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Electron tunneling in ErRh4B4
NA 2
Physica I08B (1981) 803.804 North-Holland Publishing Company
ELECTRON TUNNELING IN ErRh4B 4
C.P. Umbach and L.E. Toth Department of Chemical Engineering and Materials Science
and E.D. Dahlberg'and A.M. Goldman School of Physics and Astronomy University of Minnesota Minneapolis, MN 55455, U.S.A.
Thin film tunneling junctions of ErRh4B 4 with counterelectrodes of Mg, In, Pb and A1 have been formed using an artifical barrier of oxidized Er. Measurements of the tunneling characteristics indicate that the maximum value of 2A/kBT c for ErRh4B 4 is at least 4.2.
Ternary compounds such as ErRh4B 4 which exhibit superconductivity at one temperature and long range magnetic order at lower temperatures are unique systems for investigating the competition between superconductivity and magnetism.[l] Electron tunneling studies of ErRh4B 4 can in principle provide a number of insights into the nature of its superconductivity and reveal some aspects of the competition between magnetic ordering and superconductivit~ Efforts to investigate ErRh4B4 by this technique have been hampered by the lack of a suitable natural oxide and have required tile use of novel techniques in forming tunnel barriers on this compound. Rowell et al. [2] formed tunnel barriers by sputtering approximately i0 nm of Al over freshly sputtered ErRh4B 4. This A1 layer was subsequently oxidized to form the insulating barrier. The,value of the energy gap determined in these experiments was roughly one-half that expected of a weak-coupled BCS superconductor. When Si was used in place of the Al a very broad gap structure was observed in the tunneling characteristic from which the maximum value of 2A/kBTc for ErRh4B 4 was estimated to be 3.8 which would imply that ErRh4B4 is slightly strong coupled. Other evidence that ErRh4B4 is strong coupled is in the work of Kuwasawa et al.[3] who found 2A/kBTc = 3.9
The ErRh4B4 and Er films were sputtered sequentially in a multi-target sputtering system, while the counterelectrode was evaporated in a separate vacuum chamber. Figure 1 contains a series of plots of -dV/dI vs. V foran ErRh4B4 Er/oxide/Mg junction at various temperatures. The midpoint of the superconducting transition of the ErRh4B4 film (Tcl) was at 6.9 K. Transitions widths were the order of 0.5 K. The zero-bias resistance of this junction exhibited a maximum at 2.5 K. The temperature dependent energy gap of ErRh~B4 in this junction was obtained by correcting the voltages at which the maxima of the conductance were observed by factors given in the conductance tables complied by Bermon[5] for superconductor-insulator-normal [SIN] junctions. The value of the gap determined using this procedure increased from roughly 1.2 mV to 1.3 mV as the temperature was decreased from 5 K to below 1.5 K. A gap of 1.25 mV would result in 7~K 6~K 6~K 5K 4.~K 4~K 3.25K 3.~K 2.75 K
for Er 58Ho 42Rh4B4 using a p o i n t contact technique. "Recently, Poppe[4] f a b r i c a t e d
2.~K 2.25K
vacuum barrier junctions by pressing a Ag tip onto a single crystal of ErRh4B4 and then backing the tip away from the crystal. He found that 2A/kBTc = 3.8. Here we report on the fabrication of electron tunneling junctions on ErRh4B4 using an artificial barrier prepared by oxidizing a thin sputtered Er film. We have interpreted the data in a manner that suggests that ErRh4B4 is a strongly coupled superconductor with a maximum value of 2A/kBT c of at least 4.2. The junctions consisted of a base electrode of ErRh4B4 roughly 120-150 nm thick, a barrier of oxidized Er nominally 1-2.5 nm thic% and a counterelectrode of Mg, In, A1 or Pb.
0378-4363/81/0000-0000/$02.50 © North-HollandPublishingCompany
2.~K 1.75 K
I.~K
1~< T • 1.50K
V (mY)
Figure i. -dV/dl for an ErRh4B4/Er/Oxide/Mg junction at a variety of temperatures.
