RIXS approach to local environment around impurity atoms in diluted magnetic semiconductors and dielectrics

Journal of Electron Spectroscopy and Related Phenomena 181 (2010) 202–205

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RIXS approach to local environment around impurity atoms in diluted magnetic semiconductors and dielectrics G.S. Chang a,∗ , E.Z. Kurmaev b , L.D. Finkelstein b , A. Moewes a , A. Dinia c a b c

Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, SK, S7N 5E2 Canada Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620041 Yekaterinburg, Russia Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 DS-CNRS (UDS-ECPM), 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France

a r t i c l e

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Article history: Available online 31 May 2010 Keywords: Diluted magnetic semiconductors X-ray spectroscopy Local bonding structure

a b s t r a c t The use of resonant inelastic X-ray scattering (RIXS) to understand local environment around magnetic atoms in various diluted magnetic semiconductors (DMS) and dielectrics (DMD) has been overviewed. The overall spectroscopic results of transition-metal-doped GaAs DMS and wide band-gap oxide DMD systems show that the magnetic transition-metal dopants can occupy not only cation sites but also interstitial ones and the interactions between substituted and interstitial magnetic ions play a very important role on the nature of magnetic properties of these materials. This suggests that a careful verification of presence/absence of structural defects is required, especially for when the size of defect configuration is very small (less than few nm). © 2010 Elsevier B.V. All rights reserved.

1. Introduction The use of electron spin, in addition to electron charge, provides an additional degree of freedom to perform logic operations, store information, etc. The integration of semiconductor with magnetic functionality of electrons is a promising way to produce a new type of materials (spintronic materials) exhibiting spinpolarized transport properties. Such materials can be designed by introducing appropriate magnetic 3d transition-metal elements (Mn, Fe, or Co), at levels of a few percent, into dielectric or semiconductor, producing dilute magnetic dielectrics (DMDs) [1] and dilute magnetic semiconductors (DMSs) [2]. However, the origin of ferromagnetic properties in those systems and a lack of reproducibility in experimental results have been a matter in strong debate. In the most of DMDs, the observed ferromagnetism at room temperature (RT) is not connected with high concentration of free carriers, but rather influenced by defects (vacancies, interstitials ions and/or other structural imperfections) [3]. For DMS systems, the carrier-mediated exchange mechanism is responsible for ferromagnetic (FM) order but the lattice defects (such as interstitials) also play an important role for the formation of their magnetic properties [4]. The claim of replacement of the cation sites by magnetic dopants and absence of the structural defects is often supported by the use of X-ray diffraction (XRD)

∗ Corresponding author. Tel.: +1 306 966 2768; fax: +1 306 966 6400. E-mail addresses: [email protected], [email protected] (G.S. Chang), [email protected] (E.Z. Kurmaev). 0368-2048/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.05.001

showing no secondary phase. However, according to our previous investigation on various DMSs and DMDs, the observed magnetic properties are in well accordance with the presence of interstitial impurities and the defect configuration involving interstitial and substitutional atoms is not necessary to have a size detectable by XRD. In the present article, we provide an overview of how the transition-metal dopants occupying interstitial sites influence magnetic properties of various DMSs and DMDs. The overall results based on our previous comprehensive spectroscopic studies reflect that the influence of interstitial impurities on ferromagnetism is pretty substantial and should be taken into account by employing appropriate spectroscopic tools. In this regard, X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopy can offer an efficient way to monitor the presence of interstitial impurity atoms in these materials in dependence with the preparation conditions (thermal treatment and oxygen partial pressure). 2. Experimental The spectroscopic measurements of XAS and RIXS were carried out at Beamline 8.0.1 of the Advanced Light Source at Lawrence Berkeley National Laboratory. The 3d transition-metal (TM) L2,3 (3d4s → 2p transition) resonant X-ray emission spectra (RXES) were obtained using excitation energies (Eexc ) near L3 and L2 absorption thresholds. On the other hand, the O 1s XAS (1s → 2p) spectra and the O K␣ (2p → 1s) nonresonant X-ray emission spectra (NXES) were measured at the Eexc well above the absorption

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Fig. 1. Mn 2p XAS spectra of as-grown and post-annealed Mn:GaAs thin films at 200 and 290 ◦ C.

