Modulated microwave absorption of YBa2Cu3O7−δ thin films on LiNbO3

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0038-1098/8953.00+.00 Pergamon Press plc

Solld State Communications, Vol. 72, No. 12, pp. 1199-1201, 1989. Printed in Great Britain.

MODULATED

MICROWAVE

ABSORPTION

OF YBa2CU3OT_ 6 THIN F I L M S ON L i N b O 3

K.W. Blazey and A. HOhler" IBM Research Division, Zurich Research Laboratory, 8803 Rnschlikon, Switzerland *Applied Physics Institute, University of Giessen, 6300 Giessen, W. Germany (Received October 12, 1989, by P.Wachter)

The temperature dependence of the modulated microwave absorption of YBa2Cu307_,5 thin films is shown to exhibit a maximum near Tc where the same signals for ceramic samples go to zero. ]'he maximum is very similar to that usually found in the imaginary part of the susceptibility measured at low frequencies due to the intragranular fluxons.

Soon after the discovery of superconductivity in doped copper oxides, ) there were many reports of microwave absorption of these materials using standard electron spin resonance (ESR) spectrometers. 2-g These measurements were made on the first available samples which were powders and ceramics. Microwave absorption In these samples is dominated by the Josephson junctions connecting the grains. Here we compare those results with measurements on thin films. At low temperatures the absorption again appears to be primarily due to junctions, but near Tc the modulated microwave absorption goes through a maximum that Is very similar to that usually seen in the temperature dependence of the imaginary part of the susceptibility measured at low frequencies. This is in sharp contrast to the results In ceramics where the modulated absorption goes to zero near Tc. An ESR spectrometer measures the field derivative of the field dependence of the microwave absorption by superimposing a small ac modulation field on an external varlable field and detecting the absorption signal synchronously with this modulation field using lock-In techniques. The variation of the resulting modulated microwave absorption with external field has been widely reported for ceramics 2-g and Is shown in Fig. 1 for a thin film of YBa2Cu3OT_6 on a EiNbO 3 substrate. With the external field applied perpendicular to the film the spectrum Is very similar to that obtained for a ceramic sample, but when the field Is applied parallel to the film there is no modulated microwave absorption signal as the external field does not penetrate the sample. At temperatures well below Tc the direct microwave losses are due to damped fluxon motion. 1° In the ceramic samples the fluxons in the Josephson junctions between the grains absorb most, 11 and hysteresis Is observed on sweeping the external field due to flux trapping. 12 However, the modulated microwave absorption discussed here is not simply the derivative of this direct absorption but depends very much on the amplitude of the modulation field. 13,t4 The field dependence of the modulated signal shows open hysteresis loops with a change of sign accompanying

each field sweep reversal for small modulation fields. With Increasing modulation field the loop gradually closes and the signal only changes sign on passing through zero field. The spectrum in Fig. I is an_ example with a relatively large modulation field. This complex behavior is due to the double role of the modulation field In the Josephson junctions of the ceramic sample. One role is that it sweeps fluxons In and out of the junctions and a second is its ability to modulate the critical current flowing at the surface. The former effect causes a linear change of absorption with modulation field proportional to the number of fluxons present. This Is the dominant effect for large modulation fields > I Oe. For smaller modulation fields the signal Is dominated by a nonlinear response ts due to the modulated critical current causing either Junction decoupling 16 or fluxon plnnlng/deplnnlng 17 near the surface. This gives an additional signal at low modulation fields only, which decreases with increasing temperature, disappearing

Z

o m

/t

_.1

o

I

5

-10 MAGNETIC

FIELD

(Oe)

Fig. 1. Modulated microwave absorption over a + 10 Oe sweep of a 300 nm film of YBa2Cu307_ 6 on LINbO 3 at 4.4 K. The Incident microwave power w a s 0.t mW and the modulation field 2 0 e .

