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J. Phys. Chem. C 2008, 112, 2412-2417
Colloidal Cobalt-Doped ZnO Nanorods: Synthesis, Structural, and Magnetic Properties Thomas Bu1 sgen,† M. Hilgendorff,† S. Irsen,† F. Wilhelm,‡ A. Rogalev,‡ D. Goll,§ and M. Giersig*,† Center of AdVanced European Studies and Research (CAESAR), 53175 Bonn, Germany, European Synchrotron Research Facility (ESRF), 38000 Grenoble, France, and Max-Planck-Institut (MPI) fu¨r Metallforschung, 70569 Stuttgart, Germany ReceiVed: September 19, 2007; In Final Form: NoVember 23, 2007
The synthesis of ZnO:Co nanorods in alcoholic solutions and their structural, optical, and magnetic properties are presented. One-dimensional growth is achieved by careful control over the precursor addition and with the aid of ethylene diamine as a ligand. The nanorods are single-crystalline and show growth into the [0001] direction. They exhibit diameters of about 20 nm and lengths up to 150 nm. Using (HR)TEM, EDX, and XANES, we prove the incorporation of Co cations on Zn lattice sites. SQUID and XMCD measurements at cryogenic temperatures show paramagnetic behavior of the doped nanorods. Apparently, there are too few donor electrons to mediate ferromagnetic coupling in the highly crystalline nanorods compared to defect-rich spherical nanoparticles.
1. Introduction
2. Experimental Section
Over the past few years, extensive research has been carried out on diluted magnetic semiconductors (DMS) because of their potential applications in spintronics and optoelectronic devices.1-4 Theoretical calculations have shown that 3d-transition-metal (TM)-doped ZnO would be a good candidate to achieve Curie temperatures above room temperature.5-7 While numerous reports on ZnO:TM thin films and spherical nanoparticles exist,8-15 only few have been published on DMS nanowires;16-18 these are of special interest as building blocks for spin-based applications. Gas-phase synthesized ZnO:Co nanowires have already been fabricated successfully by several groups.19-22 Yet, only a few studies focused on the wet chemical growth of such nanostructures, which mostly requires special experimental conditions such as electric fields,23 ultrasound treatment,24 or autoclaves.25 It is well known from the synthesis of spherical nanoparticles that dopant atoms have a strong influence on the amount and size of the resulting products.12 The incorporation of impurities into the host lattice is hindered by an increase in surface energy and lattice distortion. In particular, the thermodynamically unfavored, purely kinetically driven growth of one-dimensional (1D) structures is often restrained. However, we succeeded in establishing an easy approach toward the synthesis of long ZnO:Co nanorods in alcoholic solutions under normal atmosphere. The incorporation of dopant atoms into the ZnO wurtzite lattice is verified by (high resolution) transmission electron microscopy ((HR)TEM), energydispersive X-ray analysis (EDX), and X-ray absorption nearedge spectroscopy (XANES). The macroscopic and microscopic magnetic properties of the samples have been investigated by a superconducting quantum interference device (SQUID) and X-ray magnetic circular dichroism (XMCD), respectively.
Cobalt-doped ZnO nanorods are synthesized by means of a simple hydrolysis and condensation method similar to the one we described earlier.26 Typically, 3 - x mmol (1 < x < 10) zinc acetate dihydrate (Zn(ac)2‚2H2O, Aldrich reagent grade) and x mmol cobalt acetate tetrahydrate (Co(ac)2‚4H2O, Aldrich reagent grade) are dissolved in 10 mL 1-butanol (1-BuOH, Merck p.s.) supported by 12 mmol of ethylene diamine (EDA, Merck p.s.) acting as a complexing agent. The resulting clear, orange-brown solution is put into a glass syringe. A second syringe holds a 1.2 M KOH (Aldrich A.C.S.) solution in BuOH. Both solutions are dripped slowly into a heated reaction vessel using a syringe pump at a rate of 1.5 mL‚h-1. After complete addition, the resultant suspension is boiled at 117 °C for another 30 min before being centrifuged and washed several times with tetrahydrofuran (Roth p.a.) and ethanol (Merck p.a.). The final, olive-green precipitate is dispersed in 2-propanol (Roth p.a.) for UV-vis absorption (Varian, Cary 5000) and photoluminescence (PL) spectroscopy (Horiba Jobin Yvon, Fluromax 3). The products are further characterized by TEM with an acceleration voltage of 200 kV (LEO 922-A and Zeiss Libra 200-CRISP). The composition of single nanorods is examined with the integrated EDX system (Oxford Instruments). The magnetization is measured using a SQUID (Quantum Design, MPMS-7T) at 5 K with fields of up to 7 T. For measurements, the dried ZnO:Co nanorods are suspended in sodium silicate in a small plastic tube, which is inserted into the SQUID after drying. From the obtained raw data, the diamagnetic backgrounds of ZnO, sample support, and holder, determined at 300 K, are substracted. The actual amount of Co in the samples is determined by atomic absorption spectroscopy (Perkin-Elmer, AAnalyst 800). To this end, a weighed mass of ZnO:Co nanorods is dissolved in a specific amount of diluted nitric acid and sprayed into a flame where the sample is atomized. Element-specific absorption is recorded using the 242.5 nm line of a Co lamp, which does not interfere with the absorption lines of Zn or O.
