PhysicaC 235-240 (1994) 3295-3296
North-Holland
PHYSICA 0
The superconducting proximity effect in Nb/Fe multilayers G. Verbanck, C.D. Potter, R. Schack P. Belien, V.V, Moshchalkov and Y. Brnynseraede Laboratorium voor Vaste-Stoffysika en Magnetisme, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium. We prepared Nb/Fe multilayers with varying Nb or Fe thickness by means of MBE. The x-ray diffraction patterns show good layering quality. Critical field measurements as a function of temperature and angle demonstrate that very thin Fe layers ( -~ 12A) decouple the Nb layers completely thus leading to 2D behaviour of the multilayers. The dependence of Tc as a function of Nb thickness and the critical fields are well described by the theoretical model derived for the proximity effect in superconductor/ferromagnet multilayers.
1. INTRODUCTION The interaction between superconductivity and ferromagnetism has been intensively studied during the last decade. One way to understand the character of this interaction is to investigate the proximity effect in superconductor/ferromagnet (S/F) multilayers. This work presents the experimental data which prove that, contrary to the superconducting multilayers with normal metallic separator layers, Fe layers are very efficient in decoupling superconducting sheets. As a result, a very thin metallic Fe layer ( ~ 12A) decouples completely the Nb planes in Nb/Fe multilayers.
temperature and about 5/d~cm at 10K. In order to study the influence of the Fe thickness we prepared Nb(dNb)/Fe(dFe) multilayers with dFe varying between 2A and 34A in steps of at most 2A, while dNb was kept constant at 400A. For dFe<12A the critical temperature decreases monotonically from 8.8K to 5.9K, while the parallel critical field Hc2//shows a crossover from 3D close to Tc to 2D at lower temperatures. For dFe > 12/~ Tc is much less dependent on the Fe thickness. In this regime the Nb layers are completely decoupled since Hc2// (1-T/Tc) IA ,in agreement with a 2D thin film in parallel field, and the angular critical fields are well described by the Tinkham formula for a thin film (see solid lines in fig, 1):
2. RESULTS AND DISCUSSION Hc2(O) lsin(O)i/Hc2 _L +[ttc2(0) cos(O)/Hc2/Tl 2 = 1
The Nb/Fe multilayers were prepared in a Riber MBE system by e-gun evaporation. The rate of the e-beam evaporators is stabilized within 1%. The samples were grown at 150°C on MgO(100). The top and bottom layers of the actual multilayers are always Fe. For series with varying Fe thickness the number of Nb/Fe repetitions is five, in the other case the structure consists of three Nb/Fe bilayers. The final Fe layer is capped by 20A of Nb to prevent oxidation. Low angle x-ray diffraction on similar films (i.e. thinner Nb thicknesses) show good layering quality. The high angle x-ray diffraction spectra clearly show the Nb(ll0), Nb(200) and Fe(200) Bragg reflections. The Fe(200) peak is shifted by about 2 ° from its bulk position, indicating that the Fe is stressed. Typical resistivities are 20#flcm at room
where 0 is the angle between the field and the film plane.
I~'='
dNb~O0~ d~=12~
1.5 1 10.5 ' ~=O.g4T/Tc=O.94 0
I
0
,
I
20
,
l
40
-,
-
-I
60
-,
-I
80
-,
-
IO0
Angle (degrees)
Fig. 1 Angular dependence of the upper critical field
0921-4534/94/$07.00 © 1994- ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)02213-5
G. Verbanck et al,/Physica C 235 240 (1994) 3295 3296
3296
In the theory for decoupled S/F structures by Radovic et al. ]1], the decay length of the superconductivity" into the ferromagnet is ~F = (2hVFIF/3~rI)½. where xT is the Fermi velocity, IF the mean free path and 2I the exchange splitting of Fe. Since a 12A Fe thick layer decouples the Nb planes we can put ~F= 12A. and assuming IF ~ dFe, we estimate the exchange splitting to be about 2.9 eV. This value is of thc same order as the d-d exchange splitting for Fe (=2eV) and much bigger than the s-d exchange splitting (~0.2eV). This might indicate that the proximity induced superconductivity in the Fe is carried by the d electrons, as proposed by Koorevaar et al. [2]. Fig. 2 shows the dependence of T c on dNb for multilayers with dFe-25A.
temperature for dNb-> 565A duc to a crossover from 2D to 3D behaviour.
dNb=456/~ d~=566/~
L0
- -
Radovic et al. [1 ]
0.8 z
~"
0.6
"~
0.4
0.2
"-
0.4 TflT~(Nb)
i -
0.6
-
0.8
Fig.3 Temperature dependence of parallel (solid lines) and perpendicular (dashed lines) upper critical fields for Nb(dNb)/Fe(25A) multilayer.
0.2 0
0
"%
,
I
L
300
~
i
i
I
~
400
~
~
i
I
~
500
i
~
~
I
~
600
i
i
L
I
~
,
700
Fig.2 The Tc suppresion in Nb/Fe multilayers with the fixed Fe thickness. The reduction of the critical temperature in the nmltilayers is well described by the above mentioned theory for the proximity effect of a ferromagnet. In this model, the critical temperature as well as the parallel and perpendicular critical fields are determined by dNb, the coherence length ~s=Tr~GL(0)/2 and a phenomenological parameter c [1]. By fitting the theory to the data we find e=3.4 and ~S =78/k, in agreement with the result found for Nb/Gd [3]. In fig. 3 the lines show the calculated Hc2//and Hc2 3_ using the same values for ~ and ~S, and the symbols are the measured upper critical fields. Again there is a good agreement, except some deviations at low
To conclude, we have shown that 12A Fe decouples the Nb layers completely leading to 2D bchaviour of the multilayers. For decoupled multilayers tile critical temperature and fields as a function of Nb thickness are well described by the model for tile proximity effect in superconductor/ ferromagnet structures. ACKNOWLEDGMENTS
Work supported by the Belgian High Temperature Superconducting, Concerted Action and Interuniversity Attraction Poles Programs at K.U. Leuven. G.V. is supported by the Interunwersity Institute for Nuclear Science. R.S. is a Research Fellow of the European Human Capital and Mobility Program. REFERENCES
111 Radovic et al., PRB 38, 2388 (1988) [2] P. Koorevaar et al., PRB 49, 441 (1994) 13] C. Strunk et al., PRB 49, 4053 (1994)