microwave-absorption monitoring of aging and thermal-cycling effects in cuprate superconductors

EI~gEVIER Physica C 227 (1994) 230-236

PHYSICA

Microwave-absorption monitoring of aging and thermal-cycling effects in cuprate superconductors

Ignacio Perez Naval.4ir Warfi~re Center, AircraJi Divtston, H'arminster, P-1 1,~9"4, ~ r'~

Sore Tyagi *

Department o f Physics, Drexel UniversiO', Phtladelphia. ['.4 19104, US t

Received 2 March 1994: revised manuscript received 25 .April 1994

Abstract

Using direct and magnetically modulated microwave absorption, aging and thermal-cycling effects have been studied in single crystals and thin epitaxial films of YiBa2Cu307 fabricated by laser ablation on LaAIO3 substrates. The aging is most likely due to oxygen loss from the surface layers since the aging effects can be nearly completely reversed by isothermally annealing the samples in oxygen. The relevance of these results to the performance of microwave devices is discussed.

I. Introduction

The surface resistance of good-quality single-c~'s- tal thin films of YIBa2Cu3O7 is about two orders of magnitude lower than that of copper films at the microwave X band frequencies at temperatures below about To~2, where Tc is the critical transition temperature [ 1 ]. This fact has been utilized to replace copper by the high-temperature superconductors (HTSs) in devices whose performance is largely governed by the surface resistance, Rs of the device. Since Rs for a superconductor increases quadratically with the microwave frequency, the relative advantages of replac- ing copper by the HTSs diminish above about 100 GHz and for temperatures above approximately 0.8To. Thus it is not surprising that as the technique for fabricating the HTS thin films have become more reliable, a variety of passive microwave devices, such

* Corresponding author.

as resonators, delay lines, and filters have been dem- onstrated [2]. Thermal-radiation sensors, and magnetic-field sensors based on the Josephson effect have also been reported [2]. Most of these devices had been fabricated using the low-transition temperature conventional superconductors and their performance extensively investigated well before the discov- ery of the HTS. However, their use was limited to specialized situations where the attendant complex- ity o f helium cryogenics could be justified.

To be of practical importance, such devices would have to be resistant to degradation of their superconducting properties upon thermal cycling and aging under the anticipated environment of their opera- tion. Buyuklimanli and Simmons [3] have reported studying the surface degradation of YBaCuO superconductors on exposure to air and humidity. The films and pellets were studied under normal and ac- celerated corrosion conditions of 80% relative humidity at 50:C. Formation of BaCO3 and Ba(OH )~

0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All righls reserved S S D 1 0 9 2 1 - 4 5 3 4 ( 9 4 ) 0 0 2 4 6 - C

L Perez, S. Tyagi/Physica C 227 (1994) 230-236 231

was detected. While it is true that real devices can be protected from such adverse conditions, the essential point of their study is that the rate of degradation was controlled by the grain-boundary effects. Samples with larger grain-boundary areas degrade faster than those with smaller grain-boundary areas.

Degradation of microwave properties (increased insertion loss, power-dependent surface resistance at very low power levels, etc. ) has been reported in thick- [4] and thin-[5] film devices even after one or two thermal cyclings through the transition temperature. Again the degradation is more severe in thick films than in thin films. One could therefore reasonably expect that as the film quality improves with better fabrication techniques and control parameters, the degradation of surface properties should not be a ma- jor concern. Such an expectation, however, may only be partially justifiable. It is now generally believed that "good" YBaCuO thin films (highly oriented, high Jc, correct stoichiometry, etc. ) do not deterio- rate with time under normal ambient conditions and are quite resistant to degradation under repeated thermal cycling through the transition temperature. The reason for such a view, which at best is only partially justifiable, is two-fold. First, it is true that under certain circumstances the degradation can be slowed down considerably, and second the detection of the initial stages of degradation will escape most of the conventional techniques employed to detect it. Furthermore, the claims of a lack of degradation are rarely supported by long-term quantitative investigations. For example, Brull et al. [6] have studied, over a period of fifteen months, the effects of aging and thermal cycling on the magnetic-shielding properties of good-quality thin epitaxial YBaCuO films grown on SrTiO3. The films with and without a pro- tective layer of amorphous YBaCuO were stored under normal atmospheric conditions and thermally cycled between room temperature and 77 K. It was found that the films even with an amorphous passivation layer degrade although at a much slower rate than the unprotected films.

