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Dilute U alloys: Test cases for 5f electronic structure
Journal of Magnetism and Magnetic Materials 76 & 77 (1988) 353-355 North-Holland, A m s t e r d a m
353
D I L U T E U ALLOYS: T E S T C A S E S F O R 5f E L E C T R O N I C S T R U C T U R E
F.U. H I L L E B R E C H T a, V. S E C H O V S K Y b and B.T. T H O L E c a Max-Planck-lnstitut ]'fir Festkbrperforschung, Stuttgart, Fed. Rep. Germany b Physics Department, Charles University, Prague, Czechoslovakia " Materials Science Center, Rijksuniversiteit Groningen, Netherlands
We report magnetic susceptibility and electron spectroscopic results for a dilute alloy of U and Au, A u U 0.85%. The data indicate localized U states with an effective magnetic moment of 3.1/~13 per U atom.
The physical properties of U c o m p o u n d s can be discussed either in terms of b a n d structure, or within an A n d e r s o n impurity model. The first approach implicitly assumes, that the U states including the 5f states - have a bandwidth larger than the relevant correlation energy. In the latter case the U 5f states are treated as isolated impurities at infinit separation from each other. Although U c o m p o u n d s are concentrated systems with finite separation between U sites, an approach based on an A n d e r s o n model m a y be useful because it offers a way of incorporating the C o u l o m b interaction within the 5f shell. As an example we mention the quasibinary system (UxY I x)A12 where no changes of the 5f derived signal in photoemission were observed for x = 1 to x = 0.02 [1]. This was taken to indicate that the large energy scale properties of the 5f states are dominated by single site effects, i.e. the interaction between a single 5f site and the host states, and that an A n d e r s o n model m a y indeed be an appropriate starting point for a description of UAI2 and possibly other heavy fermion compounds. However, there are c o m p o u n d s where at least part of the electronic spectrum is in agreement with b a n d structure calculations [2]. In these cases one often observes extra width which is not explained by a single particle calculation, but m a y be accounted for by including the influence of correlation [2,3]. If we want to get a more reliable standard for assessing whether - and if so, to what extent - a concentrated system resembles a dilute one, we have to compare electronic excitation spectra (and other data) for the two cases. There is, however, little information on the properties of U as a genuinely dilute impurity. We report here some selected results of an extensive study of dilute U
alloys by electron spectroscopies as well as by " l o w energy" methods such as magnetic and transport properties. In order to facilitate interpretation, we look for an alloy for which we can expect a relatively weak interaction between the 5f impurity and the host. We have shown previously [4] that this interaction can be expected to be weak if the host density of states is low in the region of the occupied U states, i.e. within the first two or three eV below the Fermi level E v. Consequently, our first choice for this experiment were noble metals as hosts, of which Au has the highest solubility for U. Samples were prepared by Ar arc melting with a starting composition of 1% U in Au. The final composition as determined by atomic emission spectroscopy was 0.85% U. The excess U accumulated on the surface of the ingot. Concentrations of other metallic impurities were below the limits of detectability, i.e. less than 10 ppm. N o secondary phases or segregation of U could be detected by scanning electron microscopy. In fig. 1 the temperature dependence of the magnetic susceptibility of A u U is shown. Also shown is the susceptibility of the Au starting material. It is temperature independent down to 20 K. The susceptibility of the alloy shows a steady increase with falling temperature. The inset shows the reciprocal of the impurity-related contribution i.e. after subtracting the diamagnetic contribution of the A u host. It closely follows a modified Curie-Weiss law with 0 = - 5 5 K. Using the chemically determined U concentration, we obtain an effective m o m e n t of 3.1/~ B per U atom. - A .first conclusion from the observed magnetic behaviour is that the U states are localized. The p h o t o e m i s s i o n s p e c t r u m of A u U is dominated by the A u 5d states, and it is difficult
0 3 0 4 - 8 8 5 3 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V.
354
F U. Hillebrecht et a L / Dilute U allov~
41 I/x ~10Bkg/~l
10
2I
E
• • .--"AuU
...'"
