Next Article in Journal
Radar Angle of Arrival System Design Optimization Using a Genetic Algorithm
Previous Article in Journal
Real-Time and High-Accuracy Arctangent Computation Using CORDIC and Fast Magnitude Estimation

Electronics 2017, 6(1), 23; https://doi.org/10.3390/electronics6010023

Article
The Recovery of a Magnetically Dead Layer on the Surface of an Anatase (Ti,Co)O2 Thin Film via an Ultrathin TiO2 Capping Layer
1
Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan
2
WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Department of Chemistry, Tohoku University, Sendai 980-8578, and Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3
Department of Chemistry, The University of Tokyo, Tokyo 113-0033, and Kanagawa Academy of Science and Technology (KAST), Kawasaki 213-0012, Japan
*
Author to whom correspondence should be addressed.
Academic Editor: Mohan Jacob
Received: 30 September 2016 / Accepted: 28 February 2017 / Published: 18 March 2017

Abstract

:
The effect of an ultrathin TiO2 capping layer on an anatase Ti0.95Co0.05O2−δ (001) epitaxial thin film on magnetism at 300 K was investigated. Films with a capping layer showed increased magnetization mainly caused by enhanced out-of-plane magnetization. In addition, the ultrathin capping layer was useful in prolonging the magnetization lifetime by more than two years. The thickness dependence of the magnetic domain structure at room temperature indicated the preservation of magnetic domain structure even for a 13 nm thick film covered with a capping layer. Taking into account nearly unchanged electric conductivity irrespective of the capping layer’s thickness, the main role of the capping layer is to prevent surface oxidation, which reduces electron carriers on the surface.
Keywords:
ferromagnetic oxide semiconductor; Co-doped TiO2; room temperature ferromagnetism; magnetically dead layer; capping layer; magnetic domain structure; surface oxidation

1. Introduction

A ferromagnetic oxide semiconductor (Ti,Co)O2 attracts much attention due to its high Curie temperature, promising for room temperature semiconductor spintronics [1,2,3,4]. In this compound, the carrier-mediated exchange interaction plays an important role evidenced by its ferromagnetism controlled via electric-field gating [5], chemical doping [6], and the Curie temperature changed by carrier density [7]. Recently, we observed the magnetic domain structure in (Ti,Co)O2 at room temperature with a magnetic force microscope [8], indicating a possibility of manipulating magnetic domains. However, a magnetically dead layer at the surface observed via X-ray magnetic circular dichroism showed significantly reduced magnetization on the surface [9], which is an obstacle for the implementation of various thin film devices.
A magnetically dead layer on a surface or interface has already been reported in various magnetic thin films. For example, NiFe films showed a magnetically dead layer on a NiFe/Cu interface caused by interfacial mixing [10]. (La,Sr)MnO3 showed a magnetic disorder on a surface, which was probably associated with a structural disorder [11]. In the case of (Ti,Co)O2, the magnetically dead layer was at least ~5 nm thick [9] and could be attributed to a surface oxidation and/or a depletion layer due to the different surfaces and bulk electronic structures observed via soft and hard X-ray photoemission spectroscopy [12]. X-ray photoemission spectroscopy under ultraviolet illumination suggested that photoinduced carriers at a depleted layer intermediate exchange coupling between Co spins [13]. These results suggest that surface depletion possibly caused by oxidation plays a major role in reduced magnetization in (Ti,Co)O2, since the oxygen vacancies serve as an electron donor to induce the ferromagnetism [6].
In order to recover the magnetically dead layer, the capping layer is known to be effective to protect surface oxidation [14,15]. In this study, we developed a nonmagnetic ultrathin TiO2 epitaxial capping layer for an anatase (Ti,Co)O2 epitaxial thin film. Only with the 2-nm-thick capping layer was the magnetization significantly improved, mainly due to enhanced out-of-plane magnetization. In addition, the magnetization lifetime was prolonged by at least two years.

