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Photoemission studies on metallic glasses using synchrotron radiation
Materials Science and Engineering, 99 (1988) 265 267
265
Photoemission Studies on Metallic Glasses Using Synchrotron Radiation* D. GREIG
Department o["Physics, University of Leeds, Leeds LS2 9JT (U.K.) B. L. GALLAGHER
Department of Physics, University of Nottingham, Nottingham NG 7 2RD (U.K.) M. A. HOWSON
Department o["Physics, UniversiO'of Leeds, Leeds LS2 9JT (U.K.) D. S.-L. LAW, D. NORMAN and F. M. QUINN
Science and Engineering Research Council, Daresbury Laboratory, Daresbury, Warrington WA4 4AD (U.K.)
Abstract
We report on the use o f UV synchrotron radiation in the energy range 40-120 eV to probe the electron states of CuZr, FeZr, CoZr and C u H f amorphous alloys. The great advantage of tunable radiation over fixed frequen 0' sources is that it allows identification of the electron states.from a knowledge o f the energy dependences o f their scattering cross-sections. This variation is particularly pronounced in the case of the d states o f zirconium which are essentially removed from the observed photoelectron spectra as the photon energy is increased .from 4 0 e V to about lOOeV. The results confirm, .for example, that for CuZr alloys it is the d states from zirconium that lie at the Fermi level. By combining these results with those obtained from a technique invoh~ing constant initial states the scattering cross-sections for the Zr 4d and Cu 3d states were investigated. This will allow the experimental photoemission data to be corrected to give a more representative picture of the electronic band structure o f these alloys, and has laid the foundation for extending the method to other newer alloy systems where the electron states have still not been identified. There have been many photoemission studies on transition metal metallic glasses using fixed frequency UV and X-ray sources [1-3], but, to our knowledge, no studies have been performed using tunable high intensity synchrotron UV radiation. The value o f a tunable source is that we can take advantage o f the differing cross-sections for photon absorption of the different transition metals. In particular the cross-section of 4d states, as in the valence band of zirconium,
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3 7, 1987. 0025-5416/88/$3.50
changes by two orders of magnitude over the energy range 20-100 eV. We have used the synchrotron radiation source at Daresbury, U.K., to carry out UV photoemission studies on a series o f metallic glasses based on transition metals. Two main alloy series were investigated. First, the alloys Cu9oZrlo, Fe9oZrlo, C09oZrto and Cu9oHf w were studied in order to look at systematic changes in the late transition metal d band. We also attempted to study N i Z r but unfortunately the alloy was destroyed by surface cleaning. However, preliminary studies were carried out on Ni92Zr8 using a fixed frequency He I line at 21.3 eV. The second series of alloys was Cu30Zr70 and Cu4oZr6o. These alloys have been studied before [3, 4] but only with fixed energy sources. F o r these alloys the tunable energy o f the synchrotron radiation was used to investigate the density of states after taking into account the varying cross-section of the Zr d states with photon energy. The samples were produced by melt spinning in a partial helium atmosphere at the University of Leeds. X-ray diffraction studies showed that all the samples were amorphous with the exception of the FegoZr~o sample for which there was evidence of a few per cent by volume of crystallinity. The valence band spectra were taken in a vacuum of about 8 x 10 1o Torr using a double-pass angle-integrated cylindrical mirror analyser. Photon energies between 40 and 150 eV selected by a plane grating m o n o c h r o m a t o r [5] were used with a resolution in the spectra of about 0.2 eV. F o r all the specimens the initial spectra were dominated by strong oxygen contaminant peaks and so the samples were cleaned in the vacuum chamber by Vigorous mechanical abrasion using a tungsten brush. Argon ion b o m b a r d m e n t was also tried but the tungsten brush was found to be much more effective. There was also the worry that Ar ÷ ion b o m b a r d m e n t ~3 Elsevier Sequoia/Printed in The Netherlands