Due to a short coherence length (0.3 to 2 nm) in the HTS, the electrodynamic response of these ma- terials is determined on the level of atomic scale dimensions. Thus even a large number of weakly superconducting regions with dimensions of the order of the coherence length will occupy a very small fraction

of any macroscopic sample. Such defects can go un- detected unless the technique employed is sensitive to the existence of a small number of weakly superconducting regions. The existence of such initial degradation will not be easily detected, for example, by the usual four-probe resistivity, AC susceptibility, and bulk-magnetization measurements.

One of the expected applications of the HTS microwave devices will be in satellite-based systems. The devices will be exposed to ionizing particle bombard- ment and y- and X-ray radiation. Such an exposure can degrade the device performance by modifying the surface resistance. Furthermore, in satellite-based systems, the closed-cycle refrigerator (cryocooler) appears to be the most likely system to be used. Un- der the vacuum conditions that exist the devices can degrade due to oxygen loss unless they are protected by a passivation layer. Thus a technique that can sense the initial stages of degradation of surface microwave properties of superconductors would be most desira- ble. Such a technique can also be used to study the efficacy of various passivation schemes. Here we re- port investigations of the aging and thermal-cycling effects on bulk single-crystal and thin epitaxial films of YBaCuO using the direct and magnetically modulated microwave absorption (MMMA) techniques.

2. Experimental

Several single-crystal samples for this study were provided by T.A. Vanderah of Naval Air Warface Center, Weapons Division, China Lake, California. Other single crystals were obtained from Superconix, St. Paul Minnesota. Studies on some of these crystals have been reported by us previously [7]. Epitaxial films 0.1-0.3 lain thick were prepared by target ablation using a pulsed excimer laser. The c-axis is oriented normal to the substrate as ascertained by X-ray diffraction. The microwave-absorption experiments were carried out using a modified Varian E-12 EPR spectrometer with an associated CTI Cryogenics closed cycle refrigerator. The samples were mounted on the end of a sapphire rod and positioned at the center of a rectangular TE,o2 microwave cavity. The microwave and the DC magnetic fields were along the c-axis of the samples. Once the samples are mounted on the sapphire rod for the microwave-absorption

232 I. Perez, S. Tyagi /Physica C 227 (1994) 230-236

experiments, they are left essentially undisturbed in the closed-cycle refrigerator. The samples were cooled down typically at a rate of 8 K/min and warmed up at a rate of 3 - 4 K/min. The temperature versus absorption data were collected during the warmup seg- ment of the thermal cycle. The typical storage pressure at room temperature in the vacuum shroud of the refrigerator was a few mTorr. Other relevant experimental details can be found elsewhere [ 7 ].

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3. Results and discussion

We have earlier reported a study of weak links in bulk single crystals using the magnetically modulated microwave-absorption technique [7]. In the follow- ing we briefly describe some of the relevant results from that study. The low-field MMMA in the Y~Ba2Cu3Ov single crystal samples, under appropri- ate conditions, is a series of well resolved lines periodic in the externally applied magnetic field [ 7-10 ]. The field periodicity of the microwave absorption is a consequence of the flux quantization through a contour containing a weak link [11]. It has been shown that a superconducting sample containing a weak link placed inside a microwave cavity acts as a self-formed SQUID-like structure in which the two SQUID junctions correspond to the same junction being crossed twice by the supercurrent [12]. The area that determines the flux-quantization condition is the effective area of a single junction, S= (22L+ w)d, where 2L is the penetration depth into the bulk on either side of the junction, w << 2L is the junction thickness, and d is the junction length. At a given temperature, there is a minimum incident power level below which no modulated absorption is seen. This power threshold effect is illustrated in Fig. 1. At incident power levels of P=0.1 mW or below no modulated-microwave absorption is observed. A single Josephson junction is activated at 0.15 mW, as the power is further increased the linewidth (the field separation between the upper and the lower lobes of a line) increases linearly with the microwave power as discussed in Ref. [7]. At P=0.85 mW, another junction is activated and a band of periodic lines appears superimposed on the first band. The lower lobes of some of the lines in this new band are indicated on the lower left-hand side of Fig. 1. Upon further in-

0'.4--0.2 ~ o12 0:4 H(Oe)

Fig. 1. Plots of magnetic field vs. modulated-microwave absorption in a single-crystal sample at 10 K at the various microwave power levels as indicated. The markers on the lower left-hand side show some of the lower lobe positions of a second periodic series with a power onset of about 0.85 mW. A field modulation of amplitude 50 mOe was employed.

