Spin-resolved core-level photoemission spectra of ferromagnetic nickel

Journal of Electron Spectroscopy and Related Phenomena 78 (1996) 329-332

Spin-resolved Core-level Photoemission Spectra of Ferromagnetic Nickel Y. Saitoh,’ S. Suga,’ A. Kakizakif T. Matsushita,’ S. Imada,’ H. Daimon,’ K. Ono? M. Fujisawa? T. Kinoshita,2 T. Ishii: J. Fuji?and K. Shimada’ Department

of Material Physics, Faculty of Engineering Science, Osaka University,

Osaka 560,

Japan

%stitute for Solid State Physics, University of Tokyo, Tokyo 106, Japan 3Department of Physics, Gakushuin University, Tokyo 171, Japan ‘Department of Physics, University of Tokyo, Tokyo 113, Japan We have measured the spin-resolved 3p and 3s core-level photoemission as well as MpVV Auger electron spectra of ferromagnetic Ni. The observed core level spin polarization spectra can be interpreted in terms of the multiplet structures based on the impurity Anderson model. In the nonresonant normal M,VV Auger-electron spectrum, positive spin polarization is clearly found not only for the “38”’ final state but also for the state with the kinetic energy beyond 62eV (binding energy of the Ni 3p core level minus the work function). The positive spin polarization of the “38” main structure is understood as the final state mixing of the 3d’ and 38 states in contrast to the previous interpretation. The positive spin polarization above 62eV is ascribed for the first time to the secondary electron excitation. 1. INTRODUCTION Ni is a typical itinerant ferromagnet, cn which many studies to probe its electronic structures have actively been done. In the corelevel photoemission spectra of Ni, multiplet and/or satellite structures are observed. The purpose of this study is to reveal the origin of the core-level satellites in the photoemission spectra and the mechanism of the M,VV Auger decay process in ferromagnetic Ni by means of spin-resolved photoemission spectroscopy (SRPES). Part of the study has already been reported 111. The spin polarization of the photoelectrons provides us information on the spindependent interactions in magnetic materials, and is one of the direct ways to clarify the origin of the complicated spectra. 2EXPERIMENTAL SRPES measurement was carried out at the BL-19B of the Photon Factory, National Laboratory for High Energy Physics. The spin polarization of the photoelectrons was analyzed by a LEED-spin-detector. The 0368-2048/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO368 - 2048 (96) 02742-9

instrumental energy resolution was set to about 1.2eV. A Ni(l10) single crystal was shaped into a picture frame with each frame being oriented along the easy magnetization axis of . A mom detailed description for the experiment was presented in Ref. 1. 3. RESULTS AND DISCUSSION 3.1. 3p and 3s core levels Figure 1 (a) shows the spin-integrated Ni 3p photoemission spe&um measured for hv=175eV. The broken line in the figure is a result of a theoretical calculation based on the impurity Anderson model [2]. In this model the electronic structure of Ni is a proximated as a mixture of 38, 3n9c and 3Qk, P where p denotes a hole in the adjacent site. According to the calculation, the main peak A has primarily the 3p53d” character and the satellite structures have mainly 3p53d9 character. Because of the large 3p-3d electrostatic interaction, the 3p53d9 final state splits into three peaks, A’, B and C, which are assigned as 3F+1D,3P+3Dand ‘P+‘F states, respectively. Figure 1 (b) shows the spin polarization of

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binding energy (ev)

bindingenergy(eV)

FIG. 1

SRPES spectra of Ni 3p core level measured for the excitation photon energy (hv) of 175eV. (a) spinintegrated photoelectron intensity (full circles) compared with an available calculation (broken curve [2]), (b) spin polarization shown with the error bars, (c) difference spectrum between the spin up and down spectra (open circles) and calculated spin-polarization spectrum (broken curve [2]). The typical error bar is indicated in the figure.