266 could alter the surface stoichiometry of the sample so that for this reason also mechanical abrasion was thought to be more satisfactory. In Figs. 1 and 2 the photoemission spectra for the alloys Fe9oZrlo, Co9oZrlo , Cu90Zrlo and Cu90Hf]o are shown for photon energies of 40 and 120 eV. The background of secondary electrons and the overlap of higher order light from the monochromator have both been suitably subtracted. For completeness we show in Fig. 3 the spectra of crystalline nickel and amorphous Ni92Zrs at 21.3 eV obtained from a fixed frequency source. We see most clearly in Fig. 2 that, while at low photon energies the Zr d band is a dominant feature of the band structure, it is completely missing at high photon energies and we can easily identify the late transition metal d band. Very broadly the spectra are all very similar to what would be expected for the complementary crystalline alloys with the Fermi level below the top of the 3d band in the iron and cobalt alloys and well above the C u d band in CuZr and CuHf. However, the parts of the spectra associated with these late transition metals do not show the sharp features seen in the d bands of crystalline iron, cobalt, nickel and copper. Thus at the Fermi energy we see a somewhat lower density of states in the amorphous phase than in the crystalline phase. This is very apparent in the comparison of the spectra for the NiZr sample and crystalline nickel. Crystalline nickel has a peak in its density of states at the Fermi energy and is ferromagnetic at room temperature while Ni92Zr8 has a much reduced density of states and is paramagnetic at room temperature. The spectra for Cu3oZr7o and Cu4oZr6o are shown in Fig. 4 for energies of 40 and 120 eV. Again the background has been suitably subtracted. Here we can see even more clearly how the Zr d band is removed from the spectra as the photon energy is increased and the cross-section for photon absorption from the Zr d states is reduced by two orders of magnitude. This shows quite decisively that the C u d and Zr d bands are distinct from each other. The different energy dependences of the cross-sections for the Zr 4d and Cu 3d states are shown more directly in Fig. 5. This figure is the result of scans where the photon energy and photoelectron energy are scanned simultaneously to examine a feature at a fixed energy - - a "constant initial s t a t e " - - b e l o w the vacuum level. The two components of Fig. 5 show clearly how the Zr d cross-section falls dramatically for photon energies above 50 eV while the C u d cross-section does not fall off until much higher energies. The cross-section is a function of the symmetry of the atomic states involved and is changed rather little by solid
j
10
o
,°[
o
-Iz
(a)
+le
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~,
_l+
Energy
6
j,
f
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o
leVI
-¢, -4 -2 E n e r g y (6V)
-12
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o
+12
(d)
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o
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Fig. 1. Photoemission spectra of Fe90Zr]o for incident photons of energy (a) 40 eV and (b) 120 eV and of (c), (d), CogoZr]0at the same two photon energies. The energy scale is relative to the Fermi energy, and the peaks at about - 6 eV are due to oxygen contamination.
~o
a e.
+
+
uo 2
1
o
o
( Q)
[nerg),
(eV)
-I+ Ibl
Energy
(e
o 2
Icl
1
Er~r'gX
leVI
(d)
(eV)
Fig. 2. Photoemission spectra of (a), (b) CugoZrjoand (c), (d) CugoHf~0 at the same two photon energies as in Fig. 1. The significant reduction in the zirconium peaks at position a with increasing photon energy should be noted. state effects, our results comparing well with the atomic results of Yeh and Lindau [6]. It was shown by Morgan arid coworkers [7, 8] that, apart from instrumental response and differing crosssections, the photoemission spectra of amorphous
267
/
101
8
6
4
2
Io
5
e
m
5 o
2
0 -e
6
(O)
-,,
Energy
-~
(Q)
o
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60
e°Energy
4o
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(eV)100
12o
(eV)
10-
iO 6 8 6 6
0~ 4 o
2 2
0
0 -e
-6
(b)
-4
o
-2
(b )
Fig. 3. Photoemission spectra of (a) pure nickel and (b) Ni92Zr 8 using a fixed frequency helium source of energy
21.3 eV. The energy scale is again relative to the Fermi energy.
