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NdNi4B and DyNi4B compounds studied by X-ray photoemission spectroscopy
PERGAMON
Solid State Communications 122 (2002) 145±149
www.elsevier.com/locate/ssc
NdNi4B and DyNi4B compounds studied by X-ray photoemission spectroscopy T. TolinÂski a,*, G. Cheøkowska b, A. Kowalczyk a a
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 PoznanÂ, Poland b Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland Received 15 January 2002; accepted 5 March 2002 by D. Van Dyck
Abstract The hexagonal NdNi4B and DyNi4B compounds were studied by X-ray photoemission spectroscopy. Both valence band and core level spectra were analyzed. A comparison of the valence bands showed that the Dy(4f) levels were well localized, whereas Nd(4f) levels overlapped strongly with the Ni(3d) peak below the Fermi level. The 3d5/2 and 3d3/2 bands of NdNi4B revealed additional satellites. The values of the 3d spin±orbit splitting ELS were equal to 22.8 eV for R Nd and 38 eV for R Dy: The coupling between the f-orbitals and the conduction states, D < 23 meV, was estimated for NdNi4B basing on the Gunnarsson± SchoÈnhammer model. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 71.20.Eh; 82.80.Pv Keywords: A. Magnetically ordered materials; D. Electronic band structure; E. Photoelectron spectroscopies
1. Introduction The series RNi4B, where R stands for rare-earth element or Y is attracting attention owing to its interesting magnetic, structural and electronic behavior. The materials belonging to the RNi4B series create a hexagonal structure of CeCo4B with space group P6/mmm. The Ni atoms occupy the crystallographic sites (2c) and (6c) and boron atoms are located in the (2d) positions. The ®lling of the Ni(3d) states, i.e. existence or absence of the magnetic moment is not well settled [1±3] for RNi4B. This question was addressed in our previous experimental and theoretical X-ray photoemission spectroscopy (XPS) studies of the electronic structure of the GdNi4B compound [1]. The magnetic properties of RNi4B series for R Nd; Dy, Gd and Ce were also studied in detail [4]. The UNi4B is claimed to exhibit spin ¯uctuations and can undergo a phase transition to an unknown magnetic order [5,6]. YNi4B surprisingly revealed superconducting properties connected with carbon admixture with a relatively large transition temperature, TC 13 K [7]. * Corresponding author. Tel.: 148-61-8695232; fax: 148-618684524. E-mail address: [email protected] (T. TolinÂski).
The electronic structure of such systems can be successfully investigated by XPS. Especially, this method provides information about the properties of the 4f level, localization and hybridization effects and characteristic binding energies. A basic theoretical model of XPS employing single impurity Anderson Hamiltonian was proposed by Gunnarsson and SchoÈnhammer [8]. This approach was derived for Cebased compounds, however, reasonable conclusions could be also inferred for other light rare-earths. In this paper, we present a comparison of electronic structures of NdNi4B and DyNi4B alloys. The electronic structure was investigated using XPS. 2. Experimental The RNi4B (R Nd or Dy) compounds were prepared by the induction melting of stoichiometric amounts of the constituent elements in a water-cooled boat, under an argon atmosphere. The ingots were inverted and melted several times to insure homogeneity. A powder X-ray diffraction technique showed that the studied samples were single-phase [4]. The XPS spectra were obtained with monochromatized Al Ka radiation at room temperature, using a PHI 5700/660
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(02)00107-2
146
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
Fig. 1. X-ray photoemission spectra of DyNi4B (a) and NdNi4B (b).
Physical Electronics Spectrometer. The energy spectra of the electrons were analyzed by a hemispherical mirror analyzer with the energy resolution of about 0.3 eV. The Fermi level EF 0 was referred to the gold 4f-levels binding energy at 84 eV. All emission spectra were measured immediately after breaking the sample in a vacuum of 10 210 Torr. The oxidation of the NdNi4B surface was checked by observing the O(1s) spectra before and after each measurement.
