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Electronic structure and magnetism of rare-earth intermetallic compounds
Physica B 327 (2003) 171–176
Electronic structure and magnetism of rare-earth intermetallic compounds A. Szytu"aa,*, A. Jezierskib, B. Penca b
a ! Poland M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, ! Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznan,
Abstract The electronic structure of R2PdSi3 (R=Ce, Nd, Tb, Dy, Ho, Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga) compounds was studied by X-ray and ultraviolet photoemission spectroscopy. For the R2PdSi3 (R=Nd, Tb–Er) compounds, the valence band is dominated by the multiplet structure of the rare-earth atoms and by the Pd 4d band at E=4 eV. In these compounds, the density of states at the Fermi level is low and the Ruderman–Kittel–Kasuya–Yosida interaction seems to play a minor role in the magnetic interactions. The valence bands of the HoTX compound are also dominated by the Ho 4f and the transition metal nd (n=3,4) states. The multiplet structure of the Ho 4f states corresponds to that of the Ho3+ ion for all these compounds. The states at the Fermi level contain contributions from Ho (5d6 s) for T=Ag and from additional nd states of the T element for compounds with T=Co, Ni, Pd and Rh. r 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 71.28.+d; 73.20.At Keywords: Rare-earth intermetallics; Electronic structure; XPS; UPS
1. Introduction The magnetic properties of rare-earth ternary intermetallic compounds have been the subject of intensive research. It is generally accepted that the collective behaviour of the magnetic rare-earth ions in these compounds is determined by an indirect interaction among localised f electrons, which are coupled by the conduction electrons according to the Ruderman–Kittel–Kasuya–Yosida (RKKY) theory [1,2]. In the RKKY theory, the *Corresponding author. Tel.: +48-12-6324888; fax: +48-126337086. E-mail address: [email protected] (A. Szytu"a).
exchange integral JðRÞ is proportional to the density of electronic states at the Fermi level. In order to investigate the influence of the electronic structure at the Fermi level on the magnetic properties, this work reports the results of X-ray and ultraviolet photoemission spectroscopy measurements of two systems R2PdSi3 (R=Ce, Nd, Tb–Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga). The X-ray photoemission spectroscopy valence band spectra are compared with ab initio electronic structure calculations using the tight-binding linear muffintin orbital (TB LMTO) method [3] and with the results of magnetic and neutron diffraction measurements that are briefly presented below.
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The neutron diffraction data reveal that R2PdSi3 compounds crystallise in the hexagonal AlB2-type of structure. In this structure, the atomic arrangement leads to two different kinds of layers in the unit cell: R layers at z=0 and Pd– Si layers at z=12: Magnetic and neutron diffraction data indicate the following magnetic properties of the R2PdSi3 compounds: *
*
*
for R=Ce below T=2.5 K, a cluster spin glass is found, while for Nd below Tc=16 K, ferromagnetic order is observed; for Tb below TN=23 K, a coexistence of a ferromagnetic spiral structure and spin-glass behaviour is observed; the rare-earth moments of the Dy, Ho and Er compounds form sine-wave modulated structures [4].
HoTX compounds crystallise in the orthorhombic TiNiSi-type structure. R, T and X atoms occupy the positions of Ti, Ni and Si atoms, respectively. The R and T atoms form columns of trigonal prisms with X atoms inside. The columns are linked by the R–R edges into infinite crimped slabs placed perpendicular to the c-axis. In the case of HoTX compounds, magnetic and neutron diffraction measurements give that: *
*
*
*
HoAgGa and HoCoSi are ferromagnets with the Curie temperatures equal to 16 and 13 K, respectively [5,6]; HoNiSi is collinear antiferromagnet with the Ne! el temperature 4.1 K. Near the Ne! el temperature, the change of the magnetic structure to the sine-wave modulated is observed [7]; HoCoSn and HoPdSn are antiferromagnets with Ne! el temperatures equal to 7.6 and 3.5 K, respectively; they exhibit modulated magnetic structures [8,9]; HoRhSi and HoRhGe are collinear antiferromagnets with a Ne! el temperature equal to 8 and 4.6 K, respectively [10,11].
Neutron diffraction data indicate that in all these compounds, the magnetic moment is localized on the rare-earth atoms.