thresholds. All spectra were recorded at RT and normalized to the number of photons falling on the sample. 3. Results and discussion 3.1. Post-annealing effect on the electronic structure of DMS (Mn:GaAs) Fig. 1 shows Mn 2p XAS spectra for Ga0.945 Mn0.055 As thin films which are as-grown and post-annealed at 200 and 290 ◦ C [5]. One can see that the fine structure (features A and B) of Mn L3 XAS is absent for the as-grown sample and arises for the two annealed samples. The more simple structure of as-grown sample is due to partial occupation of interstitials in GaAs lattice by Mn impurity atoms providing direct Mn–Mn interactions (like in pure metal). The fine structure of the post-annealed samples resembles to that of MnO which has the valency of Mn2+ [6]. The Mn 2p XAS spectra were obtained in total electron yield (TEY) mode which is surface-sensitive. Therefore, these changes can be attributed to the formation of manganese oxide on the surface of Ga0.945 Mn0.055 As during annealing. On the other hand, the Mn L2,3 RXES spectra of the Ga0.945 Mn0.055 As films (as-grown and annealed at 200 and 290 ◦ C) measured at different excitation energies (b and c in Fig. 1) are presented in Fig. 2a and b. We note that the relative I(L2 )/I(L3 ) intensity ratio of as-grown sample excited at the L2 -threshold is smaller than that of samples annealed at 200 and 290 ◦ C. The I(L2 )/I(L3 ) intensity ratio is related with the probability of radiationless L2 L3 M4,5 Coster–Kronig (C–K) transitions and the ratio of total photoabsorption coefficients (3 /2 ) for excitation energies at L2 and L3 absorption threshold [7]. Since the ratio of total photoabsorption coefficients depends only on the excitation energy, the I(L2 )/I(L3 ) intensity ratio of RXES spectra taken at the same excitation energy is determined by the C–K transitions, which are governed by the number of free d electrons around a target atom. The I(L2 )/I(L3 ) ratio of Mn atoms in conducting state (direct Mn–Mn interaction) is therefore highly suppressed which the Mn atoms in the insulating state (Mn–O interaction) shows a normal high I(L2 )/I(L3 ) ratio (see Fig. 2c). Therefore, as it is shown in Ref. [5], the increase of I(L2 )/I(L3 ) intensity ratio with annealing is due to diffusion of Mn atoms from interstitials to the surface where they are passivated by oxygen. By such a way, the antiferromagnetic Mn–Mn interactions between substituted and embedded impurity atoms are weakened which leads to an increase of the Curie temperature from 90 to 160 K.

Fig. 2. Mn L2,3 RXES spectra of as-grown and annealed Mn:GaAs films.

3.2. FM impurities in DMD thin films prepared at poor oxygen regime (Co:TiO2 ) The Co L2,3 NXES (Eexc = 820 eV) and RXES (at L2 -threshold with Eexc = 795 eV) spectra of Ti0.96 Co0.04 O2−ı thin films are presented in Fig. 3. Samples 1 and 2 exhibiting FM properties were prepared at partial oxygen pressure of 1 × 10−6 and 3.5 × 10−6 Torr, respectively (oxygen-deficient regime), while non-FM sample 3 was prepared at 1.0 × 10−4 Torr (oxygen-rich regime). As afore-

Fig. 3. (a) Co L2,3 NXES and (b) RXES spectra of Ti0.96 Co0.04 O2−ı prepared at different partial oxygen pressure.

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Fig. 4. Mn 2p XAS spectra of Zn0.8 Mn0.2 O film prepared at different temperatures.