803
804
in the zero bias conductance and in the conduc~ ance at 500 pV were observed below Tc of In, which is also, accidentally, the Neel temperature of Er203. The zero-bias conductance peak may be due to magnetic impurity scattering in the oxide barrier.[7] The finite voltage peak may be the signature of antiferromagnetic magnons in the barrier.[8]
42K t-
50
K
I-I
I
< T < 15K
I-I
ErRh4B41Er/oxide/In [
l
t
t
I
I
0
1
2
.3
4
5
V (mY) F i g u r e 2. I and - d V / d I v s . V f o r an ErRh4B4Er/ Oxide/In junction at selected temperatures. a 2A/kBTc = 4 . 2 . This correction procedure, which d e p e n d s on t h e t e m p e r a t u r e d e p e n d e n c e o f t h e q u a s i p a r t i c l e e x c i t a t i o n s , i s a t b e s t app r o x i m a t e f o r a non-BCS s u p e r c o n d u c t o r and would be e x p e c t e d t o f a i l as t h e m a g n e t i c o r d e r i n g t e m p e r a t u r e (Tc2) i s a p p r o a c h e d . The I-V c h a r a c t e r s i t i c s o f an ErRh4B4/Er/ Oxide/In junction are plotted in Figure 2 both a t 4 . 2 K and a t a t e m p e r a t u r e s l i g h t l y below 1.5 K. The t r a n s i t i o n t e m p e r a t u r e o f t h e ErRh 4B4 f i l m o f t h i s j u n c t i o n was a b o u t 6.1 K. In t h e SIN c o n f i g u r a t i o n t h e gap o f ErRh4B 4 was d e t e r m i n e d u s i n g t h e Bermon t a b l e s t o be r o u g h l y 1.1 mV. In t h e SIS c o n f i g u r a t i o n below 3.4K, t h e t r a n s i t i o n t e m p e r a t u r e o f I n , t h e gap i n c r e a s e t t from 0 . 8 5 mV t o 2 . 0 K a l m o s t l i n e a r l y up t o 1 . 0 mV a t 3.0 K. E x t r a p o l a t i o n o f t h i s c u r v e would r e s u l t i n an e n e r g y gap o f a b o u t 1.1 mV f o r ErRh4B4 a t a t e m p e r a t u r e h a l f w a y b e tween Tcl and t h e e x p e c t e d Tc2 f o r t h i s f i l m . In b o t h t h e SIN and t h e SIS c o n f i g u r a t i o n s , t h e o b s e r v e d e n e r g y gap i n t h i s j u n c t i o n would r e s u l t i n a maximum v a l u e o f 2A/kBTc o f a t l e a s t 4.2. The f a c t t h a t t h e Er 3+ i o n i s m a g n e t i c may have an e f f e c t on t h e t u n n e l i n g c h a r a c t e r i s t i c . Also i f t h e b a r r i e r i s s t o i c h i o m e t r i c Er203 i t s h o u l d a n t i f e r r o m a g n e t i c a l l y o r d e r a t 3 . 3 6 K4 . 0 K. [6] There i s no a p p a r e n t s i g n a t u r e o f ordering in this temperature range in the ErRh4B4/Er/Oxide/Mg j u n c t i o n s ( F i g . I ) . In the case of ErRh4B4/Er/Oxide/In junctions, peaks
Tunneling junctions were also fabricated with A1 and Pb counterelectrodes. The results with Al counterelectrodes were similar to those found with Mg. However, the largest value of 2A/kBT c observed in 5 junctions with Pb counte~ electrodes was only 2.5. Junctions made with Pb counterelectrodes seemed much more unstable than those made with other materials, and several of them shorted out while their derivative curves were being plotted. In summary, we have been able to fabricate electron tunneling junctions on ErRh4B4 using an oxidized Er barrier with counterelectrodes of Mg, In, AI and Pb. The I-V characteristics of these junctions, although not leakage free, have permitted an estimate of the ErRh4B 4 energy gap. ErRh4B4 appears to be strongly coupled and the maximum value of 2A/kBT c consistent with these results is at least 4.2. This work was supported by the ONR under contract NOO14-78-C-0619. REFERENCES: [i]
