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Submitted on 1 Jan 1978
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MAGNETIC AFTER EFFECTS IN A TYP-II
SUPERCONDUCTOR
H. Rogalla, C. Heiden
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, suppliment au no 8, Tome 39, aotit 1978, page C6-657
MAGNETIC AFTER EFFECTS I N A T Y P - I 1 SUPERCONDUCTOR
H. Rogalla and C. Heiden
Inst.
fl&Angew. Physik der
Justus
Liebig-UniversitBt GieBen Heirzieh
Buff-Ring
16, 0-6300GeBen, Gemany
Rdsumd.- Les effets de "bascule et reptation", phcnomane connu du comportment des matdriaux ferro-
magngtiques
/ I /soumis
iiun champ alternatif et un champ magndtique constant, existent lgalement
dans les supraconducteurs de type 11.
Abstract.- Periodically swept minor hysteresis loops of type-I1 superconductors show a creep effect,
that consists of a) a displacement and b) a tilting of the minor loop. The initial dependence of the
creep on the number of swept minor hysteresis cycle can be described by a logarithmic law, devia-
tions occur at high cycle numbers. The effect appears not to be due to simple thermal relaxation
:for not too fast sweep rates no for dependence of creep rate on the total measuring time was found
within accuracy of measurement. Moreover the flux density.gradient appears to increase with increa-
sing cycle number in the region of the sample which is subject to periodic magnetization change
indicating a more effective pinning of flux lines.
Hysteretic ferromagnetic or ferroelectric
materials often exhibit after effects, in which
domain walls move to energetically more favorable
positions 121. These effects can be observed under
yarious experimental conditions, a special one being
lreep and tilting of periodically swept minor hys-
teresis loops under the presence of a constant ma-
gnetic dc-bias field. In the case of type-II super-
conductors such a field program is used in methods
to determine the flux density profile in the mate-
rial
/ 3 , 4 , 5 / ,and the question arises whether the
swept hysteresis loops are stationary or undergo an
after effect similar to that observed in ferroma-
gnetic materials
161.
As a prerequisite for studying such an effect,
one needs a system to measure the magnetization of
the superconducting specimen under test with suf-
ficient resolution over comparatively long time
periods. Due to thermal emf's, the necessary low
drift hardly can be realized using conventional
electronic integration techniques. We therefore de-
veloped a measuring system based upon the use of a
microwave biased rf-SQUID wit,h fast response
(2
x
10'flux quantals) 171, the schematic diagram
of which is shown in figure 1.
The magnetic field H is generated with an
accuracy of 2 x
1 0 ' ~by a superconducting solenoid
fed from a precision current supply. Well defined
programs for the field sweep can be run by means of
a microprocessor, which via a 16 bit DAC provides
Fig.
1 :Scheme of measuring system
the input signal of the current generator and also
controls the operation of the whole system. An ad-
justable differential flux transformer system, con-
sisting of three separate loops a, b and c, is used
to measure the magnetization of the specimen under
test. The input loop of transformer c can be adjus-
ted such that either a differential flux proportio-
nal to the induction
Bor to the magnetization M of
the specimen is coupled to the SQUID. The resolution
of the present system is ca. IuT and the drift less
than 3yT/h, allowing magnetization measurementswith
negligible drift over a period of time of the order
of 10 h, that is presently limited only by helium
boil-off.
