Morphology Evolution and CL Property of Ni-Doped Zinc Oxide

J. Phys. Chem. C 2009, 113, 4381–4385

4381

Morphology Evolution and CL Property of Ni-Doped Zinc Oxide Nanostructures with Room-Temperature Ferromagnetism Xiaohu Huang,*,†,‡ Guanghai Li,*,† Bingqiang Cao,§ Ming Wang,3 and Changyi Hao† Key Laboratory of Material Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China, Key Laboratory of Nonferrous Metal Materials and New Processing Technology, Guilin UniVersity of Technology, Guilin 541004, P.R. China, and Fakulta¨t fu¨r Physik and Geowissenschaften, UniVersita¨t Leipzig, Linne´strasse 5, 04103 Leipzig, Germany ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 11, 2009

Ni-doped ZnO nanostructures were synthesized in situ through a pulsed-electrodeposition-assisted chemical bath deposition method, and the optical and magnetic properties of the nanostructures were studied. It was found that the morphology of the nanostructures evolved from a rodlike to a sheetlike structure because of the different growth modes, and a growth mechanism is proposed to explain these findings. A relatively strong UV emission was observed for the nanorods, whereas a relatively strong visible emission was seen for the nanosheets. Ni was successfully doped into the ZnO wurtzite lattice structure as revealed by X-ray diffraction and X-ray photoelectron spectroscopy and also verified by the cathodoluminescence characterization. Roomtemperature ferromagnetism was also observed in the Ni-doped ZnO nanostrucures. The results are helpful to tailor the physical properties of ZnO by changing its morphology and composition. 1. Introduction Currently, zinc oxide (ZnO) nanomaterials are giving rise to enormous research interest, principally for their excellent optical, catalytic, and nontoxic properties.1,2 Solution-based synthetic schemes offer inherent advantages in terms of homogeneous systems, low temperatures, and low costs compared to other methods, in addition to ecological benefits. Although various ZnO nanostructures, including nanoneedle arrays, nanotube arrays, nanosheets, and nanoporous structures, have been obtained by solution-based methods,3-7 the study on the morphology evolution of ZnO nanostructures is scarce in literature.5,7 Transitional-metal-doped ZnO (Zn1-xMxO) is one of the most promising dilute magnetic semiconductors (DMSs), which are envisioned as being promising building blocks for spintronic devices.8 Nevertheless, the chemical synthesis of Zn1-xMxO nanomaterials is rather difficult as compared to that of pure ZnO.9-11 The properties of nanostructures are strongly dependent on their shapes and sizes.12,13 Therefore, the morphologycontrolled synthesis of Zn1-xMxO is very important for exploring the applications of these materials in spintronics, optics, catalysis, and so on. In this article, we report a pulsed-electrodeposition-assisted chemical bath deposition method to synthesize Ni-doped ZnO (Zn1-xNixO) nanostructures with different morphologies, evolving from one-dimensional (1D) microrods to two-dimensional (2D) nanosheets. A potential-induced growth mechanism is proposed. The cathodoluminescence (CL) properties of the Zn1-xNixO nanostructures were studied and discussed. Hysteresis * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Chinese Academy of Sciences. ‡ Current address: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore. § Universita¨t Leipzig. 3 Guilin University of Technology, Guilin.

loop (M-H) measurements were used to check the roomtemperature magnetic properties of the Ni-doped ZnO. 2. Experimental Details The synthesis of Ni-doped ZnO nanostructures was conducted in an aqueous solution including 0.05 M Zn(NO3)2 · 6H2O, 0.005 M Ni(NO3)2 · 6H2O, and the proper amount of hexamethylenetetramine (HMT). The pH value of the solution was adjusted to about 7 by addition of appropriate amounts of ammonia. Pulsed electrodeposition was applied between a graphite anode and a clean Cu cathode coated with a thin Au film in a common two-electrode plating cell at a temperature of around 90 °C for 90 min. Both the pulse deposition time and the delay time for each pulse were 2 ms. After deposition, the samples were washed with deionized water and absolute ethanol carefully before further characterization. The phase identification of the as-obtained samples was performed by X-ray diffraction (XRD) with a Philips X’Pert power X-ray diffractometer using Cu KR (λ ) 1.542 Å) radiation. Field-emission scanning electron microscopy (FESEM, FEI Sirion 200) with energy-dispersive X-ray (EDX, Inca Oxford) analysis and high-resolution transmission electron microscopy (HRTEM, JEOL-2010) were used to examine the morphology, composition, and microstructure. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to determine the valence state of the dopant elements. The CL experiments were performed at room temperature in a CamScan CS44 scanning electron microscope with a 15 keV electron energy and a beam current of 150 pA. The wavelengthrespective energy scale of the CL spectra was calibrated prior to each set of measurements using a neon lamp. The spectral resolution of the CL spectrometer was about 0.24 meV. Magnetic measurements were performed on a Quantum Design superconducting quantum interference device (SQUID) magnetometer at 300 K.