3. Results and discussion NdNi4B and DyNi4B should differ in magnetic and electronic properties because Nd belongs to the light rare earths, while Dy represents the group of heavy rare earths. For compounds in which R is a light rare earth element J L 2 S the total R moment (gJm B) is coupled parallel to the Ni moments. By contrast, when R is a heavy rare earth element J L 1 S the total R moment is coupled antiparallel to the Ni moment. The paramagnetic±ferromagnetic phase transition temperatures obtained in our previous studies were equal to
12 and 15 K for NdNi4B and DyNi4B, respectively [4]. These TC values were extracted from the temperature dependences of magnetization. The magnetic moments were M(NdNi4B) 1.7m B and M(DyNi4B) 8.2m B and the coercive ®elds HC were very small, in the range 1±6 mT. For comparison, CeNi4B was paramagnetic and followed the Curie±Weiss law with the effective magnetic moment meff 0:52mB =f:u: [4] in accord with Ref. [9]. Fig. 1 displays the entire XPS spectrum of NdNi4B and DyNi4B compounds. A noticeable feature is a contamination of oxygen and carbon in the NdNi4B alloy, which is absent in the case of DyNi4B. A more detailed comparison of the valence band (VB) regions is shown in Fig. 2(a) and (b). The most striking observation is the peaks-rich spectrum of the DyNi4B valence band region. The peaks' positions are in good agreement with binding energies of a metallic dysprosium [10]. However, the most important difference between DyNi4B and NdNi4B appears near the Fermi level, where the ®rst compound is characterized by a strong separation of the 4f peaks and the valence band and the second one exhibits an overlapping of these bands. Hence, the 4f-electrons in the DyNi4B alloy are localized, whereas for NdNi4B hybridization occurs within the
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
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Fig. 2. Valence band region of XPS spectra for DyNi4B (a) and NdNi4B (b).
valence band. The valence band spectrum of NdNi4B (Fig. 2(b)) exhibits the domination of the Ni(3d) E 1:5 eV and Nd(4f) E 4:65 eV states. The position of the peak Nd(4f) is in agreement with the result obtained for pure Nd metal [11,12]. In the XPS valence band spectrum of a pure Nd a small intensity peak at about 1 eV is usually also observed [11]. The ®tting of the Ni(3d) band (Fig. 2(b)) is not satisfactory just below the Fermi level, therefore the small intensity Nd peak is present but cannot be separated from the Ni(3d) band. We have also experimentally observed peaks at 22.3 and 18.7 eV, which were identi®ed as Nd(5p1/2) and Nd(5p3/2), respectively. Fig. 3 shows the Nd(3d5/2,3/2) doublet of the NdNi4B compound. The spin±orbit splitting ELS is equal to 22.8 eV. The kinks indicated by arrows are connected with satellites which results from the screened Nd 3d 94f 4 ®nal states [8,13,14]. The energy scale is determined by a strong Coulomb interaction between the core hole and f electrons, which is included in the Gunnarsson±SchoÈnhammer model [8]. The satellites enable a rough estimation of the coupling D between the f-orbitals and the conduction states [8,14] on the basis of the intensity ratio r I f 4 ={I f 3 1 I f 4 }: Assuming the dependence of the intensity ratio on the D parameter like in the case of Ce [8,14±16] the D value
is about 23 meV for 3d5/2 band. Such an assumption is justi®ed because in the Gunnarsson±SchoÈnhammer model the hybridization parameter does not depend on the number of 4f electrons. Similar considerations and the small value of D suggest that the f-occupancy, nf, is close to 3. The values of the parameters resemble results for NdAuGe obtained by Szytuøa et al. [14] (ELS < 22.60 eV and the coupling energy D < 25 meV). The XPS measurements of DyNi4B compound give ELS 38 eV for the Dy(3d5/2,3/2) doublet. The understanding of the electronic properties of Ni in the RNi4B series is still incomplete [1±3]. Fig. 4 shows the shift of the Ni(2p) peaks in DyNi4B and NdNi4B compounds. Both the positions of the Ni(2p1/2) and Ni(2p3/2) bands of NdNi4B and the distance between the peaks (17.3 eV) are similar to the result for GdNi4B [1]. DyNi4B exhibits a shift of about 0.7 eV in the peak position towards higher binding energies keeping a similar mutual distance of the peaks (17.4 eV). The shift may result from different local environments. Between the Ni(2p1/2) and Ni(2p3/2) peaks a satellite is visible, which is also inherent for Ni metal. Its intensity is smaller for RNi4B (R Nd; Dy, Gd) than for Ni metal, so may re¯ect the vanishing Ni moment. Nevertheless, the existence of this satellite
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
148
Fig. 3. 3d5/2,3/2 doublet of the NdNi4B compound. Arrows indicate additional satellites. Spin±orbit splitting ELS < 22.8 eV and the coupling energy D < 23 meV for intensity ratio r(d5/2) < 0.051.
