Microwave absorption, surface resistance and magnetization: Comparative studies of YBa2Cu3O7 thin films

1252

Journal of the Less-Common

Meta& 164 & 16.5 (1990) 1252-1260

MICROWAVE ABSORPTION, SURFACE RESISTANCE AND MAGNETIZATION : ~~A~~ STUDIES OF YBafi-& THIN FILMS J. DUMAS, S. REVENAZ, C.J. LIU, R. BUDER, C. SCHLENKER Laboratoim d’Etudes des Prop&es 38042 Grenoble Cedex, France

Electroniques des Solides, CNRS,B.P. 166,

S. ORBACH, G. MijLLER, M. HEIN, N. KLEIN, H. PIEL Fachbereich Physik, Berg&he

Universitat, Wuppertal, D-5600 Wuppertal 1, West Germany

We report the results of comparative studies of field modulated microwave absorption, surface resistance and remanent magnetization performed on YBa2Cu307 thin films prepared by rf sputtering, laser ablation and electrophoresis. The microwave absorption results are related to those of remanent magnetization. The role of flux penetration at planar defects or grain boundaries is discussed in relation with the film granularity. A correlation between a small grain size and an enhanced surface resistance is shown.

1. INTRODUCHON Since the discovery of high-Tc superconducting oxides, there has been a considerable amount of experimental studies of their microwave properties with or without an externally applied magnetic field. While dc resistance measurements only tbe existence

of a superconducting

path, microwave surface resistance measurements

information

on dissipative

anomalously

large residual losses in all kinds of superconducting

superconductorsThe

mechanisms

give information mainly near and above Tc and indicate

non-resonant

also

below Te t. Such measurements

low magnetic field microwave

give

have revealed

oxides, contrary to classical absorption has been widely

studiedz. It has been ascribed to intergranular regions acting as Josephson weak-links.The magnetic properties

of YBa$u307

measurements the echoism

thin films have been investigated

via

remanent

magneti~tion

as a function of maximum applied field and temperature. They yield information on of flux penetration and flux trapping at grain boundaries or in the grains as well as

on the vortex pinning energy.3 Since all of these quantities are influenced compare in this paper the corresponding

by the granularity of the high-l;: oxides 4, we will

results

on YBa2Cu307

films

with strongly varying

degrees of granularity. 2. EXPERIMENTAL TECHNIQUES The films used in this study were deposited by three techniques: t-f-sputtering,5 laser ablation(i=l and electrophoresis*.

Films prepared by rf-sputtering were deposited on ZtO3 or SrTiO3 substrates

with post-annealing in oxygen environment at 895 “C. Films prepared by laser ablation were in-sim grown on Y stabilized Zr@, or LaAlO3. Electrophoretic layers were deposited on Ag and heated in oxygen atmosphere for 140 hours at 920 “CTable 1 summsrizes some main characteristics

of the

0 Elsevier Sequoia, Printed in The Ne~erl~ds

1253

films. X-ray diffraction studies showed that film D was untextured, the others were textured and film C was epitaxial with the c-axis perpendicular to the substrate. Films B,C show a high degree of a,b orientation. Table 1. SU~W

of fiti character&ion.

Fifm Preparation method Substrate Thickness ATc (lo%-90%) T g”se’ Ref.

Te and ATc are determined by dc magnetic measurements.

LD2 ‘2 laser ablation

spu%?g

zroz

1lJm

<2K 9OK s

0.8p,m 3K 9OK 6

SS‘ 13 (& laser ablation la.40 -0.3j.tm
ES3 b @: electrophoresis Ag -1Sptrl 20K 90K 8

ES43 (E) electm phoresls Ag -25pm 6K 94K R

The field modulated absorption measurements were performed with a Bruker ESP3CO spectrometer operating at 9.4 GHz, equipped with a TEtm rectangular cavity. The zero-field cooled state was obtained by appropriate shielding with a l.r-metal box of the remanent field of the electromagnet. The onset of the su~r~nducdng

transition was detected from the change in the reflected power

from the cavity measured by recording the leakage current which represents the imbalance of the microwave bridge. This direct signal shows changes proportional

to the surface resistance of the

sample.The in-phase modulated signal was recorded as a function of temperature

via a lock-in

amplifier simultaneously with the direct signal. In the su~rconducting

state, when the system is driven by a ~nu~id~ly

field H,,,,,d (w) , with &2x detecti

varying weak magnetic

ranging from 1 to 100 kHz, the Fourier analysis of the direct signal

at the output of the microwave bridge has been performed.

