- Email: [email protected]
Low-energy spin-polarized radioactive beams as a nano-scale probe of matter
Physica B 326 (2003) 189–195
Low-energy spin-polarized radioactive beams as a nano-scale probe of matter R.F. Kiefla,b,c,*, W.A. MacFarlaned,a, G.D. Morrise, P. Amaudruza, D. Arseneaua, H. Azumif, R. Baartmana, T.R. Bealsa,b, J. Behra, C. Bommasg, J.H. Brewera, K.H. Chowh, E. Dumonti, S.R. Dunsigere, S. Daviela, L. Greenei, A. Hatakeyamaa, R.H. Heffnere, Y. Hirayamaf, B. Hittia, S.R. Kreitzmana, C.D.P. Levya, R.I. Millera,b, M. Olivoa, R. Poutissoua a TRIUMF, 4004 Wesbrook Mall, Vancouver BC, Canada V6T 2A3 Department of Physics and Astronomy, University of British Columbia, Vancouver BC, Canada V6T 1Z1 c Canadian Institute for Advanced Research, University of British Columbia, Vancouver BC, Canada V6T 1Z1 d Chemistry Department, University of British Columbia, Vancouver, Canada e Los Alamos National Lab, MS-K764, Los Alamos, NM 87545, USA f Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan g Department of Physics, University of Bonn, Bonn, Germany h Department of Physics, University of Alberta, Edmonton, AB, Canada T6G 2J1 i Department of Physics, University of Illinois at Urbana-Champaign, IL 61801, USA b
Abstract We have commissioned a polarized low-energy 8 Li ion beam line, which together with a high-field b-NMR spectrometer, can act as sensitive new probe of thin films and interfaces. The implantation energy can be continuously adjusted from 1 to 90 keV and the maximum polarization achieved thus far is 80%. This instrument opens up new applications for b-NMR which parallel and complement efforts with low-energy muons. For example, it is possible to probe the magnetic field distribution near the surface of a material by stopping a polarized 8 Li beam in a thin overlayer of Ag. Since the 8 Li adopts a site with cubic symmetry in Ag there is no quadrupolar splitting of the resonance, and the 8 Li acts as a purely magnetic sensor. r 2002 Elsevier Science B.V. All rights reserved. Keywords: b-NMR; Nuclear magnetic resonance; Radioactive ion beams; Nuclear probes
1. Introduction
*Corresponding author. TRIUMF, Wesbrook Mall, Vancouver, BC, Canada V6T 2A3. Tel.: +604-222-7511; fax: +604-222-1074. E-mail address: kiefl@triumf.ca (R.F. Kiefl).
The defining feature of nuclear methods, such as muon spin rotation ðmSRÞ or beta detected NMR (b-NMR), is that they provide local information via the radioactive decay of the probe. This has a number of important advantages over
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 0 0 - 9
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
190
conventional methods in condensed matter physics. For example, in nuclear magnetic resonance (NMR) a relatively small nuclear polarization is generated by applying a large magnetic field, which is then tilted with a small RF magnetic field. An inductive pickup coil is used to detect the resulting precession of the nuclear magnetization. Typically NMR requires about 1018 nuclear spins and is therefore mostly a bulk probe of matter. On the other hand, in related nuclear methods, such as mSR [1] or b-NMR [2,3], a beam of highly polarized radioactive nuclei (or muons) is generated and then implanted into the material. While the polarization tends to be much higher— between 10% and 100%, the most important difference is that the time evolution of the spin polarization is monitored through the anisotropic decay properties of the nucleus or muon, which requires about 10 orders of magnitude fewer spins. For this reason nuclear methods are well suited to studies of dilute impurities, small structures or interfaces where there are few nuclear spins. The principles of b-NMR and mSR are virtually identical. Since no magnetic field is required to generate or detect the spin polarization, both methods allow measurements to be performed in any magnetic field, including zero field. So far the high signal-to-noise required for condensed matter applications has been much easier to achieve in the case of muons, compared to radioactive nuclei. However, with radioactive ion beam facilities such as ISOLDE and ISAC, it is possible to generate intense ð> 107 =sÞ highly polarized (80%) beams of low-energy radioactive nuclei [4,5]. Under these circumstances one can realize a significant enhancement in the signal-to-noise in b-NMR.
Furthermore, one has the added possibility to control the depth of implantation on an interesting length scale (5–500 nm). This is similar to the lowenergy muon beam at PSI, which has developed into an important new technique for studies of thin films and interfaces (see Ref. [6]). One particularly attractive aspect of doing similar measurements with radioactive nuclei generated at ISAC is that the beams do not need to be moderated in energy and thus are substantially more intense than lowenergy muon beams currently available. Also, the nuclear polarization in b-NMR can be observed on a much longer time scale due to the much larger nuclear lifetimes. Although, in principle any b-emitting isotope with spin can be studied with b-NMR the number of isotopes suitable for use as a probe in condensed matter is much smaller. The most essential requirements are: (1) a high production efficiency, (2) a method to efficiently polarize the nuclear spins, and (3) a high b decay asymmetry. Other desirable features are: (4) small ionic charge to reduce radiation damage on implantation, (5) a small value of spin so that the b-NMR spectra are relatively simple, and (6) a radioactive lifetime that is not much longer than a few seconds. Table 1 gives a short list of the isotopes we have identified as suitable for development at ISAC. Production rates of 106 =s are easily obtainable at ISAC. 8 Li is the easiest to polarize and was therefore selected as the first isotope to develop as a probe at ISAC for this purpose. In this paper we describe the performance of the ISAC polarizer and associated spectrometer for bNMR. Preliminary results on a gold foil and a 100 nm YBa2 Cu3 O6:95 film on a SrTiO3 substrate
Table 1 Examples of isotopes suitable for b-NMR Isotope
Spin
T1=2 (s)
g ðMHz=TÞ
b-Decay asymmetry (A)
production rate ðs1 Þ
mþ 8 Li 11 Be 15 O 17 Ne
1/2 2 1/2 1/2 1/2
2.2106 0.8 13.8 122 0.1
135.5 6.26 22 10.8
0.33 0.33 0.33 0.7 0.33
750 108 107 108 106
The production rates are projections except in the case of 8 Li: For comparison the first entry is for the low-energy muon beam at PSI from [6].
