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Neutron depolarization study of a high-Tc epitaxial thin film
436
Applied Surface Science48/49 (1991) 436-440 North-Holland
Neutron depolarization study of a high-Tc epitaxial thin film S.J. Pickart, M.L. Crow, A.C. N u n e s University of Rhode IMand, Kingston. R I 02881. USA
T.R. M c G u i r e a n d A. G u p t a I B M Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights. N Y 10598, USA
Received 13 August 1990: accepted for publication 20 August 1990
Neutron depolarization measurements on an epitaxial thin film of YBaCuO superconducting material are reported and interpreted on the basis of formation of an Abrikosov flux lattice by field cooling. Temperature and field dependences of this effect have been measured and suggest that the technique may be useful in obtaining information on flux lattice stability and pinning forces.
I. Introduction Despite the application of many different experimental techniques to the study of high-temperature superconductors [1], fundamental questions about flux line stability and pinning mechanisms in the intermediate state still remain. As a specific example, recent susceptibility measurements [2] of an epitaxiai thin film of YBaCuO grown by laser ablation indicate that critical currents are orders of magnitude larger when the film is cooled through the superconducting transition temperature with the field parallel to the film plane (i.e., normal to the c-axis) compared to field cooling perpendicular to the film plane (alon.g the c-axis); the suggestion was made that pinning of the critical currents flowing within the C u P planes of the crystal lattice were responsible for this unexpected anisotropy. Since our previous measurements [3] and those by others [4] showed that polycrystalline ceramic samples of these materials could depolarize a beam of neutrons by action of field distributions due to trapped flux, we decided to apply this technique to an epitaxial film. In this paper we report the results of our measurements to date, along with discussion of possible interpre-
tations in terms of Abrikosov flux lattice lattice behavior.
2. Experimental details The film that we studied was grown by similar methods to those measured previously [2]. It was grown by an in-situ pulsed laser ablation technique on a (100) oriented SrTiO 2 substrate to ~ 1 # m thickness. A small piece of the 2 cm diameter film was used for susceptibility measurements, which are shown in fig. 1; the observed magnetic moment, using the Bean model, translates to a critical current density of approximately 10 M A / cm 2, indicative of good film quality. The apparatus used for the neutron depolarization measurements, described elsewhere [5], is a polarized small-angle spectrometer installed at the Rhode Island Nuclear Science Center reactor. The beam is polarized and monochromatized (2.4 ,A neutron wavelength) by Bragg reflection from a Heusler alloy; the neutron spins can be rotated 1~0 ° by a Mezei flipper and the polarization of the transmitted beam analyzed by an F e - G e multilayer. In the present configuration, where ap-
0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
S.J, Pickart et aL / Neutron depolarization study of a high-T~ epi:axial thin film
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plied fields are in the horizontal plane, the beam polarization is turned from the vertical before passing through the sample and re-rotated to the vertical by guide fields. A small residual field in the magnet is necessary to maintain the neutron "olarization, but tests indicated essentially complete polarization from 50 Oe to the maximum attainable, 1700 Oe. The film could be cooled down to 8 K by a He refrigerator and the cooling field applied within the film plane or normal to it by rotating the refrigerator 90° during the field cooling process.
FIELD COOLED H (kOe) Fig. 1. Remanent magnetic moment of a 1 / t m YBaCuO film as a function of cooling field applied normal to the plane of the film.
3. Depolarization measurements: results and discussion
When the field cooling was done by applying 1700 Oe within the plane of the film, a slight depolarization was observed (fig. 2) which disappeared upon application of - 200 Oe. It is con-
00
t
!
i YBCO Single Crystal Film
1.0
i.
Cooled
;
to 8K In1700Oej.
Counting field 50Oe/i
0.8 0.95
0.6
m 0.4
g
YBCO Single Crystal Film
0.90
T ¢ 8K, Cooled to 8K in 1700Oe/I Surfecl
i
0.2
+ : Increasing Field • : Decreasing Field
0.0 0.85 ,
.
.
.
r
. . . .
i
. . . .
500 1000 Counting Field (Oe)
r
.
__
1500
Fig. 2. Polarization of the transmitted beam after passage through the YBaCuO film cooled in a 1700 d e field within the film plane, as a function of applied field at 8 K. The diagram shows the orientation of the cooling field ( H e ) and the applied ficld (HA) with respect to the film plane.
