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A new technique for measuring the microwave penetration depth in high-Tc superconducting thin films
PHYSICA ELSEVIER
PhysicaC260 (1996) 81-85
A new technique for measuring the microwave penetration depth in high-Tc superconducting thin films Bekir Akta~ a, *, Hiiseyin Zafer Durusoy b a Department of Physics, Gebze Institute of Advanced Technology, 41400 Kocaeli, Turkey b Department of Physics, Hacettepe Unioersity, Beytepe, 06532 Ankara, Turkey
Received 19 December 1995
Abstract
The electron spin resonance (ESR) technique has been used to obtain the microwave (MW) penetration depth of
YBa2Cu307 thin films. An ESR-signal-generating marker was placed in between two high-temperature superconducting (HTSC) films to probe the MW field penetrating through the films. Below the superconducting transition temperature (T~) the HTSC film started to screen the marker inside the sandwich. A meaningfully diminishing ESR signal was reproducibly recorded for various samples with different markers such as paramagnetic diphenyl pierylhydrazy (DPPH), Mn ++ impurity in MgO and a ferromagnetic (FM) permalloy film. The temperature dependence of the ESR signal intensity above and below Tc has been studied to deduce the penetration depth from the measured signal intensity. A very rapidchange of A just below Tc, slowed down later at lower temperatures and became smoothly changing. The value hab = 1020 A has been measured for c-axis films at 77 K.
Since the discovery of HTSC [1] numerous scientific studies have been undertaken to understand the nature of HTSC by using various experimental techniques. Measuring the penetration depth and determining its temperature dependence accurately have been very challenging [2-4]. Experimental techniques such as stripline resonator [5], surface impedance [6-9], and MW transmission [10] have recently been employed to obtain penetration depth values. Experimental results have been almost always controversial with conflicting results which are usually attributed to a varying sample quality. There have also been several reports interpreting penetration-depth studies in terms of s wave and d wave
* Corresponding author.
models [11-13]. The ESR spectrometer has also proven to be a very useful tool to study superconducting materials. It has been used to obtain information on the surface impedance [5,8,14], complex AC conductivity in the GHz range [15,16] and MW penetration depth [10,15,16] in either bulk or thinfilm form. The technique is basically based on the measurement of reflected or transmitted MW power [10] through the sample. However, in each case the temperature dependence of the MW cavity limits the precision of the measurement. Researchers have also tried to obtain the DC (London) penetration depth from usual ESR line-broadening of the DPPH radical adsorbed on the surface where a vortex distribution exists [17,18]. In this study, w e have tried a new, simple and very sensitive technique to measure the penetration
0921-4534f96/$15.00 © 1996Elsevier Science B.V. All rights reserved PH S0921-4534(96)001 14-1
82
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
depth. High-quality YBa2Cu30 7 (YBCO) thin films on MgO substrates have been studied through ESR spectroscopy in order to obtain the MW penetration depth as a function of temperature. First, a free radical such as DPPH has been placed between two YBCO films and placed in the ESR cavity. The ESR signal from the sandwiched marker is proportional to the average MW power in between the YBCO films. Below Tc, the penetrating MW field would be damped exponentially through the YBCO films lowering the MW power in between and on the DPPH. Therefore, the ESR signal intensity, which is the field derivative of the MW absorption, is expected to be a direct measure of the average, MW penetration depth, A, within YBCO. This phenomenon has been used to measure the varying signal intensity and to calculate the penetration depth thereof. To the best of our knowledge, this technique where a paramagnetic free radical is sandwiched between two thin superconducting films has been used for the first time. It should be noted that a paramagnetic probe has been used before [17] to obtain the DC (London) penetration depth. However, in that case the probe was adsorbed on one surface of a YBCO pellet and then EPR line broadening due to spatial flux distribution was analyzed to extract A values. YBCO films are prepared by RF magnetron sputtering on polished MgO substrates in a UHV chamber. Films were extremely smooth and epitaxial. Judged by the XRD (002)/(005) peak ratio the films have been identified as a-axis or c-axis. For the c-axis films that were used in this investigation, this ratio was typically 0.003 while Tc was 86-87 K and the critical current density ranged about 10 6 A / c m 2 at low temperatures (Fig. 1). The YBCO films were spun with photoresist immediately after deposition and cut into 3 × 3 m m pieces to fit in the sample holder. After rinsing the photoresist, the ESRsignal-generating DPPH powder has been deposited on one YBCO surface. Next, two YBCO-on-MgO pieces were pasted with a bit of silicon grease with YBCO faces looking in and MgO faces looking out. Finally, silver paste has been used to seal joining lines to prevent any MW leakage (Fig. 1, inset). Samples were studied using a commercial, 9", Varian e-line ESR spectrometer operating in the TEl02 mode. The sample was placed at the center of the rectangular cavity where the magnetic-field com-