2. Materials and Methods

An anatase Ti0.95Co0.05O2−δ epitaxial thin film was grown on a LaAlO3 step substrate via pulsed laser deposition. The LaAlO3 substrate was firstly buffered with an insulating 5-unit-cell-thick anatase TiO2 layer according to a procedure previously reported [16]. After cooling down to room temperature, the Ti0.95Co0.05O2−δ epitaxial thin film was grown on a buffer layer at 250 °C with an oxygen pressure during growth (PO2) of 1–3 × 10−6 Torr to control carrier density [6]. A nonmagnetic TiO2 capping layer (0–5 nm thick) was in situ deposited epitaxially on the Ti0.95Co0.05O2−δ film at 250 °C with a PO2 of 1 × 10−4 Torr, and was cooled down to room temperature under the same PO2. Reflection high energy electron diffraction (RHEED) was in situ monitored during growth. The thickness of the Ti0.95Co0.05O2−δ film was typically 30–40 nm unless otherwise stated. Based on X-ray diffraction measurements, all films were in a pure anatase phase and epitaxially grown in a c-axis orientation. The electrical transport properties were evaluated by four-probe and Hall effect measurements of the Hall bar patterned samples. The magnetization was measured with a superconducting quantum interference device magnetometer at 300 K in the range of ±2 T. The topographic and magnetic images of the films were observed with a magnetic force microscope at room temperature in a vacuum of 10 Pa without an external magnetic field [8].

3. Results

RHEED patterns of the buffer layer, the (Ti,Co)O2 film, and the capping layer showed a streak pattern with the flat sample surface observed via AFM (Figure 1), indicating the epitaxial growth and sharp interface between each layer.
Figure 2 shows the temperature dependence of resistivity for Ti0.95Co0.05O2−δ films with different capping layer thicknesses. The resistivity was calculated from the thickness of the Ti0.95Co0.05O2−δ film assuming the insulating capping layer because of the lower oxygen vacancy. All resistivity values were approximately equivalent. The carrier density and mobility at 300 K for non-capped, 3 nm capped, and 5 nm capped films were 4.8 × 1019 cm−3 and 5.4 cm2·V−1·s−1, 5.1 × 1019 cm−3 and 5.1 cm2·V−1·s−1, and 4.7 × 1019 cm−3 and 4.5 cm2·V−1·s−1, respectively. These small variations in carrier density and mobility, irrespective of capping layer thickness, might rule out a possibility of intensive carrier doping at the interface, as was observed in TiO2/LaAlO3 and TiO2/LaTiO3 interfaces [17].
Figure 3 shows out-of-plane and in-plane magnetization curves at 300 K for Ti0.95Co0.05O2−δ films with and without a capping layer. In case of the films without a capping layer, the out-of-plane and in-plane magnetizations were approximately the same for n = 4.5 × 1019 cm−3 (Figure 3a), while the out-of-plane magnetization was slightly larger than the in-plane magnetization for n = 6.7 × 1019 cm−3 (Figure 3b), indicating their insignificant magnetic anisotropy. In case of the films with a capping layer, the out-of-plane magnetization was nearly doubled for n = 4.5 × 1019 cm−3 (Figure 3c), representing significantly enhanced out-of-plane magnetization and perpendicular magnetic anisotropy (Figure 3c). For n = 6.7 × 1019 cm−3, the out-of-plane magnetization was slightly increased (Figure 3d). It is noted that the in-plane magnetization in saturation was scarcely changed by the capping layer. However, the difference in magnetization slope between the in-plane and out-of-plane magnetization was enhanced (insets of Figure 3), indicating enlarged perpendicular magnetic anisotropy by the capping layer.
The stability of the films without a capping layer and with a 2-nm-thick capping layer was examined as the aging of magnetization. During aging, the films were kept in a desiccator at ambient condition. Figure 4 shows the aging effect of the out-of-plane magnetization curve for the Ti0.95Co0.05O2−δ thin films (n ≈ 6–7 × 1019 cm−3) with and without a capping layer at 300 K. The film without a capping layer showed a gradual decrease in magnetization within a two-year period. On the other hands, the film with a capping layer showed almost no change in magnetization or its magnetic hysteresis even after two years. This result indicates that the capping layer protected the film surface against the surface oxidation, which causes decreased magnetization owing to reduced carrier density. Similar decreased magnetization was also reported previously in (Ti,Co)O2 nanopowders [18].
The effect of the 2-nm-thick capping layer on the magnetic domain structure in Ti0.95Co0.05O2−δ films with thicknesses from 13 nm to 38 nm was examined. Magnetization curves were almost the same with open hysteresis for all films, representing similar magnetization in a unit of Bohr magneton per Co site. The magnetic domain structure showed a weaker contrast with a smaller domain with decreasing thickness as a result of decreased thickness (Figure 5a–c), where the average domain width evaluated by a stereological method [19] was 160, 120, and 80 nm with decreasing thickness. It is noted that the magnetic domain structure was observed even for a thickness of 13 nm. On the other hand, a 15-nm-thick film without a capping layer showed smaller and irregularly shaped magnetic domains, probably caused by a magnetically dead layer on the surface and/or weak perpendicular magnetic anisotropy.