... •
A uU
/~......" "'''
T (KI
X
0
Au
~Ni~N~NKU~MNNMMW~MIXM~X
o
1;o
~R
260
'
300
temperature ( K )
Fig. 1. Magnetic susceptibility for AuU 0.85% content and for pure Au. The uncertainty is smaller than indicated by the size of the data points.
to recognize the changes caused b y the U i m p u r i ties. To e m p h a s i z e the U i n d u c e d states we show in fig. 2 difference spectra o b t a i n e d b y s u b t r a c t i n g the spectra for the A u U alloy a n d the A u host. Before s u b t r a c t i o n the p h o t o e m i s s i o n (UPS) spectra were corrected for inelastic b a c k g r o u n d a n d n o r m a l i z e d to the same total area. This is j u s t i f i e d as the change of total emission is expected to be negligible. T h e U P S difference curve exhibits sharp
U-induced changeof the DOS in AuU
uPs
BIS "....
wcQncy (calc.)
>",.
h'~ = 21 eV
.~
5 f 3 ~ S f i'
>,,.
:/
~ ". ~'..
,
-/
\
..
I ,I
I~
oc ,p
r,.s,v I =1
I
'
I -5
~
)
r~' ~
I
i
,
i
EF
,
I
i
i
I
*5
energy relative to E F (eV}
Fig. 2. Photoemission (UPS) and inverse photoemission (BIS) difference spectra for AuU 0.85%. Dashed lines show calculated vacancy-induced change of the photoemission spe&rum, and localized 5f 4 final state spectrum. Horizontal lines show zero for the difference spectra; the relative scales of UPS and BIS are arbitrary.
structures near to steep edges in the original spectra. Such structures can occur if the spectra arc not p r o p e r l y lined up. T o avoid artefacts of this type, the d a t a were taken under identical c o n d i tions from s a m p l e s m o u n t e d on the same holder and p r e p a r e d in the s a m e way. The absence of any spurious features at E v shows that such artefacts are not present in the data. T h e 5f and 6d states in U metal are within 3 eV of E v. In a dilute alloy, we still expect these states in this energy range, however s h a r p e r than in U metal. The width is in this situation governed by the h y b r i d i z a t i o n between the U and the host states. If g r o u n d a n d final states are localized, multiplet splitting m a y lead to a further increase of the width. Obviously, it is difficult to interpret the difference s p e c t r u m in this way, since it appears to be g o v e r n e d b y structures originating from A u 5d related structures in the original data. - F o r 45 eV p h o t o n energy the 5f cross section is larger by a factor of 10 than at 21 eV. However, no 5f p e a k can be identified by c o m p a r i s o n of the two difference spectra. Nevertheless, both spectra clearly are positive in the region between E l and 2 eV, showing that at least part of the U states are in this region. Below 2 eV they overlap with the A u 5d states. A first step t o w a r d s an u n d e r s t a n d i n g of these results can be o b t a i n e d if we think of the U i n d u c e d change of the total density of states as the sum of two effects. T h e first is the c h a n g e i n d u c e d by r e m o v i n g an a t o m from the host lattice, and the second is due to the i m p u r i t y which is put in its place. The d a s h e d line in fig. 2 is a calculation by v.d. Marel et al. [51 of the effect of removing an a t o m from the A u lattice on the 21 eV p h o t o e m i s sion spectrum. W e see that below - 4 . 5 eV there is g o o d a g r e e m e n t between this curve a n d the experimental one. F r o m - 4 . 5 to El:, however, there is only partial agreement, i n d i c a t i n g that the U states c o n t r i b u t e to the signal in this energy region. O n the right side of fig. 2 the difference of inverse p h o t o e m i s s i o n (BIS) spectra is shown, disp l a y i n g the U i n d u c e d changes in the u n o c c u p i e d states. T h e s p e c t r a were taken at 1487 eV electron energy, e n h a n c i n g the 5f c o n t r i b u t i o n . The alloy s p e c t r u m deviates from that of pure Au only in the range from E v to 9 eV above; at higher energies the two s p e c t r a are identical. F o r subtraction the s p e c t r a were n o r m a l i z e d to the same