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
are presented, which demonstrate the high sensitivity of the technique for detecting magnetic fields.
2. Experimental A layout of the polarized beamline and b-NMR spectrometers is given in Fig. 1. High nuclear spin polarization in the radioactive beam is achieved using a collinear optical pumping method in which polarized light from a laser is directed along the beam axis [4]. The method is well established for the case of alkalis such as 8 Li; where the neutral atom can be excited with visible laser light [4]. The first step in the procedure is to neutralize the ion beam by passing it through a Na vapor cell. The neutral beam then drifts 1:9 m in the optical pumping region in the presence of a small longitudinal magnetic holding field of 1 mT: The D1 atomic transition of neutral Li 2s2 S1=2 -2p2 P1=2 occurs at 671 nm: After about 10–20 cycles of absorption and spontaneous emission, a high degree of electronic and nuclear spin polarization is generated. The final step is to strip off the valence electron by passing it through a He gas cell. The highest nuclear polarization achieved thus far is about 80% with an overall transmission efficiency of about 20%. However, the typical operating conditions are 60% polarization and 33% transmission. The polarized 8 Li ion beam is then passed through two 451 electrostatic bending elements, which route the beam into one of three experiOsaka exp. Low field region
High field spectrometer
Optical pumping region
{
Optics bench
8 Li +
30 keV beam
Na neutralizer cell
Neutral beam monitor He re-ionizer cell
Laser optics bench
Fig. 1. Layout of the polarizer and b-NMR instruments at ISAC.
191
mental stations. Since the beam optics are all electrostatic the polarization direction is unchanged by these bends. Consequently, the beam entering the high-field spectrometer remains longitudinally polarized; whereas, at the two other stations the beam is transversely polarized. Here we are concerned mainly with the high-field spectrometer. The section just before the spectrometer has three electrostatic Einzel lenses and three adjustable collimators which control the beam spot on the sample. A schematic of the spectrometer is shown in Fig. 2. The polarized beam enters from the left and passes through a hole in the back detector before entering the last Einzel lens at the entrance to the high-homogeneity 9 T superconducting solenoid. The beam spot at the center of the magnet is a sensitive function of Einzel lens voltage, magnetic field and beam energy. Images of the 8 Li beam at the sample position were obtained using a plastic scintillator and a CCD camera. These show that the beam spot is 3–4 mm in diameter. One of the most important features of the spectrometer is that the ions can be implanted over a wide range of energies (1–90 keV), corresponding to an average implantation depth of between 6 and 400 nm; respectively. This is accomplished by placing the spectrometer on a high voltage platform which is electrically isolated from ground. The energy of implantation is controlled by adjusting the platform bias voltage. The high-field spectrometer has longitudinal geometry, such that the polarization and magnetic field are both along the beam axis. This is necessary for measurements in high magnetic fields, where both the incoming ions and outgoing betas are strongly focused by the magnet. The forward detector is on the beam/magnet axis and is located several cm downstream of the sample. In order to detect betas in the backward direction (opposite to the beam direction), it is necessary that the detector be outside the magnet since the betas are confined to the magnet axis while inside the magnet bore. Although the solid angles subtended by the two detectors in zero field are very different, they have similar detection efficiencies in high magnetic fields due to the focusing effect of the solenoid.
192
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
Fig. 2. High-field b-NMR spectrometer at ISAC. The beam enters from the left, passes through an aperture in the backward scintillation detector and is focused onto the sample in the center of the 9 T superconducting magnet. Betas are detected with two scintillation counters centered on the axis of the superconducting solenoid.
3. Results 3.1. Gold foil Fig. 3 shows a typical b-NMR resonance for 8 Li implanted in a gold foil at 300 K at the nominal
Gold Foil 1.0
A / A0
The spectrometer and final leg of the beamline to which it is connected are UHV vacuum compatible in order to avoid a buildup of residual gases on the surface of the sample. Differential pumping is used to reduce the pressure from 107 Torr upstream of the spectrometer to 1010 Torr in the main chamber. The sample cryostat is mounted on a large bellows so that it can be withdrawn from the magnet bore in order to change the sample through a load lock on top of the main vacuum chamber (see Fig. 2). Plastic scintillation detectors are used to detect the betas from 8 Li -8 Be þ ne þ e ; for which the end point energy is 13 MeV: Since beta rates can reach 107 =s; the detectors are segmented to reduce rate dependent distortions in the spectra due to dead time. The plastic scintillators and light guides are held in reentrant stainless steel housings with thin stainless steel windows, isolating the detectors from the UHV vacuum chamber, but allowing transmission of the low-energy betas.
