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Soft X-ray spectroscopy study of Mn nanoclusters on Si(1 1 1)-7×7 surface
ARTICLE IN PRESS
Physica B 351 (2004) 351–354
Soft X-ray spectroscopy study of Mn nanoclusters on Si(1 1 1)-7 7 surface T. Xiea, A. Kimuraa,*, S. Qiaob,*, K. Ioria, K. Miyamotoa, M. Taniguchia,b, M.H. Panc, J.F. Jiac, Q.K. Xuec a
Department of Physical Sciences, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan b Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan c State Key Laboratory for Surface Physics and International Center for Quantum Structures, Institute of Physics, Chinese Academy of Science, Beijing 100080, China
Abstract Mn nanoclusters have been prepared on a Si(1 1 1)-7 7 surface by substrate-induced spontaneous clustering due to delicate control of growth kinetics. An ex situ soft X-ray absorption spectroscopy (XAS) using synchrotron radiation has been used to clarify electronic structures of the nanoclusters. The small value of XAS L3 –L2 branching ratio and spectrum line-shape for the Mn clusters show that the Mn3þ and Mn4þ electronic configuration should be taken into account. r 2004 Elsevier B.V. All rights reserved. PACS: 68.65.Hb; 73.21.La; 78.70.Dm Keywords: X-ray absorption spectroscopy (XAS); X-ray magnetic circular dichroism (XMCD); Nanoclusters; Multiplet fine structures
1. Introduction Magnetic nanoclusters with diameters in the size range of 1–10 nm attract growing interest because of the great potential applications in the fields of high-density data storage, sensors and spin electronic devices. From the fundamental point of view, novel physical properties are expected due to a quantum confinement effect as the dimensions of *Corresponding authors. Fax: 081-0824-24-0719. E-mail addresses: [email protected] (A. Kimura), [email protected] (S. Qiao).
the magnetic objects become comparable to characteristic nanoscopic length such as the magnetic single domain size. Mn is one of the most fascinating elements. Although a divalent Mn ion has the highest spin magnetic moment (5 mB ) due to its half-filled character, pure Mn crystal shows a small magnetic moment below B2:3 mB (for g Mn, for example) while keeping an antiferromagnetic (AF) structure. We expect a stable magnetic structure changes when the Mn atoms are synthesized in low-dimensional forms, such as a film (2D), a wire (1D) and a cluster (0D). Recently, many experiments and theoretical
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.051
ARTICLE IN PRESS 352
T. Xie et al. / Physica B 351 (2004) 351–354
calculations have been focused on the Mn nanoclusters. However, the theorists mainly discuss the magnetic transition of small Mn clusters with atom numbers less than 10 [1–3]. The experimental results are mostly about the magnetic properties and electronic structures of free Mn nanoclusters or those deposited on highly oriented pyrolytic graphite (HOPG) and metal substrates [4,5]. However, for borrowing as many techniques from Si microlithography as possible, the semiconductor substrates are much more attractable. More recently, Li et al. [6] found that ordered identical-size nanocluster arrays can be realized on Si(1 1 1)-7 7 surface by delicate control of growth kinetics. This substrate-induced spontaneous clustering method has attracted much attention. A most striking feature is that a highquality Al nanocluster ordered array has been successfully prepared on Si(1 1 1)-7 7 and exhibit a remarkable thermal stability, which serves as an ideal template for growing magnetic nanostructures [7–9]. We have fabricated Mn nanoclusters supported by this Al nanocluster ordered arrays on Si(1 1 1)-7 7 surface. In this work, an ex situ X-ray absorption spectroscopy (XAS) measurement has been performed on the Mn nanoclusters. XAS using soft X-ray synchrotron radiation is a well-established technique for providing information on the electronic and magnetic properties of matter. A particular asset of XAS is its element specificity and the observed fine structures of XAS spectrum can be a good diagnosis of the electronic structure.