10.1021/jp810790h CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

4382 J. Phys. Chem. C, Vol. 113, No. 11, 2009

Figure 1. FE-SEM images of ZnO synthesized at (a) -1.2 and (b) -1.8 V and Zn1-xNixO synthesized at (c) -2.4 and (e) -3.6 V. (d,f) HRTEM images and SAED patterns of the Zn1-xNixO nanostructures in panels c and e, respectively. The scale bars in the insets of a and b are 500 nm and 2 µm, respectively.

3. Results and Discussion 3.1. Characterization. When the deposition potential was -1.2 or -1.8 V, ZnO nanorods arrays were obtained, as shown in Figure 1a,b. When the deposition potential was -2.4 V, Zn1-xNixO microclusters were obtained. The size of the microclusters was around 5 µm, and the microclusters were composed of microrods whose diameter was around 1 µm, as shown in Figure 1c. Figure 1d is an HRTEM image of the microrods, with the corresponding SAED pattern in the inset; these results indicate that the microrods are single-crystalline with the wurtzite ZnO structure and that the growth direction of the microrods is along [0001]. No observable defects are present in the HRTEM image. Zn1-xNixO nanosheets were obtained when the deposition potential was tuned to -3.6 V. The lateral dimension of the nanosheets was around 8 µm, and the thickness was tens of nanometers, as shown in Figure 1e. The HRTEM image and SAED pattern (Figure 1f) suggest that these nanosheets are single-crystal Zn1-xNixO and that they grow along the 〈0110〉 crystallography direction within the {0001} planes. The nickel concentration in the nanostructures was determined by energy-dispersive X-ray (EDX) spectroscopy. A typical result for the Ni-doped ZnO is shown in Figure 2a. No elemental Ni was found in the nanorods shown in Figure 1a,b, whereas Ni concentrations of 2.9 and 4.5 at. % were found in the microrods and nanosheets, respectively. Thus, there is a threshold of the applied deposition potential for the doping of Ni into ZnO. Figure 2b displays the standard diffraction peaks of ZnO (JCPDF no. 89-1397), together with the XRD patterns of the above samples: (1) S1 microrods and (2) S2 nanosheets. Except for the peaks from the Cu substrate [JCPDF no. 89-2838, labeled with asterisks (*)], all of the diffraction peaks can be indexed to wurtzite ZnO. No impurity peaks were detected, in agreement with the HRTEM and SEAD data. As in the case of Cu-doped ZnO nanowires,14 because of the smaller radius of Ni2+ ions (0.055 nm) compared to Zn2+ ions (0.060 nm) in tetrahedral coordination,15 the (002) peak is shifted to higher angle with

Huang et al.

Figure 2. (a) Typical EDX spectrum of the Ni-doped ZnO nanostructures. (b) XRD patterns of the Ni-doped ZnO S1 (microrods) and S2 (nanosheets). The peaks labeled with asterisks (*) are from the Cu substrate. The inset in b shows the shift of the (002) peak of Ni-doped ZnO compared to that of pure ZnO. The dopant concentration is 2.9 and 4.5 at. % for S1 and S2, respectively.

