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Photoelectron spectroscopy and magnetism of some gadolinium intermetallic compounds
Journal of
A~OY5
A N D COMPOUND~ ELSEVIER
Journal of Alloys and Compounds 236 (1996) 132-136
Photoelectron spectroscopy and magnetism of some gadolinium intermetallic compounds J. S z a d e a, M. N e u m a n n b alnstitute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland blnstitute of Physics, University of Osnabriick, Barbarastr. 7, 49069 Osnabriick, Germany Received 7 June 1995; in final form 15 October 1995
Abstract
X-ray photoelectron spectroscopy was used to investigate Gd intermetallic compounds: GdCu, GdAg, Gd2In and GdA12. The measured electron density of states close to the Fermi level was compared with the magnetic properties of these compounds. The exchange splittings of the gadolinium 4s and 5s levels were analysed and found to be about the same for the investigated compounds. For GdCu the present results were compared with the existing band structure calculation. Keywords: Rare earth intermetallics; X-ray photoelectron spectroscopy; Magnetic moment
1. Introduction Photoelectron spectroscopy is a widely used technique for investigating the electronic structure of rare earths and their compounds. Recently, most of research has been focused on examination of thin films of rare earths deposited on different substrates and many interesting results have been obtained, mainly from gadolinium overlayers [1,2]. The presence of gadolinium 5d electronic states near the Fermi level was verified for Gd single crystal [3] and Gd thin films [1,2]. The 5d electrons in rare earths are supposed to play an important role not only in multielectron photoexcitation processes, but also in magnetic interactions. They are involved in indirect exchange between localized 4f shell magnetic moments. The 5d electrons are known to be the main origin of the conduction electron polarization [4]. The large, excess effective magnetic moment, relative to the free ion moment, observed in many rare earth intermetallic compounds, seems to be related mainly to 5d electron polarization [5]. In this paper we present the results of X-ray photoelectron spectroscopy (XPS) investigations on four gadolinium intermetallic compounds. They exhibit different magnetic properties and we also found differences in the valence band features. In addition, we analysed the splitting of the gadolinium 4s and 5s core 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD1 0925-8388(95 )02033-0
level spectra. It is known that the s core levels are exchange split due to the interaction with the localized 4f electron spin, but a comparison with theoretical calculations yields good agreement only for the 5s shells [6]. Our results are compared with data obtained for gadolinium thin films and overlayers.
2. Experimental The measurements were made with a Perkin-Elmer (PHI) 5600 ci ESCA photoelectron spectrometer, in a vacuum of about 10 -1° Torr, using monochromatized A1 K a radiation. The energy resolution was 0.35 eV. The photoelectrons were collected normal to the sample surface. GdCu, GdAg, GdzIn and GdAI 2 samples were obtained by the Czochralski method from a cold crucible or from a levitated melt. They were either monocrystalline or consisted of large grains. The standard X-ray diffraction patterns and the Berg-Barrett method were used to check their quality. Owing to the high chemical reactivity of rare earths and their compounds, the samples were broken or scraped with a diamond file under high vacuum immediately before taking a spectrum. Some of the samples were also cleaved in air before introducing them to the U H V chamber. Soft sputtering with argon ions was then used to obtain a clean surface. No
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132--136
significant differences were found in the results obtained for the same sample prepared by the different methods. The spectra were taken within less than 20-30 min in order to obtain spectra of the gadolinium 4f and 5p peaks that had not been influenced by contamination. The shape of these peaks is very sensitive to oxidation. The Cls and Ols levels were controlled all the time and were kept as small as possible. Within the experimental error bars the compositions of the samples were in agreement with the nominal stoichiometries.
3. Results and discussion 3.1. Vale,nce band
The comparison of the valence band XPS spectra from the four different Gd compounds (Fig. 1) reveals . . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
•
Gd 4 ~ ~ A g 4(
3.0
Gd 5p3~2
2.5
/~V
/
GdAg 2.0
GdCu
-~ 1.5
Gd2In GdFII2 , i
35
30
.
.
.
.
t
25
.
.
.
.
i
20
.
.
.
.
i
.
.
.
.
15
i
I0
.
.
.
.
i
5
.
.
.
.
i
.
0
Bir~dir~ energg (eU) Fig. 1. "Valence band XPS spectra for gadolinium compounds.