[10,11]. Circularly polarized light was supplied from a twin-helical undulator, with which almost 100% polarization was obtained at the peak of the first-harmonic radiation. The Mn L2;3 XAS spectrum was obtained by means of a total photoelectron yield method by directly detecting the sample current with changing the photon energy. We have applied a magnetic field of B1:4 T with two pairs of permanent magnets normal to the sample surface for the possible observation of the X-ray magnetic circular dichroism (XMCD). The base pressure of the main chamber was 8 108 Pa: The XAS measurement was done at a temperature of 43 K:
3. Results and discussion Fig. 1 shows the observed STM image of the Mn deposited surface, where the coverage is equivalent to 1:5 ML for the film. This image has been taken with the sample bias 2:0 V: We find that the Mn clusters are grown on this surface as indicated by the bright patches. We also observe the ordered Al nanocluster arrays behind the Mn clusters as
2. Experimental The detailed preparation method of the selfassembled clusters has been published elsewhere [6,7,9]. Prior to the Mn evaporation, 0:24 ML Al has been evaporated onto a clean Si(1 1 1)-7 7 surface to fabricate the identical Al cluster ordered arrays [7,9]. Subsequently, the Mn nanoclusters were grown on the template. Ag film with 5 nm thickness was coated on the Mn nanoclusters to prevent them from oxidation. We have carried out the XAS measurement at BL25SU of SPring-8
Fig. 1. STM image (80 nm 80 nm) of Mn nanoclusters (large bright patches). In the image, the Al cluster array is observed with white small dots behind the Mn clusters. The sample bias voltage is 2:0 V:
ARTICLE IN PRESS T. Xie et al. / Physica B 351 (2004) 351–354
shown by the small, but bright dots. It has been reported that the Al atoms form a characteristic honeycomb structure occupying both the faulted and unfaulted halves of the 7 7 reconstructed unit cell [7,9]. This is consistent with the present STM image in Fig. 1. The subsequently deposited Mn atoms reside on top of the ordered Al cluster arrays. The present STM image shows that the diameters of Mn nanoclusters disperse from 2 to 5 nm: A rough estimation shows that each cluster contains 100–200 atoms: Mn L2;3 XAS spectrum of the Mn nanoclusters, where a linear background has been subtracted, is shown in Fig. 2. We find clearly the L3 and L2 core absorption edges split by a spin–orbit interaction of the excited core hole in the XAS final state. We observe several fine structures on both L3 and L2 absorption edges. On the L2 edge, we observe a distinct peak at hn ¼ 652:0 eV: There appears a sharp peak structure and shoulders at 1.2 and 3:5 eV higher energies of the L3 edge (hn ¼ 639:7 eV). These shoulders and multiplet fine structures are closely related to Mn 3d electronic states and crystal field [12]. The line shape of Mn L2;3 XAS spectrum for 0:46 ML Mn ultrathin film on Ni(1 1 0) shown as a reference in Fig. 2 can be well reproduced by Mn2þ 3d5 to 2p5 3d6 atomic multiplet calculation with a small cubic crystal field [12,13]. We realize that the observed fine structures for the nanoclusters are
Fig. 2. Experimental Mn L2;3 core absorption spectra for Mn nanoclusters (full line) and 0:46 ML Mn ultrathin film deposited on Ni(1 1 0) surface (dotted line)(Ref. [13]).
353
similar but sharper than those of the Mn film. Note that one finds the reduced intensity at the lower hn of the doublet structure on L2 edge in the spectrum of nanoclusters. We interpret these differences to the additional contribution of Mn3þ and Mn4þ in the line shape of Mn L2;3 XAS spectrum for the Mn nanoclusters [12]. The XMCD measurement was also performed to the Mn nanoclusters. And there was no XMCD signal observed. Here, we discuss a branching ratio between the L3 and L2 edges. It is well known that the L3 –L2 branching ratio, which is defined as the integrated intensity ratio R0 ¼ IðL3 Þ=IðL2 Þ; is quite sensitive to the electronic configuration [14]. According to the reported atomic multiconfiguration Dirac Fock calculation, the values of R0 for Mn2þ ; Mn3þ and Mn4þ ions are 3.25, 2.50 and 2.25, respectively [15]. Note that R0 decreases with the increase of ion number for the manganese. Although we cannot obtain the detailed electronic configuration from the absolute value of R0 ; its relative value can be a good diagnosis of the change of 3d electron number. In the present case, the estimated value of R0 is 2.0, which is much smaller than the experimental value (B4:1) for two-dimensional Mn films on Ni(1 1 0) and Cu(0 0 1) [13,16]. This small R0 value (B2:0) strongly suggests that there are Mn3þ and Mn4þ contributions in the Mn nanoclusters. We assume the kind of Mn3þ and Mn4þ characters are due to Mn–Al coupling at the interface. The Mn–Si and Mn–Ag couplings have not been included in this case. The reason why we skip the Mn 3d and Si 3p bonding comes from the observed STM image shown in Fig. 1. The Al clusters reside at the center of each half of the Si(1 1 1)-7 7 unit cell as we observe in the STM image. High-resolution STM observation combined with the pseudo-potential calculation based on the LDA have provided an optimized structure model that six Al atoms form a triangle and three Si adatoms are much displaced toward the triangle center [7,9]. Then we suspect that the displaced Si adatoms also contribute to the bonding with the Mn cluster. However, the unoccupied electronic state image taken by the STM with the sample voltage of þ1:1 V shows that the observed spots are assigned to the Al
ARTICLE IN PRESS 354
T. Xie et al. / Physica B 351 (2004) 351–354
atoms and no Si atom contributes to the triangle spots of the observed STM image. From this result, we consider that the Si dangling bond states are much suppressed in the empty electronic states, which possibly leads to a negligible coupling between the Mn and Si atoms. Also, the interaction between Mn atoms and the coated Ag layer can be excluded. This comes from the comparison between the XAS spectrum of the Mn nanoclusters and that of the Mn impurity in Ag [17]. We note that the spectral line shape of the Mn nanoclusters is much different from that of the Mn impurity, which can be reproduced by the theoretical spectrum calculated for an assumed 3d5 6 S5=2 ground state. Thus, the dominant hybridizations between the Mn–Mn and the Mn–Al are strongly suggested for the Mn nanoclusters.
