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Use of YBCO films for applications with polarized neutrons
ELSEVIER
Physica B 24l 243 (1998) 121-123
Use of YBCO films for applications with polarized neutrons M . R . F i t z s i m m o n s a'*, M . L i i t t a, R. P y n n a, H . K i n d e r b, W . P r u s s e i t b ~ Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laborato~, Los Alamos, NM 87545, USA b Physik Department der TU-Miinchen, D-85747 Garching, German),
Abstract
The use of a superconducting thin YBCO film as a Meissner screen in a cryoflipper is demonstrated, i~, 1998 Elsevier Science B.V. All rights reserved. Kevwords: Instrumentation; Polarized neutrons; YBCO
Magnetic and nuclear scattering can be determined independently by measuring the intensities of the scattered neutron beams whose polarizations are parallel and anti-parallel to the sample magnetization. On the reflectometer, SPEAR, at the Manuel Lujan Jr. Neutron Scattering Center (MLNSC), this is accomplished using a broad-band of neutron wavelengths polarized with supermirrots [1] in reflection geometry [2]. The experiments require a method for reversing the polarization of the neutron beam. One method, tried previously on SPEAR, used a Mezei rt-flipper [3] to reverse the neutron spin relative to the direction of the fields that magnetized the polarizers and sample. There are two disadvantages with this approach: a source of small angle scattering is introduced, and the process used to determine the current in the coils of the flipper is time consuming. Recently, the Meissner effect pro* Corresponding author. Tel.: (505) 665-4045: fax: (505) 6652676: e-mail: [email protected].
duced by a thin superconducting YBCO film called a Meissner screen, has been used to create a discontinuity in the magnetic field through which the neutrons travel. By reversing the current through the electromagnet magnetizing the polarizer, spin-flip of the neutron beam in the reference frame of the sample can be made to occur in a manner which is wavelength independent. The Meissner screen consisted ofa 5 cm diameter YBCO film (c-axis parallel to the surface normal) grown on a 1 mm thick single crystal substrate. The critical current densities, Jc, of three films (see Table 1) parallel to the ab-plane (in the plane of the film) exceeded 2 x 10 TM A / m z at 77 K (T~ ~ 87 K). YBCO-coated wafers were mounted separately in a Cu holder which was attached to a Displex cryostat in an ultra-high-vacuum (UHV) tee. The films were cooled to 20 K. The neutron beam entered and exited the UHV-tee through Suprasil windows. Magnetic fields generated by solenoids placed between #-metal sheets defined the quantization of the neutron spin within the tee. The solenoids in the
0921-4526/98/$19.00 g 1998 Elsevier Science B.V. All rights reserved Pll S 0 9 2 1 - 4 5 2 6 { 9 7 ) 0 0 5 2 7 - 9
122
M.R. Fitzsimmons el al. .. Physica B 241 243 (1998) 121 123
Table 1 Average flipping ratios for difl'erent films and cooling protocols Film thickness [/am]
0.40(5) 0.40(5) 1.5(5)
Substrate
LaAIO~ LaAIO3 Y-stabilized ZrO~
C o o l i n g protocol Hb = H~ = 0
H b = H~ = 25 Oe
Hh = -- H~ = - 25 Oe
18 + 3 16 + 3 12 ± 3
15 ± 3 ll + 3 8 ± 3
2.0 _+ 0.5
guide on the polarizer-side of the film were connected to a DC bipolar supply and produced a field of about Hb = + 25 Oe. The solenoids in the guide on the sample-side of the film were connected to a DC unipolar supply and produced a field of Hs = + 25 Oe in the direction parallel to the field that would be applied to a sample. The device consisting of UHV tee, guides, cryostat and Meissner screen is called a cryoflipper. A cryoflipper using superconducting Nb was developed earlier by Tasset [4]. The performance of the cryoflipper was determined from a neutron scattering experiment in which the intensity of the neutron beam was measured after reflection from two polarizing supermirrots. Two measurements were made. One when the magnetic fields on the polarizers were aligned a condition requiring the guide fields before and after the Meissner screen to be parallel, and a second when the fields were opposite. (The