F.U. Hillebrecht et al. / Dilute U alloys
intensity at 10 eV above E v. Consequently the difference curve vanishes above this energy. The fact that the U induced states are so broad is a clear indication of multiplet splitting, which indicates a highly localized 5f state. The vertical bars show a calculation for a localized 5f 3 to 5 f 4 transition; if finite lifetime is taken into account, we expect a spectrum as indicated by the dashed line. This accounts at least partially for the observed spectrum, although there is some discrepancy from 4 to 7 eV above E F. This extra intensity may come from the U 6d states which are not included in the calculation. Both photoemission and inverse photoemission indicate a low U derived signal at E v. This is in agreement with the magnetic susceptibility data which can be interpreted in terms of a stable local moment associated with each U atom. If it is possible to extract the position and shape of the 5f contribution from the UPS and BIS spectra one can determine the intra-5f Coulomb interaction from these data. The overlap of the U induced states with the Au d band does not allow an unambigous identification of the 5f state in this dilute alloy. The spectra show that the 5f Coulomb interaction is of the order of 3 eV for U in Au. It may be possible to avoid the problem of overlap
355
of the U and the host states by using e.g. Ag as a host metal. A full account of these experiments including core level spectra and resistivity data will be published elsewhere. We are grateful to Dr. H. Heckner, Zentralinstitut ftir Chemische Analysen, Kernforschungsanlage Jiilich, for performing the chemical analysis of the samples. One of us (V.S.) is grateful to the Alexander von Humboldt foundation for supporting a stay at the Institute for Festk~Srperforschung (IFF), K F A Ji)lich, during which part of this work was carried out.
References [1] J.S. Kang, J.W. Allen, M.B. Maple, M.S. Torikachvili, B. Pate, W. Ellis and I. Lindau, Phys. Rev. Lett. 59 (1987) 493. [2] D.D. Sarma, F.U. Hillebrecht and N. M~rtensson, J. Magn. Magn. Mat. 63 & 64 (1987) 509. [3] D.D. Sarma, F.U. Hillebrecht, W. Speier, N. MSrtensson and D.D. Koelling, Phys. Rev. Lett. 57 (1986) 2215. [4] F.U. Hillebrecht~ D.D. Sarma and N. Mhrtensson, Phys. Rev. B 36 (1986) 4376. [5] D. v.d. Marel, G.A. Jullianus and G.A. Sawatzky, Phys. Rev. B 32 (1985) 6331.
353
D I L U T E U ALLOYS: T E S T C A S E S F O R 5f E L E C T R O N I C S T R U C T U R E
F.U. H I L L E B R E C H T a, V. S E C H O V S K Y b and B.T. T H O L E c a Max-Planck-lnstitut ]'fir Festkbrperforschung, Stuttgart, Fed. Rep. Germany b Physics Department, Charles University, Prague, Czechoslovakia " Materials Science Center, Rijksuniversiteit Groningen, Netherlands
We report magnetic susceptibility and electron spectroscopic results for a dilute alloy of U and Au, A u U 0.85%. The data indicate localized U states with an effective magnetic moment of 3.1/~13 per U atom.
The physical properties of U c o m p o u n d s can be discussed either in terms of b a n d structure, or within an A n d e r s o n impurity model. The first approach implicitly assumes, that the U states including the 5f states - have a bandwidth larger than the relevant correlation energy. In the latter case the U 5f states are treated as isolated impurities at infinit separation from each other. Although U c o m p o u n d s are concentrated systems with finite separation between U sites, an approach based on an A n d e r s o n model m a y be useful because it offers a way of incorporating the C o u l o m b interaction within the 5f shell. As an example we mention the quasibinary system (UxY I x)A12 where no changes of the 5f derived signal in photoemission were observed for x = 1 to x = 0.02 [1]. This was taken to indicate that the large energy scale properties of the 5f states are dominated by single site effects, i.e. the interaction between a single 5f site and the host states, and that an A n d e r s o n model m a y indeed be an appropriate starting point for a description of UAI2 