133
that the position of the Gd 4f peak is hardly affected by the presence of a nonmagnetic alloyed metal. It changes from 8.3 eV for GdA12 to 8.7 eV for Gd2In; this fact indicates the low sensitivity of these electronic states to alloying gadolinium with other metals. It is worth noting that in ionic compounds, e.g. with O, F or CI, this peak is shifted to higher binding energies by as much as 2 eV [7]. The shape of the 4f peak in metallic compounds is, therefore, an indication of the presence of gadolinium oxide on the investigated surface. Our results indicate no large asymmetrical broadening, which suggests that only a low amount of oxygen was present at the surface during the measurements. In the case of GdAg the result could indicate the possible hybridization of Gd 4f and Ag 4d states, although the spatial distance between the core-like 4f electrons and also the localized Ag 4d electrons seems to be too large for the appearance of this effect. Such hybridization should affect the 4f shell spin moment, but the effective magnetic moment found in GdAg is close to that found in the isostructural GdCu (Table 1). The Ag 4d states in GdAg form a relatively narrow and symmetrical peak (after subtraction of the Gd 4f peak) with a maximum at about 6 eV and a full width at half maximum (FWHM) of 1.9 eV. It differs significantly from the pure Ag 4d spectrum, which is much broader and has its maximum at about 5 eV [8]. This could be due to the chemical interaction between those two metals and the larger distance between silver atoms in the CsCl-type GdAg structure compared with that in the pure Ag metal. A similar situation is found in GdCu, where the Cu 3d band is visible as a symmetrical peak with a maximum at 3.8 eV and FWHM of 1.9 eV. In this case we obtained a good agreement with the results of LaGraffe et al. [9] for Gd overlayers on the Cu substrate. Owing to the strong interaction (bonds) between Gd and Cu, they found the shifts in binding energy relative to the pure metals for the Gd 4f and Cu 3d levels. Their resulting binding energies are the same (within the +__0.1eV range) as we found in the GdCu compound. Comparing the experimental data with the recently calculated electronic structure of GdCu (in the CsC1-
Table 1 Selected raagnetic properties of the investigated c o m p o u n d s C o m p o u n :1
Structure
Magnetic transition (K)
Effective m o m e n t per G d atom (/-q3)
Excess effective m o m e n t (/-q3) A/x = ~ - 7.94
GdCu GdAg Gd2In
cubic CsC1 cubic CsC1 hex. Ni2In
8.49 [11] 8.59 [5] no Curie-Weiss [15]
0.55 0.65
GdAI 2
cubic M g C u 2
-TN= Tc = TN = Tc =
7.94 [18]
0
136 [5] 187 [16] 99.5 168 [17]
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132-136
134
type of structure) [10] shows a discrepancy in the binding energy of some states. A calculation performed using density functional theory and the LMTO method led to a binding energy of the occupied Gd 4f states at about 4 eV and of the Cu 3d states at about 2.7 eV. The much lower calculated binding energy of the 4f level than in the experiment is a well-known observation. The difference of 1.1 eV between the experimental and calculated binding energies of the Cu 3d levels might be explained by self energy corrections, although the deviation appears to be large. However, the interesting conclusion which was drawn from this paper concerns the d-contribution to the Gd magnetic moment which is in agreement with our earlier results of magnetic measurements [11] of a GdCu single crystal. We found the effective magnetic moment to be much larger than that calculated theoretically for the 4f shell only; the excess moment is about 0.55 p~ for the CsCl-type structure. An even larger excess moment was found in GdAg (Table 1). We propose that this moment is mainly connected with 5d electrons and that the shape of the 5d band, observed in this investigation for all compounds (Fig. 2), may be related to 5d magnetism. The exchange splitting of the Gd 5d states was experimentally found in bulk Gd [12] and in Gd overlayers [2] in the ferromagnetic state. Additionally, enhancement of 5d magnetism on the Gd surface was found, which is caused by the lower coordination
Q
V
(/) r
O t-
,, ,~ • o
2
GdAg GdCu GdAI_ Gd211~
1
0
Binding Energy (eV) Fig. 2. Valence band XPS spectra in the vicinity of the Fermi level. Intensity of spectra were normalized relative to the intensity of Gd 4f peak (in the case of GdAg after subtraction of the Ag 4d peak).