60 70

B~ -'" ~ A

~o ~o -~ ioo

T(K)

Fig. 2. Plots of microwave power absorbed, P, vs. temperature in a YBaCuO single crystal. The sample was thermally cycled 18 times between 10 K and room temperature between plots A and B recorded 15 days apart.

creasing the incident-microwave power level, additional series appear and the spectra become so complicated that it is difficult to identify individual bands of periodic lines. The appearance of new series is also seen with increasing temperature when the signal is scanned at a fixed microwave power level [7]. At about To~2, in most single-crystal samples, the observed spectra become extremely complicated due to interference of spectra from an increasing number of activated junctions. Fig. 2 shows the plots of the temperature dependence of the power absorbed from the sample placed in a rectangular resonant cavity oper-

I. Perez, S. Tyagi /Physica C 227 (1994) 230-236 233

ating in a TElo2 mode in two different experimental runs. The sample had been thermally cycled between room temperature and 10 K 18 times between the two runs over a period of 15 days. Between the runs the sample was stored at room temperature at a nominal pressure of 10- 3 Torr. Notice from Fig. 2 that below about 70 K, the change in the reflected power is neg- ligible, and at about 80 K the change is only about a few percent. This change is a result of weakly superconducting regions created at the surface as a result of possible oxygen depletion and defect generation due to thermal cycling. This view is corroborated by the temperature dependence of the modulated-microwave absorption as shown in Fig. 3. The temperature region where the directly monitored reflected power shows the largest changes between the two scans is also the region where spike-like noise is dom- inant in the corresponding modulated scans. It is now well established that the origin of this spike-like noise arises from flux motion across weak links [8,9,13]. Fig. 4 shows the modulated absorption as a function of temperature after the sample has been stored in the closed-cycle refrigerator for about five weeks without further thermal cycling. An additional peak region can be seen in the 50-70 K region. This is the range where the 60 K phase has been observed by other techniques [ 14,15 ]. At this stage of aging the samples were annealed at 450°C in flowing oxygen for 60 h. The plots of power absorbed as a function of temperature are nearly indistinguishable from curve A of Fig. 2. However, the MMMA versus H scans in the temperature ranges 20-80 K contain

v

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}

I t i

60 70 80 90

T(K) Fig. 3. Plots of modulated absorption vs. temperature corresponding to the two direct scans of Fig. 2. The sample was cooled through the transition temperature in a DC field, HDc. of 100 Oe. A field-modulation amplitude of 1.25 Oe was employed. The plot A is amplified five times relative to plot B.

g

i I i i

6O 7O 8 0 9O T(K)

Fig. 4. Plot of modulated absorption vs. temperature for the YBaCuO single-crystal sample after storage in the cryocooler for four weeks. A modulation amplitude of 4 . 1 0 e was employed to suppress the spikes in the signal.

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+ o I ; "~0.5

9 T(K) B C..'

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Fig. 5. Plots of microwave power absorbed, P, vs. temperature in the thin film sample. The labels indicate the various aging treatments: (A) as fabricated, (B) after 16 thermal cycles over 15 days, and (C) 20 thermal cycles over four weeks. The inset shows the plots of APN vs. temperature, where AP~ is the change in P as a result of aging. The upper and lower curves of the inset are obtained, respectively, from curves C and A, and B and A. The solid lines are drawn as guides to the eye.

pronounced spike-like noise present in the aged samples.