4. Discussion

The capping layer works to protect (Ti,Co)O2 films from surface oxidation. However, significantly enlarged perpendicular magnetic anisotropy might be attributed to not only the carrier density dependence of magnetic anisotropy briefly reported in previous reports [5] but also other effects such as the varied 3d orbital occupancy of Co ions in (Ti,Co)O2 caused by interfacial effects [20,21]. The influence of ferromagnetism in the capping layer (if any) on the enlarged magnetization can be ruled out because of the layer’s significantly lower thickness in comparison with that of the (Ti,Co)O2 film.

5. Conclusions

In conclusion, an ultrathin TiO2 epitaxial capping layer was developed for an anatase Ti0.95Co0.05O2−δ (001) epitaxial thin film. As a result, the magnetization was concomitantly recovered with enlarged perpendicular magnetic anisotropy, resulting in the observation of a magnetic domain structure for the 13-nm-thick Ti0.95Co0.05O2−δ film. In addition, the magnetization was almost unchanged for at least two years. One major role of the capping layer is to prevent surface oxidation, and other mechanism such as varied 3d orbital occupancy might play a role as well.

Acknowledgments

This research was in part supported by JSPS through the NEXT Program initiated by CSTP (GR029), JSPS Grant-in-Aid for Scientific Research (26105002), and CREST, JST.