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YBa2Cu307- 6 THIN FILMS ON LiNbO 3

1200

just below Tc.~4 On Increasing the modulation fields, this latter signal saturates. These two contributions to the modulated microwave absorption may be seen by measuring the signal as a function of the modulation amplitude with the result shown In Fig. 2. The hump at low m o d u lation fields is due to the modulated critical current, and the linear variation at large modulation fields Is due to the changing fluxon density. Furthermore their temperature evolution may be obtained from the curves In Fig. 2 by plotting the signal at a constant modulation amplitude as a function of temperature. Figure 3 shows that the modulated absorption component due to the critical current modulation, taken at 5 mOe, decreases nearly linearly over the entire temperature range, while that due to the changing fluxon density, taken at 2 0 e , Is nearly constant over a wide temperature range and drops to zero around Tc. The same experiment was carried out on thin films of YBa2CU3OT_ 6 deposited on single crystal LINbO 3 by planar dc magnetron sputterlngJ ° The results do not yield modulated microwave absorption signals with two clearly distinguishable components as just described for the ceramics. The variation of the signal with modulation field Is much weaker than for the ceramics by approximately one order of magnitude as shown In Fig. 4. Replottlng the data of Fig. 4 yieldsthe temperature dependence of the modulated signals at both small and large modulation as shown In Fig. 5. o The most striking feature of these results Is the maxlmum In the signal temperature dependence near Tc where they are decreasing In the ceramics. Moreover the temperature variation Is not as dependent on the magnitude of the modulation field as that of the ceramics. The modulated absorption maximum near Tc Is also observed In single crystals 1g,2° and shows that the microwave properties of thin films are more closely related to single crystals than to ceramics. The difference of the modulated absorption signal In Increasing and decreasing external magnetic fields may be taken as a guide of the hysteresis of the signal. The hysteresis defined In this way Is largest

for small modulations and reflects the Importance of the critical current flowing at the surface as Its origin. 13 This hysteresis Is always largest where the signal amplitude Is also largest, I.e. at low temperatures in ceramics s and near Tc for thin films which Indicates that the strong signals near Tc are related to the surface critical current. This Is also consistent with the curvature of the lines through the data points in Fig. 4 being somewhat similar to those in Fig. 2 at low modulations. 2O

15

(9

O 0 00 I--

,< _j 1C ,¢ z

20 40 60 80 TEMPERATURE (K)

100

Fig. 3. Temperature dependence of the two contrlbutlons to the modulated microwave absorption of a ceramic YBa2Cu3OT_6 sample, x: surface critical current component at 5 mOe modulation. *: fluxon density modulation at 2 0 e modulation.

lO"

100

o~

4.4 K

0 0 CO

10:

z

10

4.4K

i

10 102 103 104 MODULATION FIELD (mOe)

10s

Fig. 2. Variation of the modulated microwave absorption signal at 80 Oe with the amplitude of the modulation field for different temperatures of a ceramic YBa2Cu3OT_6 sample.

i

r i,ll,l

. . . . . . . .

I

,

,

, ,,,,,I

,

,

, , , , ,

10 10= 10a MODULATION FIELD (mOe) Fig. 4. Variation of the modulated microwave absorption signal at 80 Oe with the amplitude of the modulation field for different temperatures of the YBa2Cu3OT_~ film used for Fig. 1.

104

Vol. 72, No. 12

YBa2Cu307_ 6 THIN FILMS ON LiNbO 3

1201

200

150 ¢D

O

O CO

100 ,<

z

.y I OI

0

I

I

I

f

20 40 60 80 100 TEMPERATURE (K)

Fig. 5. Temperature dependence of the modulated microwave absorption of the YBa2CU3OT_6 film at * 100 mOe and x 5 0 e modulation field amplitudes. It ts also noteworthy that the maximum In the modulated absorption near Tc looks very similar to the maximum In the Imaginary part of the susceptibility X" usually found In hlgh-Tc superconductors. 21 The maximum In X" has been explained In terms of the critical-state model21,22 and Is found to move to lower temperatures with larger modulation fields as also seen In Fig. 5 In the modulated microwave absorption experiment, Indicating their common orlgtn as an Intragranular property. In conclusion It would appear that the temperature

dependence of the modulated microwave absorption In thin films shows features very similar to the Imaginary part of the susceptibility predominantly due to Intragranular fluxons. Furthermore, It has the same form as the differential of the surface Impedance measured by standard microwave techniques. 23 While this may not be so surprising It was not the case for earlier experiments on the ceramic samples whose absorption Is primarily due to intergranular fluxons and disappears on approaching Tc.

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(19Be),