number. Alloying of gadolinium with other metals can also decrease this number and, as a consequence, enhance the 5d magnetism. It should also influence the electronic structure of 5d electrons close to the Fermi level. Our measurements were performed at room temperature, where all investigated compounds were in the paramagnetic state. However, a recent calculation of Sandratskii and Kiibler [13] for Gd metal indicates that the magnetic moment, induced in the conduction band, is rather stable, even above the magnetic ordering temperature. We suppose that this is also the case for Gd intermetallic compounds and it is manifested by the excess effective moment. The relatively large contribution to the XPS spectrum in the vicinity of the Fermi level (Figs. 1 and 2) can be related to the d-like states, although the calculation for GdA12 by Magnitskaya et al. [14] revealed a significant contribution from the f states. One can expect that the f electron contribution should not change from one compound to another. Therefore, the shape of the valence bands, visible in Fig. 2, may be attributed to the d electrons. The photoexcitation sensitivity factors for s and p electrons are very low. Intensity of spectra collected in Fig. 2 were normalized relative to the intensity of the Gd 4f peak (in the case of GdAg after the subtraction of the Ag 4d peak). The largest density of states in the vicinity of E F is visible for GdCu. It might be caused by the presence of Cu 3d states which are close in energy. A similar spectrum shape was found in GdAg, which is isostructural with GdCu. It is worth noting that a pronounced band with a maximum at about 1 eV is visible in Gd2In. In this compound an unusual behaviour of magnetic properties was found [15,16]--a metamagnetic transition from ferro- to antiferromagnetic order on cooling and no Curie-Weiss behaviour at high temperatures (up to 700 K). Such properties are unusual for gadolinium compounds; one possible explanation is the presence of the narrow d band with a temperature-dependent structure. The magnetic moment formed in the exchange split d band may be responsible for the temperature changes of the total measured effective moment and no Curie-Weiss behaviour at high temperatures. In contrast, for GdA12, the shape of the measured valence band exhibits a relatively broad feature, characterized by a slightly increasing slope starting from a binding energy of 3-4 eV towards the Fermi level, which is similar to Gd metal XPS data [6]. In both cases the excess effective moment is close to zero. Our results for GdAI 2 are in agreement with the XPS data from [16]. The correlation found in our paper should be confirmed by the photoelectron experiments using lower excitation energy and a higher resolution.
J. Szade, M. Neumann I Journal of Alloys and Compounds 236 (1996) 132-136
135
Table 3 Gadolinium 5s level exchange splitting Compound
Binding energy of the main peak (with lower BE) (eV)
Exchange splitting (eV)
Intensity ratio of the satellite to the main peak
GdCu Gd2In GdAI 2 GdAg
43.5 44 43 44
3.5 3 3.5 3.5
0.8 0.7 0.7 0.7
L
v
GdCu ¢0
E
i.w
Gd 5s
Gd21n
GdR12 GdCu qgO
385
380
Bindir~ ec~rq~
375
370
,L v
(tO)
~-~IGdAg
Fig. 3. Gadolinium 4s level exchange splitting.
." " "" I
C
E
3.2. Exc,hange splitting of s core levels
Fig. 3 and Table 2 show the spectra and the collected data concerning the exchange splitting of the 4s core levels for three gadolinium compounds. In general, the value of the splitting and the intensity ratio are close to each other. The binding energy of the Gd 4s level is very close to the energy of Ag 3d states, so in the case of GdAg it was not possible to deconvolute these peaks. The observed splitting of 8 eV in GdCu, Gd2In and GdAI 2 is in agreement with results for Gd metal and GdSb [6]. This confirms that this multiplet splitting is insensitive to the chemical state of gadolinium. This also confirms that the reduction of the splitting, obtained from the atomic Slater integrals (14 eV), is a common feature for gadolinium and its compounds. It is difficult to observe the 5s level splitting owing to the low photoionization cross-section and the necessity to measure the spectra in a short time. However, the results collected in Table 3 and shown in Fig. 4 indicate the universal character of this phenomenon in
Table 2 Gadolinitm 4s level exchange splitting Compound
Binding energy of the main peak (with lower BE) (eV)
Exchange splitting (eV)
Intensity ratio of the satellite to the main peak
GdCu Gd2In GdA12
378 378 378
8 8 8
0.7 0.7 0.9
Gd21n GdAI 2
• ....-
50
45
40
Bindinq en~.-qy (~AJ) Fig. 4. Gadolinium 5s level exchange splitting•
all investigated compounds. The values of the splitting (obtained with an accuracy of about 0.5 eV) are in good agreement with the data reported for GdSb and GdF 3 [6]. They also correlate with the value of 3.9 eV obtained from the resonant photoemission investigation of Gd on Cu and calculated splittings using Slater exchange integrals and 4f spin S = 7/2 [19]. From the multiplicities of the 9S and the 7S final states, an intensity ratio of 7/9 = 0.78 was obtained which agrees with our results. Our XPS investigations of several gadolinium intermetallic compounds show an interesting correlation between the valence band shape in the vicinity of the Fermi level and the magnetic moment. More sophisticated methods using synchrotron radiation and spin polarized photoemission should be applied to the bulk compounds which are magnetically well-characterized.