* Corresponding author. E-mail:
[email protected]. † CAESAR. ‡ ESRF. § MPI.
10.1021/jp077546t CCC: $40.75 © 2008 American Chemical Society Published on Web 01/31/2008
Colloidal Cobalt-Doped ZnO Nanorods Further structural and magnetic microscopic properties have been investigated using the XANES and XMCD spectroscopies at the Co K-edge. In the case of TM, the K-edge comprises generally much more structural, electronic, and magnetic information than the L-edges. The XANES is often highly structured because it is strongly sensitive to the local environment (symmetry, bonding, ...) and to the electronic configuration of the absorbing atom. The XMCD at the K-edge, especially that of the 3d TM, consists mainly of dipole transitions (1s f 4p) and, to a lesser extent, of quadrupolar transitions(1s f 3d). Because these techniques are element-specific, they are the most appropriate tools for directly probing the electronic structure and magnetism of the diluted transition metal as has been demonstrated for wurtzite GaN:Mn.27 XANES spectra were recorded at the Co K-edge using total fluorescence yield detection mode. The main advantage of performing experiments at the K-edges is that one probes the bulk properties and not only the surface layer, as is the case for experiments at the L-edges. The XMCD signal was obtained as a direct difference of two XANES spectra recorded with rightand left-circularly polarized X-rays provided by the helical Apple-II undulator. A magnetic field up to 6 T was set parallel to the incoming X-ray beam and nearly parallel (10°) to the sample surface. The sample, prepared by drop-casting ZnO:Co nanorods on silicon substrates, was cooled down to 7 K. All measurements were performed at the ESRF beamline ID12.28 3. Results and Discussion Synthesis. The fabrication of pure ZnO nanorods in hydrophilic and hydophobic solutions has already been established by a number of different groups.29-36 The nanorods usually grow in the [0001] direction because of the anisotropy in the growth rate, often supported by the addition of ligands, blocking the other growth directions. As shown theoretically by Manna et al.,37 the {101h0} side facets of II-VI-semiconductor nanocrystals are completely covered by coordinating ligands, which donate their lone electron pair to empty cation orbitals. Yet, on the (0001) face, consisting only of metal ions, a coverage of just 75% can be achieved because of the electronic configuration of this lattice plane. The bare metal ions are additionally pushed out of the plane, leading to a very reactive site for further growth. In our case, EDA molecules act as such ligands, with their two lone electron pairs at the amine groups. Additionally, EDA complexes the cations in the precursor solution, leading to a better solubility in the used alcohol. The formation of nanorods is further aided by the slow addition of the precursors. Hence, only a few seeds are formed in the beginning of the reaction, which grow longer when further precursors are added. Dopant elements, introduced to the growth solution, hinder the formation of one-dimensional nanorods. The impurity atoms add a thermodynamical barrier to the nanocrystals’ growth as observed before by, for example, Gamelin’s group.11,12 Most likely, the first particles formed in the solution consist of only the pure host material. Additional energy has to be applied to overgrow dopant atoms, which are attached to the nanocrystal surface, resulting in a slowdown of the whole reaction. As a consequence, mostly smaller, spherical nanoparticles are obtained. However, we fabricate doped ZnO:Co nanorods by mixing the precursor solutions slowly at the elevated temperature of boiling BuOH (117 °C), providing enough thermal energy to overcome the aforementioned thermodynamical barrier. Yet, the applied energy has to be well balanced to obtain doped nanorods: Performing the reaction in shorter alcohols such as 2-propanol or ethanol at their respective boiling temperatures
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2413
Figure 1. Representative UV-vis absorbance (blue line) and photoluminescence (dashed red line) spectra of ZnO:Co nanorods showing the exciton transion in ZnO. The features around 475 nm in the PL spectrum are artifacts due to scattering of the incident beam by the nanorods’ suspension.
