Appearance of Ferromagnetism in Co-Doped CeO2 Diluted Magnetic

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Appearance of Ferromagnetism in Co-Doped CeO2 Diluted Magnetic Semiconductors Prepared by Solid-State Reaction A. Bouaine,† R. J. Green,‡ S. Colis,† P. Bazylewski,‡ G. S. Chang,*,‡ A. Moewes,‡ E. Z. Kurmaev,§ and A. Dinia*,† †

Institut de Physique et Chimie des Mat
eriaux de Strasbourg (IPCMS), UMR 7504 UDS-CNRS (UDS-ECPM), 23 rue du Loess, BP 43, F-67034, Strasbourg Cedex 2, France ‡ Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada § Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620990 Yekaterinburg, Russia ABSTRACT: We report on the magnetic and electronic properties of Co-doped CeO2 oxides prepared by a solid-state reaction. The as-prepared samples exhibit pure paramagnetic behavior, but postannealing at 450 °C for 1 h under H2/N2 atmosphere results in the appearance of ferromagnetism at room temperature. The electronic structure of as-prepared and postannealed Co-doped CeO2 diluted magnetic oxides is investigated using soft X-ray absorption and emission spectroscopy. Co L-edge resonant inelastic X-ray scattering (RIXS) spectra indicate that direct Co-Co bonds form due to the precipitation of Co atoms in CeO2. This is in agreement with zero-field-cooled and field-cooled thermal variations of the magnetization, which indicated the presence of Co clusters with size larger than 8 nm.

1. INTRODUCTION There is currently a lot of interest in the science and potential technological applications of spin-dependent transport electronics (or spintronics), in which the spin of charge carriers is exploited to provide new functionalities for microelectronic devices.1-3 The development of magnetic semiconductors with practical ordering temperatures could lead to new classes of powerful devices and circuits. Study of spintronic materials has been stimulated by theoretical development, which has shown that wide band gap semiconductors are the most promising candidates for achieving high Curie temperatures (TC).4 However, numerous studies performed on these systems have led to a controversy on the nature (intrinsic or extrinsic) of the ferromagnetism in these systems.5-12 Among promising host materials for diluted magnetic semiconductor (DMS) systems, CeO2 is an attractive transparent rare-earth oxide because its lattice parameter is similar to that of Si and withstands a strong deviation to stoichiometry upon doping while retaining its cubic structure. This means that hightemperature ferromagnetism in transition metal (TM)-doped r 2010 American Chemical Society

CeO2 DMSs facilitates the integration of spintronic components with conventional Si-based electronic devices. Recently, Tiwari et al. reported ferromagnetic properties well above room temperature (TC ≈ 875 K) for Co-doped CeO2 thin films grown on LaAlO3 (100).13 They found that the atomic magnetic moment reaches 8.2 μB/Co atom, which is the highest moment of Co observed in any Co-related oxides. This was followed by a great deal of research in TM-doped CeO2 powders14-17 and thin films.18-22 Several groups suggested that the structural defects, such as oxygen and/or Ce vacancies, are a possible origin of ferromagnetism at ambient conditions,16,20-22 as ferromagnetic behaviors were also observed even in pure CeO223-27 and CeO2 doped with nonmagnetic ions such as Cu15,28 or Ca.29 On the other hand, the absence of such influence of structural defects was also reported by others.17,19 Nevertheless, although it is often claimed that strong ferromagnetism in Co-doped CeO2 can be Received: September 15, 2010 Revised: November 29, 2010 Published: December 30, 2010 1556

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Figure 1. Magnetization hysteresis loops at 5 K and room temperature for the as-prepared 3% Co-doped CeO2 powder.

attributed to ferromagnetic exchange coupling between two Co ions mediated by oxygen vacancy, detailed characterization of local structure around the magnetic dopant ions has not been carried out. In the present Article, we extensively investigated the magnetic properties and electronic structure of Co-doped CeO2 powder samples in an effort to understand the appearance of ferromagnetism in Co-doped CeO2 DMS system upon postannealing. The Co-doped CeO2 powder samples were prepared by a solid-state reaction and underwent a postannealing at 450 °C under a H2/N2 atmosphere for 1 h. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were carried out to study the magnetic properties of the samples. Co L-edge resonant inelastic X-ray scattering (RIXS) and oxygen K-edge X-ray absorption near edge structure (XANES) measurements were performed to study the cobalt and oxygen local bonding in Co-doped CeO2. The synchrotron-based X-ray XANES and RIXS techniques provide a powerful tool for studying the change in local coordination of the constituent atoms.

