June 2011 / Published: 9 June 2011
(This article belongs to the Special Issue Multiferroics)
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Abstract

:
The 10 at.% Co-substituted BiFeO3 films (of thickness 50 nm) were successfully prepared by radio frequency (r.f.) magnetron sputtering on SrTiO3 (100) substrates with epitaxial relationships of [001](001)Co-BiFeO3//[001](001)SrTiO3. In this study, a single phase Co-substituted BiFeO3 epitaxial film was fabricated by r.f. magnetron sputtering. Sputtering conditions such as Ar, O2 gas pressure, annealing temperature, annealing atmosphere, and sputtering power were systematically changed. It was observed that a low Ar gas pressure and low sputtering power is necessary to suppress the formation of the secondary phases of BiOx. The Co-substituted BiFeO3 films were crystalized with post-annealing at 600 °C in air. The process window for single phase films is narrower than that for pure BiFeO3 epitaxial films. By substituting Fe with Co in BiFeO3, the magnetization at room temperature increased to 20 emu/cm3. This result suggests that Co-substituted BiFeO3 films can be used in spin-filter devices.

Graphical Abstract

1. Introduction

Recently, Bi-based multiferroic materials have been attracting considerable attention because of their varied applications. BiFeO3 is a well-known example of multiferroic material with a rhombohedrally distorted perovskite structure (space group: R3c). Displaying ferroelectric properties, BiFeO3 has a high transition temperature (TC) of 1100 K [1] and exhibits very large spontaneous polarization (100–150 µC/cm2) at room temperature (RT) [2,3]. With regard to magnetism, the magnetic transition temperature of BiFeO3 is above RT (TN = 653 K) [4]; however, it is antiferromagnetic and has a G-type spin configuration. In this configuration, the nearest neighboring Fe moments are aligned antiparallel to each other, and there is a six-fold degeneracy resulting in an effective “easy magnetization plane” for the orientation of the magnetic moments within the (111) plane. Owing to the Dzyaloshinskii-Moriya (DM) interaction, the symmetry allows for canting of the antiferromagnetic sublattices resulting in weak local spontaneous magnetization [5], which is macroscopically canceled by the spiral spin structure in which the antiferromagnetic axis rotates through the crystal with an incommensurate long-wavelength period of 62 nm. This spiral spin structure might be suppressed in film form and magnetic moment due to a weak ferromagnetism of ~0.1 μB/Fe atom being observed. However, this small magnetic moment is not suitable for applications such as spintronics because weak magnetic moments and spin-filter effects are difficult to detect using a magnetic sensor. Recently, it was proposed that substitution of iron atoms at the B sites with other 3d transition atoms, such as cobalt, would result in a local ferrimagnetic spin configuration or the collapse of the spiral spin structure leading to macroscopic magnetization. Indeed, the magnetic moment has been reported to increase when the Fe atom in BiFeO3 is replaced with Mn or Co atom [6,7,8]. As a result, these materials are in growing demand for use in spintronics applications. In addition, magnetization increases with increasing Co substitution in the rhombohedral structure [9]; the crystal symmetry changes from rhombohedral to tetragonal at 15–20 at.% of Co substitution [10,11]. Based on this information, in this study, 10 at.% Co-substituted BiFeO3 was used. When used for spintronics applications such as multivalued memory that uses a spin-filter structure, a few-nanometer-thick high-quality film, a flat surface/interface, and good compatibility with the thin metal film process are required. The actual spin-filter structure designed by us consists of conductive La doped SrTiO3 sub./Co-substituted BiFeO3/AlOx/metal magnetic layer/cap layer. The Co-substituted BiFeO3 layer was prepared by heat treatment for crystalizing perovskite structure and AlOx and the metal magnetic layer were deposited at ambient temperature to exclude interfacial oxidation. The schematic illustration of stacking structure of spin-filter device is shown in Figure 1. Various methods for preparing Co-substituted BiFeO3 epitaxial films, such as metalorganic chemical vapor deposition (MOCVD) [10] and chemical solution deposition (CSD) [11], have been reported. However, these chemical processes are not suitable for metal processes due to carbon impurities. One of the promising methods for preparing both oxides and metals layers are radio frequency (r.f.) sputtering [12]. However, there is no report of Co-substituted BiFeO3 thin epitaxial films fabricated by r.f. magnetron sputtering. The aim in this study is to determine the preparation conditions of single phase Co-substituted BiFeO3 [Bi(Fe0.9Co0.1)O3] epitaxial films on SrTiO3 (100) substrates by conventional r.f. magnetron sputtering.
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Figure 1. Schematic illustration of stacking structure of spin-filter device using Bi(Fe0.9Co0.1)O3.
Figure 1. Schematic illustration of stacking structure of spin-filter device using Bi(Fe0.9Co0.1)O3.
Materials 04 01087 g001