mentioned, the difference in the I(L2 )/I(L3 ) intensity ratio between NXES and RXES spectra is attributed to different 3 /2 ratio for excitation energies at L2 absorption threshold (minimum) and nonresonant regime (constant). On the other hand, it can be seen that the I(L2 )/I(L3 ) intensity ratio is reduced for FM (oxygen-deficient) samples 1 and 2 compared to that of the oxygen-rich sample 3 for both nonresonant and resonant excitation. When comparing to the Co L2,3 XES spectra of Co and CoO reference samples [8], we find that the XES spectra of nonmagnetic sample 3 are very similar to those of CoO. This similarity is not surprising because Co dopants substituting Ti atoms in the TiO2 lattice are surrounded by oxygen (in octahedral coordination). On the other hand, the Co L2,3 XES spectra of FM samples 1 and 2 are more similar to the XES spectra of Co metal than to those of CoO [8]. We therefore suppose that the changes in the local ordering of Ti0.96 Co0.04 O2 films prepared in oxygen-deficient regime are strongly related with a segregation of Co atoms in interstitial sites and thus give rise to the Co–Co interaction near the excited Co atom, which can induce ferromagnetism. Since such changes in local ordering depend on the partial oxygen pressure P(O2 ) and are not accompanied by the formation of additional phases, only a chemically sensitive local-probe method like XES provides an efficient analysis. 3.3. AFM impurity in DMD thin films grown at different growth temperatures (Mn:ZnO) Fig. 4 shows the Mn 2p XAS spectra of Zn0.8 Mn0.2 O thin films grown at 600 and 700 ◦ C. According to the measurements using a superconducting quantum interference device (not shown here), the sample grown at 600 ◦ C exhibits ferromagnetic behavior while the 700 ◦ C sample is nonmagnetic. As seen in Fig. 4, the peak positions (a–d) and the line shape of FM Zn0.8 Mn0.2 O film are similar to that of MnO [9] having the Mn valency of 2+ and therefore indicate that most of Mn atoms substitute for Zn sites. On the other hand, Mn 2p3/2 and 2p1/2 absorption lines are broadened by elevating the growth temperature to 700 ◦ C, which is more typical for the itinerant Mn 3d-states than for localized states. The ratio between a and b peaks in spectrum of sample prepared at 700 ◦ C is also changed with respect to sample prepared at 600 ◦ C towards to have more similarity with Mn metal. The effect of elevated growth temperature on the interatomic interactions between Mn impurities and their neighboring atoms can be also verified by comparing the spectral weight of the Mn L2 emission line to that of the Mn L3 line when exciting at the L2 absorption edge. This comparison is shown in Fig. 5. The I(L2 )/I(L3 ) intensity ratio of the non-FM Zn0.8 Mn0.2 O film (700 ◦ C) is much smaller than that of the FM Zn0.8 Mn0.2 O film (600 ◦ C) [9]. This

Fig. 5. Mn L2,3 RXES spectra of Zn0.8 Mn0.2 O film prepared at different temperatures.

reduction of spectral weight in L2 with respect to L3 for non-FM Mn0.8 Zn0.2 O film is largely due to radiationless C–K transitions. The RIXS spectrum of the FM Zn0.8 Mn0.2 O film resembles to that of MnO whereas non-FM Zn0.8 Mn0.2 O film (700 ◦ C) shows a limiting ratio between Mn metal and MnO, which suggests that the direct Mn–Mn bonds are formed due to the precipitation of Mn atoms in ZnO and this precipitation is enhanced by elevating the growth temperature. We conclude that the direct Mn–Mn antiferromagnetic exchange interaction suppresses the ferromagnetic ordering in the Zn1−x Mnx O thin films. 3.4. Co and Al co-doping for ferromagnetism in Co:ZnO Fig. 6 presents oxygen K-edge XES and 1s XAS spectra of Zn0.89 Co0.100 Al0.005 O prepared at partial oxygen pressure of 6 × 10−7 Torr (FM sample 1), at 3.6 × 10−5 Torr (FM sample 2) and at 5.6 × 10−4 Torr (FM sample 3) [10]. We have not found any changes in these spectra for samples prepared in oxygen-poor (1) and oxygen-rich (2 and 3) regimes. This means that free carriers in samples 2 and 3 are not connected with oxygen vacancies. The resonantly excited (at L2 -threshold) Co L2,3 XES are presented in

Fig. 6. O 1s XAS spectra and O K␣ NXES spectra of Zn0.89 5Co0.100 Al0.005 O films prepared at different partial oxygen pressure.

