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Submitted on 1 Jan 1978
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BREAKDOWN OF THE MAGNETIC SURFACE
BARRIER IN NIOBIUM BY OXYGEN DIFFUSION
G.P. van der Mey, P. Kes, D. de Klerk
To cite this version:
BREAKDOWN OF THE MAGNETIC SURFACE BARRIER IN NIOBIUM BY OXYGEN DIFFUSION
G.P. van der Mey, P.H. Kes and D. de Klerk.
Kamerlingh Onnes laboratory, Leiden, Netherlands.
Abstract.- Permeability measurements versus temperature on niobium oxidized at 330 and 200°C showed the existence of a diffusion zone with relatively large 0-concen-tration at the surface. This zone causes the breakdown of the magnetic surface irre-versibility.
1. THE OXYGEN PROFILE.- Oxidation of pure niobium at 400°C for 10 s has been applied successfully to suppress critical surface currents /!/. Evetts /2/ showed on a Pb-Tj alloy that a concentration gra-dient, accompanied by varying superconducting parameters at the surface, leads to a strong de-crease of the surface hysteresis. It is well known that interstitial 0 influences the superconducting properties of Nb appreciably /3,4/. But the maxi-mum solubility at 400°C of ^0.15 at.% cannot ex-plain the complete reduction of the barrier, whe-reas the occurence of precipitates should incwhe-rease the irreversibility strongly /3/.
In the present investigation we demonstra-te the exisdemonstra-tence of an O-diffusion larger afdemonstra-ter heating Nb strips of 30x3x0.155 mm at 330°C in an 0 atmosphere for 10 and 20 min., and 1 and 2 days. The samples will be henceforth referred to as Nb-10m/330, Nb-20m/330, Nb-ld/330, and Nb-2d/330. The RRR was > 400, the grain size ^ 1 mm; for de-tails about the sample preparation see reference 151. Zero-field permeability measurements were carried out as a function of temperature. Accor-ding to references /3/ and / 4 / T decreases strong-ly with increasing 0-concentration c.
Consequently, with increasing T, an 0-gradient at the surface leads to a layer in which T <T, while the interior is still in the Meissner
c
state. Taking into account the effect of the field penetration over an effective distance X c,(x)
de-fined by
1 T
Xg f f(x) = H" h(x')dx' (1)
we obtain for the permeability in ac fields of small amplitude and low frequency :
Here H is the applied field, h(x) the local field in the Meissner region, x is the distance from the surface, and d the half-thickness of the sample. The results below 4.5 K were independent of T, leading to u'(T < 4.5) = X ff(0)/d. Relating
T (x) to c(x) using Koch's data /4/ the 0 profiles could be determined. The circles plotted in figure 1 represent the data after correcting for X f f( x )
-X f f( 0 ) , which was obtained by solving the London
equation numerically with varying X (x).The latter follows from X2 = O /4TT).H „/H2 and data of /4/. The
o c2 c
value of X ,,(0) turns out to be ^0.2 um. The cur-err
ves in figure 1 obey
c(x) = c { 1 - erf (x/2v^t)} (3) in which c is c at the surface (^4 at % ) ; D the
diffusion coefficient, and t the effective oxida-tion time /5/. The diffusion depths /Dt are indi-cated in the figure. The value of D = 2.4 + 0.1 x 10"1 6 m2. s- 1 agrees well with data derived from
internal friction measurements /6/.
The insert in figure 1 compares the results of three samples Nb-2d/330 from two of which a
surface layer was removed by electropolishing. The positions of their surfaces are given by the bro-ken lines. The mutual agreement is convincing and demonstrates that oxidation of Nb generates a diffusion layer at the surface with concentrations up to 4 at 2. No precipitates could be observed by electron microscopy. Hence we conclude that for oxidation below 400°C supersaturated solutions of 0 may well be stable due to the very small preci-pitation rate.