2. Experimental Procedure

The Co-substituted BiFeO3 films having a thickness of 50 nm were fabricated by r.f. magnetron sputtering on single crystal SrTiO3 (100) substrates. The base pressure was below 8 × 10−5 Pa. The sputtering target of Bi(Fe0.9Co0.1)Ox consisted of Bi2O3 and Fe2O3 powder that had been sintered in air. The size of disc-shape target was 0.076 m in diameter. The sputtering conditions such as argon (Ar) and/or oxygen (O2) gas pressure, sputtering power, heating temperature in vacuum or air were systematically changed to fabricate single phase Bi(Fe0.9Co0.1)O3 films. The structure of the films was characterized by a conventional θ/2θ X-ray diffraction (XRD, Cu-Kα) pattern, and the surface morphology was investigated by atomic force microscopy (AFM). The homogeneity of the composition of the film surface was observed by field-emission scanning electron microscopy (FE-SEM). The magnetic properties at RT were measured by a superconducting quantum interference device (SQUID) magnetometer.

3. Results and Discussion

The films deposited by r.f. magnetron sputtering using Ar and Ar + O2 gases at an ambient temperature had an amorphous and/or nanocrystalline structure. In order to crystallize the perovskite structure of Bi(Fe0.9Co0.1)O3 films, the samples were annealed under both vacuum and air conditions at various temperatures. In the case of the samples annealed under vacuum conditions, secondary phases such as BiOx were formed; therefore, the experimental data of the vacuum condition is not discussed in this paper. Figure 2(a) shows the annealing temperature dependence of the θ/2θ XRD patterns for the Bi(Fe0.9Co0.1)O3 films annealed in air. The Ar gas was used for sputtering. The formation of the perovskite structure in XRD patterns was identified by the high angle peak of (003) Bi(Fe0.9Co0.1)O3, because the lattice mismatch decreased with increasing Co content. The films crystallized above the annealing temperature of 500 °C with a broad unknown peak of 2θ = 27° (indicated as blue-circle in Figure 2(a)); however, the perovskite phase was not observed at 500 °C. After annealing at 600 °C, only the perovskite peaks of (003) Bi(Fe0.9Co0.1)O3 was observed near the peaks of the (003) SrTiO3 substrate. [Figure 2(a)] At 700 °C, the secondary phases of BiOx were observed, and the peak of the perovskite structure became weak. The dependence of phase formation on annealing temperature is almost the same as that of the CSD process [13]; however; it is lower than that of the MOCVD method [10].
Materials 04 01087 g002 1024
Figure 2. (a) X-ray diffraction (XRD) patterns for the Bi(Fe0.9Co0.1)O3 films annealed at different temperatures, (b) and (c) Ar and Ar + O2 (3:1) gas pressure dependence during sputtering, (d) Input power dependences of the XRD patterns at Ar gas pressure of 0.15 Pa annealed at 600 °C, and (e) Ar and Ar + O2 gas pressure dependence of the XRD peak intensity ratio of (200) Bi2O3/(003) Bi(Fe0.9Co0.1)O3. A single phase Bi(Fe0.9Co0.1)O3 film was formed with low Ar gas pressure and annealed at 600°C in air.
Figure 2. (a) X-ray diffraction (XRD) patterns for the Bi(Fe0.9Co0.1)O3 films annealed at different temperatures, (b) and (c) Ar and Ar + O2 (3:1) gas pressure dependence during sputtering, (d) Input power dependences of the XRD patterns at Ar gas pressure of 0.15 Pa annealed at 600 °C, and (e) Ar and Ar + O2 gas pressure dependence of the XRD peak intensity ratio of (200) Bi2O3/(003) Bi(Fe0.9Co0.1)O3. A single phase Bi(Fe0.9Co0.1)O3 film was formed with low Ar gas pressure and annealed at 600°C in air.
Materials 04 01087 g002
The influence of a variety of gases and gas pressures on the formation of single phase Bi(Fe0.9Co0.1)O3 films was also investigated. Figure 2(b) and (c) show the Ar and Ar + O2 gas pressure dependence of the XRD patterns. The annealing temperature was fixed at 600 °C in air because BiOx of secondary phase has a tendency to suppress at 600 °C. Single phase Bi(Fe0.9Co0.1)O3 films formed at low Ar gas pressure, and secondary phases of (220) Bi2O3 formed at 2θ = 32° when the gas pressure increased to 0.20 Pa. This tendency of gas pressure is roughly consistent with the case of BiFeO3 films [14,15,16]. In the case of Ar + O2 gas, secondary phases of (220) Bi2O3 formed for all gas pressures [Figure 2(c)]. Figure 2(d) shows the input power dependence of the XRD patterns with an Ar gas pressure of 0.15 Pa annealed at 600 °C. It was observed that the secondary phases of BiOx increased with increasing input power. Therefore, low sputtering power is needed to obtain single phase Bi(Fe0.9Co0.1)O3. Figure 2(e) shows the Ar and Ar + O2 gas pressure dependence of the XRD peak intensity ratio of (220) Bi2O3/ (003) Bi(Fe0.9Co0.1)O3. For an Ar + O2 gas pressure of 0.10 Pa [Figure 2(c)], (220) Bi2O3 was not observed; therefore, an unknown XRD peak at 2θ = 35° was obtained as the secondary phase. The quantity of the secondary phase of Bi2O3 has a tendency to increase with increasing gas pressure. The sputtering conditions and optimal condition for obtaining single phase of Bi(Fe0.9Co0.1)O3 film was summarized in Table 1. From these results, it can be considered that the formation of single phase Bi(Fe0.9Co0.1)O3 films was sensitive to preparation processes; the single phase formed at relatively low Ar gas pressure, low sputtering power, and post-annealing at 600 °C in air.