The direct microwave absorption as a function of temperature after various aging times in a thin-film sample is shown in Fig. 5, while the corresponding scans of the modulated absorption are given in Fig. 6. In thin films, unlike in the single-crystal samples, we do not observe the clearly resolved field-periodic modulated absorption at low temperatures. The scans of the modulated microwave absorption as a function of magnetic field are shown in Fig. 7. The de- tailed nature of the hysteretic MMMA versus H plots

234 1. Perez, S. Tyagt /Physica C 227 (1994) 230-236

g

60 70 80 90 TIK)

Fig. 6. Plots of modulated absorption vs. temperature corresponding to the two direct scans of Fig. 2. The sample was cooled through the transition temperature in a DC field of 100 Oe. A field-modulation amplitude of 1.25 Oe was employed. The spectra were recorded with the DC field on during the warm up.

o= I ,

/

; ,o H(Oe)

Fig. 7. Plots of magnetic field vs. modulated-microwave absorption in the thin-film sample of Fig. 6 at 75 K. The upper and lower plots were obtained after recording curve A and C, respectively, in Fig. 6. The arrows indicate the direction of the magnetic-field scan.

has been discussed elsewhere [ 16,17 ]. Of direct relevance here is the observat ion that the M M M A signal observed in the aged film sample is characteris t ic o f granular samples that contain a large number of weak links. In our labora tory we have observed similar signals in freshly prepared YBaCuO textured thin films samples deposi ted on polycrystal l ine MgO substrates. After the aging exper iments the thin-fi lm

samples were annealed at 450°C in flowing oxygen for 10 h. As in the case of single-crystal samples, the P(H) versus T plots for the annealed samples and the as-fabricated samples are nearly indist inguisha- ble from each other. The M M M A versus H plots in the reoxygenated samples are featureless just as in the as-fabricated samples as shown in the upper plot of Fig. 7.

It has been shown that in the M M M A exper iment the HTS sample with a weak link acts as a self-formed R F S Q U I D where the two links correspond to the same junc t ion being crossed twice [12]. It is clear from Fig. 1 that the M M M A technique can detect a single weak link. However, it should be ment ioned that the possibi l i ty of the signal arising from a collec- tion of links with identical areas intercepting the magnetic flux cannot be ruled out. The technique can also detect that new junct ions are activated. The series of well resolved modula ted microwave absorp- t ion lines, per iodic in the externally appl ied magnetic field as shown in Fig. 1 were first reported in YBaCuO single crystal samples by Blazey et al. [8] . F rom this exper imental observat ion one would have hoped to construct an R F SQUID in a rather s traightforward manner . However, it turns out that the per iodic series are not reproducible over an extended period due to aging and thermal-cycling effects.

To different iate between the thermal-cycling and aging effects, it is useful to contrast the absorpt ion behavior in single-crystal and thin-fi lm samples. When exper iments are carried out with samples inside a cryocooler, some loss o f oxygen from the sample surface is to be expected, which in turn should modify the microwave surface properties. This in- deed is the case as indicated by the reoxygenation exper iments discussed above. However, in single-crystal samples, our observat ions indicate that the weak links are created, both due to oxygen loss and thermal cycling. The single-crystal samples in general contain many weak links even when they are freshly annealed. These weak links can arise due to twin boundaries, weak interlayer coupling along the c-axis, and a variety of growth related structural defects at the surface. The spectra s imilar to that shown in Fig. 1, where only a single junct ion is act ivated are not typical. However, it is possible to f ind samples, in which at very low temperatures and at low incident- microwave power levels only a few junct ions are ac-