The surface resistance

R,(T) of films Z, B,C was determined

by measuring the Q-factor of a

cylindrical copper cavity excited at 87GHz in the temperatum range 4.2-3oOK alternatively in the TEol3 and T&21 mode9. In a similar cavity lo excited in the Tl$t electrophoretic

mode

at 21.5 GHz, R, of

layers D,E was measured. A small de field was applied to the su~rconducting

sample during a separate temperature cycle. Magnetization was studied using a vibrating sample magnetometer equipped with a superconducting coil for temperatures between 4K and 300K and fields between O.lmT and 6T.The results reported here were obtained with the field perpendicular to the film. 3. EXPERIMENTAL RESULTS 3.1. Field modulated absorption and harmonic generation Figure 1 shows the reflected microwave power and the modulated signal measured simultaneously as a function of temperature for films Z, B, C. The modulated signal shows a sharp rise below a temperature T,’ slighty below the transition temperatureTc marked by a change in the temperature dependence of the power reflected by the sample placed in the EPR cavity. We find that Te- Te’ I= 2K to 4K in fdm Z, is less than IK in film B and less than 0.2K in film C.

1254

B

ii! ITC

I

-k

v

1

75

T(K)

II

I

85

9

80

I

SO

T(K)

Fig. 1. Reflected power (right scale) and modulated signal (left scalehneasumd in films Z,B,C as a function of temperature. The microwave magnetic field is parallel to the c-axis. The Fourier analysis of the power reflected by the cavity shows in all the films the generation of high-order

harmonics

of the oscillating

magnetic field fundamental

frequency.However,

the

harmonic generation phenomena are very different in films Z and B (Fig.2). As the film Z is cooled below Tc, (Ho < 0. lOe), the fundamental is first observed. Additional even and odd ~2 harmonics appear at lower temperatures and are observable down to the lowest temperatures explored (30K). In film B, close to Tc , only even harmonics are observed, while at lower temperatures additional odd components

appearThe

same behavior is found for film C. In these two films, the harmonics

amplitude becomes vanishingly dependence

small below about 8OK. In Fig.3a is reported the temperature

of the n=2 and n=4 harmonic amplitude. It reveals a complex oscillating behavior

similar to that found by Xenikos et al.11 in ac susceptibility measurements. 50

KHZ

2 5

:h 0

4 5670 III.

L

83K

$ 6E

.,

0.5 F(MHz)

1

h

I

I

I

J

100

Fig.Z.(a) Film ZUpper curve : onset of the fundamental in the Fourier spectrum of the reflected power when the system is driven by a modulation field (F = 5OkI-k); Bottom curve :Fourier spectrum at lower tempe.tature.(b) Film B.Upper curve: onset of even harmonics close to T&Bottom curve: additional odd harmonics observed at lower temperatures.(F = 12.5 kHz).

1255

When a dc field I-IOis superimposed on Hrnctl the amplitude

of the n=2 component is found to be

field independent for & c Hma. It decreases rapidly as soon as Hg>7Hrncd.We also note, for Ho >o , even close to Tc, symmetry breaking effects and additional odd harmonics generation.We have also investigated the orientation dependence of the amplitude of the n=2 component.

The observed

angular variation when a film B is rotated about a vertical axis with the c-axis in a horizontal plane is shown in Fig. 3b. We find that the n=2 harmonic amplitude is minimum when the modulation field is parallel to the c-axis.

-651 180

I -90

&me

I 90

180

8

Fig.3. (a) Temperature dependence of the amplitude of the n=2 and n==4harmonics for a film B. (b) Angular variation of the amplitude of the n= 2 component. various models have been proposed to explain the low field microwave absorption properties of these mater&.