136
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132-136
Acknowledgements Technical help from T. Albers and S. M~ihl from the Fachbereich Physik of the University of Osnabriick is greatly appreciated. The data were elaborated using software written by C. Scharfschwerdt. Financial support from the BMBF and the Deutscher Akademischer Austauschdienst ( D A A D ) is gratefully acknowledged.
References [1] D. LaGraffe, RA. Dowben and M. OneUion, Phys. Rev. B, 40 (1989) 970. [2] D. Li, C.W. Hutchings, P.A. Dowben, C. Hwang, R.-T. Wu, M. OneUion, A.B. Andrews and J.L. Erskine, J. Magn. Magn. Mater., 99 (1991) 85. [3] F.J. Himpsel and B. Reihl, Phys. Rev. B, 28 (1983) 574. [4] B.I. Min, H.J. Hansen, T. Oguchi and A.J. Freeman, J. Magn. Magn. Mater., 61 (1986) 139. [5] J. Szade, J. Less-Common Met., 136 (1987) 101.
[6] M. Campagna, G.K. Wertheim and Y. Baer, in L. Ley and M. Cardona (eds.), Photoemission in Solids H, Springer, Berlin, 1987, p. 250. [7] D. Raiser and J.R Deville, J. Electron Spectrosc. Relat Phenom., 57 (1991) 91. [8] S.Htifner in Photoemission in Solids II, edited by L.Ley and M.Cardona (Springer, Berlin, 1987), p.196. [9] D. LaGraffe, RA. Dowben and M. Onellion, Phys. Rev. B, 40 (1989) 3348. [10] A.V. Postnikov, V.R Antropov and O. Jepsen, J. Phys.: Condens. Matter, 4 (1992) 2475. [11] A. Chetkowski, E. Talik, J. Heimann and J. Szade, Physica B, 130 (1985) 231. [12] B. Kim, A,B. Andrews, J.L. Erskin, K.J. Kim and B.N. Harmon, Phys. Rev. Lett., 68 (1992) 1931. [13] L.M. Sandratskii and J. Kiibler, Europhys. Lett., 23 (1993) 661. [14] M. Magnitskaya, G.Chetkowska, G. Borstel, M. Neumann and H. Ufer, Phys. Rev. B, 49 (1994) 1113. [15] J. Szade, in preparation. [16] S.P. McAlister, J. Phys. F:, 14 (1984) 2167. [17] A. Chelkowski, E. Talik and G. Wn~trzak, Solid State Commun., 46 (1983) 759. [18] G. Chelkowska, J. Magn. Magn. Mater., 127 (1993) L37. [19] J.A. Scarfe, A.R. Law, H.P. Hughes, J.A.C. Bland, G.M. Roe and A.P. Walker, Phys. Status Solidi B, 171 (1992) 377.