Ni 3s spectra measured for hv=24OeVwith error FIG. 2 bars as in Fig 1. (a) total intensity (full circles) compared with an available calculation (broken curve [4]), (b) spin polarization, (c) spin-integrated photoemission (solid curve), spin-resolved partial intensities (dashed curves) and difference between the spin up and down spectra (open circles), all obtained after subtracting integral backgrounds.

the measured photoelectron spectrum. The spin-resolved partial intensities I, and I, are obtained from the integral intensity I and the spin polarization P according to the relations I,=I(l+P)/:! and I,=I(l-Pl/2. In order to compare our spectrum with the calculation [21, we extracted the net 3p component of I, and I, by subtracting smooth backgrounds from each spectrum. The difference spectrum, 1,-I,, thus estimated is shown in Fig. 1 (c) in comparison with the calculated spectrum. The spectral features of the spin polarization are qualitatively reproduced by the calculation. This is consistent with the interpretation that B and C correspond to the spin triplet and the spin singlet predominated final states, respectively. As for the spin polarization of main peak A, an appreciable the hybridization of some spin triplet components

of the 3#3dg state can explain the remarkably negative spin polarization of this mainly 3p53di0 final state. In Fig. 1 (a) and (c), one recognizes a noticeable difference of the energy between the experimental and the calculated spectra. The spin-resolved photoemission enables us to obtain much detailed information about the electrostatic interaction between the 3p hole and the valence electrons as well as the hybridization. Figure 2 (a) shows the spin-integrated spectrum of the Ni 3s photoemission, where result (full circles) is the experimental compared with the calculated spectrum (broken curve) 141. The observed spectrum consists of a main peak at 1lOeV and a satellite at about 6eV higher binding energy. Figure 2 fb) shows the observed spin polarization. In order to examine the spectrum

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we obtained the net 3s in mom detail, component of I, and I, in the same manner as in the case of 3p spectrum. They are presented in Fig. 2 (c). One clearly sees that the main peak has predominantly negative spin polarization and the 6eV satellite has positive spin polarization. If the main peak is resulting from a pure 3s3d” state, no spin polarization is negative spin Hence, the expected. polarization of the main peak strongly suggests an appreciable hybridization of the spin triplet component of the 3s3d9 state. The most probable explanation is that the 6eV satellite is mainly resulting from the 3s3d9 state and has more ‘D component than the 3D where the 3D component is component, energetically closer to the 3s3d” state. Then the main peak has more 3D component than the ‘D component due to the configuration interaction mixing. The recent calculation by Tanaka and Jo [31 strongly supports this interpretation. 3.2. M,VV Auger electrons In a one particle theory the valence band Auger electrons are approximated by the selfconvolution of the valence-band density of The Auger spectra of Fe is nicely states. explained by this approximation [5]. In the case of Ni, on the other hand, the Auger spectra have primarily been ascribed to localized 3d* final state from their energy position and multiplet structures; the selfconvoluted density of states cannot reproduce the Auger line shape 161. It was thought that the presence of the core hole in the Auger intermediate state induced the screening by the 3d electron as p’d*‘, and that the Auger electrons were unpolarized [7j. spin-resolved electron The Auger spectroscopy (SRAES) on Ni by means of the electron excitation by Allenspach et al. [8], has ShOWlI spin however, noticeable polarization. They attributed the observed spin polarization to the contribution of the resonant process, which is represented as 3p63d9+ 3$3d”+ 3p”3d8+cl. It is known that the 3d8 final state of the Ni 3d photoemission (6eV satellite) has positive spin polarization

-SOL(b)

j’ I

I

I

I

I

I

70 80 60 30 40 50 electron kinetic energy (eV) Present result of the Ni M,VV SRAB spectra Fig.3 excited by photons. (a) total intensity measured for hv=24CkV (full circles) and hv=95eV (open circles), (b) spin polarization spectrum shown with the error bars measured for h%240eV.