ponent of the MW is maximum and always parallel to the sample surface while the electric component is minimum and parallel to the external DC magnetic field. An Oxford continuous gas flow He cryostat was used for temperature control. The external DC magnetic field was applied to the sample surface perpendicularly. In Figs. 2 and 3, the ESR lines of DPPH sandwiched between c-axis YBCO films are shown near T~ at various temperatures. An example of the temperature-dependent intensity obtained from the second integration of one of the ESR lines is given in the inset of Fig. 2. Special attention has been paid to the calculation of the line intensity. Each line has been scaled to a standard in the second one of the double ESR cavity to avoid any error from the temperature dependence of the Q factor. As shown, the amplitude of the signal diminishes drastically with decreasing temperature just below T~ = 83 K. This sharp decrease in the ESR signal intensity below Tc is obviously due to the MW screening by the HTSC film sandwiching the paramagnetic probe. We would like to note that T~'s obtained under MW absorption are always a few K lower than those obtained through DC or low-frequency AC susceptibility measurements [19-21]. External DC magnetic field and the MW field is observed to suppress Tc by about 4 K in our case. The ESR line is distorted and shifted towards higher fields at lower temperatures as presented in
400
? ~
300
200
"~
100
50
100
150
200
250
300
Temperature (K) Fig. 1. Resistivity as a function of temperature. Note that the transition is very sharp and Tc(0) is at about 86 K. Sample geometry and field orientation are shown relative to the film plane in the inset. The marker is deposited just in the middle of the sandwich.
B. Akta}, H.Z. Durusoy / Physica C 260 (1996) 81-85
480 ¢D
i
~
i
i
i
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~
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i
T=84,86,88,90K I :... ~.:W:____'N=. ~,_
. . . . . . . . . 82.4K ......
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:~
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............................ ",.L~,.................. ~....................... [
._.~---L
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..................... ~........................ii~ ..........
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Magnetic Field(Gauss)
81..4K o
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~.:: -320
-~
3280
3285
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3290
Magnetic Field (Gauss) Fig. 2. ESR field derivative of DDPH placed in between two YBCO films at some selected temperatures. The inset gives the first and second integration (intensity) of the ESR spectra at 81.4.K
Fig. 3. This effect is a clear indication o f the setting o f vortices which cause an inhomogeneous local field on the marker [17,18]. Shifting is due to partial
30
20
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i
~
I
i
i
1
i.
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iL~
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: .........................................................
i
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screening o f the DC field in the mixed state where H~1 < H < Hc2. F o r the c-axis films this effect is more pronounced as the screening current flows in
i-
.........
~:/~\~
E
~-
~
i
i ...................................
7
-
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-
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. . . . . . . . .
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.................... i...................,.,.,.................... g f -.i:................ ~ ................................. !..................................................................
-20 3280
3285
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3295
3300
Magnetic Field (Gauss) Fig. 3. ESR spectra of DPPH at lower temperatures. Note that the lines are progressively distorted and shifted towards higher fields with decreasing temperatures. The penetrating DC field at about Tc is increasingly screened by DC supercurrents in the mixed state YBCO.