Figure 3. Typical XPS spectrum of Ni 2p in our Zn1-xNixO nanostructures.

increasing Ni dopant concentration compared to that of pure ZnO, as shown in the inset of Figure 2b. To obtain detailed information on the dopant, a fine-scanned XPS spectrum of Ni was obtained from sample S1. Figure 3 shows the Ni 2p core-level XPS spectrum. The spectrum is characterized by a Ni 2p3/2 peak at ca. 855.0 eV and a Ni 2p1/2 peak at ca. 872.6 eV; the difference in energy between them is ca. 17.6 eV. All of these details match the pattern of standard NiO.16,17 The appearance of satellite peaks (labeled “Sat.”) implies the presence of a high-spin divalent state of Ni2+ in these samples,18 as expected for Ni ions substituted at the Zn sites. 3.2. Growth Mechanism. Under an electric field, the following sequence of reactions occurs10

NO3- + H2O + 2e- f NO2- + 2OH-

(1)

Morphology and Properties of Ni-Doped ZnO Nanostructures

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4383

(Zn2+)1-x + (Ni2+)x + 2OH- f Zn1-xNixO2 + H2O

(2) The electric field provides a dynamic driving force to facilitate the growth of ZnO,10 and it is critical to the doping of Ni into the ZnO matrix. When we performed a similar experiment but applied no deposition potential, the composition of the deposit was only ZnO, with no trace of Ni dopant. Actually, Ni could be detected only when the deposition potential exceeded -2.4 V, which indicates that it is difficult to dope Ni into ZnO under a weak electrical field. Pulsed electrodeposition is a very powerful technique for fabricating single-crystalline nanowire arrays.19 The pulsed time in each pulse cycle is so short that only a small amount of Ni ions are reduced at the reaction interface during one pulse, and there is enough time for the Ni ion concentration to recover before the next deposition. Under our experimental conditions, ZnO is grown both during the pulsed time and during the delay time. This novel technique guarantees the uniform doping of Ni ions into the ZnO matrix, as confirmed by HRTEM, XRD, and XPS. The deposition potential is responsible not only for the successful doping of Ni into the ZnO matrix, but also for the morphology evolution of the nanostructures. It is well-known that ZnO is a polar crystal and produces positively Zn2+terminated (0001) and negatively O2--terminated (0001j) polar surfaces, which induce a net dipole moment along the c axis.20 The surface energies of the polar {0001} planes are higher than those of the nonpolar {011j0} and {21j1j0} planes,1 so preferential growth along the c axis is energetically favorable. Under less negative deposition potentials (say, from -1.2 to -2.4 V), the electrodepositon current is also small, because of the abovementioned crystallographic habit of ZnO, and 1D growth mode is preferred, as reported previously.10,21 The gold substrate is polycrystalline in structure, which will not induce a possible epitaxial relation with ZnO. This means that randomly oriented nuclei will be formed on the substrate during the initial deposition. This can be seen from the insets of Figure 1a,b. Thus, there is a transition from three-dimensional (3D) nucleation to 1D growth. For direct electrodepositon, relatively large deposition currents enhance the growth rate along the c axis of ZnO.21 Here, for our pulsed electrodeposition, we found that more negative deposition potentials enhanced the growth rate along both the 〈0001〉 and 〈011j0〉 directions, at least in our potential window (from -1.2 to -2.4 V). This can be seen from the fact that both the diameter and the height of the nanorods increase with increasing deposition potential (see Figure 1a,b). When the deposition potential exceeds a certain value (here, ca. -3.6 V), the growth kinetics can be dramatically different. For example, the velocity of ions will increase under a strong electrical field, so the kinetic energy of the ions will increase dramatically.19 If the energy of the ions is high enough to make the high-energy surface smoother, then the ions will diffuse to low-energy surfaces.22 Under such conditions, the growth is determined by dynamics instead of thermodynamics. The 2D growth mode is more preferred under this nonequilibrium process.21 This is the reason that the growth velocities are in the order V{011j0} > V{0001} and nanosheets were obtained under a deposition potential of -3.6 V (see Figure 1e). In fact, a similar growth of polar (0001)-surface-dominated pure or tindoped ZnO nanosheets and nanobelts has been reported.21-23 3.3. Luminescent Properties. CL is an effective technique for studying the optical properties of nanostructures. Figure 4 shows the CL spectra measured at room temperature of the above-mentioned Zn1-xNixO samples: (a) S1 and (b) S2. Two

Figure 4. CL spectra measured at 11 K for Zn1-xNixO (a) microrods and (b) nanosheets. (c,d) Visible emission parts of spectra a and b, respectively.