-0.2
0
2'0
4'0
6'0
8'0 10 0 (K) Fig. 3. Polarization of the transmitted beam after cooling the Temperature
film in a 1700 d e field normal to its surface, as a function of temperature. Diagram as in fig. 2.
S.J. Pickart etal. / Neutron depola,izatton study of a high-T, epitaxial thin film
438
ceivable that this slight initial depolarization is caused by return flux lines outside the sample, as will be discussed later. In the other case, with the cooling field normal to the film plane, significant depolarization was observed which disappeared at the superconducting transition temperature, as indicated in fig. 3. Since in the first instance an induced Abrikosov flux lattice would be parallel to the neutron spin direction and in the latter case perpendicular to it, and bearing in mind that only normal field components are effective in depolarizing neutrons, this behavior is strong evidence that such a lattice is formed within the film. This conclusion was reinforced by an experiment in which the pc,larization was measured as the film, initially cooled in the parallel configuration, was rotated away from the neutron spin direction. These results, shown in fig. 4, clearly
s og;.o_ Cooled
I
to 8K in 1700Oe~.
T=sK surface
:::;:;:2':::::::g
0.6
÷
0.4 I
'
0.2
+
.
0.0 500
0"851
Counting
I """"
1000
1500
Field// (Oe)
Fig. 5. Polarization after similar field cooling as in fig. 3. as a function of applied field. Data taken at 8 K.
0.80 I-
0.75
~ 0.70
0.65
t
YBCO Single Crystal Film 0.60
T = 8K, Counting Field 50Oe Cooled to 8K in 1700Oel;
Surface
0.55 5 10 15 20 Rotating Angle ( degrees )
25
Fig. 4. Dependence of the polarization as a function of tilt angle of the film relative to the incident neutron spin direction. The dashed line shows a simple cosine squared dependence• taking into account the slight initial depolarization.
show a decrease in polarization with tilt angle. A cosine squared depende,tce would be expected for a simple model of single neutron/flux line scat~.ering events. The observed decrease is more rapid than this, probably due to the fact that the actual situation is more complicated, as alluded to earlier. The dependence of the depolarization induced by perpendicular field cooling was investigated as a function of temperature and applied field, with the results indicated in figs. 5-7. Fig. 5 shows the data at S K, the gradually increasing polarization presumably resulting from realignment of the flux lattice along the polarization direction. Fig. 6, taken at 50 K, shows that the field required for parallel alignment is smaller, and also exhibits significant hysteresis, as expected if flux pinning effects are involved. The composite figure 7 for field dependences at 8, 30 and 50 K would allow an H - T phase diagram to be constructed for this effect for comparison with theoretical predictions.
S.J. Pickart et al. / Neutron depolartzation study of a high-T~, epitaxtal thin film
In our previous measurements of polycrystal samples [3] we checked for any influence of return fields outside the sample by performing a grazing experiment in which the beam was passed along the surface of the edge-marked sample, with no loss of polarization. When similar experiments were performed with the film, depolarization was observed, indicating that return fields could be a contributing factor. Nevertheless, we established that depolarization within the film itself was occurring by a control experiment in which a small, 2 mm hole was burned by laser in the film and the neutron beam masked to this size; no depolarization was observed. These results indicate that, before any quantitative interpretation in terms of pinning forces and activation energies can be made, the total field configurations within and in the vicinity of the film must be evaluated along the neutron path to model the depolarization caused by the flux lattice itself. Such a model could contain as parameters the core flux value,
4 3 9
1.0 m
0.8
+
0.6
=_: o=
o
+
0.4
+
+ 0.2
+
o - 8K + - 30K -- 50K
.,+
0.0
YBCEIPSingle Crystal Film
÷ [
Cooled to 8K In 1700gel
,
I
.
.
.
.
i
.
.
.
.
i
,
,
500 1000 1500 Counting Field// (Oe) Fig. 7. Summary data on polarization after field cooling normal to the plane of the film in a field of 1700 Oe, at the indicated temperatures.
F 1.0
0.8
i
¢
+
flux line density, and s;ze and tilt of coherent regions.
+ 0.6
+ +
0.4
YBCO Single Crystal Film T=50K
0.2 I 0.0
4.
Cooled to 8K in 1700oe ± surface.
i +~
+ : 1 . . . . . . ing field
I
" : Decreasing field
-0.2
s00
1 o'0o" ........ i5'00 -
Counting Field// (Oe) Fig. 6. Same as fig. 5, but with data taken at 50 K, Note the hysteresis between increasing and decreasing field measurements.