FWHM ~ 25 ppm 0.5
νMgO K ~ 100 ppm
0.0 18900
18902 18904 Frequency (kHz)
18906
Fig. 3. 8 Li b-NMR resonance in gold foil at 300 K in a magnetic field of 3 T: The asymmetry is normalized to the asymmetry off resonance A0 : The two resonances are explained in the text.
energy of 30 keV: The spectrum is obtained by applying a large static magnetic field ðH0 ¼ 3 TÞ along the initial polarization direction ðzÞ and a small RF magnetic field ðH1 p0:1 mTÞ perpendicular to H0 : One then records the beta decay asymmetry as a function of RF frequency, while a continuous beam of polarized 8 Li is implanted into the sample. When the frequency of the RF matches the Larmor frequency of the 8 Li nucleus ðgH0 Þ; the nuclear polarization precesses leading to
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
3.2. Nuclear quadrupole resonance in a thin YBa2 Cu3 O6:95 film Fig. 4 shows the b-NMR spectrum in a 100 nm thick c-axis film of YBa2 Cu3 O6:95 on a SrTiO3 substrate in an applied field of 3 T just above the superconducting transition temperature of 89 K: The implantation energy is 10 keV: The complexity compared to gold is attributed to quadrupolar splittings arising from sites with non-cubic symmetry. Assuming the electric-field-gradient tensor (EFG) is axially symmetric then in the high-field
0.10
ν0 T = 100 K > Tc, B = 3 T ELi = 8 keV
0.05
∆ (A/A0)
a reduction in the time averaged asymmetry. The measured asymmetry has been normalized to the asymmetry off resonance ðA0 Þ; which is about 0.13. The solid curve is a fit to two resonances corresponding to two crystallographic sites. The position of each resonance is a sensitive measure of the local magnetic field at the 8 Li site. In a metal the dominant fields are the applied field and the hyperfine field due to polarization of the conduction electrons at the Fermi surface. The latter is referred to as a Knight shift (see Ref. [7]), and can be estimated by comparing to the frequency in an insulator such as MgO. Both lines in Fig. 3 are within a few hundred ppm of the free Larmor frequency and thus imply the two sites have cubic symmetry. Otherwise, the electric quadrupole moment ðQ ¼ 31mBÞ of 8 Li would lead to quadrupolar splittings, which are typically large on this frequency scale. There are only three cubic sites in an FCC lattice—the substitutional (S), the octahedral interstitial (O), and the tetrahedral interstitial (T). As discussed in a related paper [8] we assign the large amplitude line to the S site and the small amplitude line to the O site. This is consistent with a previous study of 8 Li in Cu [9] which also has an FCC crystal structure. The line width of 500 Hz is remarkably narrow and corresponds to a magnetic field resolution of 0:08 mT: This is about an order of magnitude better than in a muon spin rotation experiment, but is still much greater than the intrinsic resolution of about 0:0001 mT determined by the lifetime of 8 Li: The observed line width may be due to a variety of effects such as the small nuclear magnetic dipoles in Au.
193
0.00
-0.05
-0.10 18.8
18.9 Frequency (MHz)
19.0
Fig. 4. b-NMR resonance spectrum in a 100 nm thick c axis film of YBa2 Cu3 O6:95 deposited on SrTiO3 ½1 0 0 in an applied field of 3 T: The deviation in the normalized beta decay asymmetry is plotted as a function of RF frequency for the two polarization directions. The resolved lines are attributed to stopping sites with different quadrupolar frequencies. The solid curve is a quide to the eye.
limit we expect in four equally spaced resonances centered about the Larmor frequency for each site. These correspond to the four allowed magnetic dipole transitions between the five magnetic sublevels ðm ¼ 2; 1; 0; 1; 2Þ: nðm2m 1Þ ¼ gH0 þ nQ ½m 1=2½3 cos2 y 1=2;
ð1Þ
where gH0 is the Larmor frequency, nQ ¼ 3e2 qQ=ð2Ið2I 1ÞÞ is the nuclear quadrupolar frequency and y is the angle between the symmetry axis of the EFG tensor and the magnetic field. However, when the polarization is large, only a single resonance is observable for each site [either nð221Þ or nð22 1Þ; depending on the laser helicity] since only one magnetic sublevel (m ¼ 2 or 2) is occupied appreciably. A simple example of this can be seen in SrTiO3 [10] where there is a single quadrupolar frequency. The multiple resonances in Fig. 4 are then attributed to multiple stopping sites. At least three quadrupolar frequencies can be resolved. There is also some enhanced depolarization close to the Larmor frequency. This is attributed to sites with a small quadrupolar splitting, for which the influence of the RF field is enhanced. Note for such 8 Li; the RF field fully rotates the nuclear polarization; whereas, for
194
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
centers with a large quadrupolar splitting the RF field only tilts the nuclear polarization. Significant broadening of the spectrum occurs below the superconducting transition of 89 K due to the vortex lattice as expected.
T=3K 1.0 0.8 0.6
65 K 1.0
3.3. 50 nm Ag=100 nm YBa2 Cu3 O6:95 /on SrTiO3
0.8 0.6 0.4
110 K
A / A0
1.0 0.5 0.0
290 K
1.0 0.5 0.0 18896
18900 18904 18908 Frequency ( kHz)
18912
Fig. 5. Temperature dependence of the 8 Li b-NMR resonance in a 50 nm silver film on top of 100 nm YBa2 Cu3 O6:95 on a SrTiO3 substrate in an applied field of 3 T: Above Tc the spectrum is identical to 90 nm Ag film on sapphire [8], whereas at low temperatures additional broadening of the lines is observed.