2. Experimental details The R2PdSi3 (R=Ce, Nd, Tb–Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga) samples were arc melted from the constituent metals in a cooled copper crucible in a high-purity argon atmosphere, remelted several times and then annealed at 8001C for 1 week. Their phase purities were checked by X-ray Debye–Scherrer diffraction with CoKa radiation. The diffraction data indicate that the R2PdSi3 compounds crystallise in the hexagonal AlB2-type of structure while the HoTX compounds do so in the orthorhombic TiNiSi-type one. Photoemission measurements were performed using a commercial LHS10 SPECS spectrometer with hemispherical energy analyzer using MgKa (hn ¼ 1253:6 eV) radiation and with a helium discharge lamp for ultraviolet photoemission spectroscopy (HeI: hn=21.2 eV, HeII: hn=40.8 eV). Measurements were made at room temperature in high vacuum (2 109 mbar). The total energy resolution was about 0.8 eV for Ag3d peaks. The spectrometer was calibrated using Cu2p, Ag3d and Au4f core-level photoemission spectra. Band structure calculations for Ce2PdSi3 were carried out with the use of the tight-binding linear muffintin orbital (TBLMTO) method [3] within the framework of the local spin density approximation (LSD). The scalar-relativistic approximation for band electrons and the fully relativistic treatment of the frozen potential was assumed according to von Barth and Hedin [12] with gradient corrections [13]. The self-coexistent calculations were performed in the atomic sphere approximation (ASA) for the experimental values of the lattice parameters and for 217 k-points in the irreducible edge of the Brillouin zone. 3. Results The XPS spectra of the valence bands of R2PdSi3 compounds (see Fig. 1) indicate that they are dominated by the contribution of the rareearth 4f states. At the Fermi level E ¼ 0 eV; the contribution to the density of states, except for Cecompounds, is found to be dominated by the R (5d6 s) band. In Fig. 2, we present the calculated partial and total density of states (DOS) for
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Ce2PdSi3. The 4f state of Ce atoms are close to the Fermi level, while the 4d band of Pd atoms is localized 4 eV below the Fermi level. The Si-3 s state forms a narrow peak at E=8 eV while the Si3p state forms broad bands which hybridize with the 4d band of Pd atoms. Experiments on this compound detected the Ce-4f state near the Fermi level as well as the Pd-4d band. The Ce2PdSi3 valence band spectrum shows a narrowing and mowing away of the Pd-4d states from the Fermi level (GFWHM=1.8 eV, BE= 4.0 eV) in comparison with pure Pd (GFWHM= 4.1 eV, BE=0.95 eV). The positions of the Pd-4d peaks do not change while changing the rare-earth element (see also Fig. 3 in Ref. [16]). The calculated DOSs at the Fermi level of Ce2PdSi3, equal to 6.82 states/(eV cell), is higher than that determined from the electronic specific heat g (about 100 mJ mol1K2) which leads to about 4.3 states/(eV cell) [18]. For compounds with heavier rare earths, the valence band photoelectron spectra are characterised by intense emissions related to the 4f finalstate multiplets [17]. The peaks corresponding to the final state multiplet appear to exhibit a similar shape for the valence band photoemission and are slightly shifted towards the Fermi level. The shift to the lower binding energy, in comparison with the pure rare-earth elements, is probably due to the chemical shift connected with the different chemical environment in the compounds. These data suggest that the rare-earth ions keep the 3+ valence in these compounds. This is in agreement
14
Pure R element R2PdSi3
Ce
Number of counts [a. u.]
Nd
Tb
Dy
Ho
Er
-2
0
2
4
6
8
10
12
14
Binding Energy [eV] Fig. 1. XPS valence band spectra for R2PdSi3 (R=Ce, Nd, Tb, Dy, Ho, Er) compounds and adequate spectra for the metallic rare earths from Ref. [14].
