ELSEVIER Physica C 234 (1994) 229-231

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Surface barrier effects in non-resonant microwave absorption by thin superconducting films of YBaECU307_~

Amit Rastogi ~, V.V. Srinivasu a, M.S. Hegde b, S.V. Bhat a

a Department of Physics, Indian Institute of Science, Bangalore 560 012, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India

Received 29 July 1994

Abstract

We report an unusual temperature dependence of hysteresis in the non-resonant microwave absorption signals from supercon- ducting thin films of YBa2CU3OT_6. We observe that the hysteresis increases with increasing temperature up to Tc which we interpret as evidence for the presence of surface barriers in the single crystalline films.

1. Introduction

According to Bean and Livingston [ 1 ], there exists a barrier to flux penetration and also expulsion due to the surface which keeps the flux from penetrating in a superconducting sample up to a field Hp higher than H¢~. This surface barrier (SB) arises from the interaction of a vortex with its mirror image and shielding currents on the surface. So, the surface quality plays an important role in eliciting the effects of surface barriers in magnetization measurements. In the case of an ideal planar surface, flux would not penetrate until the external applied field reaches H~, the thermodynamic critical field. But in real samples because of the surface irregularities the effect of sur- face barriers is suppressed. This leads to a penetra- tion field H v less than Hc [ 2 ]. However, in HTSC, since the value of Ginsberg-Landau parameter x is large, the thermodynamic critical field Hc>>H¢~. Thus, as pointed out by Koshelev [ 2 ], even if there are surface in_homogeneities, surface barriers should be effective.

Koneczykowski et al. [ 3 ] have reported the pres-

ence of SB in untwinned single crystals of YBa2Cu307_6. They have shown that surface modi- fication suppresses the effects of SB on magnetiza- tion. They observe two signatures ofSB: (a) penetra- tion field Hp higher than Hc~, (b) zero magnetization in reverse sweep of the external magnetic field. Re- cently Zuo et al. [ 4 ] have reported the existence of SB in Ndl.ssCeo. 15Cu204+y single crystals.

We report here the presence of SB in thin films of YBa2Cu307_ 6 observed using the technique of non- resonant microwave absorption (NRMA) [5]. We observe an unusual temperature dependence of hys- teresis in NRMA signals which we interpret as due to the effects of SB.

2. Experimental

Thin superconducting films of YBa2Cu307_ ~ were deposited by pulsed laser ablation [6 ] on Y - Z r O 2

substrates. The c-axis oriented films (To ~ 86 K) were characterized by X-ray diffraction and four-probe re- sistivity measurements.

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230 A. Rastogi et al. / Phl, sica C 234 (1994) 229-231

The NRMA signals were recorded using a Bruker ER 200D X-band EPR spectrometer equipped with an Oxford Instruments CF 1200 helium flow cryos- tat. The direction of the DC magnetic field was par- allel to c-axis of the film. The field was swept between +30 G with 4 G (I00 kHz) modulation and 12.5 mW of microwave power. Temperature of the sam- ple was varied from 77 K up to Tc.

3. Results

Fig. 1 shows the NRMA signals recorded at var- ious temperatures. The signal seems to be a combi- nation of two different contributions: a central nar- row signal arising from a weak link like the response of the twin planes and a second broad signal which could be attributed to the response of the bulk. We notice that the normal type of hysteresis in terms of displacement of signals in the magnetic field for for- ward and reverse magnetic field sweeps is absent for both of these signals. This is to be expected in the temperature range studied due to the presence of flux flow. However, the signals are displaced in vertical direction for opposite directions of the field sweep. This we interpret as a change in the intensity of the signal for opposite sweeps arising from different flux densities inside the grains. Note that the intensity of the narrow signal which is riding over the broad sig- nal is the same for both sweep directions. It is ob- served that the difference in the intensities at zero field of the broad signal which can be called the zero-

" r -

"uJ

I I I I

-30 0 30 Field (6ouss)

Fig. 1. NRMA signals for forward and reverse sweeps of the field at different temperatures.

field hysteresis (defined as the vertical separation be- tween forward and reverse sweep signals at zero field) increases with increasing temperature up to Tc while the signal intensity falls down.

