X-ray magnetic circular dichroism study of diluted ferromagnetic semiconductor Ti 1 x Co x O 2 δ. Master Thesis. Yuta Sakamoto

Transcription

1 X-ray magnetic circular dichroism study of diluted ferromagnetic semiconductor Ti 1 x Co x O 2 δ Master Thesis Yuta Sakamoto Department of Complex Science and Technology, University of Tokyo February, 29

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3 Contents 1 Introduction Diluted magnetic semiconductors Physical properties of Ti 1 x Co x O 2 δ Experimental methods Photoemission spectroscopy Resonant photoemission spectroscopy X-ray absorption spectroscopy X-ray magnetic circular dichroism X-ray magnetic circular dichroism study of Ti 1 x Co x O 2 δ Introduction Experiment Results and discussions TEY mode TFY mode Summary Photoemission study of Ti 1 x Co x O 2 δ Introduction Experiment Results and discussions Summary X-ray magnetic circular dichroism study of Ti 1 x Co x O 2 δ grown by the sputtering method Introduction Experiment Results and discussions Summary

4 6 Summary 43 7 Acknowledgement 45 4

5 Chapter 1 Introduction 1.1 Diluted magnetic semiconductors Diluted magnetic semiconductors (DMS) have been attracted much attention because of the possibilities of applications to spin electronics which aim to create novel electronics utilizing both the spins and charges of electrons [1, 2, 3]. DMS are the materials exhibiting both semiconducting properties and magnetic properties and this feature is powerful for applications [4, 5]. DMS are semiconductors dilutely doped transition metal, where small amount of cation sites are substituted by magnetic impurities [6, 7]. Many interesting phenomena have been repoted in these systems and therefore vigorous reserches have been done until now [8, 9, 1, 11, 12]. The discoveries of ferromagnetism in dilutely Mn doped InAs and GaAs at relatively high temperatures (up to 173 K) were surprizing phenomena [13]. The reports on the appearence of ferromagnetism stimulated many theoretical and experimental investigations and the origin of the ferromagnetism were cleared [14, 15]. The origin of ferromanetism in these materials is generally called carrier induced ferromagnetism. In this mechanism, short-range exchange interaction is mediated by carriers which spread in the materials as itinerant states. The clarification of its origin will enable us to design higher Tc compound. Calculations suggest wide gap semiconductors doped magnetic impurities will show high Tc [14, 15, 16, 17]. Wide gap semiconductors are semiconductors containing atoms of large electronegativity. Therefore, oxides- or nitrides- base DMS are promissing candidates to realize high Tc because oxygen and nitrogen have large electronegativities. However, it have been problem that synthesizing p-type semiconductors in oxides and nitrides is technically difficult. The conduction carriers of n-type semiconductor are 5

6 electrons in the bottom of the conduction band derived from s orbitals. Hybridization between s orbital and d orbital is very weak. In this context, room-temperature ferromagnetism reported on Co doped TiO 2 was surprizing because TiO 2 is n-type semiconductor [18]. 1.2 Physical properties of Ti 1 x Co x O 2 δ Magnetization (µ B /Co) (c) (a) (a) x= cm cm cm cm -3 H plane 3 K µ H (T) µ H (T) (b) x=.5 (c) x= cm cm cm cm -3 (b) Hall resistivity (µ cm) Electron density (cm 3 ) x= cm MCD 3 K 2kdeg. cm 1 3 K Ferromagnetic Paramagnetic x in Ti 1-xCo xo H plane µ H(T) (d) 1 Magnetization (µ B /Co).1 Figure 1.1: Various physical properties of Ti 1 x Co x O 2 δ (a)m H curve taken by SQUID magnetization measurement [18]. (b) Magnetic field dependence of Hall resistivity (blue line) and MCD in the visible light region (red circles) [2]. (c) M H curves of Ti 1 x Co x O 2 δ with various compositions [21]. (d) Phase diagram [21]. In 21, room-temperature ferromagnetism was reported in dilutely Co-doped n-type antase and rutile TiO 2 thin films fabricated by the pulsed lazer deposition method [18]. This report was the first report of ferromagnetism above room temperature in the investigations of DMS. An amazing 6

7 (a) Intensity (arbitrary units) O 2p /Ti 3d Co 3d x =.1 x =.5 x =.1 (b) Intensity (arbitrary units) CoO SrCoO 3 LiCoO 2 CoS 2 Satellite Co 2p 3/2 (c) x = Binding Energy (ev) (d) Ti.9 Co.1 O 2- Calculation Relative Binding Energy (ev) Intensity (arb. units) Ti 1-x Co x O 2 (anatase) x=.5 (a) Fluorescence (b) ( 4) Ti K-edge Energy (kev) Figure 1.2: Spectroscopic investigations of Ti 1 x Co x O 2 δ. (a) Valence band x-ray photoemission spectra [22]. The dotted and dashed lines are the spectra of Co metal and CoO, respectively. (b) Co 2p core-level spectra [22]. (c) Co K-edge x-ray absorption spectra [23]. (d) X-ray resonant scattering spectra [24]. 7

8 Figure 1.3: Soft x-ray magnetic circular dichroism spectra at Co L 2,3 edges. Left panel: results representing Co 2+ high-spin [25]. Right panel: results of the effect of annealing [26]. point of the report was using an n-type semicondutor as host semiconductor. Generally, hybridization between the s orbital and the d orbital is very weak, because of the weak overlap between orbitals. The reduced hybridizations lead to the reduced exchange integrals and Tc must drop. Thus, it is clear that we have to consider another model for describing the ferromagnetism in this material. In order to clarify the origin of ferromagnetism, many investigations have been performed on this material. Toyosaki et al performed Hall measurements on Ti 1 x Co x O 2 δ and they observed clear anomalous Hall effects [19]. They also reported qualitative consistencies between the SQUID magnetization measurements, MCD in the visible light region and anomalous Hall effect as seen in Fig. 1.1(b) [2]. They claimed that these observations support intrinsic itinerant ferromagnetism, indicating the existence of the spin polarization of carriers. Fig. 1.1(c) shows the magnetization measurements of Ti 1 x Co x O 2 δ thin films fabricated under various conditions [21]. As seen in Fig 1.1(c), Ferromagnetic behaviors are stabilized with reduced P O2 condition. Reduced P O2 condition leads to the increase of the oxygen vacancies. When oxygen vacancies are created in oxides semiconductor, each oxygen vacancy provides two electron carriers. Thus, they propose that the relationship between the ferromagnetism and partial oxygen pressure may represent 8