60 EF
DOS [states / eV]
DOS [states / eV]
EF 30 Pd
Ce
INTENSITY [ arb. units]
-2
173
30
Si 0
(a)
-8 -4 ENERGY [eV]
0
0
-8
(b)
-4 ENERGY [eV]
0
Fig. 2. (a) The contribution of Ce (6s6p5d4f), Pd (4d) and Si (3s3p) and (b) to the total DOSs of Ce2PdSi3. The Fermi level is located at E=0 eV. The dashed curve presents the DOSs convolved by Lorentzians of the half-width 0.4 eV and multiplied by the appropriate cross sections [15].
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Ga 3d3/2,5/2 Ag 4d
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HoAgGa
Ho 5p3/2 Ho 5p1/2
HoCoSi
HoCoSn
Number of counts [a. u.]
Y Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis Title
0
Sn 4d3/2,5/2
HoNiSi
HoPdSn Sn 4d3/2,5/2
HoRhGe Ge 3d3/2,5/2
HoRhSi
0
5
10
15
20
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35
40
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Binding Energy [eV] Fig. 3. XPS spectra of the valence bands of HoTX compounds and metallic holmium [19].
with the results of magnetic measurements, which give values for the effective magnetic moments close to the R3+ ion values. Fig. 3 shows the XPS valence bands of the HoTX compounds and for pure Ho. The UPS spectra for some of these compounds and for pure Ho are shown in Fig. 4. A detailed analysis of the XPS data give the following information. The valence bands of these compounds are dominated by the multiplet structure of the Ho3+ ion and the nd (n=3,4) states of the transition T metals. Also for these compounds, a shift of the multiplet state to a lower binding energy, in comparison with pure Ho, is observed. The energy shift is between 0.3 eV for HoRhSi and HoRhGe and 1.2 eV for HoAgGa and HoPdSn. This fact indicates a sensitivity of the electronic states of holmium to the other elements (T, X) in the investigated compounds. In the case of HoAgGa, the results
could indicate a possible hybridization of Ho-4f and Ag-4d. Such a hybridization should affect the 4f shell spin moment, which is smaller than the free Ho3+ ion value (3.5mB) [5]. The Ag-4d states form a relatively narrow peak (after substitution of the Ho-4f peak) with a maximum at 6.5 eV and a fullwidth at half-maximum of 1.9 eV. It differs significantly from the pure Ag-4d spectrum, which has a much broader maximum at about 5 eV [19]. In RAgSn (R=Pr,Nd) compounds, the experimental and calculated data give the peak of Ag-4d near 6.0 eV [20]. For the other compounds, the XPS and UPS spectra indicate that the binding energy of the Co-3d band is equal to 1.5 eV in HoCoSi and to 1.4 eV in HoCoSn, that the Ni-3d band in HoNiSi is equal to 2.3 eV, whose number is equal to 4.2 eV for Pd-4d in HoPdSn and that the Rh-4d band is equal to 2.4 eV in HoRhGe and to 3.0 eV in HoRhSi. In the iso-structural CeTX
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0
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Y Axis Title
HoCoSi
Number of counts [a. u.]
Y Axis Title Y Axis Title
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HoAgGa
and Ag and X=Si, Ge, Sn and Ga. The valence band spectra of all (except for R=Ce) compounds are dominated by the multiplet of the 4f final state and the nd state of the T element. The obtained results suggest that: *
HoCoSn
*
HoPdSn
*
HoRhSi
Y Axis Title
Y Axis Title
15
-5
0
5
10
15
20
Binding Energy [eV]
*
Fig. 4. UPS spectra of the valence bands of HoTX compounds and pure Ho.
compounds, the Ni-3d peak is near 1.5 eV for CeNiSn while the Pd-4d peak is at 3.5 eV in CePdSn [21]. The positions and shapes of the d bands indicate that these bands are filled and that the 3d and 4d atoms do not have localised magnetic moments. These results are in good agreement with the neutron diffraction data, which give a localised magnetic moment only on the rare-earth atoms. Near the Fermi level, the states of Ho (5d6 s) for T=Pd and Ag are observed and additional nd states of the T metal for T=Co, Ni, Rh and Pd.