ratio of the two measured intensities is called the flipping ratio.) The measurements were made separately after each of the three films became cold and for three cooling protocols. A cooling protocol refers to the condition of the magnetic fields surrounding the YBCO film when it was cooled. The first protocol involved cooling the film from room temperature~ when H b = H~ [Fig. la]. The flipping ratio measured for Film 1 after carrying out the first protocol is shown in the figure-inset. The flipping ratio varies by less than 20% in the range of ,;tmi, = 2.5, (the effective critical edge of the supermirror), to "~max = 10.5 A, (the critical edge of the glass mirror substrate). The average flipping ratio over this wavelength range is 15 ± 3, which is consistent with a polarizer producing 94% neutron polarization. The second protocol involved
1.0 _+ 0.5
cooling the sample from room temperature when H b - - - - H~, which produced a field that penetrated the film in the direction of surface normal [Fig. Ib]. The third protocol involved turning off the power supplies so the film was cooled from room temperature in the 1 0 e stray field of the laboratory, The average flipping ratios for the films and protocols are reported in Table 1. Films cooled in near-zero-field conditions or in uniform fields of about 25 Oe strength performed well, while those cooled in non-uniform fields, e.g., w h e n H b = - - H s , performed poorly. This result is consistent with the large London penetration depth, A, of YBCO, and its highly anisotropic superconducting properties. The transition between type-I and type-II behavior occurs at H~1 = 2 x 107 G nm2/~A 2 [5]. For magnetic fields applied parallel to the ab-plane, A a b ~ 140 nm [5], so Hc,,b "- 336 G - a value which was not exceeded when H b = H~. However, for fields penetrating the film along the c-axis, Ac ~ 7 0 0 G [5], so H~I~ ~ 13 G. This value was exceeded when the films were cooled when Hb = -- H~. Cooled under this condition, the films became type-II superconductors -- a vortex lattice was trapped in the films, and depolarized the neutron beam. The micronthick YBCO film performed worse than the thinner films. One reason might be a tendency of thick films to develop cracks, which may degrade J~ (the current density generated when a superconducting thin YBCO film is immersed in a 25 Oe field is about 65% of J~ for a thin film). Alternatively, a thick film may more easily trap vortices than a thin film. Thin YBCO c-axis grown films performed well as Meissner screens provided they were cooled in
123
M.R. Fitzsimmons et al. /Physica B 241 243 (1998) 121-123
(a)
. . . . . .
I Ak
Hb
a~
~
I
I
~L
Hs _
| |
,
'
~r
~T
, i
Is
15
~v
L 10 lID
r--
_
5
(b)
2
4
6 Neutron
8 10 Wavelength [A]
12
14
16
Fig. 1. Diagram showing magnetic field lines passing through the YBCO sample in its normal state for (a) field alignment parallel and (b) anti-parallel. Cross-section of the YBCO film, its substrate and the Cu holder is shown by the dashed-outline. Inset: flipping ratio measured for Film 1 cooled in a uniform 25 Oe field.
near-zero-fields or uniform fields, and the fields to which they were exposed were applied along the ab-plane of the film. The advantages of the YBCO-cryoflipper over the more conventional Nb-cryoflipper include: near-zero small angle scattering (YBCO and its substrate are single crystals), less susceptibility to the magnetic field environment during cooling (Jc is much larger for YBCO than pure Nb), affordability and ease-of-use (a closedcycle cryostat is less expensive than a liquid-He cryostat to purchase and maintain). This study was supported by the U.S. Department of Energy, BES-DMS, under Contract No.
W-7405-Eng-36. O'Brien.
Graphics
Courtesy
of Kate
References [1] D. Clemens et al., Physica B 213&214 (1995) 942. [2] M.R. Fitzsimmons et al., ICANS-12, Rutherford Appleton Laboratory Report, 94-025, 355, 1994. [3] O. Schfirpf, in: Neutron Spin Echo, vol. 128, Springer, Berlin, 1980, p. 26. [4] F. Tasset, Physica B 156&157 (1989) 627. [5] C. Kittel, in: Introduction to Solid State Physics, 7th ed, Wiley, New York, 1996, pp. 333 377.