suggests the possibility that the Ni(3d) band is not quite ®lled [1±3].
4. Conclusions The electronic structure of DyNi4B and NdNi4B compounds was examined by an XPS experiment. The
main results are summarized as follows: 1. The valence band is determined mainly by the Ni(3d), Nd(4f) and Dy(4f) bands. 2. The binding energies are near the values for pure elements. 3. The values of the spin±orbit coupling ELS obtained from the R(3d5/2,3/2) doublet splitting are equal to 22.8 eV for R Nd and 38 eV for R Dy:
Fig. 4. Ni(2p1/2,3/2) doublet of the DyNi4B (a) and NdNi4B (b). Dashed line illustrates the shift of the peaks.
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
4. The hybridization energy, D , determined experimentally from the intensity ratio of the Nd(3d5/2) peak and its satellite based on the Gunnarsson±SchoÈnhammer model is about 23 meV for NdNi4B.
References [1] T. TolinÂski, G. Chelkowska, A. Kowalczyk, submitted for publication. [2] N.M. Hong, N.P. Thuy, G. Schaudy, T. Holubar, G. Hilscher, J.J.M. Franse, J. Appl. Phys. 73 (1993) 5698. [3] P. Ravindran, S. Sankaralingam, R. Asokamani, Phys. Rev. B 52 (1995) 12921. [4] T. TolinÂski, A. Kowalczyk, A. Szlaferek, M. Timko, J. Kovac, submitted for publication. [5] H. Nakotte, L. Havela, J.P. Kuang, F.R. de Boer, K.H.J. Buschow, J.H.V.J. Brabers, T. Kuroda, K. Sugiyama, M. Date, Int. J. Mod. Phys. B 7 (1993) 838. [6] G. Cheøkowska, A. Kowalczyk, A. Szajek, A. Szlaferek, J. Magn. Magn. Mater. 236 (2001) 243.
149
[7] R. Nagarajan, L.C. Gupta, C. Mazumdar, Z. Hossain, S.K. Dhar, C. Godart, B.D. Padalia, R. Vijayaraghavan, J. Alloys Compd. 225 (1995) 571. [8] O. Gunnarsson, K. SchoÈnhammer, Phys. Rev. B 28 (1983) 4315. [9] N.M. Hong, T. Holubar, G. Hilscher, M. Vybornov, P. Rogl, IEEE Trans. Magn. 30 (1994) 4966. [10] F.R. McFeely, S.P. Kowalczyk, L. Ley, D.A. Shirley, Phys. Lett. 45A (1973) 227. [11] W.D. Schneider, Landolt-BoÈrnstein New Series III/23a, p. 358. [12] J.K. Lang, Y. Baer, P.A. Cox, J. Phys. F: Metal Phys. 11 (1981) 121. [13] J.C. Fuggle, M. Campagna, Z. Zolnierek, R. LaÈsser, A. Platau, Phys. Rev. Lett. 45 (1980) 1597. [14] A. Szytuøa, D. Fus, B. Penc, A. Jezierski, J. Alloys Compd. 317±318 (2001) 340. [15] J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. LaÈsser, Ch. Freiburg, O. Gunnarsson, K. SchoÈnhammer, Phys. Rev. B 27 (1983) 7330. [16] A. SÂlebarski, M. Neumann, S. MaÈhl, Phys. Rev. B 51 (1995) 11113.