3
Average HWHM (kHz)
The muon is the most sensitive probe of the quasi-static magnetic field distribution in the vortex state of superconductor. Furthermore, it has recently been demonstrated that low-energy muons can be used to study the magnetic field distribution near the surface of a material by using a thin film of a simple metal on top of the superconductor [11] as a stopping layer for the positive muons. One can carry out similar studies with b-NMR. Ag is a good metal for this purpose since one expects a narrow resonance, which in turn, implies high resolution for measurements of a field distribution near the surface. As a test case for this type of experiment we studied a 50 nm thick silver film evaporated on a 100 nm thick c-axis oriented film of YBa2 Cu3 O6:95 on a SrTiO3 substrate. The evolution of the spectrum in the Ag overlayer as a function of temperature is shown in Fig. 5. The implantation energy was chosen to be 5 keV in order to maximize the signal amplitude from the thin film. As a control, similar spectra were taken on 90 nm of Ag on sapphire [8]. Above 100 K the spectra for the 50 nm Ag=100 nm YBa2 Cu3 O6:95 and 90 nm Ag=Al2 O3 were essentially identical. At room temperature a single narrow line is observed in both cases while at somewhat lower temperatures (but above Tc ) a second line appears which is attributed to another site (similar to the Au foil Fig. 3). At much lower temperatures, well below Tc ; significant broadening occurs in the 50 nmAg=YBa2 Cu3 O6:95 film that is not present in the control. Fig. 6 shows the measured broadening as function of temperature. The precise origin of the low temperature broadening in Figs. 5 and 6 is not yet clear. Although we anticipate some broadening from the vortex lattice, simulations indicate this is negligible in an applied field of 3 T: In any event the data clearly show that the method is a sensitive probe of magnetic fields
40 WRF ELi = 8keV B = 3T
2
1
Tc 0 0
50
100
150 200 Temperature (K)
250
300
Fig. 6. Temperature dependence of the 8 Li b-NMR line width in 50 nm Ag=100 nm YBa2 Cu3 O6:95 SrTiO3 : The low and high frequency line widths are averaged.
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
near the interface. The observed broadening corresponds to additional magnetic fields on the order of a few gauss. We anticipate a higher sensitivity and resolution in lower applied magnetic fields where any frequency splittings (or broadening) due to different Knight shifts should scale with magnetic field. In addition the sensitivity to the vortex lattice should increase dramatically as the vortex spacing becomes comparable to the penetration depth.
4. Conclusion In conclusion, we have commissioned a novel spectrometer for b-NMR at the new ISAC facility. Test experiments have been performed which demonstrate the high frequency resolution one can obtain with low-energy b-NMR. The narrow resonances demonstrate that the 8 Li is isolated from any radiation damage. We anticipate that lowenergy 8 Li can be used in an analogous but complementary way to low-energy muons to investigate the local magnetic field distribution at interfaces and thin films, but on a different time scale.
Acknowledgements We gratefully acknowledge support from NSERC and TRIUMF for this project. Technical support was provided mainly by Rahim Abasalti. We also thank Mel Good for his design of the detectors and Kallol Mitra for calculating the field distribution near the surface of the YBCO film.
195
References [1] S.L. Lee, S.H. Kilcoyne, R. Cywinski (Eds.), Muon Science, SUSSP publications and the Institute of Physics, UK, 1999. . [2] H. Ackermann, P. Heitjans, H.-J. Stockmann, in: J. Christansen (Ed.), Hyperfine Interactions of Radioactive Nuclei, Topics in Current Physics, Vol. 31, Springer, Berlin, 1983, p. 291. [3] B. Ittermann, G. Welker, F. Kroll, F. Mai, K. Marbuch, D. Peters, Phys. Rev. B 59 (1999) 2700. [4] M. Keim, U. Georg, A. Klein, R. Neugart, M. Neuroth, S. Wilbert, P. Lievens, L. Vermeeren, The ISOLDE Collaboration, Hyperfine Interactions 97/98 (1995) 543. [5] C.D.P. Levy, A. Hatakeyama, Y. Hirayama, R.F. Kiefl, R. Baartmann, J.A. Behr, H. Izumi, D. Melconian, G.D. Morris, R. Nussbaumer, M. Olivo, M. Pearson, R. Poutissou, G.W. Wight, Proceedings EMIS-14 Conference, Victoria, BC, May 2002. [6] E. Morenzoni, in: S.L. Lee, S.H. Kilcoyne, R. Cywinski (Eds.), Muon Science, SUSSP publications and the Institute of Physics, UK, 1999; E. Morenzoni, R. Khasanov, H. Luetkens, T. Prokscha, A. Suter, N. Garifianov, H. Gluckler, . M. Birke, E. Forgan, H. Keller, J. Litterst, Ch. Niedermayer, G. Nieuwenhuys, Physica B, this issue. [7] C.P. Slichter, Principles of Magnetic Resonance, 3rd Edition, Springer, Berlin, 1989. [8] W.A. MacFarlane, et al., 8 Li b-NMR in thin metal films, Physica B, these proceedings. [9] F. Ohsumi, K. Matsuta, M. Mihara, T. Onishi, T. Miyake, M. Sasaki, K. Sato, C. Ha, A. Morishita, T. Fujio, Y. Kobayashi, M. Fukuda, T. Minamisono, Hyperfine Interactions 120/121 (1999) 419. [10] W.A. MacFarlane, et al., Quadrupolar split 8 Li b-NMR in SrTiO3 ; Physica B, these proceedings. [11] Ch. Niedermayer, E.M. Forgan, H. Gluekler, A. Hofer, E. Morenzoni, M. Pleines, T. Prokscha, T.M. Riseman, M. Birke, T.J. Jackson, J. Litterst, M.W. Long, H. Luetkens, A. Schatz, G. Schatz, Phys. Rev. Lett. 83 (1999) 3932.