2. EXPERIMENTAL SECTION Undoped and Co-doped CeO2 (with Co concentrations of 0 and 3 at. %) powders were synthesized by standard solidstate reaction method. High-purity Ce2(CO3)3 and CoCO3 powders were mixed thoroughly and sintered at 1000 °C for 10 h in air (as-prepared samples). Some samples have been additionally annealed at 450 °C for 1 h under a H2/N2 atmosphere (postannealed samples) to study the effect of annealing on the magnetic properties. The grinding process was carried out using an agate mortar to avoid any magnetic impurity contamination. The cationic composition of the undoped and Co-doped CeO2 powders was checked by means of energy dispersive X-ray spectroscopy (EDS). Measurements of the magnetic properties were carried out with a Superconducting Quantum Interference Device (SQUID) magnetometer at both room and low temperatures. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the crystalline quality, and no Co clusters or other spurious phases were detected in our samples (not shown here). The XRD data showed solely the peaks from the CeO2 structure and a very small decrease of the lattice parameter (from 5.39 to 5.36 Å) upon Co doping. However, it is noteworthy that the presence of Co clusters with few nanometers of diameter cannot be ruled out by considering the detection limit of the XRD technique and a sparse distribution

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Figure 2. Magnetization hysteresis loops at 5 K and room temperature for the 3% Co-doped CeO2 powder postannealed at 450 °C under the H2/N2 atmosphere. The inset shows the coercive fields for both temperatures.

of clusters with respect to the local character of TEM technique. The RIXS experiments, which include the measurements of X-ray absorption and emission spectra (XAS and XES, respectively), were carried out at Beamline 8.0.1 of the Advanced Light Source equipped with a fluorescence endstation.30 The emitted radiation was measured using a Rowland circle-type spectrometer. Resonant Co L2,3 (3d4s f 2p transition) XES spectra were obtained at an excitation energy (Eexc) near the L2 and L3 absorption thresholds. Additionally, O 1s XANES (1s f 2p transition) spectra were measured. All spectra were recorded at room temperature and normalized to the number of photons falling on the sample monitored by a highly transparent gold mesh. The Co 2p and O 1s XAS spectra were measured in the total electron yield and fluorescence yield modes, respectively.

3. RESULTS AND DISCUSSION 3.1. Magnetic Properties. Figure 1 shows the magnetization hysteresis loops for the as-prepared 3% Co-doped CeO2 powder (CeO2:3% Co) measured at 5 K and room temperature. The hysteresis loops show a pure paramagnetic behavior at both low and room temperatures. A linear variation of the magnetization is observed at room temperature, while a Brillouin-like behavior is observed at low temperature. This indicates that the ionic Co has a paramagnetic contribution and no ferromagnetic behavior is detected. On the other hand, the postannealed sample at 450 °C under a H2/N2 atmosphere clearly exhibits a strong ferromagnetic behavior at both low and room temperatures (see Figure 2). The most interesting result is that the saturation magnetization does not show a significant change between 5 and 300 K, indicating that the TC is well above room temperature. Wen et al.16 reported a similar ferromagnetic behavior for Co-doped CeO2 powder and attributed it to a combination of the effects of oxygen vacancies and transition metal doping. However, the macroscopic magnetization measurements as described above are not sufficient to provide explanations about the origin of ferromagnetic behavior. The saturation magnetization of our 3% Co-doped CeO2 sample is about 0.75 emu/g corresponding to 0.76 μB/Co ion and is lower than the value of 1.72 μB expected for Co metal. This indicates that a large amount of the ionic Co has been probably 1557