Solid State Communications 122 (2002) 145±149
www.elsevier.com/locate/ssc
NdNi4B and DyNi4B compounds studied by X-ray photoemission spectroscopy T. TolinÂski a,*, G. Cheøkowska b, A. Kowalczyk a a
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 PoznanÂ, Poland b Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland Received 15 January 2002; accepted 5 March 2002 by D. Van Dyck
Abstract The hexagonal NdNi4B and DyNi4B compounds were studied by X-ray photoemission spectroscopy. Both valence band and core level spectra were analyzed. A comparison of the valence bands showed that the Dy(4f) levels were well localized, whereas Nd(4f) levels overlapped strongly with the Ni(3d) peak below the Fermi level. The 3d5/2 and 3d3/2 bands of NdNi4B revealed additional satellites. The values of the 3d spin±orbit splitting ELS were equal to 22.8 eV for R Nd and 38 eV for R Dy: The coupling between the f-orbitals and the conduction states, D < 23 meV, was estimated for NdNi4B basing on the Gunnarsson± SchoÈnhammer model. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 71.20.Eh; 82.80.Pv Keywords: A. Magnetically ordered materials; D. Electronic band structure; E. Photoelectron spectroscopies
1. Introduction The series RNi4B, where R stands for rare-earth element or Y is attracting attention owing to its interesting magnetic, structural and electronic behavior. The materials belonging to the RNi4B series create a hexagonal structure of CeCo4B with space group P6/mmm. The Ni atoms occupy the crystallographic sites (2c) and (6c) and boron atoms are located in the (2d) positions. The ®lling of the Ni(3d) states, i.e. existence or absence of the magnetic moment is not well settled [1±3] for RNi4B. This question was addressed in our previous experimental and theoretical X-ray photoemission spectroscopy (XPS) studies of the electronic structure of the GdNi4B compound [1]. The magnetic properties of RNi4B series for R Nd; Dy, Gd and Ce were also studied in detail [4]. The UNi4B is claimed to exhibit spin ¯uctuations and can undergo a phase transition to an unknown magnetic order [5,6]. YNi4B surprisingly revealed superconducting properties connected with carbon admixture with a relatively large transition temperature, TC 13 K [7]. * Corresponding author. Tel.: 148-61-8695232; fax: 148-618684524. E-mail address: [email protected] (T. TolinÂski).
The electronic structure of such systems can be successfully investigated by XPS. Especially, this method provides information about the properties of the 4f level, localization and hybridization effects and characteristic binding energies. A basic theoretical model of XPS employing single impurity Anderson Hamiltonian was proposed by Gunnarsson and SchoÈnhammer [8]. This approach was derived for Cebased compounds, however, reasonable conclusions could be also inferred for other light rare-earths. In this paper, we present a comparison of electronic structures of NdNi4B and DyNi4B alloys. The electronic structure was investigated using XPS. 2. Experimental The RNi4B (R Nd or Dy) compounds were prepared by the induction melting of stoichiometric amounts of the constituent elements in a water-cooled boat, under an argon atmosphere. The ingots were inverted and melted several times to insure homogeneity. A powder X-ray diffraction technique showed that the studied samples were single-phase [4]. The XPS spectra were obtained with monochromatized Al Ka radiation at room temperature, using a PHI 5700/660
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(02)00107-2
146
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
Fig. 1. X-ray photoemission spectra of DyNi4B (a) and NdNi4B (b).
Physical Electronics Spectrometer. The energy spectra of the electrons were analyzed by a hemispherical mirror analyzer with the energy resolution of about 0.3 eV. The Fermi level EF 0 was referred to the gold 4f-levels binding energy at 84 eV. All emission spectra were measured immediately after breaking the sample in a vacuum of 10 210 Torr. The oxidation of the NdNi4B surface was checked by observing the O(1s) spectra before and after each measurement.