9 the relationship between the ferromagnetism and carriers. Fig 1.1(d) shows the phase diagram of Ti 1 x Co x O 2 δ. This phase diagram shows that the ferromagnetism is affected by both oxygen defects and Co concentrations. Spectroscopic studies are effective to clarify the electronic states which are thought to be responsible for the magnetism and therefore many spectroscopic investigations have been performed on Ti 1 x Co x O 2 δ in order to clarify the origin of ferromagnetism in this material. Fig. 1.2(a) and (b) shows the x-ray photoemission spectra of the valence band and the Co 2p core-level [22]. They performed measurements under a Nd:YAG lazer illumination to remove the effect of band bending. As seen in the spectra near the valence-band, it is obvious that Co states is not like that of Co metal, indicating the Co atoms are the ionized states. Comparing the Co 2p core-level spectra with those of the references, they concluded the Co atoms are in the Co 2+ high-spin state. In Fig 1.2 (c), Co K-edge x-ray absorption spectra are shown [23]. In this measurement, they used hard x-ray. Because x-ray absorption using hard x-rays is a very bulk-sensitive measurements, these data are reliable for determining the Co states. The results show accordance with the spectra of CoTiO 3 and a sholder structure corresponding to Co metallic states was not observed. This observation strongly suggested the imcorporation of Co atoms in the matrix of TiO 2 in the Co 2+ high-spin state. Fig. 1.2(d) shows the results of resonant x-ray scattering [24]. From the x-ray scattering, one can obtain the information about the element specific spatial distributions of atoms. The results showed the supression of the resonant behavior and they suggested the existence of large strain in Ti 1 x Co x O 2 δ. Figure 1.3 shows the results of x-ray magnetic circular dichroism (XMCD) measurements at the Co L 2,3 edges of Ti 1 x Co x O 2 δ. XMCD experiment is powerful to determine the valence and spin states of atoms because one can directly prove electronic states related to the magnetism. The XMCD spectra in the left panel shows a multiplet structure [25]. The multiplet structure is characteristic of ionized states and they concluded the Co atoms were in the Co 2+ high-spin state from comparison with calculations. Kim et al performed the annealing effect of XMCD as shown in the right panel [26]. They claimed that the XMCD signals were hardly observed in as-grown sample. When they annealed the sample below the growth temperature, the XMCD signals gradually increased and the spectral line shapes gradually came close to that of Co metal. Therefore, they concluded that the origin of the ferromagnetism in this material is the segregation of Co metal clusters. These two data are quite different and therefore it is still controvertial whether the magnetism arises from imcorporated Co atoms in the matrix of TiO 2 or not. The fact that the XMCD signals were much weaker than the results of magnetization measurements has made the problem more complicated. 9

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11 Chapter 2 Experimental methods 2.1 Photoemission spectroscopy When the incident photon with kinetic energy hν hits on the materials, a photoelectron is emitted from the sample. The relationship between the kinetic energy of the photoelectron E v kin relative to the vacuum level E vac and the binding energy E B relative to the Fermi level E F is given by the energy conservation as follows, E v kin = hν ϕ E B (2.1) where ϕ is the work function of the sample. The schematic picture of this process is displayed in Fig 2.3. In actual measurements, the E kin measured from E F is directly observed, thus the equation is simplified by E kin = hν E B (2.2) In the one-electron approximation, the binding energy given by E B = ϵ k (2.3) where ϵ k is the Hartree-Fock orbital energy with Bloch wave number k in the chemical potential µ. This relationship is called Koopman stheorem.this assumption is valid when the wave functions of both the initial and final states can be expressed by the single Slater determinants of the N- and (N- 1)-electron systems, respectively, and the one-electron wave functions do not change by the removal of the electron. If we apply this approximation, the photoemission spectrum I(E B ) can be expressed as: I(E B ) k δ(e B + ϵ k ) N( E B ). (2.4) 11

12 e- Ekin EF Photoemission spectrum Energy e- EVAC EF Valence band Φ EB Ekin VAC Ekin hν Core level Density of state (DOS) Figure 2.1: Schematic diagram of photoemission spectroscopy. When the one-electron approximation is valid, the photoemission spectrum is proportional to the DOS of the occupied one-electron states N(E). When the electron correlation effect is exist, one can not consider the electron system within the one-electron picture, because the relaxation influences the photoemission final state such as screening of photo-holes by valence electron. Thus, the energy difference between the N-electrons initial state energy E in and the (N-1)-electrons final state energy E f N 1 provides the binding energy E B, E B = E n 1 f E n i + µ. (2.5) Using Fermi s golden rules, the PES spectrum, which now corresponds to the single-particle excitation spectrum of the electron system, is expressed as: I(E B ) k Ψ n 1 f a k Ψ n i 2 δ[e B (Ef n 1 Ei n + µ)], (2.6) where Ψ n 1 f and Ψ n i denote the final and initial states, respectively, a k is the annihilation operator of the electron occupying orbital k. Considering electron correlation effect, the finite lifetime of quasi-particle also contributes to the spectral broadening. 12