4. Conclusions The paper reports on investigations of the electronic structure of R2PdSi3, R=Ce, Nd, Tb, Dy, Ho and Er and of HoTX, T=Co, Ni, Pd, Rh
175
*
*
The contribution to the density of states at EF is found to be dominated by R-5d. For HoCoX (X=Si, Sn), HoPdSn and HoRhX (X=Si, Ge) also the nd states of transition T metal are found to contribute. The density of states at the Fermi level, particularly for R2PdSi3, do not change while changing R elements (except for R=Ce). For R2PdSi3, the position of the peak corresponding to the Pd-4d state does not change while changing the R element. The position of this peak is in good agreement with the calculated one. The positions and shapes of the 3d and 4d bands indicate that these bands are filled and do not have any localized magnetic moments. This result is in good agreement with the neutron diffraction data, which indicate localized magnetic moments only on the rare-earth atoms. The hybridization of the Ho-4f and Ag-4d states is responsible for the decrease of the Ho magnetic moment in this compound. Similar hybridization effects of R-4f and Pd-4d states are observed in R2PdSi3 (R=Nd, Dy) compounds. For both compounds, the R moment is smaller than the R3+ ion value. For the other compounds, hybridization is less probable. For Ce2PdSi3, the Ce-4f states are near the Fermi level. An analysis of the Ce-3d core level indicates a trivalent character of Ce [16]. In both series of compounds, the large R3+– R3+ separation suggests that direct magnetic interactions are highly improbable. The stability of the observed magnetic ordering scheme results from indirect interactions, probably of the RKKY-type. The linear thermal dependence of the resistivity [4] indicates metallic character of R2PdSi3 compounds and the presence of conduction electrons. In the RKKY model, the critical temperature of the magnetic ordering is proportional to the de Gennes function G ¼ ðgJ 1Þ2 JðJ þ 1Þ [22]. However,
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the experimentally observed ordering temperatures of the R2PdSi3 (R=Gd–Er) compounds do not follow the de Gennes scaling. This suggests that the main interaction leading to the magnetic order in these systems is not purely of the RKKY-type but is modified, for example, by crystalline electric field effects which significantly influence the magnitudes of the critical temperature [23].
References [1] C. Kittel, in: F. Seitz, D. Turnbull, H. Ehrenreich (Eds.), Solid State Physics, Vol. 22, Wiley, New York, 1968, p. 1. [2] T. Kasuya, in: G.T. Rado, H. Suhl (Eds.), Magnetism, Vol. IIB, Academic Press, New York, 1966, p. 215. [3] O.K. Andersen, O. Jepsen, Phys. Rev. Lett. 53 (1984) 2571. [4] A. Szytu"a, et al., J. Magn. Magn. Mater. 202 (1999) 365. [5] A. Zygmunt, et al., J. Magn. Magn. Mater. 161 (1996) 127.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
J. Leciejewicz, et al., J. Alloys Compounds 240 (1996) 164. A. Szytu"a, et al., J. Magn. Magn. Mater. 191 (1999) 122. G. Andr!e, et al., J. Alloys Compounds 221 (1995) 25. G. Andr!e, et al., Acta Phys. Polon. A 85 (1994) 275. W. Ba’zela, et al., J. Magn. Magn. Mater. 50 (1985) 19. W. Ba’zela, et al., J. Phys.: Condens. Matter. 10 (1998) 2233. V.von U. Barth, L. Hedin, J. Phys. C 5 (1972) 391. D. Hu, D.C. Langreth, Phys. Scr. 32 (1985) 391. J.K. Lang, et al., J. Phys. F: Metal Phys. 11 (1981) 121. J.J. Yeh, I. Lindau, Atom. Data Nucl. Data Tables 32 (1985) 1. A.N. Chaika, et al., Phys. Rev. B 64 (2001) 125121. F. Gerken, J. Phys. F: Met. Phys. 13 (1982) 703. R. Mallik, E.V. Sampathkumaran, J. Magn. Magn. Mater. 164 (1996) L13. . S. Hufner, in: L. Ley, M. Cardona (Eds.), Photoemission in Solids II, Springer, Berlin, 1987, p. 196. A. Szytu"a, et al., J. Magn. Magn. Mater. 222 (2000) 47. S. Nohara, et al., Physics B 186–188 (1993) 403. P.G. de Gennes, J. Phys. Radium 23 (1962) 510. D.R. Noakes, G.H. Shenoy, Phys. Lett. A 91 (1982) 35.