Table
Table 1. Sputtering parameters and optimal conditions for preparing single phase of Bi(Fe0.9Co0.1)O3. The symbol (*) indicates the parameter in which the single phase was observed.
Table 1. Sputtering parameters and optimal conditions for preparing single phase of Bi(Fe0.9Co0.1)O3. The symbol (*) indicates the parameter in which the single phase was observed.
Temperature (°C)Ar+O2 gas pressure (Pa)Ar gas pressure (Pa)Input Power (W)
as-depo.0.100.1010*
5000.150.15*15
600*0.200.2020
7000.25
We confirmed that the single phase Bi(Fe0.9Co0.1)O3 film prepared at an Ar gas pressure of 0.15 Pa and annealed at 600 °C in air contained an epitaxial structure. Figure 3(a) shows the XRD profiles magnified around SrTiO3 (003). The peak position of Bi(Fe0.9Co0.1)O3 prepared by the CSD method is indicated as a dotted line [11]. The XRD peak position of Bi(Fe0.9Co0.1)O3 (003) was close to that previously reported, and the lattice parameter of a = 0.3948 nm in Bi(Fe0.9Co0.1)O3 film was almost the same as that of Bi(Fe0.9Co0.1)O3 film prepared by CSD method (a = 0.394 nm [11]). However, the peak intensity is relatively weak. Figure 3(b) shows the phi-scan measurement at Psi = 45° using the BiFeO3 (202) and SrTiO3 (202) peaks. We did not change sample configuration against beam line when measuring BiFeO3 and SrTiO3 layer. The peaks of Bi(Fe0.9Co0.1)O3 were observed to be the same as that of SrTiO3 substrate; this indicates that the Bi(Fe0.9Co0.1)O3 films were epitaxially grown on the SrTiO3 substrate with cube-on-cube: [001](001) Bi(Fe0.9Co0.1)O3//[001](001) SrTiO3.
The composition of Bi(Fe0.9Co0.1)O3 film prepared at an Ar gas pressure of 0.15 Pa and annealed at 600 °C was confirmed by plan-view FE-SEM and SEM-EDX. Figure 4 shows SEM image, and elementary mapping images of Bi, Fe, Co, and O, respectively, captured by SEM-EDX. The elemental mapping for each element did not show obvious agglomeration of elements, indicating that the larger impurity grains were not formed on surface of film. However, further investigation of smaller grains around the size of superparamagnetic limit of Co should be confirmed by transmission electron microscopy (TEM) observation.
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Figure 3. (a) XRD profile around Bi(Fe0.9Co0.1)O3 (300). The peak position of Bi(Fe0.9Co0.1)O3 prepared by CSD method is indicated by the dotted line [11]; (b) phi-scan measurement at Psi = 45° using the BiFeO3 (202) and SrTiO3 (202) peaks.
Figure 3. (a) XRD profile around Bi(Fe0.9Co0.1)O3 (300). The peak position of Bi(Fe0.9Co0.1)O3 prepared by CSD method is indicated by the dotted line [11]; (b) phi-scan measurement at Psi = 45° using the BiFeO3 (202) and SrTiO3 (202) peaks.
Materials 04 01087 g003
Materials 04 01087 g004 1024
Figure 4. (a) SEM image, and elementary mapping images of (b) Bi, (c) Fe, (d) O, and (e) Co using SEM-EDX for single phase Bi(Fe0.9Co0.1)O3 epitaxial films.
Figure 4. (a) SEM image, and elementary mapping images of (b) Bi, (c) Fe, (d) O, and (e) Co using SEM-EDX for single phase Bi(Fe0.9Co0.1)O3 epitaxial films.
Materials 04 01087 g004
The magnetic property of single phase Bi(Fe0.9Co0.1)O3 epitaxial films was measured using the SQUID magnetometer at 300 K for the in-plane direction. The result is shown in Figure 5(a) and Figure 5(b). A saturation magnetization of 20 emu/cm3 was observed along with a weak remanent magnetization. When compared with epitaxial and polycrystalline BiFeO3 films, the magnetization was clearly enhanced. This magnetization might be attributed to local ferrimagnetism and/or suppression of long-range incommensurate spin cycloids and/or moderation of short-range canted G-type antiferromagnetism. However, further investigation of the spin structure using X-ray magnetic circular dichroism (XMCD) and neutron diffraction analysis is necessary to understand the origin of the magnetism of Bi(Fe0.9Co0.1)O3 films. In order to compare our results with previous works, we plotted magnetization values as a function of Co concentration. Figure 5(c) shows Co concentration of magnetization measured at 30 and 50 kOe at 300 K for various preparation methods [7,9,17]. A magnetization of 20 emu/cm3 at a Co concentration of 10 at.% is almost consistent with previous reports that used a different preparation method. In accordance with the magneto-electric effect in the rhombohedral structure of BiFeO3 [18], the Bi(Fe0.9Co0.1)O3 film is expected to display macroscopic magnetization changes in their electric field. We plan to study this in the future.
Materials 04 01087 g005 1024
Figure 5. (a) In-plane M-H hysteresis loop for single phase Bi(Fe0.9Co0.1)O3 films measured at 300 K and (b) Expanded view of Figure (a) (c) Different Co concentration dependence of magnetization measured at 30 and 50 kOe at 300 K for various preparation methods.
Figure 5. (a) In-plane M-H hysteresis loop for single phase Bi(Fe0.9Co0.1)O3 films measured at 300 K and (b) Expanded view of Figure (a) (c) Different Co concentration dependence of magnetization measured at 30 and 50 kOe at 300 K for various preparation methods.
Materials 04 01087 g005