13 2.2 Resonant photoemission spectroscopy Resonant photoemission spectroscopy (RPES) is an effective approach to extract the PES spectrum for an impurity atom from the entire spectrum in the valence band. We can perform RPES measurements using synchrotron radiation, where photon energy hν can be continuously varied. The direct PES process of a valence 3d electron is described as: p 6 d n + hν p 6 d n 1 + e. (2.7) When the photon energy is equal to the absorption energy from the 3p core level to the valence 3d state, 3p 3d absorption and subsequent Auger decay, called super Coster-Krönig decay occur. p 6 d n + hν p 5 d n+1 p 6 d n 1 + e. (2.8) The final states of these two processes are the same electronic configurations, and quantum-mechanically interfere with each other in consequence. Thus, the photoemission intensity is resonantly enhanced and shows a so-called Fano profile [27]. This enhancement helps detecting weak signals such as photoemission from transition metal impurities in the valence band, which is difficult to obtain by normal PES. 2.3 X-ray absorption spectroscopy X-ray absorption spectroscopy (XAS) is a technique to investigate the local electronic structures in materials. We can obtain information about the valence states and the local environments surrounding each elements by XAS spectra. The element selectivity is powerful for investigating materials. XAS spectra changes between different valence states or local environments of material. This feature is advantage of XAS further to element selectivity. The photo-absorption intensity is given by I(hν) = f T i 2 δ(e i E f hν) (2.9) where T is dipole transition operator. From the XAS spectra, one can get the information about unoccupied DOS of final state because initial state is well described as isolated core level. XAS spectra well reflects the 3d electronic states in the 3d transition metal compounds including the valence states, the symmetry and the crystal-field splitting. There are two measurement modes for XAS, the transmission mode and the yields mode. In the transmission mode, the intensity of x-ray is measured 13

14 in front of and behind the sample and the ratio of the transmitted x-ray is determined. The transmission mode is standard for hard x-rays, while, for soft x-rays, the transmission mode is difficult to perform because of the strong interaction of soft x-rays with the sample. An alternative to the transmission-mode experiments has been provided by measuring the decay products of the core hole. The core hole gives rise to an avalanche of electrons, photons, and ions escaping from the surface of the sample. This is the yield-mode experiment and is standard for soft x- rays. The yield mode can be classified into the Auge electron yield, the total fluorescence yield, the ion yield and the total electron yield. The fluorescence yield mode suffers from self-absorption because of its long probing depth and data analysis may become complicated. In the Auger electron yield mode, one detects Auger electorns of a specific Auger decay channel of the core hole. The mean free path of 5 ev electron is of the order of 2 Å. Since the mean free path of photon is of the order of 1 Å, the Auger electron yield in the soft x-ray range effectively surveys the region of about 2 Å depth from the surface. Instead of Auger decay, the fluorescence decay is also used for absorption measurements. Because the fluorescence yield mode has a large detection depth (> 1Å), it is particularly suited for the studies of bulk electronic structure. When the absorption process takes place at the surface, the atoms which absorbs the x-ray can be ionized by Auger decay and can escape from the surface. Detecting the escaping ions as a function of x-ray energy, one can obtain the signals related to the absorption cross section. This is the ion yield mode. This is a highly surface sensitive method, whose detection depth is of the order of 2 Å. The total electron yield mode is the most widely used yield detection technique because of the ease of detection and the large signal. The difference from the Auger electron yield mode is that the energy of the outgoing electrons are not selected and simply the all escaping electrons are counted. Estimated detection depth of the total electron yield mode using x-rays for transition metal oxide is about 4 Å. In the present work, we have employed the total fluorescence mode and the total electron yield mode. 2.4 X-ray magnetic circular dichroism When circular poralized x-rays are irradiated on a sample under applied magnetic fields, there are differences between absorption spectra taken with right-handed (µ + ) and left-handed (µ ) circularly polarized x rays because 14

15 (a) x ray E B sample (b) 2p-3d XMCD σ + σ m d p 3/2 +3/2 +1/2-1/2-3/2 m j 2p 1/2 +1/2-1/2 m j (c) Absorption XMCD = µ + µ (µ + µ )dω A 1 µ µ + Photon Energy A 1 +2A 2 Photon Energy Photon Energy A 2 <S z > d <L z > d Figure 2.2: Schematic diagram of x-ray magnetic circular dichroism. (a) Experimetal setup for XMCD measurements. (b) Transition probability of 2p 3d absorption with circularly polarized x rays. (c) XMCD and XAS spectra of differences of transition matrix elements. Figure 2.2 shows transition matrix elements in 2p 3d absorption as an example. XMCD is an element specific measurement because of the core-level excitation and is sensitive to magnetically active component. The line shapes of XMCD spectra reflect electronic structures related to magnetism. Using the integrated intensity of the L 2,3 -edge XAS and XMCD spectra, one can separately estimate the orbital [28] and spin [29] magnetic moments by applying XMCD sum rules given by M orb = 4 L 3 +L 2 (µ + µ )dω 3 L 3 +L 2 (µ + + µ )dω (1 N d), (2.1) M spin +7M T = 6 L 3 (µ + µ )dω 4 L 3 +L 2 (µ + µ )dω (1 N L 3 +L 2 (µ + + µ d ), (2.11) )dω where M orb and M spin are the spin and orbital magnetic moments in units of µ B /atom, respectively. In this thesis, we have studied the local electronic structure of Co. Figure 2.3 shows the typical Co L 2,3 XMCD and XAS spectra, namely, the Co L 2,3 spectra of Co metal [3] and Zn 1 x Co x O [31]. The Co atoms in Zn 1 x Co x O are in the high-spin Co 2+ state. The XMCD spectrum of Co metal shows single peak, which results from the core-hole screening effect by electrons. On 15

16 the other hand, the XMCD spectrum of the Zn 1 x Co x O shows the multiplet structures, which results from the Coulomb and exchange interaction between d electrons and the crystal-field splitting. Figure 2.3: Co L 2,3 XAS and XMCD spectra. Left panel: the XAS and XMCD spectra of Co metal [3]. Right panel: the XAS and XMCD spectra of Zn 1 x Co x O [31]. 16