Electronic structure and magnetism of rare-earth intermetallic compounds A. Szytu"aa,*, A. Jezierskib, B. Penca b
a ! Poland M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, ! Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznan,
Abstract The electronic structure of R2PdSi3 (R=Ce, Nd, Tb, Dy, Ho, Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga) compounds was studied by X-ray and ultraviolet photoemission spectroscopy. For the R2PdSi3 (R=Nd, Tb–Er) compounds, the valence band is dominated by the multiplet structure of the rare-earth atoms and by the Pd 4d band at E=4 eV. In these compounds, the density of states at the Fermi level is low and the Ruderman–Kittel–Kasuya–Yosida interaction seems to play a minor role in the magnetic interactions. The valence bands of the HoTX compound are also dominated by the Ho 4f and the transition metal nd (n=3,4) states. The multiplet structure of the Ho 4f states corresponds to that of the Ho3+ ion for all these compounds. The states at the Fermi level contain contributions from Ho (5d6 s) for T=Ag and from additional nd states of the T element for compounds with T=Co, Ni, Pd and Rh. r 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 71.28.+d; 73.20.At Keywords: Rare-earth intermetallics; Electronic structure; XPS; UPS
1. Introduction The magnetic properties of rare-earth ternary intermetallic compounds have been the subject of intensive research. It is generally accepted that the collective behaviour of the magnetic rare-earth ions in these compounds is determined by an indirect interaction among localised f electrons, which are coupled by the conduction electrons according to the Ruderman–Kittel–Kasuya–Yosida (RKKY) theory [1,2]. In the RKKY theory, the *Corresponding author. Tel.: +48-12-6324888; fax: +48-126337086. E-mail address: [email protected] (A. Szytu"a).
exchange integral JðRÞ is proportional to the density of electronic states at the Fermi level. In order to investigate the influence of the electronic structure at the Fermi level on the magnetic properties, this work reports the results of X-ray and ultraviolet photoemission spectroscopy measurements of two systems R2PdSi3 (R=Ce, Nd, Tb–Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga). The X-ray photoemission spectroscopy valence band spectra are compared with ab initio electronic structure calculations using the tight-binding linear muffintin orbital (TB LMTO) method [3] and with the results of magnetic and neutron diffraction measurements that are briefly presented below.
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The neutron diffraction data reveal that R2PdSi3 compounds crystallise in the hexagonal AlB2-type of structure. In this structure, the atomic arrangement leads to two different kinds of layers in the unit cell: R layers at z=0 and Pd– Si layers at z=12: Magnetic and neutron diffraction data indicate the following magnetic properties of the R2PdSi3 compounds: *
*
*
for R=Ce below T=2.5 K, a cluster spin glass is found, while for Nd below Tc=16 K, ferromagnetic order is observed; for Tb below TN=23 K, a coexistence of a ferromagnetic spiral structure and spin-glass behaviour is observed; the rare-earth moments of the Dy, Ho and Er compounds form sine-wave modulated structures [4].
HoTX compounds crystallise in the orthorhombic TiNiSi-type structure. R, T and X atoms occupy the positions of Ti, Ni and Si atoms, respectively. The R and T atoms form columns of trigonal prisms with X atoms inside. The columns are linked by the R–R edges into infinite crimped slabs placed perpendicular to the c-axis. In the case of HoTX compounds, magnetic and neutron diffraction measurements give that: *
*
*
*
HoAgGa and HoCoSi are ferromagnets with the Curie temperatures equal to 16 and 13 K, respectively [5,6]; HoNiSi is collinear antiferromagnet with the Ne! el temperature 4.1 K. Near the Ne! el temperature, the change of the magnetic structure to the sine-wave modulated is observed [7]; HoCoSn and HoPdSn are antiferromagnets with Ne! el temperatures equal to 7.6 and 3.5 K, respectively; they exhibit modulated magnetic structures [8,9]; HoRhSi and HoRhGe are collinear antiferromagnets with a Ne! el temperature equal to 8 and 4.6 K, respectively [10,11].
Neutron diffraction data indicate that in all these compounds, the magnetic moment is localized on the rare-earth atoms.