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
84
the layers of the C u - O planes for the perpendicularly applied external field. As the temperature is decreased further the ESR line becomes completely distorted. The ESR absorption as a function of magnetic field is proportional to the square of the magnetic component of the MW field at the DPPH marker. The classical expression for the field penetration is [221 h(z)
= h(o)e -(l+i)z/8,
(1)
where z is the distance measured from the outer surface of the YBCO films and 3 is the usual skin depth which is given by 6 = (2/to/x0o-)l/2; here ois the complex AC conductivity, to is the MW frequency a n d / % is the permeability of the medium. Actually the field penetration may not be a simple exponential for a film in the mixed state with a significant areal density of vortices. Nevertheless, w e use it as a first approximation. The inverse of the real part of the exponent in Eq. (1) can be taken as the penetration depth where the MW field decays to 1 / e of its maximum value [22]. Thus the ESR line intensity would be given by I ( T ) = Ce -(2 d/a(T)),
(2)
where d is the thickness of the YBCO film and C is a temperature-dependent parameter which is proportional to the square of the magnetic component of the MW power on the outer surface of the films, the susceptibility of the DPPH at temperature T and the spectrometer gain. From Eq. (2) we can easily obtain 2d )t(T) = In C - I n I ( T ) " (3) In the limiting case, for T>> Tc we expect )tab(T> T~) ~ ~ and we get the value for C(T) as In C(T) = In I ( T > T~) for values of T above the critical temperature. Indeed, this high-temperature limit just above Tc is observed in Fig. 2. We have carefully checked this limit at various high temperatures just above T~, such as 92, 94, 96 K, to reproducibly obtain the same spectrum at all temperatures. Normally, C would depend on the Langevin function for paramagnetic DPPH radicals but this temperature dependence would be negligible in small temperature intervals around T~. Thus L n ( I ) would be almost constant above T~ and equal to In C. Having obtained In C from the spectra just above Tc, we can use Eq.
106 ~la~a
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=3
i
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0 104 0
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.......
O
.-
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(3 ........
O
. [3
.......................
g. 100 76
78
80
82
84
Temperature (K) Fig. 4. Temperature-dependent MW penetration depth of YBCO film in a DC field of about 3300 G. The inset expands the lower part of the data for clarity.
(3) to obtain )tab(T) a s a function of temperature. The resulting )tab(T) values are plotted in Fig. 4. Data for lower temperatures are plotted in the inset for clarity. The severe distortion of the ESR line made it impossible to obtain correct measurements of the line intensity at much lower temperatures. Therefore, we prefer not to attempt an analysis of the temperature dependence of )tab by employing suggested models. Instead, in this report, we would like to simply present our novel technique and our direct observations near Tc where the behavior of Aab with the temperature is very similar to previous findings in the literature [8,10,15,16,23]. Starting from hab --~ 105 ,~ at 82 K and reaching Aab -'~ 103 A at 77 K, where )tab is decreasing very smoothly and slowly, a very rapid decrease in hab is observed just below To. It should also be noted that our value for )tab, just below Tc, is very close to those found near Tc for weak coupling ( A / k T c = 1.76) [8,23]. Below T~, the effective DC magnetic field between the HTSC films is a sum of the externally applied DC magnetic field and the field from DC supercurrents. Due to vortices in YBCO this effective field changes from point to point inside the sample. This inhomogeneous, effective field causes a distortion of the ESR line shape with decreasing temperature as is clearly visible in Fig. 3. In addition, a shift in the position of the lines due to the diamagnetic response of the YBCO is observed. The local average field on the spins decreases with decreasing temperature below T~ because of a progressing screening effect at resonant field Hcl < H < Hc2. In order to avoid dis-
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
tortion of the line shape, a FM film such as a permalloy film which is directly deposited on YBCO films, can be used as the ESR signal source. In this case the strong exchange interaction between the FM spins inhibits the line broadening caused by the inhomogeneous local field and gives much stronger signals. This type of samples would allow more precise calculations to be made from the ESR signal intensity which is a direct measure of the MW power. Initial experiments with a 1000 A FM permalloy thin-film marker directly grown on YBCO with a Au buffer layer has indicated that the distortion in the FM resonance line is indeed significantly reduced. This development allows the calculation of the line intensity to be made more accurately over a much wider temperature range down to liquid He temperatures. Another advantage of the FM film as an ESR signal source is that this intrinsic signal is nearly temperature independent far below the Curie temperature of the FM material. Therefore, the proportionality parameter, C, can practically be taken as a constant over a much wider range below the Curie temperature. In our preliminary experiments, Aab values similar to those obtained with the DPPHmarked samples have also been measured with the permalloy-marked samples of YBCO films. Work is under way to study this phenomenon further with ferromagnetic markers to take more measurements and to be able to make a more rigorous interpretation of the collected data.