Author Contributions

T.F. and T.H. conceived and designed the experiments; T.S.K. performed the experiments; T.S.K. and T.F. analyzed the data; all authors wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Ogale, S.B. Dilute doping, defects, and ferromagnetism in metal oxide systems. Adv. Mater. 2010, 22, 3125–3155. [Google Scholar] [CrossRef] [PubMed]
  2. Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide. Science 2001, 291, 854–856. [Google Scholar] [CrossRef] [PubMed]
  3. Fukumura, T.; Kawasaki, M. Magnetic oxide semiconductors: On the high-temperature ferromagnetism in TiO2- and ZnO-based compounds. In Functional Metal Oxides: New Science and Novel Applications; Ogale, S.B., Venkatesan, T.V., Blamire, M., Eds.; Wiley: Weinheim, Germany, 2013; pp. 91–131. [Google Scholar]
  4. Saadaoui, H.; Luo, X.; Salman, Z.; Cui, X.Y.; Bao, N.N.; Bao, P.; Zheng, R.K.; Tseng, L.T.; Du, Y.H.; Prokscha, T.; et al. Intrinsic ferromagnetism in the diluted magnetic semiconductor Co:TiO2. Phys. Rev. Lett. 2016, 117, 227202. [Google Scholar] [CrossRef] [PubMed]
  5. Yamada, Y.; Ueno, K.; Fukumura, T.; Yuan, H.T.; Shimotani, H.; Iwasa, Y.; Gu, L.; Tsukimoto, S.; Ikuhara, Y.; Kawasaki, M. Electrically-induced ferromagnetism at room temperature in cobalt-doped titanium dioxide. Science 2011, 332, 1065–1067. [Google Scholar] [CrossRef] [PubMed]
  6. Yamada, Y.; Fukumura, T.; Ueno, K.; Kawasaki, M. Control of ferromagnetism at room temperature in (Ti,Co)O2−δ via chemical doping of electron carriers. Appl. Phys. Lett. 2011, 99, 242502. [Google Scholar] [CrossRef]
  7. Krasienapibal, T.; Fukumura, T.; Hasegawa, T. Curie temperature of Co-doped TiO2 as functions of carrier density and Co content evaluated from electrical transport and magnetization at low temperature regime. AIP Adv. 2016, 6, 055802. [Google Scholar] [CrossRef]
  8. Krasienapibal, T.S.; Inoue, S.; Fukumura, T.; Hasegawa, T. Observation of magnetic domain structure in anatase Ti1−xCoxO2−δ thin film at room temperature. Appl. Phys. Lett. 2015, 106, 202402. [Google Scholar] [CrossRef]
  9. Singh, V.R.; Ishigami, K.; Verma, V.K.; Shibata, G.; Yamazaki, Y.; Kataoka, T.; Fujimori, A.; Chang, F.-H.; Huang, D.-J.; Lin, H.-J.; et al. Ferromagnetism of cobalt-doped anatase TiO2 studied by bulk- and surface-sensitive soft X-ray magnetic circular dichroism. Appl. Phys. Lett. 2012, 100, 242404. [Google Scholar] [CrossRef]
  10. Speriosu, V.S.; Nozieres, J.P.; Gurney, B.A.; Dieny, B.; Huang, T.C.; Lefakis, H. Role of interfacial mixing in giant magnetoresistance. Phys. Rev. B 1993, 47, 11579–11582. [Google Scholar] [CrossRef]
  11. Borges, R.P.; Guichard, W.; Lunney, J.G.; Coey, J.M.D.; Ott, F.J. Magnetic and electric “dead” layers in (La0.7Sr0.3)MnO3 thin films. J. Appl. Phys. 2001, 89, 3868–3873. [Google Scholar] [CrossRef]
  12. Ohtsuki, T.; Chainani, A.; Eguchi, R.; Matsunami, M.; Takata, Y.; Taguchi, M.; Nishino, Y.; Tamasaku, K.; Yabashi, M.; Ishikawa, T.; et al. Role of Ti 3d Carriers in Mediating the Ferromagnetism of Co: TiO2 Anatase Thin Films. Phys. Rev. Lett. 2011, 106, 047602. [Google Scholar] [CrossRef] [PubMed]
  13. Yamashita, N.; Sudayama, T.; Mizokawa, T.; Yamada, Y.; Fukumura, T.; Kawasaki, M. Interplay between magnetic impurities and photo-induced carriers in surface depletion layer of anatase Ti1−xCoxO2−δ thin film probed by X-ray photoemission spectroscopy. Appl. Phys. Lett. 2010, 96, 021907. [Google Scholar] [CrossRef]
  14. Li, J.; Wang, Z.Y.; Tan, A.; Glans, P.-A.; Arenholz, E.; Hwang, C.; Shi, J.; Qiu, Z.Q. Magnetic dead layer at the interface between a Co film and the topological insulator Bi2Se3. Phys. Rev. B 2012, 86, 054430. [Google Scholar] [CrossRef]
  15. Wang, Y.-H.; Chen, W.-C.; Yang, S.-Y.; Shen, K.-H.; Park, C.; Kao, M.-J.; Tsai, M.-J. Interfacial and annealing effects on magnetic properties of CoFeB thin films. J. Appl. Phys. 2006, 99, 08M307. [Google Scholar] [CrossRef]