Physica B 24l 243 (1998) 121-123
Use of YBCO films for applications with polarized neutrons M . R . F i t z s i m m o n s a'*, M . L i i t t a, R. P y n n a, H . K i n d e r b, W . P r u s s e i t b ~ Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laborato~, Los Alamos, NM 87545, USA b Physik Department der TU-Miinchen, D-85747 Garching, German),
Abstract
The use of a superconducting thin YBCO film as a Meissner screen in a cryoflipper is demonstrated, i~, 1998 Elsevier Science B.V. All rights reserved. Kevwords: Instrumentation; Polarized neutrons; YBCO
Magnetic and nuclear scattering can be determined independently by measuring the intensities of the scattered neutron beams whose polarizations are parallel and anti-parallel to the sample magnetization. On the reflectometer, SPEAR, at the Manuel Lujan Jr. Neutron Scattering Center (MLNSC), this is accomplished using a broad-band of neutron wavelengths polarized with supermirrots [1] in reflection geometry [2]. The experiments require a method for reversing the polarization of the neutron beam. One method, tried previously on SPEAR, used a Mezei rt-flipper [3] to reverse the neutron spin relative to the direction of the fields that magnetized the polarizers and sample. There are two disadvantages with this approach: a source of small angle scattering is introduced, and the process used to determine the current in the coils of the flipper is time consuming. Recently, the Meissner effect pro* Corresponding author. Tel.: (505) 665-4045: fax: (505) 6652676: e-mail: [email protected].
duced by a thin superconducting YBCO film called a Meissner screen, has been used to create a discontinuity in the magnetic field through which the neutrons travel. By reversing the current through the electromagnet magnetizing the polarizer, spin-flip of the neutron beam in the reference frame of the sample can be made to occur in a manner which is wavelength independent. The Meissner screen consisted ofa 5 cm diameter YBCO film (c-axis parallel to the surface normal) grown on a 1 mm thick single crystal substrate. The critical current densities, Jc, of three films (see Table 1) parallel to the ab-plane (in the plane of the film) exceeded 2 x 10 TM A / m z at 77 K (T~ ~ 87 K). YBCO-coated wafers were mounted separately in a Cu holder which was attached to a Displex cryostat in an ultra-high-vacuum (UHV) tee. The films were cooled to 20 K. The neutron beam entered and exited the UHV-tee through Suprasil windows. Magnetic fields generated by solenoids placed between #-metal sheets defined the quantization of the neutron spin within the tee. The solenoids in the
0921-4526/98/$19.00 g 1998 Elsevier Science B.V. All rights reserved Pll S 0 9 2 1 - 4 5 2 6 { 9 7 ) 0 0 5 2 7 - 9
122
M.R. Fitzsimmons el al. .. Physica B 241 243 (1998) 121 123
Table 1 Average flipping ratios for difl'erent films and cooling protocols Film thickness [/am]
0.40(5) 0.40(5) 1.5(5)
Substrate
LaAIO~ LaAIO3 Y-stabilized ZrO~
C o o l i n g protocol Hb = H~ = 0
H b = H~ = 25 Oe
Hh = -- H~ = - 25 Oe
18 + 3 16 + 3 12 ± 3
15 ± 3 ll + 3 8 ± 3
2.0 _+ 0.5
guide on the polarizer-side of the film were connected to a DC bipolar supply and produced a field of about Hb = + 25 Oe. The solenoids in the guide on the sample-side of the film were connected to a DC unipolar supply and produced a field of Hs = + 25 Oe in the direction parallel to the field that would be applied to a sample. The device consisting of UHV tee, guides, cryostat and Meissner screen is called a cryoflipper. A cryoflipper using superconducting Nb was developed earlier by Tasset [4]. The performance of the cryoflipper was determined from a neutron scattering experiment in which the intensity of the neutron beam was measured after reflection from two polarizing supermirrots. Two measurements were made. One when the magnetic fields on the polarizers were aligned a condition requiring the guide fields before and after the Meissner screen to be parallel, and a second when the fields were opposite. (The ratio of the two measured intensities is called the flipping