Low-energy spin-polarized radioactive beams as a nano-scale probe of matter R.F. Kiefla,b,c,*, W.A. MacFarlaned,a, G.D. Morrise, P. Amaudruza, D. Arseneaua, H. Azumif, R. Baartmana, T.R. Bealsa,b, J. Behra, C. Bommasg, J.H. Brewera, K.H. Chowh, E. Dumonti, S.R. Dunsigere, S. Daviela, L. Greenei, A. Hatakeyamaa, R.H. Heffnere, Y. Hirayamaf, B. Hittia, S.R. Kreitzmana, C.D.P. Levya, R.I. Millera,b, M. Olivoa, R. Poutissoua a TRIUMF, 4004 Wesbrook Mall, Vancouver BC, Canada V6T 2A3 Department of Physics and Astronomy, University of British Columbia, Vancouver BC, Canada V6T 1Z1 c Canadian Institute for Advanced Research, University of British Columbia, Vancouver BC, Canada V6T 1Z1 d Chemistry Department, University of British Columbia, Vancouver, Canada e Los Alamos National Lab, MS-K764, Los Alamos, NM 87545, USA f Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan g Department of Physics, University of Bonn, Bonn, Germany h Department of Physics, University of Alberta, Edmonton, AB, Canada T6G 2J1 i Department of Physics, University of Illinois at Urbana-Champaign, IL 61801, USA b
Abstract We have commissioned a polarized low-energy 8 Li ion beam line, which together with a high-field b-NMR spectrometer, can act as sensitive new probe of thin films and interfaces. The implantation energy can be continuously adjusted from 1 to 90 keV and the maximum polarization achieved thus far is 80%. This instrument opens up new applications for b-NMR which parallel and complement efforts with low-energy muons. For example, it is possible to probe the magnetic field distribution near the surface of a material by stopping a polarized 8 Li beam in a thin overlayer of Ag. Since the 8 Li adopts a site with cubic symmetry in Ag there is no quadrupolar splitting of the resonance, and the 8 Li acts as a purely magnetic sensor. r 2002 Elsevier Science B.V. All rights reserved. Keywords: b-NMR; Nuclear magnetic resonance; Radioactive ion beams; Nuclear probes
1. Introduction
*Corresponding author. TRIUMF, Wesbrook Mall, Vancouver, BC, Canada V6T 2A3. Tel.: +604-222-7511; fax: +604-222-1074. E-mail address: kiefl@triumf.ca (R.F. Kiefl).
The defining feature of nuclear methods, such as muon spin rotation ðmSRÞ or beta detected NMR (b-NMR), is that they provide local information via the radioactive decay of the probe. This has a number of important advantages over
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 0 0 - 9
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
190
conventional methods in condensed matter physics. For example, in nuclear magnetic resonance (NMR) a relatively small nuclear polarization is generated by applying a large magnetic field, which is then tilted with a small RF magnetic field. An inductive pickup coil is used to detect the resulting precession of the nuclear magnetization. Typically NMR requires about 1018 nuclear spins and is therefore mostly a bulk probe of matter. On the other hand, in related nuclear methods, such as mSR [1] or b-NMR [2,3], a beam of highly polarized radioactive nuclei (or muons) is generated and then implanted into the material. While the polarization tends to be much higher— between 10% and 100%, the most important difference is that the time evolution of the spin polarization is monitored through the anisotropic decay properties of the nucleus or muon, which requires about 10 orders of magnitude fewer spins. For this reason nuclear methods are well suited to studies of dilute impurities, small structures or interfaces where there are few nuclear spins. The principles of b-NMR and mSR are virtually identical. Since no magnetic field is required to generate or detect the spin polarization, both methods allow measurements to be performed in any magnetic field, including zero field. So far the high signal-to-noise required for condensed matter applications has been much easier to achieve in the case of muons, compared to radioactive nuclei. However, with radioactive ion beam facilities such as ISOLDE and ISAC, it is possible to generate intense ð> 107 =sÞ highly polarized (80%) beams of low-energy radioactive nuclei [4,5]. Under these circumstances one can realize a significant enhancement in the signal-to-noise in b-NMR.
Furthermore, one has the added possibility to control the depth of implantation on an interesting length scale (5–500 nm). This is similar to the lowenergy muon beam at PSI, which has developed into an important new technique for studies of thin films and interfaces (see Ref. [6]). One particularly attractive aspect of doing similar measurements with radioactive nuclei generated at ISAC is that the beams do not need to be moderated in energy and thus are substantially more intense than lowenergy muon beams currently available. Also, the nuclear polarization in b-NMR can be observed on a much longer time scale due to the much larger nuclear lifetimes. Although, in principle any b-emitting isotope with spin can be studied with b-NMR the number of isotopes suitable for use as a probe in condensed matter is much smaller. The most essential requirements are: (1) a high production efficiency, (2) a method to efficiently polarize the nuclear spins, and (3) a high b decay asymmetry. Other desirable features are: (4) small ionic charge to reduce radiation damage on implantation, (5) a small value of spin so that the b-NMR spectra are relatively simple, and (6) a radioactive lifetime that is not much longer than a few seconds. Table 1 gives a short list of the isotopes we have identified as suitable for development at ISAC. Production rates of 106 =s are easily obtainable at ISAC. 8 Li is the easiest to polarize and was therefore selected as the first isotope to develop as a probe at ISAC for this purpose. In this paper we describe the performance of the ISAC polarizer and associated spectrometer for bNMR. Preliminary results on a gold foil and a 100 nm YBa2 Cu3 O6:95 film on a SrTiO3 substrate
Table 1 Examples of isotopes suitable for b-NMR Isotope
Spin
T1=2 (s)
g ðMHz=TÞ
b-Decay asymmetry (A)
production rate ðs1 Þ
mþ 8 Li 11 Be 15 O 17 Ne
1/2 2 1/2 1/2 1/2
2.2106 0.8 13.8 122 0.1
135.5 6.26 22 10.8
0.33 0.33 0.33 0.7 0.33
750 108 107 108 106
The production rates are projections except in the case of 8 Li: For comparison the first entry is for the low-energy muon beam at PSI from [6].
