ThisAdvanced Mgnetic and Quantum Materials Project addresses measurement needs in characterization of magnetic sensors, advanced applications of magnetic sensors, and national security. We are developing magnetoresistive arrays and readout electronics that can image magnetic fields for biomedical research, forensic analysis of magnetic recording media, and current distributions in integrated circuits for nondestructive evaluation. We also develop advanced electron tunneling barriers for superconducting Josephson junctions for future quantum computers.
Magnetic Sensors — Magnetoresistive magnetic field sensors have wide application in research and industry. Currently, cryogenic superconducting quantum interference devices (SQUIDs) are used for the measurement of ultra-low magnetic fields. They provide important information in many areas of basic research, medical magnetic field monitoring, and security. However, it is necessary to develop devices operating at room temperature that are scalable, linear, and have comparable sensitivity to realize their maximum benefit. The most promising candidates for development are magnetoresistive technologies. Magnetic noise is a critical problem in the development of these devices; such noise can be two orders of magnitude higher than the intrinsic Johnson, shot, and 1/f noise of the device. New materials, measurement techniques, and device architectures are required to characterize and reduce the effect of low- and high-frequency magnetic fluctuations.
Magnetic Forensics — In the late 1990s, in a collaboration with the National Telecommunications and Information Administration, we developed basic magnetic imaging technology to retrieve data from damaged or altered magnetic tapes and computer disks by rastering samples with a single sensor to build up an image. We first demonstrated the approach by recovering data from scraps of aircraft “black-box” tape that were too short to be played in a conventional tape deck. For several years, the Federal Bureau of Investigation (FBI) used a prototype of the magnetic imaging system. Because the system was slow, it was used only for testing and special cases. There has been a need for new, faster methods to authenticate magnetic recording tape.
Quantum Computing — Promising candidates for quantum computing
devices are quantum bits (qubits) based on Josephson junctions. These junctions
are used in magnetic detectors (SQUIDS), voltage standards, and other superconducting
electronic devices; techniques for their fabrication have been investigated in
some detail. However, qubits have far more stringent noise requirements that
are now only barely satisfied by current fabrication technology. A significant
limitation to the performance of these devices is the generation of defects in
the oxide interlayers. The structural origin of these defects is not well established,
but is likely a consequence of both the amorphous nature of the oxides and the
preparation methods.
Magnetic Sensors — In 2004 we embarked on a NIST “Competence” program with the Materials Science and Engineering Laboratory, the Physics Laboratory, and the Information Technology Laboratory on low-noise magnetic sensors. The program includes development of new amorphous and nanocrystalline materials, imaging of magnetic domains, noise measurement in sensor devices, and micromagnetic modeling.
Magnetic Forensics — We are fabricating linear arrays of 256 sensors to scan magnetic recording tape in a single pass. The sensors make use of the magnetoresistance effect, where their electrical resistance changes in response to magnetic fields detected from the tape. Software converts the sensor resistance measurements to visual images that have a resolution of about 240 dots per centimeter (1520 dpi). Systems that are of interest include cassette, VHS, and digital audio tapes.
Quantum Computing — State-of-the-art Josephson junctions employing superconducting Al or Nb electrodes and native-oxide tunnel barriers typically are fabricated by use of sputter deposition onto thermally oxidized Si wafers for microelectronics fabrication. However, spurious resonant states arising from microstructural defects in the tunnel barrier promote decoherence in Josephson qubits. These resonances may originate from defects and inhomogeneous oxidation during the tunnel barrier formation. Crystalline oxides contain three to four orders of magnitude fewer defects in the frequency range of interest for Josephson qubits. Our capability to prepare and then process samples in ultrahigh vacuum is critical to the development of defect-free, crystalline tunneling barriers. We are now developing a process to grow epitaxial Re on crystalline sapphire (Al2O3) substrates.
We have developed new “zig-zag” magnetoresistive sensors that, because of their shape, are sensitive to magnetic fields parallel to their axes but insensitive to fields perpendicular. The thin-film devices are also able to distinguish positive and negative fields because their resistance is an odd function of field.