JOURNAL DE PHYSIQUE
Colloque
C6,
supplément au n°
8,
Tome
39,
août
1978,
page
C6-651
Résumé.- Les mesures de perméabilité en fonction de la température sur du niobium oxydé à 330 et 200°C montrent l'existence d'une zone de diffusion avec une concen-tration d'O relativement grande à la surface. Cette zone provoque la diminution de la barrière magnétique de surface.
Fig. 1 : Oxygen profiles in four Nb samples oxidized at 330°c for 10
and 20 min, and 1 and 2 days. Circles : experiment; curves : eq. (3)
fitted to the data. Insert : Comparison of the oxygen profiles of
three samples Nb-2d/330 after removing a thin surface layer from two
of them. 0,
,
and A : nothing, 2.6, and 8.15 pm removed, respecti-vely.
2. BREAKDOWN OF THE SURFACE BARRIER.- Another se- tood from a modification of De Gennes' expression
ries of samples was prepared by successive oxida- for the Gibbs potential of a vortex near the sur-
tions of one strip at 200°C. The total effective face / 7 / , for the case that Hcl and Adepend on x.
oxidation times / 5 / were 5, 10, 20, 30, 40, 50, De Genne's formula was derived for K r > I and B=O,
60, 85, and 110 m. After each oxidation the magne- whereas for our type 1111 superconducting Nb h-21
tization curves in increasing and decreasing fields were measured at 4.2 K. The irreversibility ~ 4 n M
at several values of B was determined. The results ,ocb ---
a\
-
at0P m
normalized to ~ 4 p M of the sample before oxidation, O \\8
are plotted in figure 2 versus f i . The dependence \ \ Q n
on B is weak. The surface barrier starts to decrea- A\
\ O
se for
fi
5
150i
and reaches its lowest valuei
for 2 500
i.
%
a.A qualitative &planation is provided by
the strong variation of the superconducting para-
oJ
I I I I200 w 600 m A loo0
meters in the diffusion layer, in the region
a = -
x
<
4&. Above 4 f i they have already reachedtheir bulk values within a few percent. As long Fig. 2 : Relative decrease of the irreversibility
in 41rM at several values of B versus the diffusion
as 4& 5 - 25 (%680
1
/ 5 / ) , the diffusion layer depth. 0 , O , A, V : B = 1100, 1400, 1800, 2200 G,is too thin to become effective. This accounts respectively. Also shown is the dependence of X
on
6,
drawn curve. effBut we assumed that neither this, nor the fact References that B # 0, leads to significant changes. Therefore,
we tried to find a qualitative verification from
/I/ Berndt, J., Kartascheff, N., and Wenzl, H.,Z.
the relation Angew. Phys.
14
(1968) 305.4 2x
G(X,B=O) a
2
I H x 1-
f -H (bulk) 121 Evetts, J.E., Phys. Rev. (1970) 95. ",4Tr Cleff 1'
131 Desorbo, W., Phys. Rev.
132
(1963) 107.(I-exp(- ) ) } (4)
'eff 141 Koch, C.C., Scarbrough, J.O., and Kroeger,D.M.
Phys. Rev. (1974) 888.
where for our purpose we further approximated the - - . . 151 Van der Mey, G.P., Kes. P.H., and De Klerk,D. to pblished.
vortex-image interaction term f by exp(-2x/A eff) '
In figure 2 it is shown that Xeff is a monotonously 161 Powers, R.W., and Doyle M.V., J. Appl. Phys. 30 (1959) 514.
-
increasing function of6.
In addition, Hcl (x=O)171 De Gennes. P.G.. "Superconductivity in Metals
<< Hcl (bulk), depending on cs. If the thickness and ~ 1 1 0 ~ s " (~enjamii, New York) 1966, p. 76.
of the diffusion layer is sufficiently large, so that the external field has been screened off before Hcl (x) has reached its bulk value, i.e.
for 4 6
>
21eff(fi), the Gibbs potential willbe negative for all values of x. Therefore the surface barrier disappears for