Most of them are based upon the existence of intergranular

Josephson

weak-

Iinks.We ascribe the change in slope of the reflected power as a function of temperature near T, to the onset of superconductivity would correspond

in individual grains . The onset of the modulated signal at T,< Tc

to the onset of Josephson

weak-links

between

grain boundaries

or planar

defectsll. The onset of harmonics below Tc in the Fourier spectrum of the reflected power indicates that the microwave losses are modulated . In film Z, only the fundamental n = 1 is involved while even components

appear just below TC in sample B. This indicates that the response of the system is

essentially linear close to Tc < Tc in sample Z in variance with the behavior of sample B of higher quality. The linear behavior reflects flux motion with almost no pinning just below T, . The existence of even harmonics just below

Tc,in sample

B, reflects their non-linear magnetization. At

lower temperatures, both odd and even harmonics are observed in samples Z and B which show a different behavior well below TC . This indicates that samples Z are still dissipative down to the lowest temperatures of measurements (T = 30K) in variance with sample B where the amplitude of the harmonics is vanishingly

small a few degrees below TC. It is very likely that the harmonic

1256

content and its temperature

dependence reflect the temperature and field dependence of the critical

current Jc in the vicinity of Tc. Harmonic generation has been discussed either in relation with the non-linear nature of Josephson junctions13

or in the context of the Bean model 14. Our results, together with the observation

some films of Blazey-type support

in

peaks in the modulated absorption2 as a function of a dc field, rather

the Josephson weak-links model.They may be very useful for the understanding

of the

modulated signal. 3.2 Surface resistance The effective surface resistance values of in-situ grown (B,C) and strongly c-axis textured (Z) film measured at 87GHz are compared in Fig.4a. For a quantitative comparison, the film thickness has to be taken into account

if it is of the same order of magnitude as the penetration depthls.

Therefore, the corresponding intrinsic Rs values of film C are somewhat reduced especially close to and above Tc. while those for films (Z,B) remain, below Tc , about the same as in Fig. 4a. Obviously, sharp microwave transitions and low residual losses are observed for epitaxially grown films. In contrast, the polyctystalline film Z gives a much broader transition. However, the residual Rs of this film is comparable to that of film B also deposited on 2502. Since films grown on SrTiO3 and LaA103 give lower residual Rs(4.2K) values than films grown on ZI@, the role of lattice mismatch and substrate quality on the microwave performance has to be studied in more details. The temperature dependence of Rs for thick films @,E) is shown in Fig.4b.These data are taken at 21.5 GHz and have to be scaled about quadratically with frequency’ for comparison with absolute values of Fig. 4a. For such samples, Ra(77K) is at least one decade higher than for films (B,C). These additional losses reasons. Fit,

can be attributed to the strong granularity of the thick films for several

the textured layer gives a sharper transition in zero field than the untextured one.

,0-31

, 0

,

,

40

, 00

T(K)

,

1 120

m-31 0

I

I 40

I

1 80

I’

I 120

T(K)

Fig.4. (a)Surface resistance Rs (T) of films B,C and of a Z-lie film (SG2) at 87GHz. For film C, Rs is an effective value since de h in the superconducting regime.(b) Surface resistance of electrophoretic films at 21.5 GHz: lA, 1B untextured layer (type D sample), H=O, H= 13 Oe respectively; 2A, 2B textured layer (I!) , H= Heam, , H= 130e respectively. dc field parallel to the film.

1257

A superposed dc field of 130s parallel to the sample leads to enhanced losses which are stronger for the untextured temperatures

sample16. It is remarkable

below

similar experiments

that the influence of a dc field is restricted

Tc’ as indicated in Figdb and also reported in Ref.lZ.One at 87GHz on a C-like film with a superimposed

to

should note that

dc field up to 70 Oe has

revealed no additional losses within the experimental accuracy. The temperature difference

between

the onset of the microwave transition and Tc’ depends on sample quality and reaches about 2K for typical thick films. In summary, the enhanced losses of polycrystalline

layers

as compared to

epitaxial thin films are most likely due to the higher degree of granularity and to the presence of weak-links which are highly sensitive to the applied magnetic fields. 3.3 Magnetic properties Hysteresis loops, shown in Fig. 5a, have been measured at T = 10K after cooling in a zero field (< 1Ge). The remanent magnetization field Horn (Ref.3).