A~OY5
A N D COMPOUND~ ELSEVIER
Journal of Alloys and Compounds 236 (1996) 132-136
Photoelectron spectroscopy and magnetism of some gadolinium intermetallic compounds J. S z a d e a, M. N e u m a n n b alnstitute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland blnstitute of Physics, University of Osnabriick, Barbarastr. 7, 49069 Osnabriick, Germany Received 7 June 1995; in final form 15 October 1995
Abstract
X-ray photoelectron spectroscopy was used to investigate Gd intermetallic compounds: GdCu, GdAg, Gd2In and GdA12. The measured electron density of states close to the Fermi level was compared with the magnetic properties of these compounds. The exchange splittings of the gadolinium 4s and 5s levels were analysed and found to be about the same for the investigated compounds. For GdCu the present results were compared with the existing band structure calculation. Keywords: Rare earth intermetallics; X-ray photoelectron spectroscopy; Magnetic moment
1. Introduction Photoelectron spectroscopy is a widely used technique for investigating the electronic structure of rare earths and their compounds. Recently, most of research has been focused on examination of thin films of rare earths deposited on different substrates and many interesting results have been obtained, mainly from gadolinium overlayers [1,2]. The presence of gadolinium 5d electronic states near the Fermi level was verified for Gd single crystal [3] and Gd thin films [1,2]. The 5d electrons in rare earths are supposed to play an important role not only in multielectron photoexcitation processes, but also in magnetic interactions. They are involved in indirect exchange between localized 4f shell magnetic moments. The 5d electrons are known to be the main origin of the conduction electron polarization [4]. The large, excess effective magnetic moment, relative to the free ion moment, observed in many rare earth intermetallic compounds, seems to be related mainly to 5d electron polarization [5]. In this paper we present the results of X-ray photoelectron spectroscopy (XPS) investigations on four gadolinium intermetallic compounds. They exhibit different magnetic properties and we also found differences in the valence band features. In addition, we analysed the splitting of the gadolinium 4s and 5s core 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD1 0925-8388(95 )02033-0
level spectra. It is known that the s core levels are exchange split due to the interaction with the localized 4f electron spin, but a comparison with theoretical calculations yields good agreement only for the 5s shells [6]. Our results are compared with data obtained for gadolinium thin films and overlayers.
2. Experimental The measurements were made with a Perkin-Elmer (PHI) 5600 ci ESCA photoelectron spectrometer, in a vacuum of about 10 -1° Torr, using monochromatized A1 K a radiation. The energy resolution was 0.35 eV. The photoelectrons were collected normal to the sample surface. GdCu, GdAg, GdzIn and GdAI 2 samples were obtained by the Czochralski method from a cold crucible or from a levitated melt. They were either monocrystalline or consisted of large grains. The standard X-ray diffraction patterns and the Berg-Barrett method were used to check their quality. Owing to the high chemical reactivity of rare earths and their compounds, the samples were broken or scraped with a diamond file under high vacuum immediately before taking a spectrum. Some of the samples were also cleaved in air before introducing them to the U H V chamber. Soft sputtering with argon ions was then used to obtain a clean surface. No
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132--136
significant differences were found in the results obtained for the same sample prepared by the different methods. The spectra were taken within less than 20-30 min in order to obtain spectra of the gadolinium 4f and 5p peaks that had not been influenced by contamination. The shape of these peaks is very sensitive to oxidation. The Cls and Ols levels were controlled all the time and were kept as small as possible. Within the experimental error bars the compositions of the samples were in agreement with the nominal stoichiometries.
3. Results and discussion 3.1. Vale,nce band
The comparison of the valence band XPS spectra from the four different Gd compounds (Fig. 1) reveals . . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
i
•
Gd 4 ~ ~ A g 4(
3.0
Gd 5p3~2
2.5
/~V
/
GdAg 2.0
GdCu
-~ 1.5
Gd2In GdFII2 , i
35
30
.
.
.
.
t
25
.
.
.
.
i
20
.
.
.
.
i
.
.
.
.
15
i
I0
.
.
.
.
i
5
.
.
.
.
i
.
0
Bir~dir~ energg (eU) Fig. 1. "Valence band XPS spectra for gadolinium compounds.