and possibly other heavy fermion compounds. However, there are c o m p o u n d s where at least part of the electronic spectrum is in agreement with b a n d structure calculations [2]. In these cases one often observes extra width which is not explained by a single particle calculation, but m a y be accounted for by including the influence of correlation [2,3]. If we want to get a more reliable standard for assessing whether - and if so, to what extent - a concentrated system resembles a dilute one, we have to compare electronic excitation spectra (and other data) for the two cases. There is, however, little information on the properties of U as a genuinely dilute impurity. We report here some selected results of an extensive study of dilute U
alloys by electron spectroscopies as well as by " l o w energy" methods such as magnetic and transport properties. In order to facilitate interpretation, we look for an alloy for which we can expect a relatively weak interaction between the 5f impurity and the host. We have shown previously [4] that this interaction can be expected to be weak if the host density of states is low in the region of the occupied U states, i.e. within the first two or three eV below the Fermi level E v. Consequently, our first choice for this experiment were noble metals as hosts, of which Au has the highest solubility for U. Samples were prepared by Ar arc melting with a starting composition of 1% U in Au. The final composition as determined by atomic emission spectroscopy was 0.85% U. The excess U accumulated on the surface of the ingot. Concentrations of other metallic impurities were below the limits of detectability, i.e. less than 10 ppm. N o secondary phases or segregation of U could be detected by scanning electron microscopy. In fig. 1 the temperature dependence of the magnetic susceptibility of A u U is shown. Also shown is the susceptibility of the Au starting material. It is temperature independent down to 20 K. The susceptibility of the alloy shows a steady increase with falling temperature. The inset shows the reciprocal of the impurity-related contribution i.e. after subtracting the diamagnetic contribution of the A u host. It closely follows a modified Curie-Weiss law with 0 = - 5 5 K. Using the chemically determined U concentration, we obtain an effective m o m e n t of 3.1/~ B per U atom. - A .first conclusion from the observed magnetic behaviour is that the U states are localized. The p h o t o e m i s s i o n s p e c t r u m of A u U is dominated by the A u 5d states, and it is difficult
0 3 0 4 - 8 8 5 3 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V.
354
F U. Hillebrecht et a L / Dilute U allov~
41 I/x ~10Bkg/~l
10
2I
E
• • .--"AuU
...'"
... •
A uU
/~......" "'''
T (KI
X
0
Au
~Ni~N~NKU~MNNMMW~MIXM~X
o
1;o
~R
260
'
300
temperature ( K )
Fig. 1. Magnetic susceptibility for AuU 0.85% content and for pure Au. The uncertainty is smaller than indicated by the size of the data points.
to recognize the changes caused b y the U i m p u r i ties. To e m p h a s i z e the U i n d u c e d states we show in fig. 2 difference spectra o b t a i n e d b y s u b t r a c t i n g the spectra for the A u U alloy a n d the A u host. Before s u b t r a c t i o n the p h o t o e m i s s i o n (UPS) spectra were corrected for inelastic b a c k g r o u n d a n d n o r m a l i z e d to the same total area. This is j u s t i f i e d as the change of total emission is expected to be negligible. T h e U P S difference curve exhibits sharp
U-induced changeof the DOS in AuU
uPs
BIS "....
wcQncy (calc.)
>",.
h'~ = 21 eV
.~
5 f 3 ~ S f i'
>,,.
:/
~ ". ~'..
,
-/
\
..
I ,I
I~
oc ,p
r,.s,v I =1
I
'
I -5
~
)
r~' ~
I
i
,
i
EF
,
I
i
i
I
*5
energy relative to E F (eV}
Fig. 2. Photoemission (UPS) and inverse photoemission (BIS) difference spectra for AuU 0.85%. Dashed lines show calculated vacancy-induced change of the photoemission spe&rum, and localized 5f 4 final state spectrum. Horizontal lines show zero for the difference spectra; the relative scales of UPS and BIS are arbitrary.
