LT 21

Proceedings of the 21st International Conference on Low Temperature Physics
Prague, August 8-14, 1996
Part $2 - Superconductivity 1: LTS - Tunnelling phenomena, etc.

Flux Pinning and Dimensionality of Superconductivity in Multilayers
Manabu Ikebea, Yoshihisa Obi b, Hiroyuki Fujishiro a and Hiroyasu Fujimoriba Faculty of Engineering, lwate University, 4-3-5, Ueda, Morioka 020, Japanb Institute for Materials Research, Tohoku University, Aoba-ku, Sendal 980, Japan
Flux pinning mechanisms were studied for superconductor/normal metal (S/N), superconductor/superconductor (S/S')
and superconductor/insulator (S/I) multilayers. For the superconducting sublayer bib and NbTi alloy, for the metal
sublayer Ti and for the insulator sublayer AI203 were adopted, respectively. Under the parallel magnetic field, thepinning force was enhanced in quasi-two-dimensional (quasi-2D) and 2D regions of superconductivity and in thisenhanced region the peak of the pinning function Fp0t//)/Fpmax shifted toward a high reduced field h//. These
characteristics were found to be common in all types (S/N, S/S' and S/I) of the multilayers.

1. INTRODUCTION
The vortex pinning in a supeflattice superconductor
is an interesting problem because the layer structure
itself is expected to enhance the pinning force Fp. In
many materials such as high Tc oxide superconductors[1] and ct-Ti in worked NbTi alloys [2], plane-likestructures are actually at work as pinning centers. In
almost all circumstances, the surface or boundary ofdefect structures are important for the flux pinning.
Thus the layer structure is of the most fundamental
importance from the viewpoint of the flux pinning.

2. EXPERIMENTAL
Superconductor/normal metal (S/N, NbTi/Ti andNb/Ti), superconductor/superconductor (S/S', NbTi/Nb)
andsuperconductor/insulator(S/I,Nb/Al203)
multilayers were fabricated by an ff sputtering method.
The thickness of each sublayer was designed to be equal
except for Nb/Al2Oy For Nb/A1203, the thicknesses ofAl203 were changed for a fixed Nb thickness (=100/~).
The measurement of Jc was performed by a 4-terminal
resistance method mainly at 1.5K in a 9T
superconducting magnet. The magnetic field was applied
either parallel (H//) or perpendicular (I-Ix) to the layer
plane and the magnetic field was always perpendicular tothe electric current.

Fig. 1 shows the maximum value (Fp.tmax) of Fpx vs.
structural modulation wavelength ~. for NbTi/Ti, Nb/Ti
and NbTi/Nb. With increasing 7., the dimensionality ofthe superconductivity changes fzom three-dhnension(3D) to quasi-2D and from quasi-2D to 2D. Commonly
for all kinds of superlattices in Fig. 1, Fpa.max isenhanced in the quasi-2D and small ~. value 2D("shallow 2D") regions. This enhancement of Fpj. in thequasi-2D and in the "shallow 2D" region also occurs inNb/AI203 (S/I) multilayers as shown in Fig. 2. In Fig.
1 w e notice that the magnitude of Fpx is almost
independent of S layer materials (i.e., NbTi or Ni). Thisfact suggests that the layer structure really makes a main
contribution to Fpx. Comparing Fig. 1 and Fig. 2, we
notice that Fpx of Nb/A]203is almost twice as large asthat of other S/N multilayers. This comes probably
from the fact that the pinning force is given by the3. RESULTS AND DISCUSSION
We defme the pinning force density perpendicular(Fpx) and parallel (Fp/I) to the layer by the following
relations.
Fpx=(1/C) J X H//(I)F#/=(1/C) J x H xCzechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $2(2)Fig. 1. Fp•
vs. ~.for NbTi/Ti, Nb/Ti and NbTi/Nb.
The shaded region roughly corresponds to quasi-2D region.
719

I

10

I

'I'I'I

I'0o~(~!

1.5K

99EnO

E•0.6=o0LL40x9X~)&20.2I,0mI20I.