4. Conclusion In summary, soft X-ray core absorption spectrum of the Mn nanoclusters has been measured. The small value of XAS L3 –L2 branching ratio and spectrum line shape for the Mn clusters show that the Mn3þ and Mn4þ characters should be taken into account in the electronic structure.
References [1] V.S. Stepanyuk, W. Hergert, K. Wildberger, S.K. Nayak, P. Jena, Surf. Sci. 384 (1997) L892.
[2] S.K. Nayak, P. Jena, Chem. Phys. Lett. 289 (1998) 473. [3] M.R. Pederson, F. Reuse, S.N. Khanna, Phys. Rev. B 58 (1998) 5632. [4] M.B. Knickelbein, Phys. Rev. Lett. 86 (2001) 5255. [5] A.M. Keen, C. Binns, S.H. Baker, S. Mozley, C. Norris, S.C. Thornton, Surf. Sci. 352–354 (1996) 715. [6] Jian-Long Li, Jin-Feng Jia, Xue-Jin Liang, Xi Liu, JunZhong Wang, Q.-K. Xue, X.-Q. Li, J.S. Tse, Z. Zhang, S.B. Zhang, Phys. Rev. Lett. 88 (2002) 066101. [7] Jinfeng Jia, Jun-Zhong Wang, X. Liu, Qi-Kun Xue, Zhi-Qiang Li, Y. Kawazoe, S.B. Zhang, Appl. Phys. Lett. 80 (2002) 3186. [8] V.G. Kotlyar, A.V. Zotov, A.A. Saranin, T.V. Kasyanova, M.A. Cherevik, I.V.P. Sarenko, V.G. Lifshits, Phys. Rev. B 66 (2002) 165401. [9] Jin-Feng Jia, X. Liu, Jun-Zhong Wang, Jian-Long Li, X.S. Wang, Qi-Kun Xue, Zhi-Qiang Li, Zhenyu Zhang, S.B. Zhang, Phys. Rev. B 66 (2002) 165412. [10] S. Suga, S. Imada, A. Yamasaki, S. Ueda, T. Muro, Y. Saitoh, J. Magn. Magn. Mater. 233 (2001) 60. [11] Y. Saitoh, H. Kimura, Y. Suzuki, T. Nakatani, T. Matsushita, T. Muro, T. Miyahara, M. Fujisawa, K. Soda, S. Ueda, H. Harada, M. Kotsugi, A. Sekiyama, S. Suga, Rev. Sci. Instr. 71 (9) (2000) 3254. [12] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 42 (1990) 5459. [13] T. Xie, A. Kimura, T.Kanbe, S. Qiao, M. Taniguchi, T. Muro, S. Imada, S. Suga, Jpn. J. Appl. Phys. 42 (2003) 4695. [14] B.T. Thole, G. van der Laan, Phys. Rev. B 38 (1988) 3158. [15] W.G. Waddington, P. Rez, I.P. Grant, C.J. Humphreys, Phys. Rev. B 34 (1986) 1467. [16] A. Kimura, T.Kanbe, T. Xie, S. Qiao, M. Taniguchi, T. Muro, S. Imada, S. Suga, Jpn. J. Appl. Phys. 42 (2003) 4692. [17] B.T. Thole, R.D. Cowan, G.A. Sawatzky, J. Fink, J.C. Fuggle, Phys. Rev. B 31 (1985) 6856.