/j
!. k
v o
6
2
4
--6
-4
-2
o (oV)
En~gy
I~
Fig. 5. Photoemission spectra of CuaoZr70 as a function of incident photon energies relative to the Fermi energy chosen to examine features (a) 6 eV and (b) 9 eV below the vacuum level. These are known as "constant initial state" scans, which in this case are set to examine the Zr 4d and Cu 3d states respectively.
transition metals certainly represent the d density o f states o f such alloys. Indeed the cross-section is simply the Fourier transform o f the atomic d orbital for the particular species. Our results s h o w that the atomic cross-sections o f the Cu d and Zr d states are c o m parable at around 60 eV. C o n s e q u e n t l y it is in the vicinity o f this energy that the spectra are the closest representations o f the true d density o f states.
o
(o)
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6eV)
Energy
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,-6
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References
A
IO
•
¢
4
2
2
o
0
4
4
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o
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(or)
~
~ (d)
-8
-4
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.-t
~ |
~V)
Fig. 4, Photoemission spectra of (a), (b) Cu3oZr7o and (c), (d) Cu4oZr6o at the same two photon energies as in Fig. 1.
1 P. Oelhafen, in H. Beck and H.-J. Giintherodt (eds.), Glassy Metals I1, Springer, Berlin, 1983, p. 283. 2 P. Oelhafen, E. Hauser, H.-J. Giintherodt and K. H. J. Bannermann, Phys. Rev. Lett., 43 (1979) 1134. 3 A. Amamon and G. Krill, SolidState Commun., 28(1978) 957. 4 P. Oelhafen, E. Hauser and H.-J. Giintherodt, Solid State Commun., 35 (1980) 1017. 5 M. R. Howells, D. Norman, G. P. Williams and J. B. West, J. Phys. E, 11 (1985) 199. 6 J. J. Yeh and I. Lindau, Atomic Data Nucl. Data Tables, 32 (1985) 1. 7 G. J. Morgan and G. F. Weir, J. Non-Cryst. Solids, 61 62 (1984) 1319. 8 G. J. Morgan, M. A. Howson and G. F. Weir, J. Non-Cryst. Solids, 61-62 (1984) 1131.
265
Photoemission Studies on Metallic Glasses Using Synchrotron Radiation* D. GREIG
Department o["Physics, University of Leeds, Leeds LS2 9JT (U.K.) B. L. GALLAGHER
Department of Physics, University of Nottingham, Nottingham NG 7 2RD (U.K.) M. A. HOWSON
Department o["Physics, UniversiO'of Leeds, Leeds LS2 9JT (U.K.) D. S.-L. LAW, D. NORMAN and F. M. QUINN
Science and Engineering Research Council, Daresbury Laboratory, Daresbury, Warrington WA4 4AD (U.K.)
Abstract
We report on the use o f UV synchrotron radiation in the energy range 40-120 eV to probe the electron states of CuZr, FeZr, CoZr and C u H f amorphous alloys. The great advantage of tunable radiation over fixed frequen 0' sources is that it allows identification of the electron states.from a knowledge o f the energy dependences o f their scattering cross-sections. This variation is particularly pronounced in the case of the d states o f zirconium which are essentially removed from the observed photoelectron spectra as the photon energy is increased .from 4 0 e V to about lOOeV. The results confirm, .for example, that for CuZr alloys it is the d states from zirconium that lie at the Fermi level. By combining these results with those obtained from a technique invoh~ing constant initial states the scattering cross-sections for the Zr 4d and Cu 3d states were investigated. This will allow the experimental photoemission data to be corrected to give a more representative picture of the electronic band structure o f these alloys, and has laid the foundation for extending the method to other newer alloy systems where the electron states have still not been identified. There have been many photoemission studies on transition metal metallic glasses using fixed frequency UV and X-ray sources [1-3], but, to our knowledge, no studies have been performed using tunable high intensity synchrotron UV radiation. The value o f a tunable source is that we can take advantage o f the differing cross-sections for photon absorption of the different transition metals. In particular the cross-section of 4d states, as in the valence band of zirconium,
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3 7, 1987. 0025-5416/88/$3.50