References [1] J. Bednorz and K.A. Muller, Z. Phys. B 64 (1986) 189. [2] W.N. Hardy, D.A. Bonn, DC. Morgan, R. Liang and K. Zhang, Phys. Rev. Lett. 70 (1993) 3999.
85
[3] N. Kleing, N. Tellmann, H. Schulz and K. Urban, S.A. Wolf and V.Z. Kresin, Phys. Rev. Lett. 71 (1993) 3355. [4] P.A. Lee, Phys. Rev. Lett. 71 (1993) 1887. [5] D.E. Oetes, A.C. Anderson and P.M. Mankiewich, J. Supercond. 3 (1990) 252. [6] J.P. Turneaure, J. Halbritter and H.A. Schwettman, J. Supercond. 4 (1991) 341. [7] J. Owliaei, S. Sridhar and J. Talvacchio, Phys. Rev. Lett. 69 (1992) 3366. [8] J. Shibauchi, H. Kitano, K. Uchinokura, A. Maea, T. Kimura and K. Kishio, Phys. Rev. Lett. 72 (1994) 2263. [9] J.E. Sonier et al., Phys. Rev. Lett. 72 (1994) 744. [10] F.A. Miranda, W.L. Gordon, K.B. Bhasin, V.O. Hainen and J.D. Warner, J. Appl. Phys. 70 (1991) 5450. [11] P.W. Anderson and R. Schrieffer, Phys. Today 44 (1994) 54. [12] N. Bulut and D.J. Scalapino, Phys. Rev. Lett. 68 (1992) 706. [13] H.Z. Durnsoy, APS march meeting, San Jose, March 1995. [14] R. Durny, J. Hautala, S. Ducharme, B. Lee, O.G. Symko, P.C. Taylor, D.J. Zheng and J.A. Xu, Phys. Rev. B 36 (1987) 2361. [15] R.C. Taber, P. Merchant, R. Hiskes, S.A. Dicarolis and M. Narbutkovskih, J. Supercond. 5 (1992) 371. [16] B.W. Langley, S.M. Anlage, R.F.W. Pease and M.R. Beasley, Rev. Sci. Instr. 62 (1991) 1801. [17] B. Rakvin, M. Pozek and A. Dulcic, Solid State Commun. 72 (1989) 199. [18] N.S. Dalai, B. Rakvin, T.A. Mahl, A.S. BhaUa and Z.Z. Sheng, 90, Adv. Mater. Sci. and applications of High-Tc Superconductors (NASA CP-3100), Greenbelt, MD, USA, 1-6 April 1990 (Washington, DC, USA, NASA 1991) p. 83. [19] J.S. Ramachandran, M.X. Huang, S.M. Bhagat, K. Kish and S. Tyagi, Physica C 202 (1992) 151. [20] E.M. Jackson S.B. Liao, J. Silvis, A.H. Swihart, S:M. Bhagat, R. Crittenden, R.E. Glover III and M.A. Manheimer, Physica C 152 (1988) 125. [21] M.X. Huang, S.M. Bhagat, A.T. Findikoglu, T. Venkatesan, M.A. Manheimer and S. Tyagi, Physica C 193 (1992) 42. [22] T. Van Duzer and C.W. Turner, Principles of Superconductive Devices and Circuits (Elsevier, Amsterdam, 1981) p. 128. [23] S. Sridhar, D. Wu and W. Kennedy, Phys. Rev. Lett. 63 (1989) 1873.