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Figure 3. ZFC and FC variation of the magnetization as a function of temperature for the 3% Co-doped CeO2 powder postannealed at 450 °C under H2/N2 atmosphere.

reduced to Co metal, while some Co ions still occupy the Ce sites in the CeO2 matrix. While some authors have employed techniques such as tomographic atom probe (TAP)31 or X-ray magnetic circular dichroism (XMCD)32 to explore the origin of the observed ferromagnetism in other types of DMS, an easy approach that can suggest whether the observed ferromagnetism is due to metallic Co clusters or has another origin is a combination of ZFC and FC thermal variations of the magnetization. Figure 3 shows ZFC and FC curves of the magnetization (M) performed under an applied magnetic field of 100 Oe for the postannealed 3% Co-doped CeO2 powder. Interestingly, one can see that the ZFC and FC curves did not superpose even for temperatures as high as 400 K. Such behavior is a strong signature of the presence of metallic Co clusters diluted in the CeO2 matrix. The temperature-dependent magnetic behavior of metallic Co particles with a size of several nanometers is characterized by the ferromagnetic/paramagnetic transition corresponding to the blocking temperature as given below: TB ¼

KR V 25kB

ð1Þ

where V is the volume of the particles, kB is the Boltzmann constant, and KR is the effective anisotropy constant or magnetocrystalline constant (which is equal to 4.5  106 erg/cm3 for hcp Co). In the present case, the blocking temperature is not observed for measurement temperatures as high as 400 K, and from this we calculate that the Co clusters have a diameter larger than 8 nm and are likely at the origin of the room temperature ferromagnetism. This will be further supported by O 1s XANES and Co L-edge RIXS measurements, because no change in the O 1s XANES spectra was observed by doping or annealing the undoped as-prepared ceria. 3.2. O 1s XANES and Co L-Edge RIXS Spectra. Results of density functional theory (DFT) calculations have predicted strong changes in the O 2p unoccupied states for nonstoichiometric CeO2 with respect to the stoichiometric composition.33 Therefore, we used O 1s XANES measurements (which probe the O 2p unoccupied states) to analyze our undoped, Co-doped, and Co-doped/postannealed CeO2 samples. The spectra, shown in Figure 4, do not show any noticeable change. This reflects that no additional oxygen vacancies are introduced by Co doping or

Figure 4. O 1s XANES spectra of undoped and 3% Co-doped CeO2.

Figure 5. Co L2 RXES spectra of 3% Co-doped CeO2 (a) and reference samples (b).

postannealing than are intrinsically present in as-prepared undoped ceria. Although this does not rule out the possibility of native vacancies having some sort of role in the ferromagnetic behavior, the O 1s XANES results and the absence of ferromagnetism in undoped CeO2 and Co-doped CeO2 without a postannealing suggest that Co definitely plays a role in the ferromagnetism, and the concentration of oxygen vacancies may not be high enough to form a delocalized impurity band for magnetic percolation in our samples. In addition, we investigated the electronic structure of Co dopant atoms in an effort to understand local bonding features around magnetic dopants. Figure 5 depicts the Co L-edge RIXS spectra (excited at the Co L2-threshold) of Co-doped CeO2. Dramatic changes are found in the spectrum of ferromagnetic postannealed sample with respect to the paramagnetic asprepared sample, as the I(L2)/I(L3) intensity ratio is found to be reduced by a factor of 2. This reduction of the L2 emission line with respect to L3 line for ferromagnetic postannealed CeO2:3% Co sample is largely due to radiationless Coster-Kronig (C-K) transitions observed in pure 3d transition metals. The intensity of these C-K transitions is correlated to the number of free charge carriers that can absorb the decay energy, and thus increased 1558

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sites possess the same local symmetry). The lower panel shows that the RXES spectrum of ferromagnetic postannealed sample is best described as a combination of the substitutional/interstitial Co atoms found in the paramagnetic sample and the spectrum from a Co metal reference. Here, the same experimental spectrum from the upper panel is combined with that taken from Co metal, and the direct sum agrees very well with the experimental spectrum of ferromagnetic CeO2:Co sample. Thus, again it is obvious that the nonferromagnetic sample contains Co coordinated with oxygen atoms in its local environment, while the ferromagnetic sample contains a significant amount of cobalt atoms in metallic aggregates. This reiterates that the presence of Co clusters is responsible for the observed ferromagnetism, and that substitutional/interstitial positioning of the Co in the lattice does not lead to ferromagnetic behavior.