and shows enhancement of not only the intensity but also the spin polarization near the 3p core level excitation threshold. Figure 3 shows the present result of the spin-integrated M,VV Auger-electron spectra with the excitation photon energy of 95eV and 240eV (a) and the observed spin polarization at hv=24OeV (b). An almost identical result of the spin polarization was obtained at hv=95eV. The vertical broken lines in these figures indicate the electron kinetic energy (E,) of 62eV corresponding to the situation in which two resultant holes in the final state of the M,VV Auger process remain at Fermi energy (Er). The spin polarization of the photoexcited Auger electrons qualitatively agrees with that of the electron excited Auger electrons within the experimental error. When we subtract the spectral background by the same method as in Ref. 8, the effective spin polarization is almost comparable with Possible contribution of the each other. resonant pm is, therefore, not the main source of the spin polarization of the Auger electrons by the electron excitation. In the Auger spectra of Cu, atomic like sharp 3d8 multiplet structures are observed. The broader structures of Ni are thought to

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arise from the hybridization effect. On the basis of the impurity Anderson model, the intermediate state has the Auger configurations of 3$3dpg and 3p53P’$ and the final state has the configurations of 3d’~ 3d8$ and 3d9$ ( The weight of the 3de initial state is much smaller than those of the 3d9E and 3d’“z2states and the 3d* state is neglected for the moment 1. The intermediate state is just the final state of the 3p photoemission. In the 3p SRPES the contribution of the 3p53d9 states cannot be ignored compared with the 3p53d10 state, as we have just found. Hence, the positive spin polarization around 56eV strongly suggests the contribution of the 3d’ final state component because the 3d8 final state does not give rise to the spin polarization. It is natural to understood as the positive spin polarization around 56eV results from the hybridization between the 3d’v state and the 3d8g2 state. The decrease of the spin polarization around 62eV is understood by considering the contributions of band like part of the 3d9z3 where the resultant 2 holes are 01 different sites. The peak of the selfconvolution curve locates around 58eV and its spin polarization can be estimated at most as large as the valence-band spin polarization, which is almost equal to the background level. Above the main Auger line, another positive spin polarization peak is observed, whereas the spin-integrated intensity shows only a broad hump. Similar features have been observed in some ferromagnets and discussed in relation to the resonant 3p+3d excitation [8]. We can no longer adopt such an argument for explaining the origin of the hump structure. It is certain that a state with large positive spin polarization is excited, although its probability is not high. The effective spin polarization at 67eV is estimated to be 23*10% in our analysis. The sources of such a spin polarization is very restricted for Ni. Among them, the spin polarization of the secondary electrons in Ni has been found to reach a maximum of -15% at the lowest kinetic energy (see upper side of Fig. 4). Assuming the excitation of such electrons following the 3d+3p core hole decay as shown in Fig. 4, we

A schematic explanation of the spin polarization Q34 Assuming the exdtation of secondary above +62eV. electrons follows the 3d4p core hole decay, we can qualitatively explain the experimental result.

can explain the degree of the spin polarization. This interpretation is not incompatible with the LVV SRAES, in which the pronounced peak of the spin polarization is not clearly observed above the LjVV main line [81. Since the L,VV Auger structure is observed in the high energy region of the LJVV peak, it the is difficult to deconvolute spin polarization above the &VV Auger peak. For detailed understanding of this behavior, spinresolved measurement of photo-excited LVV Auger electrons are required. REFERENCES Y. Saitoh et al., Phys. Rev. B (in press). B.T. Thole and G. van der Laan, Phys. Rev. Lett. 67,3306 (1991). A. Tanaka and T. Jo, J. Phys. Sot. Jpn. (in press). G. van der Laan et al., Phys. Rev. B46,7221 (1992). K. Schriider et al., Solid State Gnnmun, 55, 377 (1985). P.A. Bennett et al., Phys. Rev. B27, 2194 (1983). L.A. Feldkamp and L.C. Davis, Phys. Rev. Lett. 43,151 (1979). R. Allenspach et al., Phys. Rev. B35, 4801 (1987).