also results in long nanorods, but less dopant is built in the host lattice because the provided thermal energy is not high enough to incorporate the impurity atoms. However, performing the reaction in boiling octanol (194 °C) leads to a relatively high dopant concentration in the formed particles. Yet, these are spherical in shape, most likely due to fast nucleation of many seeds and less protection by adsorbed ligands. The incorporation of Co is directly observed by the olivegreen coloration of the product. While the dry Co(ac)2‚H2O powder and its alcoholic solution are pink because of the hydroxylic groups surrounding the cations, the color changes to orange-brown upon addition of the amine. The EDA chelates the transition-metal octahedral, increasing the energetic splitting of the 3d orbitals and giving rise to an absorption at shorter wavelengths. Incorporated in the wurtzite lattice on Zn sites, the Co cations are located in a tetrahedral environment of oxygen anions. The ligand field splitting in the tetrahedral geometry is about half as large as that in the octahedral one, whereby the absorption onset is red-shifted, seen from the green complement color of ZnO:Co. Optical Spectroscopy. From the absorption onset of corresponding UV-vis spectra, as shown exemplarily in Figure 1 (blue line), the band gap energy can be estimated. It matches the bulk value, and since the exciton Bohr radius of ZnO is much smaller than the diameter of the nanorods there is no indication of quantum confinement. The PL spectrum (Figure 1, dashed red line) shows a strong transition at 371 nm, which belongs to the direct exciton recombination. However, the visible luminescence from defect states, usually seen in pure ZnO with a maximum at about 550 nm, cannot be observed for the doped ZnO:Co nanorods. This might be due to several reasons: Because of good crystallinity, there are only a few defect sites in the wurtzite lattice, which would act as electron traps shifting the photoluminescence into the visible range. Anyway, there are always dangling bonds on the surface. These, however, are passivated by EDA molecules acting as coordinating ligands because of their lone electron pairs on nitrogen.38 Besides, the dopant atoms also play a role in the photoluminescence behavior of the ZnO nanorods since Co2+ ions act as deep traps in IIVI materials and open up nonradiative relaxation pathways.11,12 Structural Investigation. The TEM image in Figure 2 of the prepared nanostructures in BuOH at 117 °C shows rods with an average diameter of 20 nm and lengths of up to 150 nm.
2414 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Figure 2. TEM survey of ZnO:Co nanorods prepared in BuOH with a dopant concentration of 1 atom % determined by EDX.
Bu¨sgen et al.
Figure 4. HRTEM picture of a ZnO:Co nanorod showing good crystallinity and [0001] growing direction. Inset: Corresponding FFT image from which the c-lattice constant of 507 ( 11 pm is deduced.
Figure 3. TEM picture of a single ZnO:Co nanorod. The frame indicates the area taken for the HRTEM picture shown in Figure 4.
Figure 5. EDX spectra of ZnO:Co nanorods shown in Figures 2 and 4. From comparison of the K-edge signals, a Co/Zn ratio of 1/99 is deduced. Cu and C signals arise from the TEM grid, Fe from the TEM objective pole shoe, and K and Si from remains of the used base and silicon sealing grease.