TABLE 1: UV and Visible Emission Peaks in the CL Spectra of the Ni-Doped ZnO Nanostructures at 11 K Ni concentration UV emission (nm) visible emission (nm) emission 1 emission 2 emission 3

microrods

nanosheets

2.9 at. % 382

4.5 at. % 382

533 579 648

538 594 656

emission bands, UV emission and visible emission, can be seen in the spectra. For S1, the UV emission is stronger than the visible emission, whereas for S2, the UV emission is weakened, and the visible emission is stronger. The UV emissions of the two samples are almost centered at about 382 nm, which comes from the combinations of the excitons.3,24 Compared to the band edge of pure ZnO,25 the UV emissions of Ni-doped ZnO with dopant concentrations of 2.9 and 4.5 at. % are red-shifted by about 54 meV, and similar results were observed in Ni-doped ZnO films and nanowires.26,27 Within this range of dopant concentration, the band edge of Ni-doped ZnO changes slightly and is red-shifted by about 64 meV compared to that of pure ZnO according to optical adsorption results,25 which is in agreement with our results. This red shift is caused by the sp-d exchange interaction.25 The visible emissions can be divided into three emission bands (emission 1, emission 2, and emission 3) by Gauss fitting, as shown in Figure 4c,d, and the results are listed in Table 1. According to previous studies, these visible emissions, especially emission 1 and emission 2, are usually ascribed to deep-level defects, such as oxygen vacancies, zinc interstitial atoms, and so on.3,24,28 The variations in the relative intensities of the UV emission and the visible emission in the CL spectra are related to the different defect concentrations in S1 and S2.28 Emission 3 in the visible range is located at about 648 nm (1.91 eV in energy) and 656 nm (1.89 eV in energy) for S1 and S2, respectively. The emission at ∼640-650 nm in ZnO was proposed to be due to oxygen-rich samples in previous reports.29,30 The samples were oxygen-poor in our experiments, and there were fewer oxygen vacancies in S1 than in S2. However, emission 3 is much weaker for S1 than for S2, as shown in Figure 4c,d. Therefore, the possibility that emission 3 originated from defects can be excluded. Studies on the optical

4384 J. Phys. Chem. C, Vol. 113, No. 11, 2009

Figure 5. M-H curves measured at 300 K for Zn1-xNixO (a) microrods and (b) nanosheets.

absorption of Co2+ and Ni2+ in ZnO suggest that there are some characteristic absorption peaks due to the crystal-field transition in tetrahedrally bonded Co2+ and Ni2+,31-34 and Yang’s group observed this kind of transition in Co-doped ZnO nanowires through luminescence measurements.11 However, to the best of our knowledge, there are still no reports on luminescence evidence on this kind of transition in Ni-doped ZnO nanostructures. According to previous studies, Ni-doped ZnO shows an absorption peak near 1.9 eV due to the 3T1(F) f 3T1(P) transition of Ni2+.31,34 Obviously, the more Ni2+ ions in ZnO, the stronger this absorption peak will be.11 The Ni concentration is larger in S2 than in S1, and the emission 3 is stronger from S2 than from S1, which further confirms that emission 3 originates from the 3 T1(P) f 3T1(F) transition of tetrahedral Ni2+ ions in ZnO. The appearance of the Ni2+-related emission can be regarded as an indication of successful doping. Actually, we did photoluminescence (PL) measurements on our samples at room temperature, but unfortunately no Ni2+related emission was observed in the PL spectra. The main reason might be the difference in the penetration depth of the incident photons and the injected electrons. In photoexcitation, the penetration depth of the photons with a wavelength of 325 nm is about 60 nm.35 In electron-beam excitation, the penetration depth of electrons in ZnO is estimated to be about 1.35 µm at the present accelerating voltage (15 kV).35 Therefore, compared to CL emission, PL emission tends to be influenced by surface defects, and these defects can act as quenching centers, which is responsible for the disappearance of the Ni2+-related emission in PL. 3.4. Ferromagnetism at 300 K. According to theoretical predictions,8 the successful doping of Ni in ZnO enables the material to exhibit ferromagnetism at room temperature. The magnetic properties of Ni-doped ZnO were investigated by SQUID at 300 K. Figure 5a,b shows typical M-H curves of Ni-doped ZnO microrods and nanosheets, respectively. The data were extracted from the diamagnetic background, which origi-