2. Experimental details The R2PdSi3 (R=Ce, Nd, Tb–Er) and HoTX (T=Co, Ni, Rh, Pd, Ag; X=Si, Ge, Sn, Ga) samples were arc melted from the constituent metals in a cooled copper crucible in a high-purity argon atmosphere, remelted several times and then annealed at 8001C for 1 week. Their phase purities were checked by X-ray Debye–Scherrer diffraction with CoKa radiation. The diffraction data indicate that the R2PdSi3 compounds crystallise in the hexagonal AlB2-type of structure while the HoTX compounds do so in the orthorhombic TiNiSi-type one. Photoemission measurements were performed using a commercial LHS10 SPECS spectrometer with hemispherical energy analyzer using MgKa (hn ¼ 1253:6 eV) radiation and with a helium discharge lamp for ultraviolet photoemission spectroscopy (HeI: hn=21.2 eV, HeII: hn=40.8 eV). Measurements were made at room temperature in high vacuum (2 109 mbar). The total energy resolution was about 0.8 eV for Ag3d peaks. The spectrometer was calibrated using Cu2p, Ag3d and Au4f core-level photoemission spectra. Band structure calculations for Ce2PdSi3 were carried out with the use of the tight-binding linear muffintin orbital (TBLMTO) method [3] within the framework of the local spin density approximation (LSD). The scalar-relativistic approximation for band electrons and the fully relativistic treatment of the frozen potential was assumed according to von Barth and Hedin [12] with gradient corrections [13]. The self-coexistent calculations were performed in the atomic sphere approximation (ASA) for the experimental values of the lattice parameters and for 217 k-points in the irreducible edge of the Brillouin zone. 3. Results The XPS spectra of the valence bands of R2PdSi3 compounds (see Fig. 1) indicate that they are dominated by the contribution of the rareearth 4f states. At the Fermi level E ¼ 0 eV; the contribution to the density of states, except for Cecompounds, is found to be dominated by the R (5d6 s) band. In Fig. 2, we present the calculated partial and total density of states (DOS) for
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2
4
6
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Ce2PdSi3. The 4f state of Ce atoms are close to the Fermi level, while the 4d band of Pd atoms is localized 4 eV below the Fermi level. The Si-3 s state forms a narrow peak at E=8 eV while the Si3p state forms broad bands which hybridize with the 4d band of Pd atoms. Experiments on this compound detected the Ce-4f state near the Fermi level as well as the Pd-4d band. The Ce2PdSi3 valence band spectrum shows a narrowing and mowing away of the Pd-4d states from the Fermi level (GFWHM=1.8 eV, BE= 4.0 eV) in comparison with pure Pd (GFWHM= 4.1 eV, BE=0.95 eV). The positions of the Pd-4d peaks do not change while changing the rare-earth element (see also Fig. 3 in Ref. [16]). The calculated DOSs at the Fermi level of Ce2PdSi3, equal to 6.82 states/(eV cell), is higher than that determined from the electronic specific heat g (about 100 mJ mol1K2) which leads to about 4.3 states/(eV cell) [18]. For compounds with heavier rare earths, the valence band photoelectron spectra are characterised by intense emissions related to the 4f finalstate multiplets [17]. The peaks corresponding to the final state multiplet appear to exhibit a similar shape for the valence band photoemission and are slightly shifted towards the Fermi level. The shift to the lower binding energy, in comparison with the pure rare-earth elements, is probably due to the chemical shift connected with the different chemical environment in the compounds. These data suggest that the rare-earth ions keep the 3+ valence in these compounds. This is in agreement
14
Pure R element R2PdSi3
Ce
Number of counts [a. u.]
Nd
Tb
Dy
Ho
Er
-2
0
2
4
6
8
10
12
14
Binding Energy [eV] Fig. 1. XPS valence band spectra for R2PdSi3 (R=Ce, Nd, Tb, Dy, Ho, Er) compounds and adequate spectra for the metallic rare earths from Ref. [14].