3. Results and discussion NdNi4B and DyNi4B should differ in magnetic and electronic properties because Nd belongs to the light rare earths, while Dy represents the group of heavy rare earths. For compounds in which R is a light rare earth element J L 2 S the total R moment (gJm B) is coupled parallel to the Ni moments. By contrast, when R is a heavy rare earth element J L 1 S the total R moment is coupled antiparallel to the Ni moment. The paramagnetic±ferromagnetic phase transition temperatures obtained in our previous studies were equal to
12 and 15 K for NdNi4B and DyNi4B, respectively [4]. These TC values were extracted from the temperature dependences of magnetization. The magnetic moments were M(NdNi4B) 1.7m B and M(DyNi4B) 8.2m B and the coercive ®elds HC were very small, in the range 1±6 mT. For comparison, CeNi4B was paramagnetic and followed the Curie±Weiss law with the effective magnetic moment meff 0:52mB =f:u: [4] in accord with Ref. [9]. Fig. 1 displays the entire XPS spectrum of NdNi4B and DyNi4B compounds. A noticeable feature is a contamination of oxygen and carbon in the NdNi4B alloy, which is absent in the case of DyNi4B. A more detailed comparison of the valence band (VB) regions is shown in Fig. 2(a) and (b). The most striking observation is the peaks-rich spectrum of the DyNi4B valence band region. The peaks' positions are in good agreement with binding energies of a metallic dysprosium [10]. However, the most important difference between DyNi4B and NdNi4B appears near the Fermi level, where the ®rst compound is characterized by a strong separation of the 4f peaks and the valence band and the second one exhibits an overlapping of these bands. Hence, the 4f-electrons in the DyNi4B alloy are localized, whereas for NdNi4B hybridization occurs within the
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147
Fig. 2. Valence band region of XPS spectra for DyNi4B (a) and NdNi4B (b).
valence band. The valence band spectrum of NdNi4B (Fig. 2(b)) exhibits the domination of the Ni(3d) E 1:5 eV and Nd(4f) E 4:65 eV states. The position of the peak Nd(4f) is in agreement with the result obtained for pure Nd metal [11,12]. In the XPS valence band spectrum of a pure Nd a small intensity peak at about 1 eV is usually also observed [11]. The ®tting of the Ni(3d) band (Fig. 2(b)) is not satisfactory just below the Fermi level, therefore the small intensity Nd peak is present but cannot be separated from the Ni(3d) band. We have also experimentally observed peaks at 22.3 and 18.7 eV, which were identi®ed as Nd(5p1/2) and Nd(5p3/2), respectively. Fig. 3 shows the Nd(3d5/2,3/2) doublet of the NdNi4B compound. The spin±orbit splitting ELS is equal to 22.8 eV. The kinks indicated by arrows are connected with satellites which results from the screened Nd 3d 94f 4 ®nal states [8,13,14]. The energy scale is determined by a strong Coulomb interaction between the core hole and f electrons, which is included in the Gunnarsson±SchoÈnhammer model [8]. The satellites enable a rough estimation of the coupling D between the f-orbitals and the conduction states [8,14] on the basis of the intensity ratio r I f 4 ={I f 3 1 I f 4 }: Assuming the dependence of the intensity ratio on the D parameter like in the case of Ce [8,14±16] the D value
is about 23 meV for 3d5/2 band. Such an assumption is justi®ed because in the Gunnarsson±SchoÈnhammer model the hybridization parameter does not depend on the number of 4f electrons. Similar considerations and the small value of D suggest that the f-occupancy, nf, is close to 3. The values of the parameters resemble results for NdAuGe obtained by Szytuøa et al. [14] (ELS < 22.60 eV and the coupling energy D < 25 meV). The XPS measurements of DyNi4B compound give ELS 38 eV for the Dy(3d5/2,3/2) doublet. The understanding of the electronic properties of Ni in the RNi4B series is still incomplete [1±3]. Fig. 4 shows the shift of the Ni(2p) peaks in DyNi4B and NdNi4B compounds. Both the positions of the Ni(2p1/2) and Ni(2p3/2) bands of NdNi4B and the distance between the peaks (17.3 eV) are similar to the result for GdNi4B [1]. DyNi4B exhibits a shift of about 0.7 eV in the peak position towards higher binding energies keeping a similar mutual distance of the peaks (17.4 eV). The shift may result from different local environments. Between the Ni(2p1/2) and Ni(2p3/2) peaks a satellite is visible, which is also inherent for Ni metal. Its intensity is smaller for RNi4B (R Nd; Dy, Gd) than for Ni metal, so may re¯ect the vanishing Ni moment. Nevertheless, the existence of this satellite
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
148
Fig. 3. 3d5/2,3/2 doublet of the NdNi4B compound. Arrows indicate additional satellites. Spin±orbit splitting ELS < 22.8 eV and the coupling energy D < 23 meV for intensity ratio r(d5/2) < 0.051.