L Perez, S. Tyagi/Physica C 227 (1994) 230-236 235

tivated. In such cases, the reproducibility of both the discrete field-scan absorption spectra at low temperatures and the near-To behavior of the direct and modulated-absorption signals is governed both by the number of thermal cycles the sample is subjected to, and the time of storage in the cryocooler. The field- scan spectra at low temperatures ( 10-40 K) are reproducible over about a week and after 10-12 thermal cyclings through the transition temperature. However, changes in the temperature scan similar to those shown in Fig. 2 are easily detectible after such treatments. Some samples were cycled only twice over a 24 h period to check the reproducibility of various signals. Over this period the signals are reproducible within the experimental error. However, when the same samples are cycled between 150 and 10 K six to eight times over a period of 24 h, changes in the field- scan and temperature-scan spectra become quite ev- ident. This is an indication that both the oxygen loss due to storage under vacuum and thermal cycling contribute to defect generation in the sample. One possible effect of thermal cycling could be the weak- ening of crystalline interlayer coupling giving rise to weak links. For example, Blazey et al. [ 18 ] have reported spontaneous cleaving of a YBaCuO single crystal sample as a result of thermal cycling. In a less severe case where the crystal does not cleave, we speculate that a reduction in the weak coupling along the c-axis between the micaceous layers of the crystal sample takes place due to thermally induced stress giving rise to weakly superconducting regions. The weak links thus created can survive to the lowest temperatures. Such weak links are to be contrasted with those that are created due to local depletion of oxygen. At very low temperatures the critical current of oxygen-depleted regions can attain values large enough so that such regions do not contribute to the modulated-microwave absorption. This may explain the pronounced noise in the MMMA signal in the 20- 80 K range which cannot be removed by reoxygenation. In thin films, the effects of aging are mostly due to oxygen loss. The change in the microwave power absorbed A/N, as a result of aging shown in Fig. 5 becomes detectable after storage at room temperature in the cryocooler for about a week. Unlike in the case of single-crystal samples, for thin films the number of thermal cycles do not change this time notice- ably. The initial effects of aging become most appar-

ent in a small (10-15 K) temperature region below Tc. As the aging continues, the temperature range in which AP~ is nonzero extends to lower temperatures. This can be seen in the plots of APN versus temperature in Fig. 5.

In published reports of the various HTS-based microwave devices the aging effects are rarely discussed. Furthermore, the change APN as shown in the inset of Fig. 5 is only about one percent at the liquid- nitrogen temperatures where most of the passive microwave devices are usually studied. This has led to the general perception that the devices perform re- producibly over extended periods of time. Such a perception may not be very sound.

The microwave devices reported in the literature can be divided into two broad categories. In the first category are the passive microwave devices exploit- ing the fact that the microwave surface resistance of HTS films is orders of magnitude lower than that of copper. These devices include resonators, filters, and delay lines. The performance of such devices is usually characterized by measurements of the Q value which is inversely related to the microwave losses in the device. The Q value measurement techniques usually employ signal-averaging procedures which will mask the increased noise, precisely the signature of the initial modification of the microwave surface resistance due to generation of weak links. Thus in sit- uation where the output signal from a passive microwave thin-film device can be processed to eliminate the increased noise due to weak links, the initial degradation may not be of any great practical consequence. However, it will be deleterious in cases where signal averaging is not possible, for example, in the case of a band pass filter used at the input end of sat- eUite-communication systems whose performance is limited by radio-frequency interference. In such cases the output from the band pass filter is directly fed to the input of the front end amplifier which is a deli- cate high electron mobility transistor. The noise from the band pass filter will result in gain compression and noise temperature increase.

The second category of thin-film microwave devices are based on the Josephson effect. A SQUID magnetometer is one such example. The primary contribution to the 1/ fnoise in such devices is the hopping of single fluxons or bundles of fluxons between trapping sites in the body of the superconduc-

236 L Perez, S. "l~vagi /Physica C 227 (1994) 230-236

tor [ 19 ]. Thus the performance of such devices, unlike the conventional SQUIDs, will also be subject to deterioration with time unless passivation schemes can be devised to retard the creation and growth of weak-link regions with aging and repeated thermal cycling. To test the efficacy of such schemes, the direct microwave absorption in conjunction with the modulated-microwave absorption technique which is sensitive to very small modifications of the microwave surface resistance should prove to be very useful. It the case of a SQUID sensor, the noise measurements are routinely used to characterize the device. Thus it can be argued that in the case of such sensitive devices, the noise measurements themselves can be used to monitor the effects of aging and thermal cycling. However, in the microwave technique the ef- fectiveness of the passivation layer can be evaluated before any patterning or any other device-fabrication and packaging steps are taken. APN in the case of the direct microwave absorption and noise measurements in the case of MMMA around 77 K could pos- sibly serve as quantitative measurements of aging and thermal-cycling effects.

Acknowledgements

One of us (ST) gratefully acknowledges the award of a fellowship under the US Navy-ASEE Summer Faculty Research Program. Thanks are also due to S.M. Bhagat and members of his research group for many useful discussions.

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microwave-absorption monitoring of aging and thermal-cycling effects in cuprate superconductors

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