changes by two orders of magnitude over the energy range 20-100 eV. We have used the synchrotron radiation source at Daresbury, U.K., to carry out UV photoemission studies on a series o f metallic glasses based on transition metals. Two main alloy series were investigated. First, the alloys Cu9oZrlo, Fe9oZrlo, C09oZrto and Cu9oHf w were studied in order to look at systematic changes in the late transition metal d band. We also attempted to study N i Z r but unfortunately the alloy was destroyed by surface cleaning. However, preliminary studies were carried out on Ni92Zr8 using a fixed frequency He I line at 21.3 eV. The second series of alloys was Cu30Zr70 and Cu4oZr6o. These alloys have been studied before [3, 4] but only with fixed energy sources. F o r these alloys the tunable energy o f the synchrotron radiation was used to investigate the density of states after taking into account the varying cross-section of the Zr d states with photon energy. The samples were produced by melt spinning in a partial helium atmosphere at the University of Leeds. X-ray diffraction studies showed that all the samples were amorphous with the exception of the FegoZr~o sample for which there was evidence of a few per cent by volume of crystallinity. The valence band spectra were taken in a vacuum of about 8 x 10 1o Torr using a double-pass angle-integrated cylindrical mirror analyser. Photon energies between 40 and 150 eV selected by a plane grating m o n o c h r o m a t o r [5] were used with a resolution in the spectra of about 0.2 eV. F o r all the specimens the initial spectra were dominated by strong oxygen contaminant peaks and so the samples were cleaned in the vacuum chamber by Vigorous mechanical abrasion using a tungsten brush. Argon ion b o m b a r d m e n t was also tried but the tungsten brush was found to be much more effective. There was also the worry that Ar ÷ ion b o m b a r d m e n t ~3 Elsevier Sequoia/Printed in The Netherlands
266 could alter the surface stoichiometry of the sample so that for this reason also mechanical abrasion was thought to be more satisfactory. In Figs. 1 and 2 the photoemission spectra for the alloys Fe9oZrlo, Co9oZrlo , Cu90Zrlo and Cu90Hf]o are shown for photon energies of 40 and 120 eV. The background of secondary electrons and the overlap of higher order light from the monochromator have both been suitably subtracted. For completeness we show in Fig. 3 the spectra of crystalline nickel and amorphous Ni92Zrs at 21.3 eV obtained from a fixed frequency source. We see most clearly in Fig. 2 that, while at low photon energies the Zr d band is a dominant feature of the band structure, it is completely missing at high photon energies and we can easily identify the late transition metal d band. Very broadly the spectra are all very similar to what would be expected for the complementary crystalline alloys with the Fermi level below the top of the 3d band in the iron and cobalt alloys and well above the C u d band in CuZr and CuHf. However, the parts of the spectra associated with these late transition metals do not show the sharp features seen in the d bands of crystalline iron, cobalt, nickel and copper. Thus at the Fermi energy we see a somewhat lower density of states in the amorphous phase than in the crystalline phase. This is very apparent in the comparison of the spectra for the NiZr sample and crystalline nickel. Crystalline nickel has a peak in its density of states at the Fermi energy and is ferromagnetic at room temperature while Ni92Zr8 has a much reduced density of states and is paramagnetic at room temperature. The spectra for Cu3oZr7o and Cu4oZr6o are shown in Fig. 4 for energies of 40 and 120 eV. Again the background has been suitably subtracted. Here we can see even more clearly how the Zr d band is removed from the spectra as the photon energy is increased and the cross-section for photon absorption from the Zr d states is reduced by two orders of magnitude. This shows quite decisively that the C u d and Zr d bands are distinct from each other. The different energy dependences of the cross-sections for the Zr 4d and Cu 3d states are shown more directly in Fig. 5. This figure is the result of scans where the photon energy and photoelectron energy are scanned simultaneously to examine a feature at a fixed energy - - a "constant initial s t a t e " - - b e l o w the vacuum level. The two components of Fig. 5 show clearly how the Zr d cross-section falls dramatically for photon energies above 50 eV while the C u d cross-section does not fall off until much higher energies. The cross-section is a function of the symmetry of the atomic states involved and is changed rather little by solid
j
10
o
,°[
o
-Iz
(a)
+le
-e
~,
_l+
Energy
6
j,
f
-12
(c~
-z
-io
-II
o
leVI
-¢, -4 -2 E n e r g y (6V)
-12
-io
4
~.