suggests the possibility that the Ni(3d) band is not quite ®lled [1±3].
4. Conclusions The electronic structure of DyNi4B and NdNi4B compounds was examined by an XPS experiment. The
main results are summarized as follows: 1. The valence band is determined mainly by the Ni(3d), Nd(4f) and Dy(4f) bands. 2. The binding energies are near the values for pure elements. 3. The values of the spin±orbit coupling ELS obtained from the R(3d5/2,3/2) doublet splitting are equal to 22.8 eV for R Nd and 38 eV for R Dy:
Fig. 4. Ni(2p1/2,3/2) doublet of the DyNi4B (a) and NdNi4B (b). Dashed line illustrates the shift of the peaks.
T. TolinÂski et al. / Solid State Communications 122 (2002) 145±149
4. The hybridization energy, D , determined experimentally from the intensity ratio of the Nd(3d5/2) peak and its satellite based on the Gunnarsson±SchoÈnhammer model is about 23 meV for NdNi4B.
References [1] T. TolinÂski, G. Chelkowska, A. Kowalczyk, submitted for publication. [2] N.M. Hong, N.P. Thuy, G. Schaudy, T. Holubar, G. Hilscher, J.J.M. Franse, J. Appl. Phys. 73 (1993) 5698. [3] P. Ravindran, S. Sankaralingam, R. Asokamani, Phys. Rev. B 52 (1995) 12921. [4] T. TolinÂski, A. Kowalczyk, A. Szlaferek, M. Timko, J. Kovac, submitted for publication. [5] H. Nakotte, L. Havela, J.P. Kuang, F.R. de Boer, K.H.J. Buschow, J.H.V.J. Brabers, T. Kuroda, K. Sugiyama, M. Date, Int. J. Mod. Phys. B 7 (1993) 838. [6] G. Cheøkowska, A. Kowalczyk, A. Szajek, A. Szlaferek, J. Magn. Magn. Mater. 236 (2001) 243.
149
[7] R. Nagarajan, L.C. Gupta, C. Mazumdar, Z. Hossain, S.K. Dhar, C. Godart, B.D. Padalia, R. Vijayaraghavan, J. Alloys Compd. 225 (1995) 571. [8] O. Gunnarsson, K. SchoÈnhammer, Phys. Rev. B 28 (1983) 4315. [9] N.M. Hong, T. Holubar, G. Hilscher, M. Vybornov, P. Rogl, IEEE Trans. Magn. 30 (1994) 4966. [10] F.R. McFeely, S.P. Kowalczyk, L. Ley, D.A. Shirley, Phys. Lett. 45A (1973) 227. [11] W.D. Schneider, Landolt-BoÈrnstein New Series III/23a, p. 358. [12] J.K. Lang, Y. Baer, P.A. Cox, J. Phys. F: Metal Phys. 11 (1981) 121. [13] J.C. Fuggle, M. Campagna, Z. Zolnierek, R. LaÈsser, A. Platau, Phys. Rev. Lett. 45 (1980) 1597. [14] A. Szytuøa, D. Fus, B. Penc, A. Jezierski, J. Alloys Compd. 317±318 (2001) 340. [15] J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. LaÈsser, Ch. Freiburg, O. Gunnarsson, K. SchoÈnhammer, Phys. Rev. B 27 (1983) 7330. [16] A. SÂlebarski, M. Neumann, S. MaÈhl, Phys. Rev. B 51 (1995) 11113.