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
are presented, which demonstrate the high sensitivity of the technique for detecting magnetic fields.
2. Experimental A layout of the polarized beamline and b-NMR spectrometers is given in Fig. 1. High nuclear spin polarization in the radioactive beam is achieved using a collinear optical pumping method in which polarized light from a laser is directed along the beam axis [4]. The method is well established for the case of alkalis such as 8 Li; where the neutral atom can be excited with visible laser light [4]. The first step in the procedure is to neutralize the ion beam by passing it through a Na vapor cell. The neutral beam then drifts 1:9 m in the optical pumping region in the presence of a small longitudinal magnetic holding field of 1 mT: The D1 atomic transition of neutral Li 2s2 S1=2 -2p2 P1=2 occurs at 671 nm: After about 10–20 cycles of absorption and spontaneous emission, a high degree of electronic and nuclear spin polarization is generated. The final step is to strip off the valence electron by passing it through a He gas cell. The highest nuclear polarization achieved thus far is about 80% with an overall transmission efficiency of about 20%. However, the typical operating conditions are 60% polarization and 33% transmission. The polarized 8 Li ion beam is then passed through two 451 electrostatic bending elements, which route the beam into one of three experiOsaka exp. Low field region
High field spectrometer
Optical pumping region
{
Optics bench
8 Li +
30 keV beam
Na neutralizer cell
Neutral beam monitor He re-ionizer cell
Laser optics bench
Fig. 1. Layout of the polarizer and b-NMR instruments at ISAC.
191
mental stations. Since the beam optics are all electrostatic the polarization direction is unchanged by these bends. Consequently, the beam entering the high-field spectrometer remains longitudinally polarized; whereas, at the two other stations the beam is transversely polarized. Here we are concerned mainly with the high-field spectrometer. The section just before the spectrometer has three electrostatic Einzel lenses and three adjustable collimators which control the beam spot on the sample. A schematic of the spectrometer is shown in Fig. 2. The polarized beam enters from the left and passes through a hole in the back detector before entering the last Einzel lens at the entrance to the high-homogeneity 9 T superconducting solenoid. The beam spot at the center of the magnet is a sensitive function of Einzel lens voltage, magnetic field and beam energy. Images of the 8 Li beam at the sample position were obtained using a plastic scintillator and a CCD camera. These show that the beam spot is 3–4 mm in diameter. One of the most important features of the spectrometer is that the ions can be implanted over a wide range of energies (1–90 keV), corresponding to an average implantation depth of between 6 and 400 nm; respectively. This is accomplished by placing the spectrometer on a high voltage platform which is electrically isolated from ground. The energy of implantation is controlled by adjusting the platform bias voltage. The high-field spectrometer has longitudinal geometry, such that the polarization and magnetic field are both along the beam axis. This is necessary for measurements in high magnetic fields, where both the incoming ions and outgoing betas are strongly focused by the magnet. The forward detector is on the beam/magnet axis and is located several cm downstream of the sample. In order to detect betas in the backward direction (opposite to the beam direction), it is necessary that the detector be outside the magnet since the betas are confined to the magnet axis while inside the magnet bore. Although the solid angles subtended by the two detectors in zero field are very different, they have similar detection efficiencies in high magnetic fields due to the focusing effect of the solenoid.
192
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
Fig. 2. High-field b-NMR spectrometer at ISAC. The beam enters from the left, passes through an aperture in the backward scintillation detector and is focused onto the sample in the center of the 9 T superconducting magnet. Betas are detected with two scintillation counters centered on the axis of the superconducting solenoid.
3. Results 3.1. Gold foil Fig. 3 shows a typical b-NMR resonance for 8 Li implanted in a gold foil at 300 K at the nominal
Gold Foil 1.0
A / A0
The spectrometer and final leg of the beamline to which it is connected are UHV vacuum compatible in order to avoid a buildup of residual gases on the surface of the sample. Differential pumping is used to reduce the pressure from 107 Torr upstream of the spectrometer to 1010 Torr in the main chamber. The sample cryostat is mounted on a large bellows so that it can be withdrawn from the magnet bore in order to change the sample through a load lock on top of the main vacuum chamber (see Fig. 2). Plastic scintillation detectors are used to detect the betas from 8 Li -8 Be þ ne þ e ; for which the end point energy is 13 MeV: Since beta rates can reach 107 =s; the detectors are segmented to reduce rate dependent distortions in the spectra due to dead time. The plastic scintillators and light guides are held in reentrant stainless steel housings with thin stainless steel windows, isolating the detectors from the UHV vacuum chamber, but allowing transmission of the low-energy betas.