Figure

5b shows

Mr has been studied as a function of the maximum applied Mr as a function of Horn for various

comparison, some values of Mr on electrophoretic

samples.

For

layers are also shown. Mr values obtained with

large Horn are typically two orders of magnitude larger for epitaxial films than for granular films deposited on Zr@ or prepared by electrophoresis.The

low field values of Mr (I-&,m< 10 Ge) are

much larger for granular films than for epitaxial films. The remanent magnetization has also been studied as a function of temperature. At low temperatures (Tad2),

Mr obeys approximately a linear law :Mr (T) = Mr (O)( l- y T/T, ) as shown in Fig. 6a for

samples Z and B. The slope y is found to be dependent on Horn . Fig. 6b shows the slope y as a function of Hem for films Z and C.The tempemtum dependence of Mr is usually attributed to flux creep. In a first approximation , y is related to the vortex pinning energy Uo through the relation :

y = (kTJ Uo ) In (o@ O) , where wg is a characteristic frequency of vortex vibration and o related to the characteristic time scale of the measurementWe

have evaluated the order of magnitude of Uo

with In (@ o) = 20. Values in the range 50-90 meV ( shown in the right scale of Figure 6b) are found for large Horn. The large differences

found for Mr obtained with large Horn in granular films (Z, D, E) and

epitaxial films (C) am due to the geometry of macroscopic currents appearing in the critical state. Within the Bean model17 , Mr = J&30, r being some characteristic length. While r is expected to be the average Elm radius in epitaxial films, it is the average grain size in granular films. In the epitaxial film C, one obtains Jc > 107Acm-* at 10K consistent with the transport valuet*. Assuming that the intragrain critical current is in all samples of the order of 107Acm-2, one obtains an average grain size ranging from a few pm to

1OOpm in different samples. For small Hour, the larger Mr found in

granular samples is due to an easier flux penetration at grain boundaries. The dependence of yen Hour may be accounted for by two mechanisms. In film Z, it may be due to a stronger pinning at grain boundaries (small Hour) than in the grains (large w).T’his not seem to apply for epitaxial film C.

picture does

1258

v , v ,m~,, -20000

-10000

0

10000

20000

Fig. 5,(a) Hysteresis 10op~for a POE film 2 and an epitaxialfilm C. (b) mmanent magnedmtion vs maximum applied field for different samples; T= 10K.

0 0.1 lo

H,m(Oef O3

106

Fig.a(a) Normalized remanent election vs reduced temperature; (b) slope of the curve Mr vs T at low temperatures vs maximum applied field at IOK for films B, C, Z, D, E. 4. DISCUSSION The three types of physical properties reported here are stmngly connected to the ~~~~ the samples as ascertained

by

the comparative

study of

of

Tc - Te’, Rs and Mr in different

samples.The onset of the modulated signal at Te’ is correlated to the dc field dependence of Rs near T,.Tbe sharp drop of Rs near Te is dominated by intragrain properties while the gradual decrease below Te ’ is dominated by intergranular regions acting as weak-links which lead to additional lossesSince

the residual microwave

losses may be related to grain boundaries,

one expects a

relation between Mr obtained for large II@ and Rs. Figure 7a shows Rs( at 87GHz) as a function of Mr measured at 1OK , for values of Rs measured at 4.2K and 77K.. A clear relation between both quantities appearThis relation corroborates that the limitations of Rs in tbe high-T, superconductors are at least partially due to defects like grain boundaries.Epitaxial

films give higher Jc and lower

R,because they contain less weak-links. In the same context, Fig. 7b shows Rs at 21.5 GHz as a

1259

function of grain size in electrophoretic boundaries.

layers.This figure shows that low R~require less grain

The role of pinning remains to be elucidated.

inhomogeneous

High Mr and high Jc values need

pinning but low Rs values do not requite pinning.