The sharp tips are most likely formed during the final stage of the synthesis, when the concentration of free precursor ions in the solution drops. In Figure 3, a magnified image of a single nanorod is shown. One can see that the rod is a single crystal with a homogeneous diameter over the whole particle. Neither grain boundaries nor major crystal dislocations are observable. The HRTEM picture and its corresponding FFT (Figure 4) prove the high crystallinity and [0001] growth direction of the nanorods. The c-lattice constant is estimated to be 507 (11 pm, similar to the one in pure ZnO. There are no indications of secondary phases or impurities visible in the (HR)TEM pictures, suggesting that all dopant atoms are homogeneously incorporated into the ZnO rods and that no Co clustering occurred. The composition of single nanorods is determined by an integrated EDX detector. In Figure 5, a typical spectrum is shown. For a better visualization of the small signals, a logarithmic scale is chosen. By comparing the peak areas of corresponding K-edge signals, the ratio of Co and Zn is determined. The Co concentration of the prepared nanorods varies between 1 and 9 atom % depending on the Co/Zn precursor ratio and reaction conditions. In general, less Co is found than provided in the precursor solution, which also indicates the hindering of dopant incorporation into the host lattice. The other signals originate from the used carbon-coated
copper TEM grid (C and Cu), the pole shoe (Fe) of the TEM, and remains of the used base (K) and silicon grease (Si) sealing the reaction vessel. Further structural investigations by XANES, performed at the K-edge of Co, confirm that the cobalt atoms are wellincorporated into the host lattice. In Figure 6, the XANES of the 1 atom % Co-doped ZnO nanorods, measured at 7 K, is given in comparison to metallic Co and the oxides CoO and Co3O4 (taken from ref 39). The XANES of the 7 atom % doped sample looks essentially the same. The shape of the sample’s absorption spectrum is completely different from the one of Co metal. This already indicates that the Co atoms do not form pure metallic clusters. The position of the absorption edge fits almost exactly to that of CoO. This fact confirms the 2+ oxidation state of the incorporated dopant. However, the presence of a strong pronounced pre-peak feature at 7.71 keV and the pre-edge shoulder at 7.715 keV speak against the formation of pure CoO particles as a secondary phase. The pre-edge shoulder arises principally from dipolar transitions to Co 4p states and from quadrupolar transitions to Co 3d states. It is well known that the impurity Co d bands are located in the band gap and nearly the Fermi level. Because the 3d impurity band is strongly hybridized with 4p states, we observe then a pre-peak in the K-edge that corresponds to transitions into the unocuppied band impurity states
Colloidal Cobalt-Doped ZnO Nanorods
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Figure 7. XMCD intensity vs the applied magnetic field for the XMCD signals at 7.710 keV (pre-peak, blue line) and 7.726 keV (K-edge, red line) for 7 atom % doped ZnO nanorods measured at 7 K.
Figure 6. Top: XANES of ZnO:Co nanorods perform Co K-edge in comparison to reference data of Co, CoO, and Co3O4 taken from ref 39 Copyright 2006 Elsevier Limited. All spectra are normalized to unity; reference spectra are shifted vertically for clarity. Bottom: The corresponding XMCD for 1 atom % (solid red line) and 7 atom % (dashed blue line) doped ZnO nanorods in relative units toward the normalized XANES. Vertical lines are guides to the eye.
located just nearly above the Fermi level. The tetrahedral crystal field surrounding the absorbing Co atom within the wurtzite lattice splits the Co d-orbitals into an energetically lower, doubly occupied e and a higher lying t2 set, which carries three single electrons. Because the Co 4p-orbitals also have t2 symmetry in the tetrahedral environment induced by the ligands, a p-dhybridization of the Co 3dt2 and Co 4pt2 states is possible.43 In higher symmetric environments, like the octahedral, the hybridization is weaker so that the pre-edge signal is less pronounced.40 However, we cannot rule out the presence of CoO completely because we cannot perform X-ray magnetic linear dichroism measurements, since the casted nanorods have a random c-axis orientation on the substrate. From all of the results presented, one can conclude that the dopant atoms are well-incorporated in the host lattice as Co2+ on Zn lattice sites. Since there is no indication of metallic Co, the possibility of Co clusters as the source of magnetism can be ruled out. Magnetic Characterization. XANES experiments with leftand right-circular polarized light in a magnetic field, which is parallel or antiparallel to the incident beam, result in different absorption intensities. This dichroism is due to the interaction of the angular moment of the incident photon with the spin and orbital momentum of the magnetic sample. The XMCD signal thus gives insight into the magnetic configuration of the examined sample. XMCD signals for ZnO nanorods with two different doping levels (1 and 7 atom %) are shown in the lower part of Figure 6. Both exhibit two main features: one at 7.726 keV, the energetic position of the K-edge, and the other at 7.710 keV, the so-called pre-peak. At the K-edge, mostly 1s-4p dipolar transitions occur. However, we cannot rule out a certain amount of quadropolar 1s-3d transitions, but their contribution should be very small.41 Therefore, the XMCD at the K-edge probes mainly the orbital
Figure 8. Magnetization per Co atom at 5 K for different ZnO:Co nanorods with 1, 5, and 9 atom % Co, respectively. The raw data is corrected by substracting the diamagnetic background, obtained at 300 K for each sample, respectively.
