摘要
Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films.
| 原文 | English |
|---|---|
| 文章編號 | 152507 |
| 期刊 | Applied Physics Letters |
| 卷 | 94 |
| 發行號 | 15 |
| DOIs | |
| 出版狀態 | Published - 2009 |
All Science Journal Classification (ASJC) codes
- 物理與天文學（雜項）
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於: Applied Physics Letters, 卷 94, 編號 15, 152507, 2009.
研究成果: Article › 同行評審
TY - JOUR
T1 - Carrier-mediated ferromagnetism in single crystalline (Co, Ga)-codoped ZnO films
AU - Lu, Zhonglin
AU - Hsu, Hua Shu
AU - Tzeng, Yonhua
AU - Huang, Jung Chun Andrew
N1 - Funding Information: Lu Zhonglin 1,2 Hsu Hua-Shu 3 Tzeng Yonhua 4 Huang Jung-Chun-Andrew 1 a) 1 Department of Physics and Institute of Innovations and Advanced Studies (IIAS), National Cheng Kung University , Tainan 701, Taiwan 2 Department of Physics, Southeast University , Nanjing 211189, People’s Republic of China 3 Department of Applied Physics, National Ping Tung University of Education , Ping Tung 900, Taiwan 4 Department of Electrical Engineering Institute of Microelectronics and Institute of Innovations and Advanced Studies (IIAS), National Cheng Kung University , Tainan 701, Taiwan a) Electronic mail: [email protected] . 13 04 2009 94 15 152507 17 03 2009 28 03 2009 16 04 2009 2009-04-16T16:18:45 2009 American Institute of Physics 0003-6951/2009/94(15)/152507/3/ $25.00 Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films. 96-2120-M-006-001 10804017 Diluted magnetic semiconductors (DMSs) have attracted a great deal of interest owing to their spintronics applications because the charge and spin of the carriers can be simultaneously controlled. 1 With both semiconducting and magnetic properties, DMS materials are ideal sources of spin-polarized carriers and can easily be integrated with semiconductor devices. 2 The ferromagnetic ordering temperature for III-V–based DMSs, such as Mn-doped GaAs, is well below room temperature (RT) because the Curie temperature is typically below 160 K. 3 RT ferromagnetism in ZnO has been theoretically predicted to be realizable by 3 d transition-metal (TM) doping, 4 and so many thin films and nanoparticles of ZnO-based DMSs have been reported upon. 5–15 Ferromagnetism above RT has been observed in ZnO DMSs with 5–9,11–14 or even without 15 TM doping. However, the presented data were plagued by instability and a lack of reproducibility. In some cases, the ferromagnetism has been attributed to uncontrolled ferromagnetic clusters or secondary phases, 9 which are absent from single crystalline specimens. 10 In some cases, high-temperature ferromagnetism could have been caused by defects that are present in the TM-doped ZnO, as explained by the bound magnetic polaron model, 11 which are not true DMSs, as originally envisaged. In a true DMS, the spins of the magnetic dopant retain remanent alignment under the influence of spin-polarized free carriers. 6 Recently, careful theoretical studies indicate substitutional Co in ZnO has a weak preference for antiferromagnetic ordering and stabilization of ferromagnetism can be achieved through n -type doping. 16 Behan et al. 12 reported on carrier-mediated ferromagnetism and charge carrier spin polarization in the metallic Al-doped ZnCoO film, which they claimed to be a genuine magnetic semiconductor. However, the Al ions tend to be interstitial and their role in Al-doped ZnCoO films still remains controversial 13 and, therefore, more careful and systematic studies of the local structural and magnetic characteristics are desired. In the present work, we report our experimental studies of metallic and magnetic (Co, Ga)-codoped ZnO single crystalline films and demonstrate their carrier-mediated ferromagnetism at RT. Zn 0.95 Co 0.05 O (ZnCoO) and Zn 0.93 Co 0.05 Ga 0.02 O (ZnCoGaO) thin films with a thickness of 500 Å were prepared in molecular beam epitaxy (MBE) system with a base pressure of 5 × 10 − 10 Torr on Al 2 O 3 ( 11 2 ¯ 0 ) substrate at 400 ° C using the multilayer δ -doping technique. 