Summary
With this and previous work we have shown that polycrystal and epitaxiai thin film high-temperature superconductors can depolarize neutrons, and that the depolarization is clearly attributable to formation of Abrikosov flux lattices. Interpretation of the observed depolarization requires numerical modelling of the flux paths both within and outside the samples, efforts which are underway. With this modelling, and with more extensive measurements of temperature, field, and thickness dependence in these films, we believe that useful information about flux lattice stability and pinning forces in thin epitaxial films can be obtained.
440
S.J. Ptckart et a L / Neutron depolarization study of a htgh- T, epttaxial thin film
Acknowledgements W e wish the a c k n o w l e d g e the assistance o f H. Lilienthal in m a k i n g the m a g n e t i c m e a s u r e m e n t s a n d B. Fu in a c q u i r i n g the n e u t r o n data. T h i s r e s e a r c h w a s s u p p o r t e d in p a r t by N S F G r a n t N o s . D M R 8704671 a n d 89023466.
References It] A. Malozemoff. in: Physical Properties of High Temperature Superconductors. Ed D. Ginsburg (World Scientific, Singapore. 1989) p. 71.
[2] T.R. McGuire, D. Dimos, A. Gupta. G. Koren and R.B. Laibowitz. J. Appl. Phys. 67 (1990) 5070; T.R. McGuire, A. Gupta, G. Koren and R. Gross, IEEE Trans. Mag. 26. to be published. [3] A.C. Nunes. S.J. Pickart, M.L. Crow and P..J. Goyette, J. .Appl. Phys. 64 (1988) 5815; M.L. Crow, R.J. Goyette, A.C. Nunes. S.J. Pickart, T.R. McGuire. S. Shinde and T.M. Shaw, J. Appl. Phys. 67 (1990) 4542. 14] R.J. Papoular and G. Collin, Physica B 156/157 (1988) 871; Phys Rev. B 38 (1988) 768. [5] S. Hasanain, S.J. Pickart and A.C. Nunes. Kerntechnik S 44 (1984) 5815.
Applied Surface Science48/49 (1991) 436-440 North-Holland
Neutron depolarization study of a high-Tc epitaxial thin film S.J. Pickart, M.L. Crow, A.C. N u n e s University of Rhode IMand, Kingston. R I 02881. USA
T.R. M c G u i r e a n d A. G u p t a I B M Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights. N Y 10598, USA
Received 13 August 1990: accepted for publication 20 August 1990
Neutron depolarization measurements on an epitaxial thin film of YBaCuO superconducting material are reported and interpreted on the basis of formation of an Abrikosov flux lattice by field cooling. Temperature and field dependences of this effect have been measured and suggest that the technique may be useful in obtaining information on flux lattice stability and pinning forces.
I. Introduction Despite the application of many different experimental techniques to the study of high-temperature superconductors [1], fundamental questions about flux line stability and pinning mechanisms in the intermediate state still remain. As a specific example, recent susceptibility measurements [2] of an epitaxiai thin film of YBaCuO grown by laser ablation indicate that critical currents are orders of magnitude larger when the film is cooled through the superconducting transition temperature with the field parallel to the film plane (i.e., normal to the c-axis) compared to field cooling perpendicular to the film plane (alon.g the c-axis); the suggestion was made that pinning of the critical currents flowing within the C u P planes of the crystal lattice were responsible for this unexpected anisotropy. Since our previous measurements [3] and those by others [4] showed that polycrystalline ceramic samples of these materials could depolarize a beam of neutrons by action of field distributions due to trapped flux, we decided to apply this technique to an epitaxial film. In this paper we report the results of our measurements to date, along with discussion of possible interpre-
tations in terms of Abrikosov flux lattice lattice behavior.