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cient to detect the defect configuration consisting of few interstitial and substitutional atoms. Therefore, a careful verification of the presence of interstitial impurities and their direct interaction with substitutional atoms is crucial in explaining ferromagnetism (or absence of ferromagnetism) in DMS and DMS system. This overview of our previous investigation clearly shows that the combined approach using XAS and XES provides a very efficient local probe of electronic structure of 3d-impurity atoms in DMS and DMD systems and can distinguish the transition-metal atoms substituting cation sites and occupying interstitial sites. According these results, one can conclude that for the systems involving Mn dopants, the sample synthesis is necessary to do at comparatively low temperatures to prevent the formation of Mn segregation with AFM interactions competing with FM interactions. In case of systems doped with Co atoms, the interaction between substitutional and interstitial atoms is FM-type and the Co-clustering induces free carriers which enhance FM exchange interactions. Therefore, the samples need to be prepared under oxygen-poor environment to avoid the AFM superexchange via oxygen and bonding of some free carriers. Acknowledgements

Fig. 7. (a) Co L2,3 RXES spectra of Zn0.895 Co0.100 Al0.005 O films prepared at different partial oxygen pressure and (b) the detailed spectra in the vicinity of L2 emission line.

Fig. 7 [10]. The I(L2 )/I(L3 ) intensity ratio is 1.14, 1.33 and 1.34 for samples 1, 2 and 3, respectively. The comparatively high values of this intensity ratio (close to that of CoO (1.45) [8]) suggest that most of the Co dopants occupy substitutional sites. Sample 1 prepared in oxygen-deficient regime has the lowest I(L2 )/I(L3 ) intensity ratio which evidences the formation of some Co-clusters. On the other hand, I(L2 )/I(L3 ) intensity ratio for samples 2 and 3 prepared in oxygen-rich regime is less than that of CoO. This means that a significant number of free d-carriers still present in the Co and Al co-doped sample. 4. Conclusion The overall spectroscopic results suggest that the interstitial occupancy of magnetic dopants significantly influences the observed magnetic properties of DMSs and DMDs. It should be noted that the interstitial-related defect configuration is not necessary to be a cluster with noticeable size. X-ray diffraction is commonly being used to verify the perfect replacement of cation sites by magnetic dopants, but its spectral resolution is not suffi-

We gratefully acknowledge the Russian Academy of Sciences Program (Project 01.2.006 13395), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair program. This work is also partly supported by the Russian Science Foundation for Basic Research (Projects 08-0200148 and 06-02-16733). A.D. would like to thank the Region Alsace for financial support. References [1] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173. [2] T. Jungwirth, J. Sinova, J. Maˇsek, J. Kuˇcera, A.H. MacDonald, Rev. Mod. Phys. 78 (2006) 809. [3] K.A. Kikoin, J. Magn. Magn. Mater. 321 (2009) 702. [4] Y. Takeda, M. Kobayashi, T. Okane, T. Ohkochi, J. Okamoto, Y. Saitoh, K. Kobayashi, H. Yamagami, A. Fujimori, A. Tanaka, J. Okabayashi, M. Oshima, S. Ohya, P.N. Hai, M. Tanaka, Phys. Rev. Lett. 100 (2008) 247202. [5] G.S. Chang, E.Z. Kurmaev, L.D. Finkelstein, H.K. Choi, W.O. Lee, Y.D. Park, T.M. Pedersen, A. Moewes, J. Phys.: Condens. Matter 19 (2007) 076215. [6] J.-S. Kang, G. Kim, S.C. Wi, S.S. Lee, S. Choi, S.-L. Cho, S.W. Han, K.H. Kim, H.J. Shin, A. Sekiyama, S. Kasai, S. Suga, B.I. Min, Phys. Rev. Lett. 94 (2005) 147202. [7] E.Z. Kurmaev, A.L. Ankudinov, J.J. Rehr, L.D. Finkelstein, P.F. Karimov, A. Moewes, J. Electron Spectrosc. Relat. Phenom. 148 (2005) 1. [8] G.S. Chang, E.Z. Kurmaev, D.W. Boukhvalov, L.D. Finkelstein, D.H. Kim, T.-W. Noh, A. Moewes, T.A. Callcott, J. Phys.: Condens. Matter 18 (2006) 424. [9] G.S. Chang, E.Z. Kurmaev, S.W. Jung, H.-J. Kim, G.-C. Yi, S.-I. Lee, M.V. Yablonskikh, T.M. Pedersen, A. Moewes, L.D. Finkelstein, J. Phys.: Condens. Matter 19 (2007) 276210. [10] G.S. Chang, E.Z. Kurmaev, D.W. Boukhvalov, L.D. Finkelstein, A. Moewes, H. Bieber, S. Colis, A. Dinia, J. Phys.: Condens. Matter 21 (2009) 056002.