100

10-l B z 10-2

Fig.7 (a) Surface resistance at 87GHz at 4.2K (0) and 77K (0) as a function of remanent magnetization measured at 10K after application of a field Hum= 2T 1 to the film. For samplesD and E, R,(87GHz) is estimated from Rs(21.5 GHz) assuming a square frequency law. (b)Surface resistance at 21SGHz and 77K for electrophoretic layers with average grain size determined by counting the number of grains in a given surface area 5. CONCLUSION Our comparative

study of modulated absorption,surface

resistance, magnetization

of films of

different quality has shown strong correlations due to the granularity of the samples.Further should include a detailed comparison

of the microwave

and magnetic properties

work

in the same

temperature range.The combination of the three techniques, should improve the understanding of basic microwave and magnetic properties and give some insight into the present limitations of the anomalously high losses in these materials. ACKNOWLEDGEMENTS This work was partially supported by an ECC-SCIENCE Contract SC-0038C(CD). The authors gratefully acknowledge

B. Roas and L. Schultz (Siemens AG, Forschunglaboratorien,

Erlangen)

for providing laser ablated films on LaA103 and B. Stritzker for the films prepared at KFA, Julich. They have also benefited from the contribution of J. Marcus for films prepared by rf-sputtering. REFERENCES 1) G. Mtiller, in : Proc. of the 4th Workshop on r-f superconductivity, 1989),KEK Report, Vol. 21, ed.Y. Kojima, p. 267 (1989).

T&tuba,

Japan (Aug.

2) A.M. Portis, in : Springer Series in Solid State Science, Vol. 90, eds. J.G. Bednorz and ‘K. A. Mtiller ( Springer Verlag, 1990) p.278.

1260

3) R. Buder, C. Filippini, H. Guyot, C. Liu, J. Marcus, S. Revenaz, P.L. Reydet, C. Schlenker, in : Studies of High Temperature Superconductors, Vol. 6, ed. A.V. Nadikar, (Nova Science Pub. 1990), in print. 4) J.R. Clem, Physica C153-155 (1988) 50. 5) J.25 Bruy&re, J. Marcus, P.L. Reydet, C. Filippini, C. Schlenker, Mat. Res. Bull. 23 (1988) 6) J. Frlihlingsdorf, W. Zander, B. Stritzker, Solid State Comm. 67 (1988) 965. 7) B. Roas, L. Schultz, G. E&es,

Appl. Phys. Lett. 53 (1988) 1557.

8) M. Hein, G. Mtiller, H. Piel, L. Ponto, J. Appl. Phys. 66 (1989) 5940. 9) N. Klein, G. Mtlller, H. Piel, B. Roas, L. Schultz, U. Klein, M. Peiniger, Appl. Phys.Lett. 54 (1989) 757. 10) G. Milller, D.J. Brauer, R. Eujen, M. Hein, N. Klein, H. Piel, L. Ponto, IEEE Trans. on Magnetics 25 (1989) 2402. 11) D. G. Xenikos, T.R. Lemberger, Phys. Rev.B41 (1990) 869. 12) S. Revenaz, J. Dumas, S. Tyagi, P.L. Reydet, C. Liu, Solid State Comm. 72 (1989) 1009. 13) C. Jeffries, Q. Lam, Y. Kim, C. Kim, A. Zettl, Phys. Rev. B 39 (1989) 11526; M. Golosovsky, D. Davidov, C. Rettori, A. Stem, Phys. Rev. B40 (1989) 9299. 14) L. Li, R.H. John, G.C. Spalding, C.J. Lobb, M.Tinkham, Phys. Rev. 40 (1989) 10936. 15) N. Klein, H. Chaloupka, G. Miiller, S. Orbach, H. Piel, B. Roas, L. Schultz,U. Klein, M. Peiniger, J. Appl. Phys. 67 (1990) , in print. 16) M. Hein, S. Kraut, G. Miiller, D. Opie, H. Piel, L. Ponto, D. Wehler, M. Becks, U. Klein, M. Peiniger, Proc. of ICM’BO Topical Conf. HTSC Materials Aspects, Garmisch-Pattenkirchen, (May 1990). 17) C.P. Bean, Rev. Mod. Phys. 36 (1964) 31. 18) B. Roas, L. Schultz, G. Saemann-Ischenko,

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