133
that the position of the Gd 4f peak is hardly affected by the presence of a nonmagnetic alloyed metal. It changes from 8.3 eV for GdA12 to 8.7 eV for Gd2In; this fact indicates the low sensitivity of these electronic states to alloying gadolinium with other metals. It is worth noting that in ionic compounds, e.g. with O, F or CI, this peak is shifted to higher binding energies by as much as 2 eV [7]. The shape of the 4f peak in metallic compounds is, therefore, an indication of the presence of gadolinium oxide on the investigated surface. Our results indicate no large asymmetrical broadening, which suggests that only a low amount of oxygen was present at the surface during the measurements. In the case of GdAg the result could indicate the possible hybridization of Gd 4f and Ag 4d states, although the spatial distance between the core-like 4f electrons and also the localized Ag 4d electrons seems to be too large for the appearance of this effect. Such hybridization should affect the 4f shell spin moment, but the effective magnetic moment found in GdAg is close to that found in the isostructural GdCu (Table 1). The Ag 4d states in GdAg form a relatively narrow and symmetrical peak (after subtraction of the Gd 4f peak) with a maximum at about 6 eV and a full width at half maximum (FWHM) of 1.9 eV. It differs significantly from the pure Ag 4d spectrum, which is much broader and has its maximum at about 5 eV [8]. This could be due to the chemical interaction between those two metals and the larger distance between silver atoms in the CsCl-type GdAg structure compared with that in the pure Ag metal. A similar situation is found in GdCu, where the Cu 3d band is visible as a symmetrical peak with a maximum at 3.8 eV and FWHM of 1.9 eV. In this case we obtained a good agreement with the results of LaGraffe et al. [9] for Gd overlayers on the Cu substrate. Owing to the strong interaction (bonds) between Gd and Cu, they found the shifts in binding energy relative to the pure metals for the Gd 4f and Cu 3d levels. Their resulting binding energies are the same (within the +__0.1eV range) as we found in the GdCu compound. Comparing the experimental data with the recently calculated electronic structure of GdCu (in the CsC1-
Table 1 Selected raagnetic properties of the investigated c o m p o u n d s C o m p o u n :1
Structure
Magnetic transition (K)
Effective m o m e n t per G d atom (/-q3)
Excess effective m o m e n t (/-q3) A/x = ~ - 7.94
GdCu GdAg Gd2In
cubic CsC1 cubic CsC1 hex. Ni2In
8.49 [11] 8.59 [5] no Curie-Weiss [15]
0.55 0.65
GdAI 2
cubic M g C u 2
-TN= Tc = TN = Tc =
7.94 [18]
0
136 [5] 187 [16] 99.5 168 [17]
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132-136
134
type of structure) [10] shows a discrepancy in the binding energy of some states. A calculation performed using density functional theory and the LMTO method led to a binding energy of the occupied Gd 4f states at about 4 eV and of the Cu 3d states at about 2.7 eV. The much lower calculated binding energy of the 4f level than in the experiment is a well-known observation. The difference of 1.1 eV between the experimental and calculated binding energies of the Cu 3d levels might be explained by self energy corrections, although the deviation appears to be large. However, the interesting conclusion which was drawn from this paper concerns the d-contribution to the Gd magnetic moment which is in agreement with our earlier results of magnetic measurements [11] of a GdCu single crystal. We found the effective magnetic moment to be much larger than that calculated theoretically for the 4f shell only; the excess moment is about 0.55 p~ for the CsCl-type structure. An even larger excess moment was found in GdAg (Table 1). We propose that this moment is mainly connected with 5d electrons and that the shape of the 5d band, observed in this investigation for all compounds (Fig. 2), may be related to 5d magnetism. The exchange splitting of the Gd 5d states was experimentally found in bulk Gd [12] and in Gd overlayers [2] in the ferromagnetic state. Additionally, enhancement of 5d magnetism on the Gd surface was found, which is caused by the lower coordination
Q
V
(/) r
O t-
,, ,~ • o
2
GdAg GdCu GdAI_ Gd211~
1
0
Binding Energy (eV) Fig. 2. Valence band XPS spectra in the vicinity of the Fermi level. Intensity of spectra were normalized relative to the intensity of Gd 4f peak (in the case of GdAg after subtraction of the Ag 4d peak).
number. Alloying of gadolinium with other metals can also decrease this number and, as a consequence, enhance the 5d magnetism. It should also influence the electronic structure of 5d electrons close to the Fermi level. Our measurements were performed at room temperature, where all investigated compounds were in the paramagnetic state. However, a recent calculation of Sandratskii and Kiibler [13] for Gd metal indicates that the magnetic moment, induced in the conduction band, is rather stable, even above the magnetic ordering temperature. We suppose that this is also the case for Gd intermetallic compounds and it is manifested by the excess effective moment. The relatively large contribution to the XPS spectrum in the vicinity of the Fermi level (Figs. 1 and 2) can be related to the d-like states, although the calculation for GdA12 by Magnitskaya et al. [14] revealed a significant contribution from the f states. One can expect that the f electron contribution should not change from one compound to another. Therefore, the shape of the valence bands, visible in Fig. 2, may be attributed to the d electrons. The photoexcitation sensitivity factors for s and p electrons are very low. Intensity of spectra collected in Fig. 2 were normalized relative to the intensity of the Gd 4f peak (in the case of GdAg after the subtraction of the Ag 4d peak). The largest density of states in the vicinity of E F is visible for GdCu. It might be caused by the presence of Cu 3d states which are close in energy. A similar spectrum shape was found in GdAg, which is isostructural with GdCu. It is worth noting that a pronounced band with a maximum at about 1 eV is visible in Gd2In. In this compound an unusual behaviour of magnetic properties was found [15,16]--a metamagnetic transition from ferro- to antiferromagnetic order on cooling and no Curie-Weiss behaviour at high temperatures (up to 700 K). Such properties are unusual for gadolinium compounds; one possible explanation is the presence of the narrow d band with a temperature-dependent structure. The magnetic moment formed in the exchange split d band may be responsible for the temperature changes of the total measured effective moment and no Curie-Weiss behaviour at high temperatures. In contrast, for GdA12, the shape of the measured valence band exhibits a relatively broad feature, characterized by a slightly increasing slope starting from a binding energy of 3-4 eV towards the Fermi level, which is similar to Gd metal XPS data [6]. In both cases the excess effective moment is close to zero. Our results for GdAI 2 are in agreement with the XPS data from [16]. The correlation found in our paper should be confirmed by the photoelectron experiments using lower excitation energy and a higher resolution.