moment of the 4p states on the core-hole site. Calculations with the tight-binding approximation show that this orbital moment of the 4p states is induced mainly by the 3d-orbital moment due to p-d hybridization.42 Hence, the spin-orbit coupling of the 3d states plays the dominant role in generating the XMCD signal at the K-edge. Likewise, the Co 3d orbitals also gain some p character because of this hybridization. Therefore, a transition into these states is possible as well giving rise to the aforementioned prepeak feature. Because of 3d spin imbalance and the spin-orbit interaction, a pronouced XMCD signal is observed. We recall that at the K-edge the XMCD signal is purely of orbital origin and is therefore a measure of the orbital magnetism. As shown in Figure 7, both XMCD signals exhibit a linear dependence on the applied magnetic field, indicating paramagnetic behavior of the ZnO:Co nanorods. The measured effect is a mixture of linear and nonlinear components because the ZnO:Co nanorods are spread on the substrate building an anisotropic layer. In contrast to SQUID, which measures the macroscopic magnetism of the sample including the substrate and the holder, XMCD probes the microscopic magnetic properties of the Co atoms only. Since there is no indication for a ferromagnetic coupling between the transition-metal cations at this microscopic level, one can rule out the possibility of a superparamagnetic behavior of weakly ferromagnetic particles. SQUID measurements confirm the paramagnetic behavior of the samples. As shown in Figure 8, there is no ferromagnetic hysteresis visible for any of the investigated ZnO:Co nanorod samples, varying in the amount of incorporated dopant. We can
2416 J. Phys. Chem. C, Vol. 112, No. 7, 2008 rule out the possibility that small CoO clusters are responsible for the overall magnetic behavior. We synthesized small CoO nanoparticles with a diameter of 2 nm under the same reaction conditions without adding the zinc precursor. Their magnetic behavior is purely antiferromagnetic at 5 K and paramagnetic at 300 K as shown in the Supporting Information. There is a distinct dependency of the maximum attainable magnetization of the ZnO:Co samples on the Co percentage: While it is almost unchanged in the samples with 1 and 5 atom %, it decreases for high dopant concentrations. This behavior can be explained by the fact that for high concentrations it is more probable for dopant atoms to occupy next-nearest lattice sites. Such Co pairs couple by super-exchange over the intermediate oxygen atom in an antiferromagnetic way. This antiferromagnetism suppresses the magnetization because the external magnetic field is not strong enough to flip one spin of these antiparallel aligned pairs. The magnetism of DMS can be explained by different mechanisms such as carrier-mediated interaction5,44 or RKKY exchange.45,46 However, especially for semiconductors with low carrier densities such as oxides, the magnetic polaron mechanism is applicable.1 In this model, the spins of magnetic dopants incorporated into the semiconductor lattice interact through a donor-impurity band, formed by lattice defects such as oxygen vacancies (VO). Coey et al.4 explain the spin-alignment of the 3d transition-metal cations by the coupling of their spins, which are antiparallel to the spin of donor electrons residing in hydrogenic orbits. Due to this coupling, all spins within this expanded orbit are aligned. Because of the overlap of different orbits, an impurity band is formed, aligning a huge number of 3d magnetic spins parallel, resulting in ferromagnetism. In the framework of this theory, not only the dopant concentration but also the number of donor electrons must be quite large in order to obtain ferromagnetism. Nanoparticles possess a lot of such defect sites because of their comparatively large surface, which, naturally, has many defects in the form of unsaturated bonds. Rubi et al. demonstrate that the magnetism of Co-doped ZnO powders can be switched reversibly from ferro- to paramagnetic behavior by annealing in either oxygen-poor or oxygen-rich atmosphere, resulting in the generation or cancellation of VO, respectively.47 Contrary to small particles, bulk single crystals of ZnO:Co, grown at near-equilibrium, exhibit a very low defect concentration and show no ferromagnetism but behave paramagnetically.48 These