Huang et al. nated from the metal substrate,36 and the corresponding raw data are shown in the inset. The obvious hysteresis loops indicate the existence of ferromagnetism at room temperature, consistent with previous studies.10 In addition, it can be seen that the data fluctuate in the M-H curve of the nanosheets, indicating weaker ferromagnetism in the nanosheets than the microrods. The possibility that the observed ferromagnetism originates from the metallic Ni or its oxides can be precluded. There is no evidence for the presence of zero-valence-state Ni in the XPS profiles. The Curie temperature of NiO (TC < 5 K) is very low,37 and Ni2O and Ni2O3 are paramagnetic materials, so their ferromagnetism could not be observed at room temperature. Previous studies suggested that oxygen vacancies play an important role in mediating the long-range magnetic exchange coupling in DMSs.38 Based on the F-center exchange mechanism, oxygen vacancies could couple with adjacent magnetic ions to form F-centers, and the overlap of a sufficiently large number of F-centers would lead to long-range magnetic exchange coupling.38 Recently, structual inhomogeneity was found to result in an enhancement of the room-temperature ferromagnetism of DMSs.14 A large amount of oxygen vacancies are present in our Ni-doped ZnO nanostructures, as indicated by the CL data. Therefore, the observed ferromagnetism at room temperature is considered to be mainly due to the coexistence of the magnetic dopants and oxygen vacancies in Ni-doped ZnO microrods and nanosheets, and the different dopant and oxygen concentrations have an influence on the strength of the magnetism. 4. Conclusion Wurtzite Ni-doped ZnO nanostructures were synthesized in situ through a facile chemical route. The morphology of the nanostructures evolved from microrods to nanosheets through adjustment of the deposition potential toward more negative values, which was ascribed to the potential-induced transition from thermodynamic to dynamic control of the growth. Doping can be achieved only if the potential exceeds a threshold value. The CL properties of the nanostructures were studied and were found to be closely related to the morphology and dopant concentration of the nanostructures. It was found that CL can be used to identify the appearance of the Ni2+-related emission, which is solid evidence of the successful doping of Ni into the ZnO matrix. Room-temperature ferromagnetism was observed in the Ni-doped ZnO nanostructures. The results presented here provide a route to fabricate Ni-doped ZnO nanostructures with controllable morphology and luminescent properties and shed some light on how to identify the magnetic dopant in ZnO through luminescent techniques. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 10474098). References and Notes (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (2) Fan, H. J.; Bertram, F.; Dadgar, A.; Christen, J.; Krost, A.; Zacharias, M. Nanotechnology 2004, 15, 1401. (3) Chang, Y. C.; Chen, L. J. J. Phys. Chem. C 2007, 111, 1268. (4) Li, L.; Pan, S. S.; Dou, X. C.; Zhu, Y. G.; Huang, X. H.; Yang, Y. W.; Li, G. H.; Zhang, L. D. J. Phys. Chem. C 2007, 111, 7288. (5) Cao, B. Q.; Cai, W. P. J. Phys. Chem. C 2008, 112, 680. (6) Li, G. R.; Dawa, C. R.; Bu, Q.; Liu, X. H.; Ke, Z. H.; Hong, H. E.; Zheng, F. L.; Yao, C. Z.; Liu, G. K.; Tong, Y. X. J. Phys. Chem. C 2007, 111, 1919. (7) Xu, L. F.; Guo, Y.; Liao, Q.; Zhang, J. P.; Xu, D. S. J. Phys. Chem. B 2005, 109, 13519. (8) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019.