In Fig. 2 we have plotted the normalized hysteresis (defined as the ratio of the zero field hysteresis to the signal intensity ) against ( To- T)0.5, T being the tem- perature of the sample. The normalized hysteresis in- creases exponentially as we approach To. The solid line in Fig. 2 is the fit to the equation

y=6.1 exp[ - 1.7 (T~ - T) °'5 ] . ( 1 )

4. Discussion

Microwave power dissipation in single crystals and single crystalline thin film samples is due to the fluxon motion [ 7]. The free or weakly pinned fluxons are driven back and forth by induced microwave cur- rents. In this process the fluxons dissipate power. Thus free fuxons are responsible for the absorption. At low enough temperatures, pinning forces dominate and so we consider a fraction of fluxon density contrib- uting to the absorption. However, at higher temper- atures closer to T¢, thermal energy of the fluxons is sufficient enough to overcome pinning forces and so one can assume that all the fluxons are participating in the process of absorption.

Because of pinning, fluxons get trapped during in- creasing field. This leads to a hysteresis in NRMA signals. Usually as the temperature increases, fluxons

c

m

t b

- r

m ¢3

E o

Z

0.5 _T)1/2 3.0 (Tc

Fig. 2. Fit of Eq. (1) (solid line) to the normalized vertical hys- teresis (see text).

A. Rastogi et al. / Physica C 234 (1994) 229-231 231

get more and more thermal energy so as to surpass the pinning energy. Then there is lesser flux trapping and hence lesser hysteresis. This hysteresis due to bulk pinning leads to signals for forward and reverse mag- netic field sweeps at different values of the field (lat- eral hysteresis). Such hysteresis is absent in our sam- ple in the temperature range studied indicating negligible bulk pinning. Instead, a displacement of the signals along the y-axis (vertical hysteresis) is ob- served and it increases with the increase in tempera- ture. This hysteresis, is in fact, due to the difference in the dissipations for forward and reverse field sweeps caused by the different flux densities in the sample. The explanation for this unusual behaviour can be given in terms of surface barrier effects.

Let us consider a free flux density B such that B = H + 4xM where M is the magnetization and H the external magnetic field. As mentioned in Section 1, zero magnetization in the reverse field sweep is a sig- nature of SB. It has been reported mostly in un- twinned samples [3,4]. However, in laser ablated films there exist twin planes. The field may penetrate these planes though it may not have penetrated the bulk. Then the fluxon-density-created field inside the sample would exceed ]-/el and the field penetrates the bulk also. But this field could block the flux escape [ 1 ] in decreasing field sweep. In such cases magnet- ization will not be zero in decreasing fields but will have a different value from that in increasing fields. This difference in magnetization shows up as a ver- tical hysteresis in our NRMA signals.

Bean and Livingston [ 1 ] have suggested that the origin of surface barriers lies in two kinds of interac- tions: ( 1 ) repulsive interaction between the applied field and a fluxon near the surface which varies as e x p ( - x / 2 ) , where ;t is the penetration depth and x is the distance of the fluxon from the surface and (2) attractive interaction between the fluxon and its im- age, which varies as exp( - 2x/2). For high-T~ super- conductors the penetration depth 2oc ( 1 - T/Tc)-°5 [ 8 ]. Therefore the interactions are temperature de- pendent. As 2 increases with the increase in temper- ature, the interactions get stronger leading to an in- crease in the hysteresis as observed here in the case of thin single crystalline fdms of YBa2Cu3OT_~.

The fit ofEq. ( 1 ) to the experimental points shown in Fig. 2 indicates the essential correctness of this ap-

proach. The surface barrier effects could get sup- pressed if the surface has inhomogeneities. However, near To, since the size of vortices diverges [9], the influence of the defects becomes comparatively much smaller. Hence SB would be effective close to Tc de- spite the presence of surface defects.

In summary, we have observed the effects of sur- face barriers in thin single crystalline films of YBa2Cu3OT_a using the technique of NRMA. We find that there is a vertical hysteresis (unlike lateral hys- teresis due to bulk pinning) which increases unu- sually with the increase in temperature of the sample. Vertical hysteresis is believed to be due to the differ- ence in magnetizations in forward and reverse field sweeps which is one of the two signatures of SB. We do not observe the second signature viz. flux penetra- tion at a field Hp higher than H~I. This could be be- cause H¢1 ~ 0 due to the high demagnetization factor for rectangular shaped films and secondly due to the presence of twin planes in the film which behave as 'gates" for the flux entry.

Acknowledgements

The authors would like to thank the Council of sci- entific and Industrial Research and Department of Science and Technology, India for the financial support.

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