17,18 A 50 Å pure ZnO film was grown on the substrate as a buffer layer before the film was deposited. Pure (99.995%) ZnO and Co in a ratio of 20:1 were evaporated from two independent e-beam sources, while Ga from a Knudsen effusion cell was codeposited on the ZnO layer. The typical growth pressure of the doped films was below 5 × 10 − 8 Torr and the deposition rate was about 0.02 Å/s. The film structure and crystalline quality were determined by in situ high-energy electron diffraction (RHEED) and high-resolution x-ray diffraction at the BL17B beamline of the Taiwan Light Source (TLS) in Hsinchu, Taiwan. The x-ray-absorption spectra of the Zn, Co, and Ga K -edges were obtained in the wiggler-C beamline of TLS to determine the electronic state and local environment of the Zn, Co, and Ga in the doped films. Magnetization studies were carried out using a superconducting quantum interference device magnetometer. Electrical transport including Hall measurements was carried out using a four terminal van der Pauw configuration. Because the neglectable lattice mismatch ( < 0.1 % ) between ZnO(0001) and Al 2 O 3 ( 11 2 ¯ 0 ) , 19 epitaxial growth of high quality ZnCoO and ZnCoGaO films were realized on Al 2 O 3 ( 11 2 ¯ 0 ) substrates. Clear c -oriented ZnO streaky in situ RHEED patterns were observed from both films, indicating a flat surface morphology, which is suitable for multilayer devices. High-resolution x-ray measurements also confirm that both films exhibit a strong c -axis texture without any second phase. The typical ϕ scan from a ( 10 1 ¯ 1 ) plane of the ZnCoGaO thin film was also performed to verify its single crystalline characteristics, as shown in Fig. 1 . The sixfold symmetry of the ZnO hexagonal structure crystal basal plane is clearly revealed by the six peaks separated by 60°. The full width at half maximum of the peaks is about 1.5°, revealing very good epitaxy. The out-of-plane and in-plane orientational relationships are ZnCoGaO [ 0001 ] ∥ sapphire [ 11 2 ¯ 0 ] and ZnCoGaO [ 11 2 ¯ 0 ] ∥ sapphire [0001]. The x-ray absorption near-edge structure (XANES) is highly sensitive to the presence of TM clusters in host oxides. Figure 2(a) shows the Co K -edge XANES spectra of the samples as well as those of the standard Co metal and the oxide for comparison. In contrast to the marked shoulder around 7712 eV for the Co metal, the Co K -edge spectra of both the samples show clear 1 s to 3 d pre-edge features around 7709 eV, which are characteristic of Co 2 + substitution for Zn 2 + in ZnO. 9 Extended x-ray absorption fine structure (EXAFS) is also adopted to clarify the local structures around the Co atoms. Figure 2(b) plots the radial distribution function (RDF), the Fourier transform amplitude of EXAFS, at the Co, Ga, and Zn K -edge for both the samples. For the Zn RDF, the first and second peaks observed can be identified as Zn–O and Zn–Zn bonding, respectively. The RDFs for both the films at the Co K -edge are very similar to those of the Zn K -edge spectra, implying that Co ions have similar local structures, as do the Zn ions in ZnO. The results suggest that most Co actually substitutes for Zn without the formation of any detectable metallic Co or other secondary phase. In addition, Similar RDF from Ga K -edge EXAFS for the ZnCoGaO thin films was also found, revealing most of the Ga should replace Zn in the ZnCoO lattice rather than being aggregate or interstitial. Measurements of the resistivity and Hall effect of the ZnCoO and ZnCoGaO films were made, and the data are shown in Table I . Clearly, the resistivity ρ of the ZnCoO film greatly decreases from 9.0 × 10 − 3 Ω cm to 3.2 × 10 − 4 Ω cm as the Ga doping, because of a rapid increase in both the carrier concentration n c and the Hall mobility μ . The low resistivity and high mobility suggest the carriers are weakly localized in the films. In order to further study the electric properties of the films, the Fermi temperature T F and the mean free path λ at RT were estimated with equation T F = ℏ 2 ( 3 π 2 n c ) 2 / 3 / ( 2 m e k B ) and λ = ℏ ( 3 π 2 ) 1 / 3 / ( n c 2 / 3 e 2 ρ ) , respectively, where ℏ is the Planck constant, m e is the electron mass, k B is the Boltzmann constant, and e is the electron charge. 12 Clearly, for both the films, T F is higher than RT and λ is much greater than the lattice spacing (3 Å), which satisfy the criteria for identifying the metallic regime of the ZnO semiconductor. 12,20 The magnetizations of the ZnCoO and ZnCoGaO films were measured and data were also shown in Table I . The magnetic moment without Ga doping is only 0.35 μ B / Co 2 + , and increases greatly to 0.85 μ B / Co 2 + with Ga doping. It seems the enhanced ferromagnetism can be attributed to the significant increase in the free carrier concentration, which was predicted to be effective to strengthen the ferromagnetic interaction in Co doped ZnO films. 