FWHM ~ 25 ppm 0.5
νMgO K ~ 100 ppm
0.0 18900
18902 18904 Frequency (kHz)
18906
Fig. 3. 8 Li b-NMR resonance in gold foil at 300 K in a magnetic field of 3 T: The asymmetry is normalized to the asymmetry off resonance A0 : The two resonances are explained in the text.
energy of 30 keV: The spectrum is obtained by applying a large static magnetic field ðH0 ¼ 3 TÞ along the initial polarization direction ðzÞ and a small RF magnetic field ðH1 p0:1 mTÞ perpendicular to H0 : One then records the beta decay asymmetry as a function of RF frequency, while a continuous beam of polarized 8 Li is implanted into the sample. When the frequency of the RF matches the Larmor frequency of the 8 Li nucleus ðgH0 Þ; the nuclear polarization precesses leading to
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
3.2. Nuclear quadrupole resonance in a thin YBa2 Cu3 O6:95 film Fig. 4 shows the b-NMR spectrum in a 100 nm thick c-axis film of YBa2 Cu3 O6:95 on a SrTiO3 substrate in an applied field of 3 T just above the superconducting transition temperature of 89 K: The implantation energy is 10 keV: The complexity compared to gold is attributed to quadrupolar splittings arising from sites with non-cubic symmetry. Assuming the electric-field-gradient tensor (EFG) is axially symmetric then in the high-field
0.10
ν0 T = 100 K > Tc, B = 3 T ELi = 8 keV
0.05
∆ (A/A0)
a reduction in the time averaged asymmetry. The measured asymmetry has been normalized to the asymmetry off resonance ðA0 Þ; which is about 0.13. The solid curve is a fit to two resonances corresponding to two crystallographic sites. The position of each resonance is a sensitive measure of the local magnetic field at the 8 Li site. In a metal the dominant fields are the applied field and the hyperfine field due to polarization of the conduction electrons at the Fermi surface. The latter is referred to as a Knight shift (see Ref. [7]), and can be estimated by comparing to the frequency in an insulator such as MgO. Both lines in Fig. 3 are within a few hundred ppm of the free Larmor frequency and thus imply the two sites have cubic symmetry. Otherwise, the electric quadrupole moment ðQ ¼ 31mBÞ of 8 Li would lead to quadrupolar splittings, which are typically large on this frequency scale. There are only three cubic sites in an FCC lattice—the substitutional (S), the octahedral interstitial (O), and the tetrahedral interstitial (T). As discussed in a related paper [8] we assign the large amplitude line to the S site and the small amplitude line to the O site. This is consistent with a previous study of 8 Li in Cu [9] which also has an FCC crystal structure. The line width of 500 Hz is remarkably narrow and corresponds to a magnetic field resolution of 0:08 mT: This is about an order of magnitude better than in a muon spin rotation experiment, but is still much greater than the intrinsic resolution of about 0:0001 mT determined by the lifetime of 8 Li: The observed line width may be due to a variety of effects such as the small nuclear magnetic dipoles in Au.
193
0.00
-0.05
-0.10 18.8
18.9 Frequency (MHz)
19.0
Fig. 4. b-NMR resonance spectrum in a 100 nm thick c axis film of YBa2 Cu3 O6:95 deposited on SrTiO3 ½1 0 0 in an applied field of 3 T: The deviation in the normalized beta decay asymmetry is plotted as a function of RF frequency for the two polarization directions. The resolved lines are attributed to stopping sites with different quadrupolar frequencies. The solid curve is a quide to the eye.
limit we expect in four equally spaced resonances centered about the Larmor frequency for each site. These correspond to the four allowed magnetic dipole transitions between the five magnetic sublevels ðm ¼ 2; 1; 0; 1; 2Þ: nðm2m 1Þ ¼ gH0 þ nQ ½m 1=2½3 cos2 y 1=2;
ð1Þ
where gH0 is the Larmor frequency, nQ ¼ 3e2 qQ=ð2Ið2I 1ÞÞ is the nuclear quadrupolar frequency and y is the angle between the symmetry axis of the EFG tensor and the magnetic field. However, when the polarization is large, only a single resonance is observable for each site [either nð221Þ or nð22 1Þ; depending on the laser helicity] since only one magnetic sublevel (m ¼ 2 or 2) is occupied appreciably. A simple example of this can be seen in SrTiO3 [10] where there is a single quadrupolar frequency. The multiple resonances in Fig. 4 are then attributed to multiple stopping sites. At least three quadrupolar frequencies can be resolved. There is also some enhanced depolarization close to the Larmor frequency. This is attributed to sites with a small quadrupolar splitting, for which the influence of the RF field is enhanced. Note for such 8 Li; the RF field fully rotates the nuclear polarization; whereas, for
194
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
centers with a large quadrupolar splitting the RF field only tilts the nuclear polarization. Significant broadening of the spectrum occurs below the superconducting transition of 89 K due to the vortex lattice as expected.
T=3K 1.0 0.8 0.6
65 K 1.0
3.3. 50 nm Ag=100 nm YBa2 Cu3 O6:95 /on SrTiO3
0.8 0.6 0.4
110 K
A / A0
1.0 0.5 0.0
290 K
1.0 0.5 0.0 18896
18900 18904 18908 Frequency ( kHz)
18912
Fig. 5. Temperature dependence of the 8 Li b-NMR resonance in a 50 nm silver film on top of 100 nm YBa2 Cu3 O6:95 on a SrTiO3 substrate in an applied field of 3 T: Above Tc the spectrum is identical to 90 nm Ag film on sapphire [8], whereas at low temperatures additional broadening of the lines is observed.