PhysicaC260 (1996) 81-85
A new technique for measuring the microwave penetration depth in high-Tc superconducting thin films Bekir Akta~ a, *, Hiiseyin Zafer Durusoy b a Department of Physics, Gebze Institute of Advanced Technology, 41400 Kocaeli, Turkey b Department of Physics, Hacettepe Unioersity, Beytepe, 06532 Ankara, Turkey
Received 19 December 1995
Abstract
The electron spin resonance (ESR) technique has been used to obtain the microwave (MW) penetration depth of
YBa2Cu307 thin films. An ESR-signal-generating marker was placed in between two high-temperature superconducting (HTSC) films to probe the MW field penetrating through the films. Below the superconducting transition temperature (T~) the HTSC film started to screen the marker inside the sandwich. A meaningfully diminishing ESR signal was reproducibly recorded for various samples with different markers such as paramagnetic diphenyl pierylhydrazy (DPPH), Mn ++ impurity in MgO and a ferromagnetic (FM) permalloy film. The temperature dependence of the ESR signal intensity above and below Tc has been studied to deduce the penetration depth from the measured signal intensity. A very rapidchange of A just below Tc, slowed down later at lower temperatures and became smoothly changing. The value hab = 1020 A has been measured for c-axis films at 77 K.
Since the discovery of HTSC [1] numerous scientific studies have been undertaken to understand the nature of HTSC by using various experimental techniques. Measuring the penetration depth and determining its temperature dependence accurately have been very challenging [2-4]. Experimental techniques such as stripline resonator [5], surface impedance [6-9], and MW transmission [10] have recently been employed to obtain penetration depth values. Experimental results have been almost always controversial with conflicting results which are usually attributed to a varying sample quality. There have also been several reports interpreting penetration-depth studies in terms of s wave and d wave
* Corresponding author.
models [11-13]. The ESR spectrometer has also proven to be a very useful tool to study superconducting materials. It has been used to obtain information on the surface impedance [5,8,14], complex AC conductivity in the GHz range [15,16] and MW penetration depth [10,15,16] in either bulk or thinfilm form. The technique is basically based on the measurement of reflected or transmitted MW power [10] through the sample. However, in each case the temperature dependence of the MW cavity limits the precision of the measurement. Researchers have also tried to obtain the DC (London) penetration depth from usual ESR line-broadening of the DPPH radical adsorbed on the surface where a vortex distribution exists [17,18]. In this study, w e have tried a new, simple and very sensitive technique to measure the penetration
0921-4534f96/$15.00 © 1996Elsevier Science B.V. All rights reserved PH S0921-4534(96)001 14-1
82
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
depth. High-quality YBa2Cu30 7 (YBCO) thin films on MgO substrates have been studied through ESR spectroscopy in order to obtain the MW penetration depth as a function of temperature. First, a free radical such as DPPH has been placed between two YBCO films and placed in the ESR cavity. The ESR signal from the sandwiched marker is proportional to the average MW power in between the YBCO films. Below Tc, the penetrating MW field would be damped exponentially through the YBCO films lowering the MW power in between and on the DPPH. Therefore, the ESR signal intensity, which is the field derivative of the MW absorption, is expected to be a direct measure of the average, MW penetration depth, A, within YBCO. This phenomenon has been used to measure the varying signal intensity and to calculate the penetration depth thereof. To the best of our knowledge, this technique where a paramagnetic free radical is sandwiched between two thin superconducting films has been used for the first time. It should be noted that a paramagnetic probe has been used before [17] to obtain the DC (London) penetration depth. However, in that case the probe was adsorbed on one surface of a YBCO pellet and then EPR line broadening due to spatial flux distribution was analyzed to extract A values. YBCO films are prepared by RF magnetron sputtering on polished MgO substrates in a UHV chamber. Films were extremely smooth and epitaxial. Judged by the XRD (002)/(005) peak ratio the