17 Chapter 3 X-ray magnetic circular dichroism study of Ti 1 x Co x O 2 δ 3.1 Introduction The predictions of possible high T c in wide gap semiconductors such as oxides and nitrides dilutely doped with transition metal stimulated a number of investigations and the ferromagnetism above room temperature in the nitrides- and oxides-base DMS have been reported [32, 33, 34, 35]. After the reports of room-temperature ferromagnetism in these systems, it has been reported that a number of results indicated the extrinsic origin of the ferromagnetism such as segregations of magnetic impurities. It has been still unclear whether reports of room-temperature ferromagntism in nitridesand oxides-base semiconductors are intrinsic or not. X-ray magnetic circular dichroism (XMCD) is a effective measurement to clarify this issue and therefore a number of XMCD investigations have been performed on oxides- and nitrides-base DMS which has been repoted to exhibit the room-temperature ferromagnetism [36, 37, 38, 39, 4]. However, these results are still inconsistent between them. The results of extrinsic origin such as segregations were also reported. In Ti 1 x Co x O 2 δ, discrepancies were reported. There are two quite different reports, supporting the intrinsic nature of ferromagnetism and indicating the segregations of Co metallic clusters. Mamiya et al performed Co L 2,3 XMCD measurements on Ti.97 Co.3 O 2 δ [25]. The results suggested that the Co atoms substituted for Ti sites in TiO 2 and they suggested that the ferromagnetism is intrinsic. However, the observed intensities of the XMCD 17

18 is too weak to conclude the intrinsic ferromagnetism. Kim et al investigated the effect of annealing on Ti 1 x Co x O 2 δ below the growth temperature [26]. First, they performed XMCD measurements on the as-grown Ti.9 Co.1 O 2 δ thin film and observed highly reduced XMCD signals. Then, they performed XMCD measurements with increasing annealing time (2-min, 1-min and 2-min). They observed the enhancements of XMCD intensities with increasing annealing time and the XMCD spectra gradually became similar to the metallic signal. Thus, they suggested that the segregations of Co metalic clusters were responsible for the ferromagnetism in Ti 1 x Co x O 2 δ. These XMCD results of Ti 1 x Co x O 2 δ were not consistent with the magnetization measurements of as-grown samples which exhibited the ferromagnetic behaviors with relatively large saturation magnetizations ( 1 µ B /Co). Based on these reports, we have performed the XMCD measurements on Ti 1 x Co x O 2 δ in order to clarify the electronic states related to magnetism. 3.2 Experiment Epitaxial thin films of rutile Ti 1 x Co x O 2 δ were provided by Prof. Kawasaki s group (Tohoku University). The thin films were synthesized by the pulsed laser deposition method on r-sapphire substrates at 4 C. Details of the sample fabrications can be found in Ref [21]. Ferromagnetism of the samples were confirmed by SQUID measurements, Hall measurements and MCD measurements in the visible light region. The conditions of sample fabrications were different Co concentration x =.3,.5,.1 at different oxygen pressure conditions P O2 = 1 6, 1 7 Torr in order to control the oxygen vacancy concentration δ. In this thesis, we call the sample fabricated under P O2 = 1 6 Torr as low-δ and fabricated under P O2 = 1 7 Torr as high-δ. The M-H curves of the samples are shown in Fig The film thickness were 7-12 nm. We have performed all the measurements without surface cleaning in order to avoid possible destructions of the sample surfaces. The experiments were performed at BL-11A of National Synchrotron Radiation Research Center, BL-23SU of SPring-8, and BL-16A of Photon Factory. The XAS and XMCD measurements at BL-11A of National Synchrotron Radiation Research Center [41] were performed in the total fluorescence yield mode and the total electron yield mode. The resolution of the monochromater was E/ E > 1 and the degree of circular polarization of light was 55%. Absorption spectra were obtained by switching the magnetic field. The x-rays were irradiated perpendicular to the sample surface and external magnetic fields were applied to 3 against the light. 18

19 The XAS and XMCD measurements at BL23-SU of SPring-8 [42] were taken in the total electron yield mode. The resolution of the monochromater was E/ E > 1 and the degree of circular polarization of light was larger than 9%. Absorption spectra of µ + and µ were obtained by reversing the photon helicity. The x-rays were irradiated perpendicular to the sample surface and external magnetic fields were also applied perpendicular to the sample surface. The XAS and XMCD measurements at BL-16A of photon factory were performed in the total electron yield mode. The resolution of the monochromater was E/ E > 1 and the degree of circular polarization of light was larger than 9%. Absorption spectra of µ + and µ were obtained by reversing the photon helicity. The x-rays were irradiated perpendicular to the sample surface and external magnetic fields were also applied perpendicular to the sample surface. Magnetization (µ B /Co) 4 (a) x= cm cm cm cm -3-2 H plane 3 K µ H (T) (b) x=.5 (c) x= cm cm cm cm µ H (T) Figure 3.1: M H curves of rutile Ti 1 x Co x O 2 δ samples [21]. 19

20 3.3 Results and discussions TEY mode XAS (a. u.) (a) x =.1 high-δ T = 3 K B = 1 T rutile Ti 1-x Co x O 2-δ (b) µ + x =.5 high-δ µ (c) Photon Energy (ev) (d) Photon Energy (ev) XAS (a. u.) x =.3 high-δ x =.5 low-δ Photon Energy (ev) Photon Energy (ev) Figure 3.2: Co L 2,3 XAS spectra of Ti 1 x Co x O 2 δ taken at each helicity in the TEY mode. (a) x =.1 high-δ. (b) x =.5 high-δ. (c) x =.3 high-δ. (d) x =.5 low-δ. Figure 3.2 shows Co L 2,3 XAS spectra of Ti 1 x Co x O 2 δ thin films fabricated under various conditions for parallel and antiparalell directions of helicity relative to the magnetic field taken in the TEY mode. The backgrounds have been subtracted and the spectra have been normalized to the peak height of L 3 absorption edge of the µ + + µ spectra. All the XAS spectra show multiplet features. The multiplet features are derived from the Coulomb and exchange interaction between d electrons and the splitting due to crystal field and therefore the multiplet features represent ionic states of atoms. All the spectra show differences in the absorption taken with differ- 2