[2]
[3]
[4] [5]
[6]
[7]
[8]
Fertig, W.A., Johnston, D.C., DeLong L.E., McCallum, R.W., Maple, M.B., and Matthias, B.T., Destruction of Superconductivity at the Onset of Long-Range Magnetic Order in the Compound ErRh4B4, Phys. Rev. Lett. 38 (1977) 987-990. Rowell, J.M., Dynes, R.C., Schmidt, P.H., Ion Damage, Critical Current and Tunneling Studies of ErRh4B 4 Films in Suhl, Harry and Maple, M. Brian (eds.) Superconductivity in d- and f-Band Metals (Academic Press, New York, 1980), 409-417. Kuwasawa, Y., Rinderer, L., and Matthias, B.T., Josephson Effect of a Superconducting Ferromagnet, J. Low Temp. Phys. 37 (1979) 179-188. Poppe, U., private commun, from F. Pobel. Bermon, S., The BCS Differential Conductance for a Metal-lnsulator-Superconductor Tunneling Junction, Technical Report, Dept of Physics, U. of Illinois, (March 1964). Moon, R.M~, Koehler, W.C., Child, H.R., and Raubenheimer, L.J., Magnetic Structures of Er203 and Yb203, Phys. Rev. 176 ('1968) 72~ 31 and references therein. Rowell, J.M., Tunneling Anomalies in B u r s t e i n , E l i a s and L u n d q u i s t , S t i g . (edsO T u n n e l i n g Phenomena i n S o l i d s , (Plenum New York, 1969) 385-404. T s u i , D.C., Detz, R . E . , and W a l k e r , L. R., M u l t i p l e Magnon E x c i t a t i o n i n NiO by E l e c t r o n T u n n e l i n g , Phys. Rev. L e t t . 27 (1971). 1729-1739.
Physica I08B (1981) 803.804 North-Holland Publishing Company
ELECTRON TUNNELING IN ErRh4B 4
C.P. Umbach and L.E. Toth Department of Chemical Engineering and Materials Science
and E.D. Dahlberg'and A.M. Goldman School of Physics and Astronomy University of Minnesota Minneapolis, MN 55455, U.S.A.
Thin film tunneling junctions of ErRh4B 4 with counterelectrodes of Mg, In, Pb and A1 have been formed using an artifical barrier of oxidized Er. Measurements of the tunneling characteristics indicate that the maximum value of 2A/kBT c for ErRh4B 4 is at least 4.2.
Ternary compounds such as ErRh4B 4 which exhibit superconductivity at one temperature and long range magnetic order at lower temperatures are unique systems for investigating the competition between superconductivity and magnetism.[l] Electron tunneling studies of ErRh4B 4 can in principle provide a number of insights into the nature of its superconductivity and reveal some aspects of the competition between magnetic ordering and superconductivit~ Efforts to investigate ErRh4B4 by this technique have been hampered by the lack of a suitable natural oxide and have required tile use of novel techniques in forming tunnel barriers on this compound. Rowell et al. [2] formed tunnel barriers by sputtering approximately i0 nm of Al over freshly sputtered ErRh4B 4. This A1 layer was subsequently oxidized to form the insulating barrier. The,value of the energy gap determined in these experiments was roughly one-half that expected of a weak-coupled BCS superconductor. When Si was used in place of the Al a very broad gap structure was observed in the tunneling characteristic from which the maximum value of 2A/kBTc for ErRh4B 4 was estimated to be 3.8 which would imply that ErRh4B4 is slightly strong coupled. Other evidence that ErRh4B4 is strong coupled is in the work of Kuwasawa et al.[3] who found 2A/kBTc = 3.9