409I60,80It 20dAI203(A)Fig. 2. Fp.Lmax vs. d,~o s (=AI203 sublayer thickness)for Nb/AI203. The superconductivity is 3D ford/azo.a--5/~, quasi-2D for =10]~ and 2D for d/a2o3Z20/L
spatial derivative of that condensation energy U (Fp ,c8U/&v). Because of the absence of the proximity effect,
the order parameter should change spatially more steeply
in S/I system than SIN system at the boundaries.
Fig. 3 and Fig. 4 show the pinning function f,,L(=Fp.t/Fp.tmax) vs. reduced parallel field h a (=Ht/H~/) for
NbTi/Ti and NbTi/Nb. In Fig. 3, the peak of F~I shifts
toward higher h in quasi-2D (X--200A) and 2D Q.=300Aand 500A). The similar shift of the peak position related
to the dimensionality was also noticed for Nb/Ti [3]. We
have firstly noticed this type of the transition of thepinning function for Nb/AIzO 3 [4,5]. In contrast, we can
not clearly recognize the corresponding transition for
NbTi/Nb (SIS')multilayers. Takahashi-Tachiki [6] andKoorevaar et al. [7] suggested a complicated phase diagram of SIS' multilayers under Ha; the center positionof vortices is expected to jump between the two kindI "-,t.SKo~O

90xt,%e 9oE•0.6fX~a00.8, o Lo~E-dZid~rr-r----r--o4)_x~9)bo 9~

x@0 eg,"~0.4
LL0--l~pJ.max

r O A~

NbTi/TiI

00.2,&o~,~'*o~ .

J0.2x(A)o'~*/ oo

"1 x 9 120**no~..~

60/ 9I ,200280i - " ~ ' ~ """'~'. a400g0.60.81H///Hc2#
Fig. 4. fp.Lmax vs. h//for NbTi/Nb multilaycrs.
00.4

superconducting sublayers in a complex manner. This
complexity of file vortex site may masked out the simple transition of the pinning function peak pervasively
observed in superconductor/non-superconductor multilayers. The pinning force enhancement and the shift ofthe pinning function peak in the quasi-2D and the "shallow 2D" regions were not observed in Hi configuration(Fpu). This result also suggests that the observed anomalies in the vortex pinning really originate from the layerstructure.

4. CONCLUSION
1) The pinning force Fpl perpendicular to the layer (illparallel magnetic field) is enhanced in the quasi-2D mid
2D regions of superconductivity. This enhancement iscommonly observed in superconductor/nomml metal,superconductor/insulator
and also superconductor/superconductor multilayers.
2) In case of the enhanced Fp.t in quasi-2D and small X
2D regions, the peak of the pinning function shows acharacteristic shift toward a high field. In
superconductor/superconductor multilayers, however, thecorresponding shift of the peak cannot be confimled,
presumably because of complex motion of the vortex
center site in the S/S' system under parallel fields.
[1] M. Taehiki and S. T a k a h a s h i : Solid State Com. 70

LL~'a&X~lREFERENCES

ee

0.2

(3gzFp•NbTi/Nbe.

t O0009x d~ett.~0.40ImI

0.4
0.6
H///Hc2//x~ffx(A)
ox

50
I O0

*200

a300
500

oI0.8

Fig. 3. fp.Lmax vs. hi/for NbTi/Ti multilayers.

720

O~el~&0OX,~,10u_0OXA

0.889o ~,,o o :,,,, ,,1.5K(1991) 291.
[2] R.G. ltamphire and M.T. Taylor: J. Phys. ~ (1972) 89.
[3] Y. Obi, M. Ikebe, Y. Muto and It. Fujimori: Supplement
to Trans. JIM 29 (1988) 313.
[4] Y. Obi, M. Ikebe and 1t. Fujimori: Jpn. J. Appl. Phys.
31 (1992) 1334.
[5] M. Ikebe, Y. Obi, H. Fujishiro and It. Fujimori: Jpn. J.
Appl. Phys. 32 (1993) 55.
[6] M. Tachiki and S. Takahashi: Solid State Com. 72(1989) 1083.
[7] P. Koorevaar, W. Maj, P.tl. Kes and J. Aarts: Phys. Rev.
B47 (1993) 934.

Czech. J. Phys. 46 (1996), Suppl. $2