Physica B 351 (2004) 351–354
Soft X-ray spectroscopy study of Mn nanoclusters on Si(1 1 1)-7 7 surface T. Xiea, A. Kimuraa,*, S. Qiaob,*, K. Ioria, K. Miyamotoa, M. Taniguchia,b, M.H. Panc, J.F. Jiac, Q.K. Xuec a
Department of Physical Sciences, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan b Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan c State Key Laboratory for Surface Physics and International Center for Quantum Structures, Institute of Physics, Chinese Academy of Science, Beijing 100080, China
Abstract Mn nanoclusters have been prepared on a Si(1 1 1)-7 7 surface by substrate-induced spontaneous clustering due to delicate control of growth kinetics. An ex situ soft X-ray absorption spectroscopy (XAS) using synchrotron radiation has been used to clarify electronic structures of the nanoclusters. The small value of XAS L3 –L2 branching ratio and spectrum line-shape for the Mn clusters show that the Mn3þ and Mn4þ electronic configuration should be taken into account. r 2004 Elsevier B.V. All rights reserved. PACS: 68.65.Hb; 73.21.La; 78.70.Dm Keywords: X-ray absorption spectroscopy (XAS); X-ray magnetic circular dichroism (XMCD); Nanoclusters; Multiplet fine structures
1. Introduction Magnetic nanoclusters with diameters in the size range of 1–10 nm attract growing interest because of the great potential applications in the fields of high-density data storage, sensors and spin electronic devices. From the fundamental point of view, novel physical properties are expected due to a quantum confinement effect as the dimensions of *Corresponding authors. Fax: 081-0824-24-0719. E-mail addresses: [email protected] (A. Kimura), [email protected] (S. Qiao).
the magnetic objects become comparable to characteristic nanoscopic length such as the magnetic single domain size. Mn is one of the most fascinating elements. Although a divalent Mn ion has the highest spin magnetic moment (5 mB ) due to its half-filled character, pure Mn crystal shows a small magnetic moment below B2:3 mB (for g Mn, for example) while keeping an antiferromagnetic (AF) structure. We expect a stable magnetic structure changes when the Mn atoms are synthesized in low-dimensional forms, such as a film (2D), a wire (1D) and a cluster (0D). Recently, many experiments and theoretical
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.051
ARTICLE IN PRESS 352
T. Xie et al. / Physica B 351 (2004) 351–354
calculations have been focused on the Mn nanoclusters. However, the theorists mainly discuss the magnetic transition of small Mn clusters with atom numbers less than 10 [1–3]. The experimental results are mostly about the magnetic properties and electronic structures of free Mn nanoclusters or those deposited on highly oriented pyrolytic graphite (HOPG) and metal substrates [4,5]. However, for borrowing as many techniques from Si microlithography as possible, the semiconductor substrates are much more attractable. More recently, Li et al. [6] found that ordered identical-size nanocluster arrays can be realized on Si(1 1 1)-7 7 surface by delicate control of growth kinetics. This substrate-induced spontaneous clustering method has attracted much attention. A most striking feature is that a highquality Al nanocluster ordered array has been successfully prepared on Si(1 1 1)-7 7 and exhibit a remarkable thermal stability, which serves as an ideal template for growing magnetic nanostructures [7–9]. We have fabricated Mn nanoclusters supported by this Al nanocluster ordered arrays on Si(1 1 1)-7 7 surface. In this work, an ex situ X-ray absorption spectroscopy (XAS) measurement has been performed on the Mn nanoclusters. XAS using soft X-ray synchrotron radiation is a well-established technique for providing information on the electronic and magnetic properties of matter. A particular asset of XAS is its element specificity and the observed fine structures of XAS spectrum can be a good diagnosis of the electronic structure.