The ErRh4B4 and Er films were sputtered sequentially in a multi-target sputtering system, while the counterelectrode was evaporated in a separate vacuum chamber. Figure 1 contains a series of plots of -dV/dI vs. V foran ErRh4B4 Er/oxide/Mg junction at various temperatures. The midpoint of the superconducting transition of the ErRh4B4 film (Tcl) was at 6.9 K. Transitions widths were the order of 0.5 K. The zero-bias resistance of this junction exhibited a maximum at 2.5 K. The temperature dependent energy gap of ErRh~B4 in this junction was obtained by correcting the voltages at which the maxima of the conductance were observed by factors given in the conductance tables complied by Bermon[5] for superconductor-insulator-normal [SIN] junctions. The value of the gap determined using this procedure increased from roughly 1.2 mV to 1.3 mV as the temperature was decreased from 5 K to below 1.5 K. A gap of 1.25 mV would result in 7~K 6~K 6~K 5K 4.~K 4~K 3.25K 3.~K 2.75 K
for Er 58Ho 42Rh4B4 using a p o i n t contact technique. "Recently, Poppe[4] f a b r i c a t e d
2.~K 2.25K
vacuum barrier junctions by pressing a Ag tip onto a single crystal of ErRh4B4 and then backing the tip away from the crystal. He found that 2A/kBTc = 3.8. Here we report on the fabrication of electron tunneling junctions on ErRh4B4 using an artificial barrier prepared by oxidizing a thin sputtered Er film. We have interpreted the data in a manner that suggests that ErRh4B4 is a strongly coupled superconductor with a maximum value of 2A/kBT c of at least 4.2. The junctions consisted of a base electrode of ErRh4B4 roughly 120-150 nm thick, a barrier of oxidized Er nominally 1-2.5 nm thic% and a counterelectrode of Mg, In, A1 or Pb.
0378-4363/81/0000-0000/$02.50 © North-HollandPublishingCompany
2.~K 1.75 K
I.~K
1~< T • 1.50K
V (mY)
Figure i. -dV/dl for an ErRh4B4/Er/Oxide/Mg junction at a variety of temperatures.
803
804
in the zero bias conductance and in the conduc~ ance at 500 pV were observed below Tc of In, which is also, accidentally, the Neel temperature of Er203. The zero-bias conductance peak may be due to magnetic impurity scattering in the oxide barrier.[7] The finite voltage peak may be the signature of antiferromagnetic magnons in the barrier.[8]
42K t-
50
K
I-I
I
< T < 15K
I-I
ErRh4B41Er/oxide/In [
l
t
t
I
I
0
1
2
.3
4
5
V (mY) F i g u r e 2. I and - d V / d I v s . V f o r an ErRh4B4Er/ Oxide/In junction at selected temperatures. a 2A/kBTc = 4 . 2 . This correction procedure, which d e p e n d s on t h e t e m p e r a t u r e d e p e n d e n c e o f t h e q u a s i p a r t i c l e e x c i t a t i o n s , i s a t b e s t app r o x i m a t e f o r a non-BCS s u p e r c o n d u c t o r and would be e x p e c t e d t o f a i l as t h e m a g n e t i c o r d e r i n g t e m p e r a t u r e (Tc2) i s a p p r o a c h e d . The I-V c h a r a c t e r s i t i c s o f an ErRh4B4/Er/ Oxide/In junction are plotted in Figure 2 both a t 4 . 2 K and a t a t e m p e r a t u r e s l i g h t l y below 1.5 K. The t r a n s i t i o n t e m p e r a t u r e o f t h e ErRh 4B4 f i l m o f t h i s j u n c t i o n was a b o u t 6.1 K. In t h e SIN c o n f i g u r a t i o n t h e gap o f ErRh4B 4 was d e t e r m i n e d u s i n g t h e Bermon t a b l e s t o be r o u g h l y 1.1 mV. In t h e SIS c o n f i g u r a t i o n below 3.4K, t h e t r a n s i t i o n t e m p e r a t u r e o f I n , t h e gap i n c r e a s e t t from 0 . 