21 Normalized XMCD (a. u.) Normalized XMCD(a. u.) (a) TEY rutile Ti1-xCoxO2-δ Photon Energy (ev) (c) TEY x =.1 high-δ T = 3 K 1 T.5 T.2 T x =.3 high-δ T = 3 K.5 T 1. T 2. T 1. T Photon Energy (ev) Normalized XMCD (a. u.) Normalized XMCD (a. u.) (b) TEY Photon Energy (ev) (d) TEY x =.5 high-δ T =3 K 1 T.5 T.1 T x =.5 low-δ T = 2 K 1. T 2. T 3. T 6. T Photon Energy (ev) Figure 3.3: Normalized XMCD spectra taken at various magnetic fields in the TEY mode. (a) x =.1 high-δ. (b) x =.5 high-δ. (c) x =.3 high-δ. (d) x =.5 low-δ. 21

22 Moment (µ B / Co) Moment (µ B / Co) (a) x =.1 high-δ rutile Ti 1-x Co x O 2-δ Magnetic field (T) (c) T = 3 K T = 3 K x =.3 high-δ Magnetic field (T) Moment (µ B / Co) Moment (µ B / Co) (b) Magnetic field (T) (d) T = 3 K x =.5 high-δ T = 3 K x =.5 low-δ Magnetic field (T) Figure 3.4: Estimated magnetic moment from the XMCD spectra using XMCD sum rules. (a) x =.1 high-δ. (b) x =.5 high-δ. (c) x =.3 high-δ. (d) x =.5 low-δ. 22

23 ent helicities. These differences are XMCD and one can obtain the XMCD spectra to take the difference between µ + and µ at each photon energies. Figure 3.3 shows the normalized XMCD spectra of Ti 1 x Co x O 2 δ taken at various magnetic fields. The spectra have been normalized to the peak height of the L 3 absorption edge. The XMCD spectra of the x =.5 sample and the x =.3 sample show multiplet features. The observation of the ionic states suggests the incorporation of Co atoms in the matrix of TiO 2. The XMCD spectrum of the x =.1 large-δ sample shows a single-peak structure without multiplet structures. A possible origin of single-peak structure is the supercomposition of ionic spectra having various peak positions due to various crystal-field splitting derived from possible multiple Co ion sites in the crystal. Another possible origin of the single-peak structure is metallic signal. We therefore suggest that the x =.1 large-δ sample has disordered ionic Co cordinations or segregation of Co metalic clusters. Thus, we conculde that low Co density x =.3 and x =.5 samples incorporate Co atoms in the matrix of TiO 2 and the high Co density x =.1 samples had the tendency to have multiple Co ion sites in the TiO 2 matrix or the metallic precipitations. Figure 3.4 shows the magnetic field dependence of the magnetic moment of Co estimated from the XMCD spectra using the XMCD sum rules. The estimated magnetic moments represent only the magnetic moment of Co ions without any extrinsic contributions. One can thus obtain the M -H curve from the magnetic field dependence of XMCD intensity. From the extrapolation of magnetic moment to H = T, one can obtain the remanent magnetizations. All the samples show the existence of remanent magnetization, which indicates the existence of ferromagnetic components intrinsic to the samples. The estimated remanent moments of all samples were in the range of µ B /Co. The suturation moments measured by SQUID magnetization measurements were µ B /Co. Alothough we can detect the ionic Co atoms responsible for the ferromagnetism, these results are still incomplete because the detected magnetizations were too small compared with the bulk magnetization. 23

24 3.3.2 TFY mode (a) rutile Ti 1-x Co x O 2-δ (b) T = 3 K B = 1T µ + µ XAS (a. u.) x =.5 high-δ x =.3 high-δ (c) Photon Energy (ev) 8 (d) Photon Energy (ev) 8 XAS (a. u.) x =.5 low-δ x =.3 low-δ Photon Energy (ev) Photon Energy (ev) 8 Figure 3.5: Co L 2,3 XAS spectra of Ti 1 x Co x O 2 δ taken at each helicity taken in the TFY mode. (a) x =.5 high-δ. (b) x =.3 high-δ. (c) x =.5 low-δ. (d) x =.3 low-δ. In the previous section, we showed the behaviors of XMCD in the surface region and the observed XMCD was very small, and therefore the results were still controvertial. We consider that these inconsistencies were derived from the effect of surface. Therefore, we performed XMCD measurements with greater bulk sensitivity. If one detects XAS by fluorescence emitted from materials, one can obtain bulk information because the mean-free path of photons is much longer than that of electrons. In this section, we show the results of XMCD detected by the bulk-sensitive total fluorescence yield method. The detection depth of the total fluorescence yield mode is about 1 nm and the thicknesses of samples are about 1 nm. Therefore, we can detect the entire samples by the TFY mode. We performed XMCD measurements taken in the TFY mode on the Ti 1 x Co x O 2 δ thin films with various composition from the same batches as in the previous Section. Fig. 3.5 shows the Co L 2,3 XMCD and XAS spectra taken in the TFY mode. Clear XMCD is observed and it is clear that the intensities of XMCD are much stronger than those taken in the TEY mode. 24

25 Normalized XMCD (a. u.) 776 (c) Normalized XMCD (a. u.) (a) T = 3 K x =.5 high-δ Photon x =.5 Energy low-δ (ev) 1 T.5T.1T 784 (b) x =.3 high-δ 776 (d) rutile Ti 1-x Co x O 2-δ Photon Energy (ev) x =.3 low-δ Photon Energy (ev) Photon Energy (ev) Figure 3.6: Normalized XMCD spectra taken at various magnetic fields in the TFY mode. (a) x =.5 high-δ. (b) x =.3 high-δ. (c) x =.5 low-δ. (d) x =.3 low-δ. These results indicate difference between surface and bulk and the enhanced XMCD signals indicate that the magnetization in the bulk region is larger than that in the surface region. The XAS spectra of the samples with high oxygen vacancy concentrations show almost the same spectral line shapes, however, the line shapes show single peak feature, which indicates multiple Co ion sites or metallic Co as discussed in the previous section. For the low oxygen vacancy concentraton δ, the spectra show the multiplet features characteristic of the Co 2+ ions. The x =.5 low-δ sample show clear multiplet features and the x =.3 low-δ sample show relatively weak multiplet features. Therefore, one can coclude that Co atoms in the x =.5 low-δ sample are incorporated in the TiO 2 matrix and multiple Co ion sites or segregation of Co metallic nano-clusters may occur in the high-δ samples. Next, we show comparison of the line shapes of the XMCD spectra between different samples. Figure 3.6 shows the normalized XMCD spectra taken at various magnetic fields. The spectra of the high-δ samples show a single peak structure, which indicates multiple Co ion sites or metallic Co states. In the low-δ region, the spectra show multiplet structure, which is derived from localized nature of Co 3d electrons. However, the multiplet 25