4. Conclusions

We fabricated Bi(Fe0.9Co0.1)O3 (50 nm) films on SrTiO3 (100) substrates using r.f. magnetron sputtering, while systematically changing various parameters such as Ar + O2 and Ar gas pressure, sputtering power, and annealing temperature to obtain single phase Bi(Fe0.9Co0.1)O3 films. The process window for preparing the single phase is very narrow, and the key requirement for obtaining single phase is the suppression of the formation of secondary phases of BiOx. The single phase and epitaxial structure of Bi(Fe0.9Co0.1)O3 films were obtained using low Ar gas pressure, low sputtering power, and an annealing temperature of 600°C in air. The epitaxial structure and homogeneity of composition were confirmed by XRD patterns and SEM-EDX, respectively. The saturation magnetization of Bi(Fe0.9Co0.1)O3 films increased to 20 emu/cm3 at RT.

Acknowledgements

This work was supported (in part) by Global COE Program “Materials Integration, Tohoku University,” MEXT, Japan, by Strategic International Cooperative Program (Joint Research Type), Japan Science and Technology Agency “Advanced spintronic materials and transport phenomena (ASPIMATT)”, Grant-in-Aid for challenging Exploratory Research (No. 23656025) and by Tohoku University Exploratory Research Program for Young Scientists (TU-ERYS).

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MDPI and ACS Style

Begum, H.A.; Naganuma, H.; Oogane, M.; Ando, Y. Fabrication of Multiferroic Co-Substituted BiFeO3 Epitaxial Films on SrTiO3 (100) Substrates by Radio Frequency Magnetron Sputtering. Materials 2011, 4, 1087-1095. https://doi.org/10.3390/ma4061087

AMA Style

Begum HA, Naganuma H, Oogane M, Ando Y. Fabrication of Multiferroic Co-Substituted BiFeO3 Epitaxial Films on SrTiO3 (100) Substrates by Radio Frequency Magnetron Sputtering. Materials. 2011; 4(6):1087-1095. https://doi.org/10.3390/ma4061087

Chicago/Turabian Style

Begum, Husne Ara, Hiroshi Naganuma, Mikihiko Oogane, and Yasuo Ando. 2011. "Fabrication of Multiferroic Co-Substituted BiFeO3 Epitaxial Films on SrTiO3 (100) Substrates by Radio Frequency Magnetron Sputtering" Materials 4, no. 6: 1087-1095. https://doi.org/10.3390/ma4061087

APA Style

Begum, H. A., Naganuma, H., Oogane, M., & Ando, Y. (2011). Fabrication of Multiferroic Co-Substituted BiFeO3 Epitaxial Films on SrTiO3 (100) Substrates by Radio Frequency Magnetron Sputtering. Materials, 4(6), 1087-1095. https://doi.org/10.3390/ma4061087

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