2. Experimental details The film that we studied was grown by similar methods to those measured previously [2]. It was grown by an in-situ pulsed laser ablation technique on a (100) oriented SrTiO 2 substrate to ~ 1 # m thickness. A small piece of the 2 cm diameter film was used for susceptibility measurements, which are shown in fig. 1; the observed magnetic moment, using the Bean model, translates to a critical current density of approximately 10 M A / cm 2, indicative of good film quality. The apparatus used for the neutron depolarization measurements, described elsewhere [5], is a polarized small-angle spectrometer installed at the Rhode Island Nuclear Science Center reactor. The beam is polarized and monochromatized (2.4 ,A neutron wavelength) by Bragg reflection from a Heusler alloy; the neutron spins can be rotated 1~0 ° by a Mezei flipper and the polarization of the transmitted beam analyzed by an F e - G e multilayer. In the present configuration, where ap-
0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
S.J, Pickart et aL / Neutron depolarization study of a high-T~ epi:axial thin film
50 25
, ; D;
i
i
i,
1
~" 2o E [
is
•
In
I
I
I
/ YB°2CusOt-B FILM I/~m THICK T-SK
1
I
I
I
2
5
1
I
I
I
1
I
I
4
5
6
"r
8
9
Io
437
plied fields are in the horizontal plane, the beam polarization is turned from the vertical before passing through the sample and re-rotated to the vertical by guide fields. A small residual field in the magnet is necessary to maintain the neutron "olarization, but tests indicated essentially complete polarization from 50 Oe to the maximum attainable, 1700 Oe. The film could be cooled down to 8 K by a He refrigerator and the cooling field applied within the film plane or normal to it by rotating the refrigerator 90° during the field cooling process.
FIELD COOLED H (kOe) Fig. 1. Remanent magnetic moment of a 1 / t m YBaCuO film as a function of cooling field applied normal to the plane of the film.
3. Depolarization measurements: results and discussion
When the field cooling was done by applying 1700 Oe within the plane of the film, a slight depolarization was observed (fig. 2) which disappeared upon application of - 200 Oe. It is con-
00
t
!
i YBCO Single Crystal Film
1.0
i.
Cooled
;
to 8K In1700Oej.
Counting field 50Oe/i
0.8 0.95
0.6
m 0.4
g
YBCO Single Crystal Film
0.90
T ¢ 8K, Cooled to 8K in 1700Oe/I Surfecl
i
0.2
+ : Increasing Field • : Decreasing Field
0.0 0.85 ,
.
.
.
r
. . . .
i
. . . .
500 1000 Counting Field (Oe)
r
.
__
1500
Fig. 2. Polarization of the transmitted beam after passage through the YBaCuO film cooled in a 1700 d e field within the film plane, as a function of applied field at 8 K. The diagram shows the orientation of the cooling field ( H e ) and the applied ficld (HA) with respect to the film plane.
-0.2
0
2'0
4'0
6'0
8'0 10 0 (K) Fig. 3. Polarization of the transmitted beam after cooling the Temperature
film in a 1700 d e field normal to its surface, as a function of temperature. Diagram as in fig. 2.
S.J. Pickart etal. / Neutron depola,izatton study of a high-T, epitaxial thin film
438
ceivable that this slight initial depolarization is caused by return flux lines outside the sample, as will be discussed later. In the other case, with the cooling field normal to the film plane, significant depolarization was observed which disappeared at the superconducting transition temperature, as indicated in fig. 3. Since in the first instance an induced Abrikosov flux lattice would be parallel to the neutron spin direction and in the latter case perpendicular to it, and bearing in mind that only normal field components are effective in depolarizing neutrons, this behavior is strong evidence that such a lattice is formed within the film. This conclusion was reinforced by an experiment in which the pc,larization was measured as the film, initially cooled in the parallel configuration, was rotated away from the neutron spin direction. These results, shown in fig. 4, clearly
s og;.o_ Cooled
I
to 8K in 1700Oe~.
T=sK surface
:::;:;:2':::::::g
0.6
÷
0.4 I
'
0.2
+
.
0.0 500
0"851
Counting
I """"
1000
1500
Field// (Oe)
Fig. 5. Polarization after similar field cooling as in fig. 3. as a function of applied field. Data taken at 8 K.
0.80 I-
0.75
~ 0.70
0.65
t
YBCO Single Crystal Film 0.60
T = 8K, Counting Field 50Oe Cooled to 8K in 1700Oel;
Surface
0.55 5 10 15 20 Rotating Angle ( degrees )
25
Fig. 4. Dependence of the polarization as a function of tilt angle of the film relative to the incident neutron spin direction. The dashed line shows a simple cosine squared dependence• taking into account the slight initial depolarization.
show a decrease in polarization with tilt angle. A cosine squared depende,tce would be expected for a simple model of single neutron/flux line scat~.ering events. The observed decrease is more rapid than this, probably due to the fact that the actual situation is more complicated, as alluded to earlier. The dependence of the depolarization induced by perpendicular field cooling was investigated as a function of temperature and applied field, with the results indicated in figs. 5-7. Fig. 5 shows the data at S K, the gradually increasing polarization presumably resulting from realignment of the flux lattice along the polarization direction. Fig. 6, taken at 50 K, shows that the field required for parallel alignment is smaller, and also exhibits significant hysteresis, as expected if flux pinning effects are involved. The composite figure 7 for field dependences at 8, 30 and 50 K would allow an H - T phase diagram to be constructed for this effect for comparison with theoretical predictions.