26 structure of the x =.3 low-δ sample looks weaker than that of the x =.5 low-δ sample. Therefore, it seems that there were multiple Co ion sites or coexistence of the ionic Co and segregations of Co metallic clusters in the x =.3 low-δ sample. The spectra of the x =.5 low-δ sample show clear multiplet features. One can safely say that the majority of Co atoms in the x =.5 low-δ sample are ionized, indicating good inconporation of Co atoms in the matrix of TiO 2. We note that we have probed all the regions of the material within the detection depth of TFY mode. The XAS and XMCD results are thus consistent with each other and Co atoms in the x =.5 low-δ sample are mostly ionized. XMCD Intensity (%) x =.5 high-δ x =.3 high-δ x =.3 low-δ x =.5 low-δ magnetic field (T) 1. Figure 3.7: Magnetic field dependence of XMCD intensity at the L 3 absorption edge. To plot the magnetic field dependence of XMCD, one can get the information about M vs H relation. Fig. 3.7 show magnetic field dependence of the XMCD intensities. The XMCD intensity did not change significantly at H = 1.,.5 T but decreased at.1 T. This behavior is consistent with the SQUID magnetization measurements as shown in Section 3.1. The deduced magnetic moments are also nearly consistent with the magnetization measurements. Thus, we can detect the magnetization of the Co ions responsible for the ferromagnetism in these materials. To summarize the data measured by the TFY method, clear enhancement of XMCD intencities was observed compared to the TEY mode and the observed XMCD signals were nearly consistent with the SQUID magnetization 26

27 measurements. The results of the x =.5 low-δ sample indicate ionized Co atoms indicating incorporation of Co atoms into the matrix of TiO 2. The results of the x =.3 low-δ sample indicate relatively weak effects of multiple Co ion sites or the coexistence of metallic and ionized atoms. The results of the x =.5 high-δ sample and the x =.3 high-δ sample suggest multiple Co ion sites or metallic Co resulting from the segregations of Co metallic clusters. magnetic moment (µ B / Co) magnetic moment (µ B / Co) x =.5 high-δ TFY SQUID TEY Magnetic field (T) rutile Ti 1-x Co x O 2-δ 1. T = 3 K SQUID TFY x =.5 low-δ TEY Magnetic field (T) magnetic moment (µ B / Co) magnetic moment (µ B / Co) x =.3 high-δ TFY SQUID TEY Magnetic field (T) x =.3 low-δ TFY SQUID Magnetic field (T) Figure 3.8: M-H relation of SQUID magnetization and magnetization estimated from XMCD spectra. Figure 3.8 shows the M -H curves of the SQUID magnetization measurements and the magnetization estimated from XMCD spectra taken in the TFY mode. The magnetization estimated from XMCD spectra was similar to that estimated from the SQUID measurement. The M -H curves are also consistent. Thus, we can conclude that the Co ions in the bulk region is responsible for the ferromagnetism in this material. The magnetization estimated from XMCD spectra of the x =.5 low-δ sample was a little smaller than that of magnetization measurements and the others were a little larger. Understanding the reason for these discrepancies is difficult under the present situation because it is difficult to make quantitative argument at 27

28 Co L 3 edge rutile Ti 1-x Co x O 2-δ x =.5 P O2 = 1-7 Torr TEY TFY XAS (a. u.) Photon Energy (ev) 784 Figure 3.9: Comparison of XAS spectra taken in the TEY mode and the TFY mode. this moment. XMCD taken in the TFY mode have the self-absorption effect. Figure 3.9 shows the comparison of the spectral line shapes taken in the TEY and TFY modes. The energy of the multiplet features are almost the same, although it is not completely same because of the self-absorption effect of the TFY mode. The same crystal splitting represent the same local environment. This fact indicates that the basic electronic structure is the same between the surface and bulk regions. 3.4 Summary We have performed Co L 2,3 XMCD measurements on Ti 1 x Co x O 2 δ. First, we performed measurements in the total electron yield mode (TEY) mode. All the XAS spectra of samples showed multiplet structures, which indicated the ionic states of Co atoms. All the samples showed XMCD. The XMCD spectra except for the x =.1 high-δ sample showed multiplet structures. This indicate that the Co ions are reponsible for the magnetism of the x =.3 and the x =.5 samples. We have also studied the magnetic field dependence of XMCD. The observed M vs H relation showed ferromagnetic behaviors. However, the observed remanent magnetization was much smaller than that obtained for SQUID magnetization measurements. Next, we have performed Co L 2,3 XMCD measurements on Ti 1 x Co x O 2 δ 28

29 in the total fluorescence yield (TFY) mode. Clear enhancement of the XMCD was observed in the all samples. The XAS and the XMCD spectra of the x =.5 low-δ sample showed clear multiplet features chracteristic of ionized Co atoms. The XAS and the XMCD results of the other samples showed effects of multiple Co ion sites or the coexistence of metallic and ionic states. The estimated magnetic moments were nearly consistent with that of the SQUID magnetization and the M vs H curves were also nearly consistent between the SQUID and XMCD measurements. This fact suggests that the Co atoms in the bulk region were responsible for the magnetizations of this material. From comparison of the XAS in the TEY and TFY modes of x =.5 low-δ, it was indicated that the local electronic structures in the bulk region and in the surface region were almost the same. 29