ratio.) The measurements were made separately after each of the three films became cold and for three cooling protocols. A cooling protocol refers to the condition of the magnetic fields surrounding the YBCO film when it was cooled. The first protocol involved cooling the film from room temperature~ when H b = H~ [Fig. la]. The flipping ratio measured for Film 1 after carrying out the first protocol is shown in the figure-inset. The flipping ratio varies by less than 20% in the range of ,;tmi, = 2.5, (the effective critical edge of the supermirror), to "~max = 10.5 A, (the critical edge of the glass mirror substrate). The average flipping ratio over this wavelength range is 15 ± 3, which is consistent with a polarizer producing 94% neutron polarization. The second protocol involved
1.0 _+ 0.5
cooling the sample from room temperature when H b - - - - H~, which produced a field that penetrated the film in the direction of surface normal [Fig. Ib]. The third protocol involved turning off the power supplies so the film was cooled from room temperature in the 1 0 e stray field of the laboratory, The average flipping ratios for the films and protocols are reported in Table 1. Films cooled in near-zero-field conditions or in uniform fields of about 25 Oe strength performed well, while those cooled in non-uniform fields, e.g., w h e n H b = - - H s , performed poorly. This result is consistent with the large London penetration depth, A, of YBCO, and its highly anisotropic superconducting properties. The transition between type-I and type-II behavior occurs at H~1 = 2 x 107 G nm2/~A 2 [5]. For magnetic fields applied parallel to the ab-plane, A a b ~ 140 nm [5], so Hc,,b "- 336 G - a value which was not exceeded when H b = H~. However, for fields penetrating the film along the c-axis, Ac ~ 7 0 0 G [5], so H~I~ ~ 13 G. This value was exceeded when the films were cooled when Hb = -- H~. Cooled under this condition, the films became type-II superconductors -- a vortex lattice was trapped in the films, and depolarized the neutron beam. The micronthick YBCO film performed worse than the thinner films. One reason might be a tendency of thick films to develop cracks, which may degrade J~ (the current density generated when a superconducting thin YBCO film is immersed in a 25 Oe field is about 65% of J~ for a thin film). Alternatively, a thick film may more easily trap vortices than a thin film. Thin YBCO c-axis grown films performed well as Meissner screens provided they were cooled in
123
M.R. Fitzsimmons et al. /Physica B 241 243 (1998) 121-123
(a)
. . . . . .
I Ak
Hb
a~
~
I
I
~L
Hs _
| |
,
'
~r
~T
, i
Is
15
~v
L 10 lID
r--
_
5
(b)
2
4
6 Neutron
8 10 Wavelength [A]
12
14
16
Fig. 1. Diagram showing magnetic field lines passing through the YBCO sample in its normal state for (a) field alignment parallel and (b) anti-parallel. Cross-section of the YBCO film, its substrate and the Cu holder is shown by the dashed-outline. Inset: flipping ratio measured for Film 1 cooled in a uniform 25 Oe field.
near-zero-fields or uniform fields, and the fields to which they were exposed were applied along the ab-plane of the film. The advantages of the YBCO-cryoflipper over the more conventional Nb-cryoflipper include: near-zero small angle scattering (YBCO and its substrate are single crystals), less susceptibility to the magnetic field environment during cooling (Jc is much larger for YBCO than pure Nb), affordability and ease-of-use (a closedcycle cryostat is less expensive than a liquid-He cryostat to purchase and maintain). This study was supported by the U.S. Department of Energy, BES-DMS, under Contract No.
W-7405-Eng-36. O'Brien.
Graphics
Courtesy
of Kate
References [1] D. Clemens et al., Physica B 213&214 (1995) 942. [2] M.R. Fitzsimmons et al., ICANS-12, Rutherford Appleton Laboratory Report, 94-025, 355, 1994. [3] O. Schfirpf, in: Neutron Spin Echo, vol. 128, Springer, Berlin, 1980, p. 26. [4] F. Tasset, Physica B 156&157 (1989) 627. [5] C. Kittel, in: Introduction to Solid State Physics, 7th ed, Wiley, New York, 1996, pp. 333 377.