Figure 6. Co L3 RXES spectra of 3% Co-doped CeO2 and multiplet calculations.

C-K transition intensity indicates increased metallicity.34,35 A second reason for the reduction in intensity of this L2 line is a decrease in the amount of scattering via dd-excitations; in a more metallic sample, the 3d states are more delocalized, and thus scattering intensities into these states are diminished with respect to regular nonresonant fluorescence. As shown in the figure, the RIXS spectrum of paramagnetic as-prepared CeO2:3% Co sample is similar to the spectrum of CoO (strong L2 line due to strong scattering and weak C-K transitions), whereas the ferromagnetic sample shows a limited ratio between Co metal (minimal scattering but strong C-K transitions) and CoO, which suggests that direct Co-Co bonds form due to the precipitation of Co atoms in CeO2, induced by a postannealing process. It is necessary to point out that the important decrease of the I(L2)/I(L3) ratio is induced only by heat treatment in the H2/N2 atmosphere without any changes in the chemical composition of the samples. Taking into account the local character of X-ray transitions, it is likely that such a decrease can be attributed to the reconfiguration of local environment of the excited Co-atom. Again looking into the spectra of reference samples (CoO and Co metal), also shown in Figure 5, we can conclude that the changes in the I(L2)/I(L3) ratio are due to Co clustering after postannealing in H2/N2 atmosphere. Figure 6 shows Co L-edge resonant XES (RXES) spectra (excited at the L3 absorption threshold) of the same samples, along with ligand field multiplet calculations and a reference spectrum of Co metal for comparison. The RXES excitation energy of 779 eV is denoted by the vertical arrow on the calculated XAS spectrum (see inset of Figure 6), and the RXES spectra are plotted on an energy axis relative to this excitation energy. One can see that the RXES spectra of the two CeO2:Co samples are somewhat different, but both agree well with the respective simulated spectrum shown below. In the upper panel, a ligand field multiplet calculation with a cubic crystal field of strength (10Dq = -0.65 eV) is shown and provides the best fit with the experimental data. This result shows that, in the case of paramagnetic as-prepared sample, the Co atoms reside within the cubic host lattice, either at interstitial or substitutional sites (both

4. CONCLUSION We have successfully induced room temperature ferromagnetism in Co-doped CeO2 diluted magnetic oxides after postannealing under the H2/N2 atmosphere at 450 °C. We have used resonant inelastic X-ray scattering to demonstrate that the local electronic environment of Co is mainly composed of Co atoms instead of oxygen, indicating the presence of metallic Co clusters. This observation of metallic clusters is also supported by the temperature-dependent measurements of the magnetization, which indicated the presence of Co clusters larger than 8 nm. For as-prepared, paramagnetic samples, we have shown that there are no metallic Co clusters and that the Co atoms reside entirely at substitutional/interstitial locations within the host lattice. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (G.S.C.); aziz.dinia@ipcms. u-strasbg.fr (A.D.).

’ ACKNOWLEDGMENT This work was done with partial support of the Russian Science Foundation for Basic Research (Project No. 11-0200022). We gratefully acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair program, and we thank the staff of the Advanced Light Source at Lawrence Berkeley National Laboratory for assistance. ’ REFERENCES (1) Ohno, H. Science 1998, 281, 951. (2) Prinz, G. A. Science 1998, 282, 1660. (3) Kikkawa, J. M.; Smorchkova, I. P.; Samarth, N.; Awschalom, D. D. Science 1997, 277, 1284. (4) Dielt, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (5) Dinia, A.; Schmerber, G.; M
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