30

31 Chapter 4 Photoemission study of Ti 1 x Co x O 2 δ 4.1 Introduction The electronic structure of material gives the valuable information about the physical properties of material. In order to understand the ferromagnetism in Ti 1 x Co x O 2 δ, Quilty et al performed XPS measurements on Ti 1 x Co x O 2 δ [22]. The measurements were performed under a Nd:YAG laser illumination in order to remove the effect of band bending. They observed the changes of the spectra under the lazer illumination, indicating the relaxations of the effect of band bending. The valence-band spectra and Co 2p core-level spectra consistently suggested that the electronic states of Co atoms are high-spin Co 2+. They also observed the systematic chemical potential shifts in all core levels as shown in Fig. 4.1 and they concluded that the core-level shifts were derived from exchange splitting, indicating the intrinsic ferromagnetic interaction in Ti 1 x Co x O 2 δ. From this result, it was indicated that doped Co atoms forms the impurity states in the band gap. 4.2 Experiment Epitaxial thin films of Ti 1 x Co x O 2 δ were provided by Prof. Kawasaki s group (Tohoku University). The thin films were synthesized by the pulsed laser deposition method on r-sapphire substrate at 4 C in various conditions. These thin films were the same batches as the samples studied in 31

32 Intensity (arbitrary units) Ti 2p x =. x =.1 x =.5 x =.1 O 1s Binding Energy (ev) Figure 4.1: X-ray photoemission spectra of Ti 1 x Co x O 2 δ [22]. Chapter 3. We performed all the measurements without surface cleaning in order to avoid possible destructions of the sample surfaces. The x-ray photoemission spectroscopy (XPS) measurements were performed using the photon energies of ev (Mg Kα). The measurements were performed under the base pressure of 1 9 Torr at room temperature. Photoelectron were corrected using a Scienta SES-1 electron-energy analyzer operated in the transmission mode. The energy resolution was about 8 mev. Binding energies were calibrated to the Au 4f core-level peaks of a gold reference sample. The Co 2p 3d RPES were performed at BL-23SU of SPring-8 using a Gammadata Scienta SES-22 electron-energy analyzer operated in the transmission mode. The monochromator resolution was E/ E > 1. The spectra were taken at room temperature in a vacuum below Torr. The total resolution of the spectrometer including temperature broadening was 2 mev. Binding energies were calibrated to the Au 4f core-level peaks. 4.3 Results and discussions From the XMCD measurements, it was clarified that the ferromagnetism is weakened in the surface region. In this chapter, we present the results of the XPS and RPES measurements using soft x-rays. The detection depth is 32

33 Intensity (a. u.) (a) x =.1 high-δ x =.1 low-δ x =.5 high-δ x =.5 low-δ rutile Ti 1-x Co x O 2-δ Co 2p (b) CoO SrCoO 3 LiCoO 2 CoS 2 Co 2p 3/ Binding Energy (ev) Relative Binding Energy (ev) (c) Ti 2p (d) O 1s Intensity (a. u.) Binding Energy (ev) Binding Energy (ev) Figure 4.2: X-ray photoemission spectra of Ti 1 x Co x O 2 δ. (a) Co 2p corelevel spectra. (b) Co 2p core-level spectra of various compounds which exhibit various electronic structures. (c) Ti 2p core-level spectra. (d) O 1s core-level spectra. 33

34 n m l ± «ª Å Æ Ç j k l h e i e f gd c d < ƒ u ˆ uš Œ bž u ˆ u šƒ œ ˆžuŸ u A B<C!" # $&% ' ()* +, -./ :<; D E<FG H IKJML N O P QSRN O T USV W XY ZX[ δ \&]_^``ba δ o p q r p sut v w t v<xymvz { x 4} z<~ È ÉËÊ ÌˆÍ¹Î Ï Ð ÑˆÒÔÓÕ ² ¹ º» ²<µ º ¼½¼¾ À Á ² µ ¾Ã Ä ²<³ Figure 4.3: Resonant photoemission measurements of Ti.9 Co.1 O 2 δ. (a) valence-band spectra. (b) valence-band spectra near the E F. (c) Ti L 2,3 XAS spectra. 34

35 about 2 nm. Figure 4.1 (a) shows Co 2p core-level spectra of Ti 1 x Co x O 2 δ thin films fabricated under various growth conditions. The spectra have been normalized to the peak top. Satellite structures are observed at the higher binding energy side of the main peaks. The satellite structures give the information about the electronic structure. The Co 2p core-level spectra of various compounds, which exhibit a variety of electronic states, are shown in Fig. 4.1 (b) as references. These spectra include high-spin Co 2+ (CoO), lowspin Co 3+ (LiCoO 2 ), low-spin Co 2+ (CoS 2 ) and low- or intermemediated-spin Co 4+ (SrCoO 3 ). The spectrum of CoO shows the best resemblance to the spectrum of Ti 1 x Co x O 2 δ. The high-spin Co 2+ is consistent with the results of XAS. Figure 4.1 (c) shows the O 1s and Ti 2p core-level photoemission spectra. The chemical potential shift are observed between the different samples. The x =.5 sample lie in the lower binding energy region and the dependence on the oxygen vacancy concentration is hardly observed. The x =.1 samole shows shifts between the different oxygen vacancy concentrations, but the shifts are relatively small. Figure 4.2 shows the Ti 2p 3d resonant photoemission measurements of the x =.1 high-δ sample. The spectra have been normalized to the intensities of incident light. The resonant energy is determined from the Ti L 2,3 XAS spectrum and we use the x-rays, 455, and 46.5 ev. Clear enhancements of the partial density of states (DOS) of Ti was observed in the valence-band as seen in Fig. 4.2 (a). Figure 4.2 (b) shows the RPES spectra near the E F region. The enhancement of the DOS at E F was observed. This enhancement may be originated from the oxygen defect level as discussed in Ref. [43, 44, 45]. This observation suggests when the oxygen vacancies were introduced in Ti 1 x Co x O 2 δ, the oxygen defects states are created near the E F. 4.4 Summary We have performed XPS and RPES measurements on Ti 1 x Co x O 2 δ. First, we performed XPS measurements. The Co 2p core-level spectra showed high-spin Co 2+. It was consistent with the results of the XAS measurements. All the core level spectra showed the chemical potential shifts. The dependence on oxygn vacancy concentrations were hardly observed in the chemical potential shift. We have performed Ti 2p 3d RPES on the x =.1 high-δ sample. A clear enhancement of the partial DOS of Ti was observed in the valence 35