structures near to steep edges in the original spectra. Such structures can occur if the spectra arc not p r o p e r l y lined up. T o avoid artefacts of this type, the d a t a were taken under identical c o n d i tions from s a m p l e s m o u n t e d on the same holder and p r e p a r e d in the s a m e way. The absence of any spurious features at E v shows that such artefacts are not present in the data. T h e 5f and 6d states in U metal are within 3 eV of E v. In a dilute alloy, we still expect these states in this energy range, however s h a r p e r than in U metal. The width is in this situation governed by the h y b r i d i z a t i o n between the U and the host states. If g r o u n d a n d final states are localized, multiplet splitting m a y lead to a further increase of the width. Obviously, it is difficult to interpret the difference s p e c t r u m in this way, since it appears to be g o v e r n e d b y structures originating from A u 5d related structures in the original data. - F o r 45 eV p h o t o n energy the 5f cross section is larger by a factor of 10 than at 21 eV. However, no 5f p e a k can be identified by c o m p a r i s o n of the two difference spectra. Nevertheless, both spectra clearly are positive in the region between E l and 2 eV, showing that at least part of the U states are in this region. Below 2 eV they overlap with the A u 5d states. A first step t o w a r d s an u n d e r s t a n d i n g of these results can be o b t a i n e d if we think of the U i n d u c e d change of the total density of states as the sum of two effects. T h e first is the c h a n g e i n d u c e d by r e m o v i n g an a t o m from the host lattice, and the second is due to the i m p u r i t y which is put in its place. The d a s h e d line in fig. 2 is a calculation by v.d. Marel et al. [51 of the effect of removing an a t o m from the A u lattice on the 21 eV p h o t o e m i s sion spectrum. W e see that below - 4 . 5 eV there is g o o d a g r e e m e n t between this curve a n d the experimental one. F r o m - 4 . 5 to El:, however, there is only partial agreement, i n d i c a t i n g that the U states c o n t r i b u t e to the signal in this energy region. O n the right side of fig. 2 the difference of inverse p h o t o e m i s s i o n (BIS) spectra is shown, disp l a y i n g the U i n d u c e d changes in the u n o c c u p i e d states. T h e s p e c t r a were taken at 1487 eV electron energy, e n h a n c i n g the 5f c o n t r i b u t i o n . The alloy s p e c t r u m deviates from that of pure Au only in the range from E v to 9 eV above; at higher energies the two s p e c t r a are identical. F o r subtraction the s p e c t r a were n o r m a l i z e d to the same
F.U. Hillebrecht et al. / Dilute U alloys
intensity at 10 eV above E v. Consequently the difference curve vanishes above this energy. The fact that the U induced states are so broad is a clear indication of multiplet splitting, which indicates a highly localized 5f state. The vertical bars show a calculation for a localized 5f 3 to 5 f 4 transition; if finite lifetime is taken into account, we expect a spectrum as indicated by the dashed line. This accounts at least partially for the observed spectrum, although there is some discrepancy from 4 to 7 eV above E F. This extra intensity may come from the U 6d states which are not included in the calculation. Both photoemission and inverse photoemission indicate a low U derived signal at E v. This is in agreement with the magnetic susceptibility data which can be interpreted in terms of a stable local moment associated with each U atom. If it is possible to extract the position and shape of the 5f contribution from the UPS and BIS spectra one can determine the intra-5f Coulomb interaction from these data. The overlap of the U induced states with the Au d band does not allow an unambigous identification of the 5f state in this dilute alloy. The spectra show that the 5f Coulomb interaction is of the order of 3 eV for U in Au. It may be possible to avoid the problem of overlap
355
of the U and the host states by using e.g. Ag as a host metal. A full account of these experiments including core level spectra and resistivity data will be published elsewhere. We are grateful to Dr. H. Heckner, Zentralinstitut ftir Chemische Analysen, Kernforschungsanlage Jiilich, for performing the chemical analysis of the samples. One of us (V.S.) is grateful to the Alexander von Humboldt foundation for supporting a stay at the Institute for Festk~Srperforschung (IFF), K F A Ji)lich, during which part of this work was carried out.
References [1] J.S. Kang, J.W. Allen, M.B. Maple, M.S. Torikachvili, B. Pate, W. Ellis and I. Lindau, Phys. Rev. Lett. 59 (1987) 493. [2] D.D. Sarma, F.U. Hillebrecht and N. M~rtensson, J. Magn. Magn. Mat. 63 & 64 (1987) 509. [3] D.D. Sarma, F.U. Hillebrecht, W. Speier, N. MSrtensson and D.D. Koelling, Phys. Rev. Lett. 57 (1986) 2215. [4] F.U. Hillebrecht~ D.D. Sarma and N. Mhrtensson, Phys. Rev. B 36 (1986) 4376. [5] D. v.d. Marel, G.A. Jullianus and G.A. Sawatzky, Phys. Rev. B 32 (1985) 6331.