J. Szade, M. Neumann I Journal of Alloys and Compounds 236 (1996) 132-136
135
Table 3 Gadolinium 5s level exchange splitting Compound
Binding energy of the main peak (with lower BE) (eV)
Exchange splitting (eV)
Intensity ratio of the satellite to the main peak
GdCu Gd2In GdAI 2 GdAg
43.5 44 43 44
3.5 3 3.5 3.5
0.8 0.7 0.7 0.7
L
v
GdCu ¢0
E
i.w
Gd 5s
Gd21n
GdR12 GdCu qgO
385
380
Bindir~ ec~rq~
375
370
,L v
(tO)
~-~IGdAg
Fig. 3. Gadolinium 4s level exchange splitting.
." " "" I
C
E
3.2. Exc,hange splitting of s core levels
Fig. 3 and Table 2 show the spectra and the collected data concerning the exchange splitting of the 4s core levels for three gadolinium compounds. In general, the value of the splitting and the intensity ratio are close to each other. The binding energy of the Gd 4s level is very close to the energy of Ag 3d states, so in the case of GdAg it was not possible to deconvolute these peaks. The observed splitting of 8 eV in GdCu, Gd2In and GdAI 2 is in agreement with results for Gd metal and GdSb [6]. This confirms that this multiplet splitting is insensitive to the chemical state of gadolinium. This also confirms that the reduction of the splitting, obtained from the atomic Slater integrals (14 eV), is a common feature for gadolinium and its compounds. It is difficult to observe the 5s level splitting owing to the low photoionization cross-section and the necessity to measure the spectra in a short time. However, the results collected in Table 3 and shown in Fig. 4 indicate the universal character of this phenomenon in
Table 2 Gadolinitm 4s level exchange splitting Compound
Binding energy of the main peak (with lower BE) (eV)
Exchange splitting (eV)
Intensity ratio of the satellite to the main peak
GdCu Gd2In GdA12
378 378 378
8 8 8
0.7 0.7 0.9
Gd21n GdAI 2
• ....-
50
45
40
Bindinq en~.-qy (~AJ) Fig. 4. Gadolinium 5s level exchange splitting•
all investigated compounds. The values of the splitting (obtained with an accuracy of about 0.5 eV) are in good agreement with the data reported for GdSb and GdF 3 [6]. They also correlate with the value of 3.9 eV obtained from the resonant photoemission investigation of Gd on Cu and calculated splittings using Slater exchange integrals and 4f spin S = 7/2 [19]. From the multiplicities of the 9S and the 7S final states, an intensity ratio of 7/9 = 0.78 was obtained which agrees with our results. Our XPS investigations of several gadolinium intermetallic compounds show an interesting correlation between the valence band shape in the vicinity of the Fermi level and the magnetic moment. More sophisticated methods using synchrotron radiation and spin polarized photoemission should be applied to the bulk compounds which are magnetically well-characterized.
136
J. Szade, M. Neumann / Journal of Alloys and Compounds 236 (1996) 132-136
Acknowledgements Technical help from T. Albers and S. M~ihl from the Fachbereich Physik of the University of Osnabriick is greatly appreciated. The data were elaborated using software written by C. Scharfschwerdt. Financial support from the BMBF and the Deutscher Akademischer Austauschdienst ( D A A D ) is gratefully acknowledged.
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