films have been identified as a-axis or c-axis. For the c-axis films that were used in this investigation, this ratio was typically 0.003 while Tc was 86-87 K and the critical current density ranged about 10 6 A / c m 2 at low temperatures (Fig. 1). The YBCO films were spun with photoresist immediately after deposition and cut into 3 × 3 m m pieces to fit in the sample holder. After rinsing the photoresist, the ESRsignal-generating DPPH powder has been deposited on one YBCO surface. Next, two YBCO-on-MgO pieces were pasted with a bit of silicon grease with YBCO faces looking in and MgO faces looking out. Finally, silver paste has been used to seal joining lines to prevent any MW leakage (Fig. 1, inset). Samples were studied using a commercial, 9", Varian e-line ESR spectrometer operating in the TEl02 mode. The sample was placed at the center of the rectangular cavity where the magnetic-field com-
ponent of the MW is maximum and always parallel to the sample surface while the electric component is minimum and parallel to the external DC magnetic field. An Oxford continuous gas flow He cryostat was used for temperature control. The external DC magnetic field was applied to the sample surface perpendicularly. In Figs. 2 and 3, the ESR lines of DPPH sandwiched between c-axis YBCO films are shown near T~ at various temperatures. An example of the temperature-dependent intensity obtained from the second integration of one of the ESR lines is given in the inset of Fig. 2. Special attention has been paid to the calculation of the line intensity. Each line has been scaled to a standard in the second one of the double ESR cavity to avoid any error from the temperature dependence of the Q factor. As shown, the amplitude of the signal diminishes drastically with decreasing temperature just below T~ = 83 K. This sharp decrease in the ESR signal intensity below Tc is obviously due to the MW screening by the HTSC film sandwiching the paramagnetic probe. We would like to note that T~'s obtained under MW absorption are always a few K lower than those obtained through DC or low-frequency AC susceptibility measurements [19-21]. External DC magnetic field and the MW field is observed to suppress Tc by about 4 K in our case. The ESR line is distorted and shifted towards higher fields at lower temperatures as presented in
400
? ~
300
200
"~
100
50
100
150
200
250
300
Temperature (K) Fig. 1. Resistivity as a function of temperature. Note that the transition is very sharp and Tc(0) is at about 86 K. Sample geometry and field orientation are shown relative to the film plane in the inset. The marker is deposited just in the middle of the sandwich.
B. Akta}, H.Z. Durusoy / Physica C 260 (1996) 81-85
480 ¢D
i
~
i
i
i
i
~
i
i
i
T=84,86,88,90K I :... ~.:W:____'N=. ~,_
. . . . . . . . . 82.4K ......
320
•?
82.8K
:~
I
:0 5
¢:°
i
]
2
i ..,_3
o
0~~J
°" ~
o
............................ ",.L~,.................. ~....................... [
._.~---L
.......
o
~.................
i 3270
~.°y 81.5K 0
'T=BI.4K
..................... ~........................ii~ ..........
:+ 00~ ~ ;0°i
160
83
3278
3285
3292
3300
Magnetic Field(Gauss)
81..4K o
oog. ""
-160
°0
~
~.:: -320
-~
3280
3285
3295
3290
Magnetic Field (Gauss) Fig. 2. ESR field derivative of DDPH placed in between two YBCO films at some selected temperatures. The inset gives the first and second integration (intensity) of the ESR spectra at 81.4.K
Fig. 3. This effect is a clear indication o f the setting o f vortices which cause an inhomogeneous local field on the marker [17,18]. Shifting is due to partial
30
20
i
i
~
I
i
i
1
i.
81.2K
iL~
81.0K
: .........................................................
i
°,~
t
screening o f the DC field in the mixed state where H~1 < H < Hc2. F o r the c-axis films this effect is more pronounced as the screening current flows in
i-
.........
~:/~\~
E
~-
~
i
i ...................................
7
-
v
-
~
i
. . . . . . . . .
79K
10
0
°Flll
Or~
-10
.................... i...................,.,.,.................... g f -.i:................ ~ ................................. !..................................................................
-20 3280
3285
3290
3295
3300
Magnetic Field (Gauss) Fig. 3. ESR spectra of DPPH at lower temperatures. Note that the lines are progressively distorted and shifted towards higher fields with decreasing temperatures. The penetrating DC field at about Tc is increasingly screened by DC supercurrents in the mixed state YBCO.