60 EF
DOS [states / eV]
DOS [states / eV]
EF 30 Pd
Ce
INTENSITY [ arb. units]
-2
173
30
Si 0
(a)
-8 -4 ENERGY [eV]
0
0
-8
(b)
-4 ENERGY [eV]
0
Fig. 2. (a) The contribution of Ce (6s6p5d4f), Pd (4d) and Si (3s3p) and (b) to the total DOSs of Ce2PdSi3. The Fermi level is located at E=0 eV. The dashed curve presents the DOSs convolved by Lorentzians of the half-width 0.4 eV and multiplied by the appropriate cross sections [15].
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Ga 3d3/2,5/2 Ag 4d
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HoAgGa
Ho 5p3/2 Ho 5p1/2
HoCoSi
HoCoSn
Number of counts [a. u.]
Y Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis TitleY Axis Title
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Sn 4d3/2,5/2
HoNiSi
HoPdSn Sn 4d3/2,5/2
HoRhGe Ge 3d3/2,5/2
HoRhSi
0
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Binding Energy [eV] Fig. 3. XPS spectra of the valence bands of HoTX compounds and metallic holmium [19].
with the results of magnetic measurements, which give values for the effective magnetic moments close to the R3+ ion values. Fig. 3 shows the XPS valence bands of the HoTX compounds and for pure Ho. The UPS spectra for some of these compounds and for pure Ho are shown in Fig. 4. A detailed analysis of the XPS data give the following information. The valence bands of these compounds are dominated by the multiplet structure of the Ho3+ ion and the nd (n=3,4) states of the transition T metals. Also for these compounds, a shift of the multiplet state to a lower binding energy, in comparison with pure Ho, is observed. The energy shift is between 0.3 eV for HoRhSi and HoRhGe and 1.2 eV for HoAgGa and HoPdSn. This fact indicates a sensitivity of the electronic states of holmium to the other elements (T, X) in the investigated compounds. In the case of HoAgGa, the results
could indicate a possible hybridization of Ho-4f and Ag-4d. Such a hybridization should affect the 4f shell spin moment, which is smaller than the free Ho3+ ion value (3.5mB) [5]. The Ag-4d states form a relatively narrow peak (after substitution of the Ho-4f peak) with a maximum at 6.5 eV and a fullwidth at half-maximum of 1.9 eV. It differs significantly from the pure Ag-4d spectrum, which has a much broader maximum at about 5 eV [19]. In RAgSn (R=Pr,Nd) compounds, the experimental and calculated data give the peak of Ag-4d near 6.0 eV [20]. For the other compounds, the XPS and UPS spectra indicate that the binding energy of the Co-3d band is equal to 1.5 eV in HoCoSi and to 1.4 eV in HoCoSn, that the Ni-3d band in HoNiSi is equal to 2.3 eV, whose number is equal to 4.2 eV for Pd-4d in HoPdSn and that the Rh-4d band is equal to 2.4 eV in HoRhGe and to 3.0 eV in HoRhSi. In the iso-structural CeTX
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5
10
Y Axis Title
HoCoSi
Number of counts [a. u.]
Y Axis Title Y Axis Title
20
HoAgGa
and Ag and X=Si, Ge, Sn and Ga. The valence band spectra of all (except for R=Ce) compounds are dominated by the multiplet of the 4f final state and the nd state of the T element. The obtained results suggest that: *
HoCoSn
*
HoPdSn
*
HoRhSi
Y Axis Title
Y Axis Title
15
-5
0
5
10
15
20
Binding Energy [eV]
*
Fig. 4. UPS spectra of the valence bands of HoTX compounds and pure Ho.
compounds, the Ni-3d peak is near 1.5 eV for CeNiSn while the Pd-4d peak is at 3.5 eV in CePdSn [21]. The positions and shapes of the d bands indicate that these bands are filled and that the 3d and 4d atoms do not have localised magnetic moments. These results are in good agreement with the neutron diffraction data, which give a localised magnetic moment only on the rare-earth atoms. Near the Fermi level, the states of Ho (5d6 s) for T=Pd and Ag are observed and additional nd states of the T metal for T=Co, Ni, Rh and Pd.