8 5 mV t o 2 . 0 K a l m o s t l i n e a r l y up t o 1 . 0 mV a t 3.0 K. E x t r a p o l a t i o n o f t h i s c u r v e would r e s u l t i n an e n e r g y gap o f a b o u t 1.1 mV f o r ErRh4B4 a t a t e m p e r a t u r e h a l f w a y b e tween Tcl and t h e e x p e c t e d Tc2 f o r t h i s f i l m . In b o t h t h e SIN and t h e SIS c o n f i g u r a t i o n s , t h e o b s e r v e d e n e r g y gap i n t h i s j u n c t i o n would r e s u l t i n a maximum v a l u e o f 2A/kBTc o f a t l e a s t 4.2. The f a c t t h a t t h e Er 3+ i o n i s m a g n e t i c may have an e f f e c t on t h e t u n n e l i n g c h a r a c t e r i s t i c . Also i f t h e b a r r i e r i s s t o i c h i o m e t r i c Er203 i t s h o u l d a n t i f e r r o m a g n e t i c a l l y o r d e r a t 3 . 3 6 K4 . 0 K. [6] There i s no a p p a r e n t s i g n a t u r e o f ordering in this temperature range in the ErRh4B4/Er/Oxide/Mg j u n c t i o n s ( F i g . I ) . In the case of ErRh4B4/Er/Oxide/In junctions, peaks
Tunneling junctions were also fabricated with A1 and Pb counterelectrodes. The results with Al counterelectrodes were similar to those found with Mg. However, the largest value of 2A/kBT c observed in 5 junctions with Pb counte~ electrodes was only 2.5. Junctions made with Pb counterelectrodes seemed much more unstable than those made with other materials, and several of them shorted out while their derivative curves were being plotted. In summary, we have been able to fabricate electron tunneling junctions on ErRh4B4 using an oxidized Er barrier with counterelectrodes of Mg, In, AI and Pb. The I-V characteristics of these junctions, although not leakage free, have permitted an estimate of the ErRh4B 4 energy gap. ErRh4B4 appears to be strongly coupled and the maximum value of 2A/kBT c consistent with these results is at least 4.2. This work was supported by the ONR under contract NOO14-78-C-0619. REFERENCES: [i]
[2]
[3]
[4] [5]
[6]
[7]
[8]
Fertig, W.A., Johnston, D.C., DeLong L.E., McCallum, R.W., Maple, M.B., and Matthias, B.T., Destruction of Superconductivity at the Onset of Long-Range Magnetic Order in the Compound ErRh4B4, Phys. Rev. Lett. 38 (1977) 987-990. Rowell, J.M., Dynes, R.C., Schmidt, P.H., Ion Damage, Critical Current and Tunneling Studies of ErRh4B 4 Films in Suhl, Harry and Maple, M. Brian (eds.) Superconductivity in d- and f-Band Metals (Academic Press, New York, 1980), 409-417. Kuwasawa, Y., Rinderer, L., and Matthias, B.T., Josephson Effect of a Superconducting Ferromagnet, J. Low Temp. Phys. 37 (1979) 179-188. Poppe, U., private commun, from F. Pobel. Bermon, S., The BCS Differential Conductance for a Metal-lnsulator-Superconductor Tunneling Junction, Technical Report, Dept of Physics, U. of Illinois, (March 1964). Moon, R.M~, Koehler, W.C., Child, H.R., and Raubenheimer, L.J., Magnetic Structures of Er203 and Yb203, Phys. Rev. 176 ('1968) 72~ 31 and references therein. Rowell, J.M., Tunneling Anomalies in B u r s t e i n , E l i a s and L u n d q u i s t , S t i g . (edsO T u n n e l i n g Phenomena i n S o l i d s , (Plenum New York, 1969) 385-404. T s u i , D.C., Detz, R . E . , and W a l k e r , L. R., M u l t i p l e Magnon E x c i t a t i o n i n NiO by E l e c t r o n T u n n e l i n g , Phys. Rev. L e t t . 27 (1971). 1729-1739.