Magnetization curves of a vanadium sample
obtained with this system are shown in figure 2.The
behaviour of a small minor loop starting at pointA
of the virgin branch
1for linear periodic field
I LOO 500
Fig. 2 : Magnetization c u r v e s of a c i r c u l a r v a n a d i u m rod (99.96 % p u r i t y , outgassed t o some degree a t 1420°C i n UHV of c a . t o r r )
sweeps between f i x e d v a l u e s HI and H2 can be seen from t h e i n s e t , where. t h e I . , l o . , and 100. c y c l e i s
d e p i c t e d i n a B(H)-plot. Flux c r e e p i s c l e a r l y v i - s i b l e i n t h e displacement of t h e loops, and i s observed a l s o f o r o t h e r dc-bias f i e l d s along bran- ches 1 and 2 of t h e m a g n e t i z a t i o n curve. The c r e e p r a t e i s found t o v a r y along t h e minor h y s t e r e s i s loop, an e f f e c t t h a t becomes more pronounced f o r l a r g e r amplitudes of t h e a c - f i e l d 181. This l e a d s t o a t i l t i n g of t h e minor loop which was always obser- ved t o be connected t o a d e c r e a s e of t h e t o t a l f l u x d e n s i t y v a r i a t i o n i n t h e sample during t h e f i e l d sweep. Pinning of f l u x l i n e s obviously becomes more e f f e c t i v e i n t h i s process.
F i g u r e 3 shows t h e displacement
AB = B(H1,n)
-
B(HI, 1) of t h e i n n e r loop end p o i n tI 10 100
Number of Loops
-
AB .Il n ( n ) f o r t h e f i r s t , 100 c y c l e s . D e v i a t i o n s from
t h i s r e l a t i o n s h i p l e a d i n g t o a slower v a r i a t i o n of AB can be observed a t h i g h e r n-values. Apart from a d e v i a t i o n a t t h e h i g h e s t sweep r a t e
( I d ~ / d t
1
= 139 A/cm s ) which may be due t o sample h e a t i n g no s y s t e m a t i c dependence of t h e c r e e p r a t e dAB/dln(n) on I d ~ / d t l i s observed. The measuring time t h e r e f o r e does n o t appear t o p l a y a r o l e i n t h e observed a f t e r e f f e c t . This and t h e above mentioned f a c t , t h a t t h e o v e r a l l f l u x d e n s i t y g r a d i e n t becomes s t e e p e r i n t h e volume f r a c t i o n of t h e sample, i n which t h e p e r i o d i c m a g n e t i z a t i o n change t a k e s p l a c e , i s c o n t r a r y t o r e s u l t s o b t a i n e d f o r p u r e l y thermal a c t i v a t e d r e l a x a t i o n p r o c e s s e s a s f o r i n s t a n c e i n t h e f l u x c r e e p experiments using c y l i n d r i c a l tubes / 9 , 1 0 / . The p e r i o d i c f l u x s h u t t l i n g obviously a i d s t o t h e rearrangement of t h e f l u x l i n e s i n t h e sample. The dependence of t h i s p r o c e s s on m a t e r i a l parame- t e r s and t o what e x t e n t thermal motion comes i n t o p l a y i s s u b j e c t of f u r t h e r i n v e s t i g a t i o n s .References
/ I / NQel, L . , J. Physique (1959) 215
/ 2 / See e.g. K n e l l e r , E . , i n "Ferromagnetismus"
(Springer, B e r l i n , G t t i n g e n , Heidelberg) (1962), chap. 41
/3/ Campbell, A.M., 3 . Phys. (1969) 1492 141 R o l l i n s , R.W., Kiipfer, H. and Gey, W., 3 . Appl.
Phys.
9
(1974) 5392/5/ E c k e r t , D. and Handstein, A., Phys. S t a t u s S o l i d i (a)
2
(1976) 171/ 6 / For r e c e n t r e s u l t s s e e P o r t e s e i l , J.L., Physica 93B (1978) 201
-
/7/ Rogalla, H. and Heiden, C . , Appl. Phys.
14
(1977) 161181 Rogalla, H. and Heiden, C., Phys. L e t t .
63A
(1977) 63/ 9 / K i m , Y.B., Hempstead, C.F. and S t r n a d , A.R., Phys. Rev.
131
(1963) 2486/ l o / Antesberger, G. and Ullmaier, H., P h i l o s Mag. 29 (1974) 1101
-
Fig. 3 : Loop displacement AB(n) f o r d i f f e r e n t sweep r a t e s