36 band. The enhancement of the DOS at E F was observed. This observation suggested when oxygen vacancies were introduced in Ti 1 x Co x O 2 δ oxygen vacancies states were created near E F. %[aps,prl,preprint,superscriptaddress]revtex4 36

37 Chapter 5 X-ray magnetic circular dichroism study of Ti 1 x Co x O 2 δ grown by the sputtering method 5.1 Introduction In Chapters 3 and 4, we have studied rutile Ti 1 x Co x O 2 δ thin films synthesyzed by the pulsed lazer deposition method. In this Chapter, we have performed Co L 2,3 XMCD measurements on rutile Ti 1 x Co x O 2 δ thin films synthesyzed by the sputtering method. In industrial applications, prevailing methods such as sputtering method are favorable and a number of studies have been performed [46, 47, 48, 49, 5, 51, 52]. However, these reports focused on the magnetization measurements and the characterizations of samples were inadequate to prove the intrinsic nature of ferromagnetism. Recently, rutile Ti 1 x Co x O 2 δ thin films synthesized by sputtering method were characterized by magnetization measurements, magneto-optical effect and anomalous Hall effect as shown in Fig. 5.1 [53]. Figure 5.1(a) show the magnetization measurements. The enhancements of saturation magnetizations were observed. The saturation magnetizations were nearly as large as 3 µ B /Co, which was consistent with the full moment of Co assuming the high spin Co 2+. The results of MCD measurements in the visible light region is shown in Fig 5.1(b). MCD signals were observed and the specctral features were similar to those of the PLD-grown films. In Fig 5.1(c), magnetic field dependence of Hall measurements are shown. As seen in Fig 5.1(c), anomalous Hall effects were observed and the conductivities satisfied a scall- 37

38 (a) 3 M [µ B /Co] K H plane x=.5.1 MS [µb/co] T [K] µ H [T] (b) MCD [kdeg./cm] [1 5 cm -1 ] 3K 1 µ H=1T H plane x= Photon energy [ev] (c) H [µω cm] ρ H [µω cm] ρ x=.5.1 3K H plane x=.5 3K 2K 1K µ H [T] σ xx [Ω -1 cm -1 ] x=.1 3K 2K 1K µ H [T] AH [Ω -1 cm -1 ] ρ Figure 5.1: Physical properties of Ti 1 x Co x O 2 δ grown by the sputtering method. (a) SQUID magnetization measurement. (b) MCD in the visible light region. (c) Magetic field dependences of Hall resistivity [53]. ing law. These observations suggested the intrinsic nature of ferromagnetism in these thin films. In order to characterize the electronic states of Co and its magnetism, we have performed x-ray magnetic circular magnetic circular dichroism on well-characterized Ti 1 x Co x O 2 δ thin films grown by the sputtering method. 5.2 Experiment Epitaxial thin films of rutile Ti 1 x Co x O 2 δ were provided by Prof. Kawasaki s group (Tohoku University). Rutile Ti 1 x Co x O 2 δ epitaxial thin films were synthesized by the dc magnetron sputtering method on r-sapphire substrates at 4 C using CoO and TiO 2 targets. The thin films were fabricated with various Co concentrations (x =.5,.1) and the base pressure of the deposition chamber was Torr. The thickness of thin films was 8-11 nm. Ditails of sample fabrication can be found in Ref.[53] The XAS and XMCD measurements at BL-11A of National Synchrotron Radiation Research Center [41] were performed in the total fluorescence yield mode and the total electron yield mode. The resolution of the monochromater was E/ E > 1 and the degree of circular polarization of light was 55%. Absorption spectra were obtained by switching the magnetic field. The x-rays were irradiated perpendicular to the sample surface and external 38

39 TEY x =.5 x =.1 XAS (a. u.) rutile Ti 1-x Co x O 2-δ grown by sputtering method Intensity (a. u.) µ + µ Photon Energy (ev) x = Photon Energy (ev) x =.1 XMCD (a. u.) Intensity (a. u.) Photon Energy (ev) Photon Energy (ev) 8 85 Figure 5.2: Co L 2,3 XAS and XMCD spectra of sputter-grown rutile Ti 1 x Co x O 2 δ thin films taken in the TEY mode. Left panel: x =.5. Right panel: x =.1 magnetic fields were applied to 3 against the light. 5.3 Results and discussions Figure 5.2 shows the Co L 2,3 XAS and XMCD spectra of Ti 1 x Co x O 2 δ (x =.5,.1) samples fabricated by the sputtering method taken in the total electron yield (TEY) mode. The backgrounds have been sabtracted. The XAS spectra show the differences in the absorption taken with different helicities and these differences are XMCD. One can obtain the XMCD spectra to take the difference between µ + and µ at each photon energies. The line shapes of the XAS spectra show multiplet features, which indicates ionic states as discussed in Chapter 3. Figure 5.3 shows the Co L 2,3 XAS and XMCD spectra of Ti 1 x Co x O 2 δ samples taken in the total fluorescence yield (TFY) mode. The XAS spectra show circular dichroism behaviors and the intensities of XMCD are much stronger than those observed by the TEY mode. The enhanced XMCD 39

40 x =.5 TFY x =.1 XAS (a. u.) rutile Ti 1-x Co x O 2-δ grown by sputtering method XAS (a. u.) µ µ x =.5 Photon Energy (ev) Photon Energy (ev) x =.1 XMCD (a. u.) XMCD (a. u.) XMCD Photon Energy (ev) Photon Energy (ev) Figure 5.3: Co L 2,3 XAS and XMCD spectra of sputter-grown rutile Ti 1 x Co x O 2 δ taken in the TFY mode. Left panel: x =.5. Right panel: x =.1 Magnetic moment (µ B /Co) x =.5 SQUID TFY TEY Magnetic moment (µ B /Co) x =.1 SQUID rutile Ti 1-x Co x O 2-δ grown by sputtering method TFY TEY Magnetic field (T) Magnetic field (T) Figure 5.4: M-H curves of the SQUID magnetization measurements and magnetization estimated from XMCD spectra. Left panel: x =.5. Right panel: x =.1. 4