4. Conclusions The paper reports on investigations of the electronic structure of R2PdSi3, R=Ce, Nd, Tb, Dy, Ho and Er and of HoTX, T=Co, Ni, Pd, Rh
175
*
*
The contribution to the density of states at EF is found to be dominated by R-5d. For HoCoX (X=Si, Sn), HoPdSn and HoRhX (X=Si, Ge) also the nd states of transition T metal are found to contribute. The density of states at the Fermi level, particularly for R2PdSi3, do not change while changing R elements (except for R=Ce). For R2PdSi3, the position of the peak corresponding to the Pd-4d state does not change while changing the R element. The position of this peak is in good agreement with the calculated one. The positions and shapes of the 3d and 4d bands indicate that these bands are filled and do not have any localized magnetic moments. This result is in good agreement with the neutron diffraction data, which indicate localized magnetic moments only on the rare-earth atoms. The hybridization of the Ho-4f and Ag-4d states is responsible for the decrease of the Ho magnetic moment in this compound. Similar hybridization effects of R-4f and Pd-4d states are observed in R2PdSi3 (R=Nd, Dy) compounds. For both compounds, the R moment is smaller than the R3+ ion value. For the other compounds, hybridization is less probable. For Ce2PdSi3, the Ce-4f states are near the Fermi level. An analysis of the Ce-3d core level indicates a trivalent character of Ce [16]. In both series of compounds, the large R3+– R3+ separation suggests that direct magnetic interactions are highly improbable. The stability of the observed magnetic ordering scheme results from indirect interactions, probably of the RKKY-type. The linear thermal dependence of the resistivity [4] indicates metallic character of R2PdSi3 compounds and the presence of conduction electrons. In the RKKY model, the critical temperature of the magnetic ordering is proportional to the de Gennes function G ¼ ðgJ 1Þ2 JðJ þ 1Þ [22]. However,
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the experimentally observed ordering temperatures of the R2PdSi3 (R=Gd–Er) compounds do not follow the de Gennes scaling. This suggests that the main interaction leading to the magnetic order in these systems is not purely of the RKKY-type but is modified, for example, by crystalline electric field effects which significantly influence the magnitudes of the critical temperature [23].
References [1] C. Kittel, in: F. Seitz, D. Turnbull, H. Ehrenreich (Eds.), Solid State Physics, Vol. 22, Wiley, New York, 1968, p. 1. [2] T. Kasuya, in: G.T. Rado, H. Suhl (Eds.), Magnetism, Vol. IIB, Academic Press, New York, 1966, p. 215. [3] O.K. Andersen, O. Jepsen, Phys. Rev. Lett. 53 (1984) 2571. [4] A. Szytu"a, et al., J. Magn. Magn. Mater. 202 (1999) 365. [5] A. Zygmunt, et al., J. Magn. Magn. Mater. 161 (1996) 127.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
J. Leciejewicz, et al., J. Alloys Compounds 240 (1996) 164. A. Szytu"a, et al., J. Magn. Magn. Mater. 191 (1999) 122. G. Andr!e, et al., J. Alloys Compounds 221 (1995) 25. G. Andr!e, et al., Acta Phys. Polon. A 85 (1994) 275. W. Ba’zela, et al., J. Magn. Magn. Mater. 50 (1985) 19. W. Ba’zela, et al., J. Phys.: Condens. Matter. 10 (1998) 2233. V.von U. Barth, L. Hedin, J. Phys. C 5 (1972) 391. D. Hu, D.C. Langreth, Phys. Scr. 32 (1985) 391. J.K. Lang, et al., J. Phys. F: Metal Phys. 11 (1981) 121. J.J. Yeh, I. Lindau, Atom. Data Nucl. Data Tables 32 (1985) 1. A.N. Chaika, et al., Phys. Rev. B 64 (2001) 125121. F. Gerken, J. Phys. F: Met. Phys. 13 (1982) 703. R. Mallik, E.V. Sampathkumaran, J. Magn. Magn. Mater. 164 (1996) L13. . S. Hufner, in: L. Ley, M. Cardona (Eds.), Photoemission in Solids II, Springer, Berlin, 1987, p. 196. A. Szytu"a, et al., J. Magn. Magn. Mater. 222 (2000) 47. S. Nohara, et al., Physics B 186–188 (1993) 403. P.G. de Gennes, J. Phys. Radium 23 (1962) 510. D.R. Noakes, G.H. Shenoy, Phys. Lett. A 91 (1982) 35.