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
84
the layers of the C u - O planes for the perpendicularly applied external field. As the temperature is decreased further the ESR line becomes completely distorted. The ESR absorption as a function of magnetic field is proportional to the square of the magnetic component of the MW field at the DPPH marker. The classical expression for the field penetration is [221 h(z)
= h(o)e -(l+i)z/8,
(1)
where z is the distance measured from the outer surface of the YBCO films and 3 is the usual skin depth which is given by 6 = (2/to/x0o-)l/2; here ois the complex AC conductivity, to is the MW frequency a n d / % is the permeability of the medium. Actually the field penetration may not be a simple exponential for a film in the mixed state with a significant areal density of vortices. Nevertheless, w e use it as a first approximation. The inverse of the real part of the exponent in Eq. (1) can be taken as the penetration depth where the MW field decays to 1 / e of its maximum value [22]. Thus the ESR line intensity would be given by I ( T ) = Ce -(2 d/a(T)),
(2)
where d is the thickness of the YBCO film and C is a temperature-dependent parameter which is proportional to the square of the magnetic component of the MW power on the outer surface of the films, the susceptibility of the DPPH at temperature T and the spectrometer gain. From Eq. (2) we can easily obtain 2d )t(T) = In C - I n I ( T ) " (3) In the limiting case, for T>> Tc we expect )tab(T> T~) ~ ~ and we get the value for C(T) as In C(T) = In I ( T > T~) for values of T above the critical temperature. Indeed, this high-temperature limit just above Tc is observed in Fig. 2. We have carefully checked this limit at various high temperatures just above T~, such as 92, 94, 96 K, to reproducibly obtain the same spectrum at all temperatures. Normally, C would depend on the Langevin function for paramagnetic DPPH radicals but this temperature dependence would be negligible in small temperature intervals around T~. Thus L n ( I ) would be almost constant above T~ and equal to In C. Having obtained In C from the spectra just above Tc, we can use Eq.
106 ~la~a
~< ~-~
10 s
g
=3
i
[]
i
ID
0 104 0
~'em~ratum
loo0
.......
O
.-
[3 .......
(K)
(3 ........
O
. [3
.......................
g. 100 76
78
80
82
84
Temperature (K) Fig. 4. Temperature-dependent MW penetration depth of YBCO film in a DC field of about 3300 G. The inset expands the lower part of the data for clarity.
(3) to obtain )tab(T) a s a function of temperature. The resulting )tab(T) values are plotted in Fig. 4. Data for lower temperatures are plotted in the inset for clarity. The severe distortion of the ESR line made it impossible to obtain correct measurements of the line intensity at much lower temperatures. Therefore, we prefer not to attempt an analysis of the temperature dependence of )tab by employing suggested models. Instead, in this report, we would like to simply present our novel technique and our direct observations near Tc where the behavior of Aab with the temperature is very similar to previous findings in the literature [8,10,15,16,23]. Starting from hab --~ 105 ,~ at 82 K and reaching Aab -'~ 103 A at 77 K, where )tab is decreasing very smoothly and slowly, a very rapid decrease in hab is observed just below To. It should also be noted that our value for )tab, just below Tc, is very close to those found near Tc for weak coupling ( A / k T c = 1.76) [8,23]. Below T~, the effective DC magnetic field between the HTSC films is a sum of the externally applied DC magnetic field and the field from DC supercurrents. Due to vortices in YBCO this effective field changes from point to point inside the sample. This inhomogeneous, effective field causes a distortion of the ESR line shape with decreasing temperature as is clearly visible in Fig. 3. In addition, a shift in the position of the lines due to the diamagnetic response of the YBCO is observed. The local average field on the spins decreases with decreasing temperature below T~ because of a progressing screening effect at resonant field Hcl < H < Hc2. In order to avoid dis-
B. Akta~, H.Z. Durusoy / Physica C 260 (1996) 81-85
tortion of the line shape, a FM film such as a permalloy film which is directly deposited on YBCO films, can be used as the ESR signal source. In this case the strong exchange interaction between the FM spins inhibits the line broadening caused by the inhomogeneous local field and gives much stronger signals. This type of samples would allow more precise calculations to be made from the ESR signal intensity which is a direct measure of the MW power. Initial experiments with a 1000 A FM permalloy thin-film marker directly grown on YBCO with a Au buffer layer has indicated that the distortion in the FM resonance line is indeed significantly reduced. This development allows the calculation of the line intensity to be made more accurately over a much wider temperature range down to liquid He temperatures. Another advantage of the FM film as an ESR signal source is that this intrinsic signal is nearly temperature independent far below the Curie temperature of the FM material. Therefore, the proportionality parameter, C, can practically be taken as a constant over a much wider range below the Curie temperature. In our preliminary experiments, Aab values similar to those obtained with the DPPHmarked samples have also been measured with the permalloy-marked samples of YBCO films. Work is under way to study this phenomenon further with ferromagnetic markers to take more measurements and to be able to make a more rigorous interpretation of the collected data.
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