  16. Krasienapibal, T.S.; Fukumura, T.; Hirose, Y.; Hasegawa, T. Improved room temperature electron mobility in self-buffered anatase TiO2 epitaxial thin film grown at low temperature. Jpn. J. Appl. Phys. 2014, 53, 090305. [Google Scholar] [CrossRef]
  17. Takahashi, K.S.; Hwang, H.Y. Carrier doping in anatase TiO2 film by perovskite overlayer deposition. Appl. Phys. Lett. 2008, 93, 082112. [Google Scholar] [CrossRef]
  18. Silvestre, A.J.; Pereira, L.C. J.; Nunes, M.R.; Monteiro, O.C. Ferromagnetic order in aged Co-doped TiO2 anatase nanopowders. J. Nanosci. Nanotechnol. 2012, 12, 6850–6854. [Google Scholar] [CrossRef] [PubMed]
  19. Hubert, A.; Schafer, R. Magnetic Domains; Springer: Heidelberg, Germanay, 1998. [Google Scholar]
  20. Pesquera, D.; Herranz, G.; Barla, A.; Pellegrin, E.; Bondino, F.; Magnano, E.; Sánchez, F.; Fontcuberta, J. Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films. Nat. Commun. 2012, 3, 1189. [Google Scholar] [CrossRef] [PubMed]
  21. Peng, J.J.; Song, C.; Li, F.; Gu, Y.D.; Wang, G.Y.; Pan, F. Restoring the magnetism of ultrathin LaMnO3 films by surface symmetry engineering. Phys. Rev. B 2016, 94, 214404. [Google Scholar] [CrossRef]
Figure 1. Reflection high energy electron diffraction (RHEED) patterns of (a) the buffer layer; (b) the (Ti,Co)O2 film; (c) the capping layer. (d) Typical topographic image of the sample surface.
Figure 1. Reflection high energy electron diffraction (RHEED) patterns of (a) the buffer layer; (b) the (Ti,Co)O2 film; (c) the capping layer. (d) Typical topographic image of the sample surface.
Electronics 06 00023 g001
Figure 2. Temperature dependence of resistivity for Ti0.95Co0.05O2−δ epitaxial thin films with the capping layer of nonmagnetic TiO2 epitaxial thin films. The capping layer thickness was 0, 3, and 5 nm.
Figure 2. Temperature dependence of resistivity for Ti0.95Co0.05O2−δ epitaxial thin films with the capping layer of nonmagnetic TiO2 epitaxial thin films. The capping layer thickness was 0, 3, and 5 nm.
Electronics 06 00023 g002
Figure 3. Out-of-plane (red) and in-plane (blue) magnetization curves at 300 K for anatase Ti0.95Co0.05O2−δ epitaxial thin films with different carrier density. (a) n = 4.5 × 1019 cm−3 and (b) n = 6.7 × 1019 cm−3 without capping layer; (c) n = 4.5 × 1019 cm−3 and (d) n = 6.7 × 1019 cm−3 with a 2-nm-thick capping layer. Insets denote magnified views at around a zero magnetic field.
Figure 3. Out-of-plane (red) and in-plane (blue) magnetization curves at 300 K for anatase Ti0.95Co0.05O2−δ epitaxial thin films with different carrier density. (a) n = 4.5 × 1019 cm−3 and (b) n = 6.7 × 1019 cm−3 without capping layer; (c) n = 4.5 × 1019 cm−3 and (d) n = 6.7 × 1019 cm−3 with a 2-nm-thick capping layer. Insets denote magnified views at around a zero magnetic field.
Electronics 06 00023 g003
Figure 4. The aging of out-of-plane magnetization curves at 300 K for Ti0.95Co0.05O2−δ thin films (a) without a capping layer and (b) with a 2-nm-thick capping layer. Insets denote magnified views at around a zero magnetic field.
Figure 4. The aging of out-of-plane magnetization curves at 300 K for Ti0.95Co0.05O2−δ thin films (a) without a capping layer and (b) with a 2-nm-thick capping layer. Insets denote magnified views at around a zero magnetic field.
Electronics 06 00023 g004
Figure 5. Magnetic images at 300 K for Ti0.95Co0.05O2−δ thin films with different film thicknesses (ac) with a 2-nm-thick capping layer and (d) without a capping layer.
Figure 5. Magnetic images at 300 K for Ti0.95Co0.05O2−δ thin films with different film thicknesses (ac) with a 2-nm-thick capping layer and (d) without a capping layer.
Electronics 06 00023 g005
Electronics EISSN 2079-9292 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top