Morphology and Properties of Ni-Doped ZnO Nanostructures

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4385

(9) Wang, X. F.; Xu, J. B.; Zhang, B.; Yu, H. G.; Wang, J.; Zhang, X. X.; Yu, J. G.; Li, Q. AdV. Mater. 2006, 18, 2476. (10) Cui, J. B.; Gibson, U. J. J. Phys. Chem. B 2005, 109, 22074. (11) Yuhas, B. D.; Zitoun, D. O.; Pauzauskie, P. J.; He, R. R.; Yang, P. D. Angew. Chem., Int. Ed. Engl. 2006, 45, 420. (12) Schwartzberg, A. M.; Olson, T. Y.; Talley, C. E.; Zhang, J. Z. J. Phys. Chem. B 2006, 110, 19935. (13) Ye, C. H.; Bando, Y.; Fang, X. S.; Shen, G. Z.; Golberg, D. J. Phys. Chem. C 2007, 111, 12673. (14) Xing, G. Z.; Yi, J. B.; Tao, J. G.; Liu, T.; Wong, L. M.; Zhang, Z.; Li, G. P.; Wang, S. J.; Ding, J.; Sum, T. C.; Huan, C. H. A.; Wu, T. AdV. Mater. 2008, 20, 3521. (15) Pearton, S. J.; Norton, D. P.; Ip, K.; Heo, Y. W.; Steiner, T. Prog. Mater. Sci. 2005, 50, 293. (16) Matienzo, L. J.; Yin, L. I.; Grim, S. O.; Swartz, W. E. Inorg. Chem. 1973, 12, 2762. (17) Li, C. P.; Proctor, A.; Hercules, D. M. Appl. Spectrosc. 1984, 38, 880. (18) Liu, X. F.; Yu, R. H. J. Appl. Phys. 2007, 102, 083917. (19) Huang, X. H.; Li, L.; Luo, X.; Zhu, X. G.; Li, G. H. J. Phys. Chem. C 2008, 112, 1468. (20) Wang, Z. L.; Kong, X. Y.; Zou, J. M. Phys. ReV. Lett. 2003, 91, 185502. (21) Cao, B. Q.; Cai, W. P.; Zeng, H. B.; Duan, G. T. J. Appl. Phys. 2006, 99, 073516. (22) Fan, H. J.; Barnard, A. S.; Zacharias, M. Appl. Phys. Lett. 2007, 90, 143116. (23) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Li, Y.; Xiao, Z. D. Chem. Lett. 2005, 34, 436.

(24) Zeng, H. B.; Cai, W. P.; Liu, P. S.; Xu, X. X.; Zhou, H. J.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (25) Venkataprasad Bhat, S.; Deepak, F. L. Solid State Commun. 2005, 135, 345. (26) Qiu, D. J.; Wu, H. Z.; Feng, A. M.; Lao, Y. F.; Chen, N. B.; Xu, T. N. Appl. Surf. Sci. 2004, 222, 263. (27) He, J. H.; Lao, C. S.; Chen, L. J.; Davidovic, D.; Wang, Z. L. J. Am. Chem. Soc. 2005, 127, 16376. (28) Djurisˇic´, A. B.; Leung, Y. H. Small 2006, 2, 944. (29) Studenikin, S. A.; Golego, N.; Cocivera, M. J. Appl. Phys. 1998, 84, 2287. (30) Onga, H. C.; Du, G. T. J. Cryst. Growth 2004, 265, 471. (31) Weakliem, H. A. J. Chem. Phys. 1962, 36, 2117. (32) Koidl, P. Phys. ReV. B 1977, 15, 2493. (33) Radovanovic, P. V.; Norberg, N. S.; McNally, K. E.; Gamelin, D. R. J. Am. Chem. Soc. 2002, 124, 15192. (34) Radovanovic, P. V.; Gamelin, D. R. Phys. ReV. Lett. 2003, 91, 157202. (35) Ichimiya, M.; Horii, T.; Hirai, T.; Sawada, Y.; Minamiguchi, M.; Ohno, N.; Ashida, M.; Itoh, T. J. Phys.: Condens. Matter 2006, 18, 1967. (36) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharma, R.; Ahuja, R.; Guillen, J. M. O.; Johansson, B.; Gehring, G. A. Nat. Mater. 2003, 2, 673. (37) Schwartz, D. A.; Kittilstved, K. R.; Gamelin, D. R. Appl. Phys. Lett. 2004, 85, 1395. (38) Coey, J. M. D.; Douvalis, A. P.; Fitzgerald, C. B.; Venkatesan, M. Appl. Phys. Lett. 2004, 84, 1332.

JP810790H