[10,11]. Circularly polarized light was supplied from a twin-helical undulator, with which almost 100% polarization was obtained at the peak of the first-harmonic radiation. The Mn L2;3 XAS spectrum was obtained by means of a total photoelectron yield method by directly detecting the sample current with changing the photon energy. We have applied a magnetic field of B1:4 T with two pairs of permanent magnets normal to the sample surface for the possible observation of the X-ray magnetic circular dichroism (XMCD). The base pressure of the main chamber was 8 108 Pa: The XAS measurement was done at a temperature of 43 K:
3. Results and discussion Fig. 1 shows the observed STM image of the Mn deposited surface, where the coverage is equivalent to 1:5 ML for the film. This image has been taken with the sample bias 2:0 V: We find that the Mn clusters are grown on this surface as indicated by the bright patches. We also observe the ordered Al nanocluster arrays behind the Mn clusters as
2. Experimental The detailed preparation method of the selfassembled clusters has been published elsewhere [6,7,9]. Prior to the Mn evaporation, 0:24 ML Al has been evaporated onto a clean Si(1 1 1)-7 7 surface to fabricate the identical Al cluster ordered arrays [7,9]. Subsequently, the Mn nanoclusters were grown on the template. Ag film with 5 nm thickness was coated on the Mn nanoclusters to prevent them from oxidation. We have carried out the XAS measurement at BL25SU of SPring-8
Fig. 1. STM image (80 nm 80 nm) of Mn nanoclusters (large bright patches). In the image, the Al cluster array is observed with white small dots behind the Mn clusters. The sample bias voltage is 2:0 V:
ARTICLE IN PRESS T. Xie et al. / Physica B 351 (2004) 351–354
shown by the small, but bright dots. It has been reported that the Al atoms form a characteristic honeycomb structure occupying both the faulted and unfaulted halves of the 7 7 reconstructed unit cell [7,9]. This is consistent with the present STM image in Fig. 1. The subsequently deposited Mn atoms reside on top of the ordered Al cluster arrays. The present STM image shows that the diameters of Mn nanoclusters disperse from 2 to 5 nm: A rough estimation shows that each cluster contains 100–200 atoms: Mn L2;3 XAS spectrum of the Mn nanoclusters, where a linear background has been subtracted, is shown in Fig. 2. We find clearly the L3 and L2 core absorption edges split by a spin–orbit interaction of the excited core hole in the XAS final state. We observe several fine structures on both L3 and L2 absorption edges. On the L2 edge, we observe a distinct peak at hn ¼ 652:0 eV: There appears a sharp peak structure and shoulders at 1.2 and 3:5 eV higher energies of the L3 edge (hn ¼ 639:7 eV). These shoulders and multiplet fine structures are closely related to Mn 3d electronic states and crystal field [12]. The line shape of Mn L2;3 XAS spectrum for 0:46 ML Mn ultrathin film on Ni(1 1 0) shown as a reference in Fig. 2 can be well reproduced by Mn2þ 3d5 to 2p5 3d6 atomic multiplet calculation with a small cubic crystal field [12,13]. We realize that the observed fine structures for the nanoclusters are
Fig. 2. Experimental Mn L2;3 core absorption spectra for Mn nanoclusters (full line) and 0:46 ML Mn ultrathin film deposited on Ni(1 1 0) surface (dotted line)(Ref. [13]).
353
similar but sharper than those of the Mn film. Note that one finds the reduced intensity at the lower hn of the doublet structure on L2 edge in the spectrum of nanoclusters. We interpret these differences to the additional contribution of Mn3þ and Mn4þ in the line shape of Mn L2;3 XAS spectrum for the Mn nanoclusters [12]. The XMCD measurement was also performed to the Mn nanoclusters. And there was no XMCD signal observed. Here, we discuss a branching ratio between the L3 and L2 edges. It is well known that the L3 –L2 branching ratio, which is defined as the integrated intensity ratio R0 ¼ IðL3 Þ=IðL2 Þ; is quite sensitive to the electronic configuration [14]. According to the reported atomic multiconfiguration Dirac Fock calculation, the values of R0 for Mn2þ ; Mn3þ and Mn4þ ions are 3.25, 2.50 and 2.25, respectively [15]. Note that R0 decreases with the increase of ion number for the manganese. Although we cannot obtain the detailed electronic configuration from the absolute value of R0 ; its relative value can be a good diagnosis of the change of 3d electron number. In the present case, the estimated value of R0 is 2.0, which is much smaller than the experimental value (B4:1) for two-dimensional Mn films on Ni(1 1 0) and Cu(0 0 1) [13,16]. This small R0 value (B2:0) strongly suggests that there are Mn3þ and Mn4þ contributions in the Mn nanoclusters. We assume the kind of Mn3þ and Mn4þ characters are due to Mn–Al coupling at the interface. The Mn–Si and Mn–Ag couplings have not been included in this case. The reason why we skip the Mn 3d and Si 3p bonding comes from the observed STM image shown in Fig. 1. The Al clusters reside at the center of each half of the Si(1 1 1)-7 7 unit cell as we observe in the STM image. High-resolution STM observation combined with the pseudo-potential calculation based on the LDA have provided an optimized structure model that six Al atoms form a triangle and three Si adatoms are much displaced toward the triangle center [7,9]. Then we suspect that the displaced Si adatoms also contribute to the bonding with the Mn cluster. However, the unoccupied electronic state image taken by the STM with the sample voltage of þ1:1 V shows that the observed spots are assigned to the Al
ARTICLE IN PRESS 354
T. Xie et al. / Physica B 351 (2004) 351–354
atoms and no Si atom contributes to the triangle spots of the observed STM image. From this result, we consider that the Si dangling bond states are much suppressed in the empty electronic states, which possibly leads to a negligible coupling between the Mn and Si atoms. Also, the interaction between Mn atoms and the coated Ag layer can be excluded. This comes from the comparison between the XAS spectrum of the Mn nanoclusters and that of the Mn impurity in Ag [17]. We note that the spectral line shape of the Mn nanoclusters is much different from that of the Mn impurity, which can be reproduced by the theoretical spectrum calculated for an assumed 3d5 6 S5=2 ground state. Thus, the dominant hybridizations between the Mn–Mn and the Mn–Al are strongly suggested for the Mn nanoclusters.