16 The AHE, which signifies the degree of spin polarization of carriers, has been considered to be important in identifying a true DMS. 6,7 The Hall resistivity ( ρ xy ) in ferromagnets is expressed as a sum of the ordinary and the anomalous Hall term, ρ xy = R 0 B + R S μ 0 M ⊥ (where R 0 is the ordinary Hall coefficient; B is the magnetic induction; R S is the anomalous Hall coefficient; μ 0 represents the magnetic permeability and M ⊥ is the perpendicular magnetization). ρ xy is linear at high field, as expected, with a small vertical shift at low field, indicating typical AHE in oxide-based DMSs. 12,21,22 Subtracting the linear normal Hall effect term yields a clear signature that corresponds to the AHE, which is shown in Figs. 3(a) and 3(b) for the ZnCoO and ZnCoGaO films, respectively. The noise in the ρ AHE -H curve is caused by the low value of ρ AHE , which is typically one order of magnitude less than ρ xy . Notably, the shape of the ρ AHE -H curve is consistent with that of the out-of-plane magnetization curve, as anticipated, since the ρ AHE is proportional to M ⊥ , further confirming the intrinsic ferromagnetism. To study AHE quantitatively, ρ AHE was transformed to anomalous Hall conductivity [ σ AHE = ρ AHE 2 / ( ρ 2 + ρ AHE 2 ) ] , which is considered to be an essential measure of the strength of AHE. 23 Notably, σ AHE is only 0.0022 Ω − 1 cm − 1 for the ZnCoO film and it increases greatly to 0.078 Ω − 1 cm − 1 for the ZnCoGaO film, suggesting that enhanced carrier spin polarization, associated with Ga doping, strengthens spin scattering. The spin scattering is considered to be due to the interaction between the spin moment of free carrier and orbital moment from Co doping. The values of σ AHE are still much lower than those of some III-V based DMSs but are comparable with those previously reported for rutile Co : TiO 2 . 21 Our experimental observation is well consistent with the recent theoretical study of carrier-mediated magnetism in Co doped ZnO. 16 Although the ratio of carrier density to the number of Co ions can reaches only 0.3 in the ZnCoGaO film, the trend of strengthened ferromagnetic interaction by additional free carrier is clear, which strongly support the theoretical investigation. Finally, we note that the moment of the ZnCoGaO film ( 0.85 μ B / Co 2 + ) is much lower than the expected value of 3 μ B / Co 2 + for tetrahedrally coordinated high-spin Co 2 + ions, and the AHE is also far weaker than that of III-V p -type DMSs. Further experimental and theoretical studies are then necessary. Investigations for magnetic circular dichroism, neutron diffraction, and spin detection through tunnel junctions are underway. In conclusion, (Co, Ga)-codoped ZnO single crystalline films have been grown by MBE. Substantial ferromagnetism and AHE at RT reveal the intrinsic ferromagnetism and the presence of spin polarization of charge carriers in the films. Both the saturation magnetization and σ AHE can be significant enhanced by increasing the carrier concentration or conductivity evidence that the ferromagnetism is carrier mediated. (Co, Ga)-codoped ZnO DMSs are promising for future applications in RT spintronic devices. This work is supported by the National Science Council of Taiwan, ROC under Grant No. 96-2120-M-006-001, and the Center for Micro-Nano Technology, National Cheng Kung University. Z.L.L. acknowledges the National Science Foundation of China (NSFC Grant No. 10804017). Table I. Electrical and magnetic parameters of the ZnCoO samples with and without Ga codoping at 300 K. Sample ρ ( Ω cm ) n c ( cm − 3 ) μ ( cm 2 V − 1 s − 1 ) T F (K) λ (Å) M S ( μ B / Co 2 + ) σ AHE ( Ω − 1 cm − 1 ) ZnCoO 9.0 × 10 − 3 4.0 × 10 19 17.4 496 12.1 0.35 0.0022 ZnCoGaO 3.2 × 10 − 4 5.7 × 10 20 34.3 2911 57.9 0.85 0.078 FIG. 1. A typical ϕ scan from a ( 10 1 ¯ 1 ) plane of the ZnCoGaO thin film. FIG. 2. (a) Normalized absorption spectra of the ZnCoO and ZnCoGaO thin films on Co K -edges, and (b) the RDF of Co and Ga from Fourier transform magnitude of EXAFS of Co and Ga K -edges, respectively. FIG. 3. Anomalous Hall resistivity for the (a) ZnCoO and (b) ZnCoGaO thin films at 300 K, the magnetization measured with the field applied perpendicular to the films is also shown for comparison.
PY - 2009
Y1 - 2009
N2 - Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films.
AB - Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films.
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UR - http://www.scopus.com/inward/citedby.url?scp=65249106552&partnerID=8YFLogxK
U2 - 10.1063/1.3120557
DO - 10.1063/1.3120557
M3 - Article
AN - SCOPUS:65249106552
SN - 0003-6951
VL - 94
JO - Applied Physics Letters
JF - Applied Physics Letters
IS - 15
M1 - 152507
ER -