S.J. Pickart et al. / Neutron depolartzation study of a high-T~, epitaxtal thin film
In our previous measurements of polycrystal samples [3] we checked for any influence of return fields outside the sample by performing a grazing experiment in which the beam was passed along the surface of the edge-marked sample, with no loss of polarization. When similar experiments were performed with the film, depolarization was observed, indicating that return fields could be a contributing factor. Nevertheless, we established that depolarization within the film itself was occurring by a control experiment in which a small, 2 mm hole was burned by laser in the film and the neutron beam masked to this size; no depolarization was observed. These results indicate that, before any quantitative interpretation in terms of pinning forces and activation energies can be made, the total field configurations within and in the vicinity of the film must be evaluated along the neutron path to model the depolarization caused by the flux lattice itself. Such a model could contain as parameters the core flux value,
4 3 9
1.0 m
0.8
+
0.6
=_: o=
o
+
0.4
+
+ 0.2
+
o - 8K + - 30K -- 50K
.,+
0.0
YBCEIPSingle Crystal Film
÷ [
Cooled to 8K In 1700gel
,
I
.
.
.
.
i
.
.
.
.
i
,
,
500 1000 1500 Counting Field// (Oe) Fig. 7. Summary data on polarization after field cooling normal to the plane of the film in a field of 1700 Oe, at the indicated temperatures.
F 1.0
0.8
i
¢
+
flux line density, and s;ze and tilt of coherent regions.
+ 0.6
+ +
0.4
YBCO Single Crystal Film T=50K
0.2 I 0.0
4.
Cooled to 8K in 1700oe ± surface.
i +~
+ : 1 . . . . . . ing field
I
" : Decreasing field
-0.2
s00
1 o'0o" ........ i5'00 -
Counting Field// (Oe) Fig. 6. Same as fig. 5, but with data taken at 50 K, Note the hysteresis between increasing and decreasing field measurements.
Summary
With this and previous work we have shown that polycrystal and epitaxiai thin film high-temperature superconductors can depolarize neutrons, and that the depolarization is clearly attributable to formation of Abrikosov flux lattices. Interpretation of the observed depolarization requires numerical modelling of the flux paths both within and outside the samples, efforts which are underway. With this modelling, and with more extensive measurements of temperature, field, and thickness dependence in these films, we believe that useful information about flux lattice stability and pinning forces in thin epitaxial films can be obtained.
440
S.J. Ptckart et a L / Neutron depolarization study of a htgh- T, epttaxial thin film
Acknowledgements W e wish the a c k n o w l e d g e the assistance o f H. Lilienthal in m a k i n g the m a g n e t i c m e a s u r e m e n t s a n d B. Fu in a c q u i r i n g the n e u t r o n data. T h i s r e s e a r c h w a s s u p p o r t e d in p a r t by N S F G r a n t N o s . D M R 8704671 a n d 89023466.
References It] A. Malozemoff. in: Physical Properties of High Temperature Superconductors. Ed D. Ginsburg (World Scientific, Singapore. 1989) p. 71.
[2] T.R. McGuire, D. Dimos, A. Gupta. G. Koren and R.B. Laibowitz. J. Appl. Phys. 67 (1990) 5070; T.R. McGuire, A. Gupta, G. Koren and R. Gross, IEEE Trans. Mag. 26. to be published. [3] A.C. Nunes. S.J. Pickart, M.L. Crow and P..J. Goyette, J. .Appl. Phys. 64 (1988) 5815; M.L. Crow, R.J. Goyette, A.C. Nunes. S.J. Pickart, T.R. McGuire. S. Shinde and T.M. Shaw, J. Appl. Phys. 67 (1990) 4542. 14] R.J. Papoular and G. Collin, Physica B 156/157 (1988) 871; Phys Rev. B 38 (1988) 768. [5] S. Hasanain, S.J. Pickart and A.C. Nunes. Kerntechnik S 44 (1984) 5815.