4. Conclusion In summary, soft X-ray core absorption spectrum of the Mn nanoclusters has been measured. The small value of XAS L3 –L2 branching ratio and spectrum line shape for the Mn clusters show that the Mn3þ and Mn4þ characters should be taken into account in the electronic structure.
References [1] V.S. Stepanyuk, W. Hergert, K. Wildberger, S.K. Nayak, P. Jena, Surf. Sci. 384 (1997) L892.
[2] S.K. Nayak, P. Jena, Chem. Phys. Lett. 289 (1998) 473. [3] M.R. Pederson, F. Reuse, S.N. Khanna, Phys. Rev. B 58 (1998) 5632. [4] M.B. Knickelbein, Phys. Rev. Lett. 86 (2001) 5255. [5] A.M. Keen, C. Binns, S.H. Baker, S. Mozley, C. Norris, S.C. Thornton, Surf. Sci. 352–354 (1996) 715. [6] Jian-Long Li, Jin-Feng Jia, Xue-Jin Liang, Xi Liu, JunZhong Wang, Q.-K. Xue, X.-Q. Li, J.S. Tse, Z. Zhang, S.B. Zhang, Phys. Rev. Lett. 88 (2002) 066101. [7] Jinfeng Jia, Jun-Zhong Wang, X. Liu, Qi-Kun Xue, Zhi-Qiang Li, Y. Kawazoe, S.B. Zhang, Appl. Phys. Lett. 80 (2002) 3186. [8] V.G. Kotlyar, A.V. Zotov, A.A. Saranin, T.V. Kasyanova, M.A. Cherevik, I.V.P. Sarenko, V.G. Lifshits, Phys. Rev. B 66 (2002) 165401. [9] Jin-Feng Jia, X. Liu, Jun-Zhong Wang, Jian-Long Li, X.S. Wang, Qi-Kun Xue, Zhi-Qiang Li, Zhenyu Zhang, S.B. Zhang, Phys. Rev. B 66 (2002) 165412. [10] S. Suga, S. Imada, A. Yamasaki, S. Ueda, T. Muro, Y. Saitoh, J. Magn. Magn. Mater. 233 (2001) 60. [11] Y. Saitoh, H. Kimura, Y. Suzuki, T. Nakatani, T. Matsushita, T. Muro, T. Miyahara, M. Fujisawa, K. Soda, S. Ueda, H. Harada, M. Kotsugi, A. Sekiyama, S. Suga, Rev. Sci. Instr. 71 (9) (2000) 3254. [12] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 42 (1990) 5459. [13] T. Xie, A. Kimura, T.Kanbe, S. Qiao, M. Taniguchi, T. Muro, S. Imada, S. Suga, Jpn. J. Appl. Phys. 42 (2003) 4695. [14] B.T. Thole, G. van der Laan, Phys. Rev. B 38 (1988) 3158. [15] W.G. Waddington, P. Rez, I.P. Grant, C.J. Humphreys, Phys. Rev. B 34 (1986) 1467. [16] A. Kimura, T.Kanbe, T. Xie, S. Qiao, M. Taniguchi, T. Muro, S. Imada, S. Suga, Jpn. J. Appl. Phys. 42 (2003) 4692. [17] B.T. Thole, R.D. Cowan, G.A. Sawatzky, J. Fink, J.C. Fuggle, Phys. Rev. B 31 (1985) 6856.