observations strengthen the concept of an impurity-band exchange by magnetic polarons evoked by VO in the ZnO:Co nanoparticles. From these theoretical and experimental results, it becomes evident that not only the dopant concentration but also the amount of defect sites has to be substantial enough to get a ferromagnetic coupling of the magnetic spins. In the framework of the polaron model, the defect concentration has to exceed the polaron percolation threshold. The polaron radius can be estimated to be rH ) γa0 with the atomic Bohr radius a0 ) 53 pm and γ ) (me/m*). In ZnO, the high-frequency dielectric constant ) 4.0 and the effective electron mass m* ) 0.28me result in rH ) 0.76 nm and a polaron volume of 1.84 nm3. Percolation occurs when the polarons fill about 16% of space.49 From this, a critical donor concentration of ncrit ) 8.7 × 1025 m-3 can be deduced. Considering the oxygen density of ZnO to be nO ) 3.94 × 1028 m-3, the polaron threshold is δp ) ncrit/nO ) 2.2 × 10-3. Because of the careful growth process, that is, the slow addition of precursor solutions to the reaction vessel in the presence of EDA molecules, the formed ZnO:Co nanorods
Bu¨sgen et al. become highly crystalline and surface-bound defects are passivated by the ligand. Therefore, the defect concentration is quite low as the above-mentioned TEM investigations and optical spectroscopy confirm. We believe that due to this fact there are too few donor electrons in the samples to form extended defect bands, which would align the 3d magnetic spins in large domains. Therefore, only paramagnetism is observed. 4. Summary In this study, the synthesis of Co-doped ZnO nanorods with high aspect ratios is demonstrated using a simple wet chemical solution route under normal atmosphere. Despite the energetic barrier for the incorporation of dopant atoms into the host lattice, the 1D growth of ZnO:Co is retained because of the presence of EDA and the careful control exerted over the precursor addition. TEM investigation shows that the formed nanorods are highly crystalline and [0001]-oriented. EDX demonstrates the integration of the dopant into the ZnO nanostructures. Further structural investigations with XANES, performed at the Co K-edge, prove the incorporation of Co ions into the wurtzite host lattice on Zn lattice sites. The visible, defect-related luminescence usually observed in ZnO nanoparticles is quenched completely, indicating high crystallinity of the samples and passivation of surface-bound defects by the coordinating ligand EDA. XMCD and SQUID measurements show only paramagnetic behavior of the samples at cyrogenic temperatures. However, the maximum attainable magnetization decreases for high Co concentrations, most likely because of antiferromagnetic coupling of Co atoms occupying neighboring Zn lattice sites. The fact that no ferromagnetism is observed in our wetchemical synthesized nanorods probably results from the low defect concentration. Therefore, no continuous impurity band is formed by overlapping polarons in the nanocrystals, which would align all magnetic spins. Future research will be focused on the dependence of cobalt and defect concentration on the magnetic properties of these ZnO:Co nanorods. Acknowledgment. AAS measurements were performed by S. Maurer at the University of Applied Sciences Bonn-RheinSieg. We thank G. Ctistis for helpful discussions. Financial support by the European Union under grant no. MRTN-CT2004-005567 and the priority program SPP 1165 of the Deutsche Forschungsgemeinschaft is acknowledged. Supporting Information Available: TEM picture of CoO clusters, formed under the same synthetic conditions but without adding the zinc precursor, and magnetization at 5 K and 300 K for CoO clusters shown in the TEM picture. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pearton, S. J.; Heo, W. H.; Ivill, M.; Norton, D. P.; Steiner, T. Semicond. Sci. Technol. 2004, 19, R59. (2) Chambers, S. A.; Droubay, T. C.; Wang, C. M.; Rosso, K. M.; Heald, S. M.; Schwartz, D. A.; Kittilstved, K. R.; Gamelin, D. R. Mater. Today 2006, 9, 28. (3) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molna´r, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (4) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173. (5) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019.
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