_l+
E,-++p9 y
(b}
o
+12
(d)
-Io
-e
.i+
-2
-4
[rlBrgy
o
(ev)
-2
o
(@V)
Fig. 1. Photoemission spectra of Fe90Zr]o for incident photons of energy (a) 40 eV and (b) 120 eV and of (c), (d), CogoZr]0at the same two photon energies. The energy scale is relative to the Fermi energy, and the peaks at about - 6 eV are due to oxygen contamination.
~o
a e.
+
+
uo 2
1
o
o
( Q)
[nerg),
(eV)
-I+ Ibl
Energy
(e
o 2
Icl
1
Er~r'gX
leVI
(d)
(eV)
Fig. 2. Photoemission spectra of (a), (b) CugoZrjoand (c), (d) CugoHf~0 at the same two photon energies as in Fig. 1. The significant reduction in the zirconium peaks at position a with increasing photon energy should be noted. state effects, our results comparing well with the atomic results of Yeh and Lindau [6]. It was shown by Morgan arid coworkers [7, 8] that, apart from instrumental response and differing crosssections, the photoemission spectra of amorphous
267
/
101
8
6
4
2
Io
5
e
m
5 o
2
0 -e
6
(O)
-,,
Energy
-~
(Q)
o
40
60
e°Energy
4o
ea
eOEnergy I~ (eV)
(eV)100
12o
(eV)
10-
iO 6 8 6 6
0~ 4 o
2 2
0
0 -e
-6
(b)
-4
o
-2
(b )
Fig. 3. Photoemission spectra of (a) pure nickel and (b) Ni92Zr 8 using a fixed frequency helium source of energy
21.3 eV. The energy scale is again relative to the Fermi energy.
/j
!. k
v o
6
2
4
--6
-4
-2
o (oV)
En~gy
I~
Fig. 5. Photoemission spectra of CuaoZr70 as a function of incident photon energies relative to the Fermi energy chosen to examine features (a) 6 eV and (b) 9 eV below the vacuum level. These are known as "constant initial state" scans, which in this case are set to examine the Zr 4d and Cu 3d states respectively.
transition metals certainly represent the d density o f states o f such alloys. Indeed the cross-section is simply the Fourier transform o f the atomic d orbital for the particular species. Our results s h o w that the atomic cross-sections o f the Cu d and Zr d states are c o m parable at around 60 eV. C o n s e q u e n t l y it is in the vicinity o f this energy that the spectra are the closest representations o f the true d density o f states.
o
(o)
t~
6eV)
Energy
4
,-6
(b)
~ ~ Energy (eV)
o
References
A
IO
•
¢
4
2
2
o
0
4
4
(C)
-4
,4
En4~'gy
o
-4
(or)
~
~ (d)
-8
-4
~9)'
.-t
~ |
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Fig. 4, Photoemission spectra of (a), (b) Cu3oZr7o and (c), (d) Cu4oZr6o at the same two photon energies as in Fig. 1.
1 P. Oelhafen, in H. Beck and H.-J. Giintherodt (eds.), Glassy Metals I1, Springer, Berlin, 1983, p. 283. 2 P. Oelhafen, E. Hauser, H.-J. Giintherodt and K. H. J. Bannermann, Phys. Rev. Lett., 43 (1979) 1134. 3 A. Amamon and G. Krill, SolidState Commun., 28(1978) 957. 4 P. Oelhafen, E. Hauser and H.-J. Giintherodt, Solid State Commun., 35 (1980) 1017. 5 M. R. Howells, D. Norman, G. P. Williams and J. B. West, J. Phys. E, 11 (1985) 199. 6 J. J. Yeh and I. Lindau, Atomic Data Nucl. Data Tables, 32 (1985) 1. 7 G. J. Morgan and G. F. Weir, J. Non-Cryst. Solids, 61 62 (1984) 1319. 8 G. J. Morgan, M. A. Howson and G. F. Weir, J. Non-Cryst. Solids, 61-62 (1984) 1131.