3
Average HWHM (kHz)
The muon is the most sensitive probe of the quasi-static magnetic field distribution in the vortex state of superconductor. Furthermore, it has recently been demonstrated that low-energy muons can be used to study the magnetic field distribution near the surface of a material by using a thin film of a simple metal on top of the superconductor [11] as a stopping layer for the positive muons. One can carry out similar studies with b-NMR. Ag is a good metal for this purpose since one expects a narrow resonance, which in turn, implies high resolution for measurements of a field distribution near the surface. As a test case for this type of experiment we studied a 50 nm thick silver film evaporated on a 100 nm thick c-axis oriented film of YBa2 Cu3 O6:95 on a SrTiO3 substrate. The evolution of the spectrum in the Ag overlayer as a function of temperature is shown in Fig. 5. The implantation energy was chosen to be 5 keV in order to maximize the signal amplitude from the thin film. As a control, similar spectra were taken on 90 nm of Ag on sapphire [8]. Above 100 K the spectra for the 50 nm Ag=100 nm YBa2 Cu3 O6:95 and 90 nm Ag=Al2 O3 were essentially identical. At room temperature a single narrow line is observed in both cases while at somewhat lower temperatures (but above Tc ) a second line appears which is attributed to another site (similar to the Au foil Fig. 3). At much lower temperatures, well below Tc ; significant broadening occurs in the 50 nmAg=YBa2 Cu3 O6:95 film that is not present in the control. Fig. 6 shows the measured broadening as function of temperature. The precise origin of the low temperature broadening in Figs. 5 and 6 is not yet clear. Although we anticipate some broadening from the vortex lattice, simulations indicate this is negligible in an applied field of 3 T: In any event the data clearly show that the method is a sensitive probe of magnetic fields
40 WRF ELi = 8keV B = 3T
2
1
Tc 0 0
50
100
150 200 Temperature (K)
250
300
Fig. 6. Temperature dependence of the 8 Li b-NMR line width in 50 nm Ag=100 nm YBa2 Cu3 O6:95 SrTiO3 : The low and high frequency line widths are averaged.
R.F. Kiefl et al. / Physica B 326 (2003) 189–195
near the interface. The observed broadening corresponds to additional magnetic fields on the order of a few gauss. We anticipate a higher sensitivity and resolution in lower applied magnetic fields where any frequency splittings (or broadening) due to different Knight shifts should scale with magnetic field. In addition the sensitivity to the vortex lattice should increase dramatically as the vortex spacing becomes comparable to the penetration depth.
4. Conclusion In conclusion, we have commissioned a novel spectrometer for b-NMR at the new ISAC facility. Test experiments have been performed which demonstrate the high frequency resolution one can obtain with low-energy b-NMR. The narrow resonances demonstrate that the 8 Li is isolated from any radiation damage. We anticipate that lowenergy 8 Li can be used in an analogous but complementary way to low-energy muons to investigate the local magnetic field distribution at interfaces and thin films, but on a different time scale.
Acknowledgements We gratefully acknowledge support from NSERC and TRIUMF for this project. Technical support was provided mainly by Rahim Abasalti. We also thank Mel Good for his design of the detectors and Kallol Mitra for calculating the field distribution near the surface of the YBCO film.
195
References [1] S.L. Lee, S.H. Kilcoyne, R. Cywinski (Eds.), Muon Science, SUSSP publications and the Institute of Physics, UK, 1999. . [2] H. Ackermann, P. Heitjans, H.-J. Stockmann, in: J. Christansen (Ed.), Hyperfine Interactions of Radioactive Nuclei, Topics in Current Physics, Vol. 31, Springer, Berlin, 1983, p. 291. [3] B. Ittermann, G. Welker, F. Kroll, F. Mai, K. Marbuch, D. Peters, Phys. Rev. B 59 (1999) 2700. [4] M. Keim, U. Georg, A. Klein, R. Neugart, M. Neuroth, S. Wilbert, P. Lievens, L. Vermeeren, The ISOLDE Collaboration, Hyperfine Interactions 97/98 (1995) 543. [5] C.D.P. Levy, A. Hatakeyama, Y. Hirayama, R.F. Kiefl, R. Baartmann, J.A. Behr, H. Izumi, D. Melconian, G.D. Morris, R. Nussbaumer, M. Olivo, M. Pearson, R. Poutissou, G.W. Wight, Proceedings EMIS-14 Conference, Victoria, BC, May 2002. [6] E. Morenzoni, in: S.L. Lee, S.H. Kilcoyne, R. Cywinski (Eds.), Muon Science, SUSSP publications and the Institute of Physics, UK, 1999; E. Morenzoni, R. Khasanov, H. Luetkens, T. Prokscha, A. Suter, N. Garifianov, H. Gluckler, . M. Birke, E. Forgan, H. Keller, J. Litterst, Ch. Niedermayer, G. Nieuwenhuys, Physica B, this issue. [7] C.P. Slichter, Principles of Magnetic Resonance, 3rd Edition, Springer, Berlin, 1989. [8] W.A. MacFarlane, et al., 8 Li b-NMR in thin metal films, Physica B, these proceedings. [9] F. Ohsumi, K. Matsuta, M. Mihara, T. Onishi, T. Miyake, M. Sasaki, K. Sato, C. Ha, A. Morishita, T. Fujio, Y. Kobayashi, M. Fukuda, T. Minamisono, Hyperfine Interactions 120/121 (1999) 419. [10] W.A. MacFarlane, et al., Quadrupolar split 8 Li b-NMR in SrTiO3 ; Physica B, these proceedings. [11] Ch. Niedermayer, E.M. Forgan, H. Gluekler, A. Hofer, E. Morenzoni, M. Pleines, T. Prokscha, T.M. Riseman, M. Birke, T.J. Jackson, J. Litterst, M.W. Long, H. Luetkens, A. Schatz, G. Schatz, Phys. Rev. Lett. 83 (1999) 3932.