In general, anisotropic magnetoresistive sensors must be biased, with their magnetization at an angle with respect to the current direction. Our new sensors are fabricated in a zig-zag pattern that pins the magnetization at alternating positive and negative 45 degrees to the direction of the current flow (see figure 1). This novel approach provides a built-in magnetization bias determined by the corrugation of the edges. Since the angle of magnetization of the magnetic domains is controlled only by the sensor shape, and not by adjacent biasing fields, the devices may be scaled to nanometric dimensions. When the sensor is exposed to a magnetic field oriented in the direction of the current, the angle of magnetization relative to the current in both sets of the domains decreases. This results in an increase of electrical resistance in each domain and in the entire device. For magnetic fields in the negative current direction, the magnetization angle increases and the resistance decreases. For magnetic fields in the perpendicular axis, the magnetization angle increases in one set of magnetic domains and decreases in the other, resulting in no net change in device resistance.
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Magnetization of a 10 mm long zig-zag shaped magnetic element. |
We conducted high-resolution magnetic imaging measurements on erased audio tape for the FBI as part of its participation in a study by the National Archives and Records Administration (NARA) on the feasibility of recovering audio from an 18.5 minute gap in a tape from the Nixon White House.
NARA’s 6-millimeter-wide test tapes had been recorded and then erased in a manner similar to that of the Nixon tapes. As described by NARA, the test tapes were “recorded on an original Nixon White House Sony 800B tape recorder, then erased on Rosemary Woods’ UHER 5000.” Tests were made of both an erased audio recording and an erased blank section of tape. The FBI could find no trace of the recording or erasure marks using a standard imaging technique with a ferromagnetic fluid. However, we were able to detect an extra noise band on the outside edge of the erased section of the audio tape that was not evident on the erased section of blank tape. The noise band was scanned at high resolution and converted to an audio signal. It consisted of only very low frequency sounds, with no trace of audible speech. In a press release dated May 8, 2003, Archivist of the United States John W. Carlin said, “I am fully satisfied that we have explored all of the avenues to attempt to recover the sound on this tape. The candidates were highly qualified and used the latest technology in their pursuit.”
We developed a real-time magnetic imaging system that enables criminal investigators to see signs of tampering in audiotapes — erasing, over-recording, and other alterations — while listening to the tapes. An upgraded system was delivered to the FBI in Sept. 2006. The upgrades included increasing the resolution by a factor of 4 (to 256 sensors from 64), the capability to scan both VHS and cassette tapes, on-the-fly image capture, complete computer control of the cassette tape transport, and the capability to digitize the audio directly from the acquired image (either real-time or in post-processing). A sample image is shown in the figure.
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Magnetic image of ~ 2 seconds of audio cassette tape where an overrecorded event occurred. On the top left is new data; the top right is old data. The erased gap in center top is the area between the erase head and write head. |
At the heart of the technology is a cassette player modified with an array of 256 customized magnetic sensors that detects and maps the microscopic magnetic fields on audiotapes as they are played. The array is connected to a desktop computer programmed to convert the magnetic data into a displayable image. Authentic, original tapes produce images with non-interrupted, predictable patterns, whereas erase and record functions produce characteristic “smudges” in an image that correlate to “pops” and “thumps” in the audio signal. The original markings specific to different types of tape players are not present on tape copies. An examiner can use this new system to help determine the authenticity of a tape and also help determine whether that tape is a copy. The benefits of the system are its speed in correlating sounds with magnetic marks on tape, and the fact that it makes an image without damaging the tape.
In collaboration with the Quantum Information and Measurements Project, we studied the growth of tunnel junction base electrodes to investigate the role that crystalline quality plays in device performance. We found that there is a strong correlation between the morphology of oxidized base electrodes and the lowering of subgap currents, and through the use of epitaxial seed layers, junctions with even lower subgap currents were obtained. In order to accomplish this, we engineered Josephson junctions with increasingly improved crystalline quality and were able to correlate the microstructure of the junctions with low-frequency transport measurements. Our data indicate a strong correlation between improved crystallinity of the tunnel barrier and reduced subgap leakage currents.
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Comparison of spectra taken from qubits with amorphous (a) and crystalline (b) barriers. |
In order to test the properties of these materials, we have fabricated quantum bits using the crystalline Al2O3 as a tunnel barrier. We have found an 80% reduction in the density of the spectral splittings that indicate the existence of two-level fluctuators (TLFs) in amorphous tunnel barriers (see the figure). The residual 20% TLFs can be attributed to interfacial effects that may be further reduced by different electrode materials. These results show that decoherence sources in the tunnel barrier of Josephson qubits can be identified and eliminated.
R. R. Owings, F. C. S. da Silva, D. P. Pappas, “Investigation of a thin film magnetic moment reference material,” NIST Journal of Research, in press, (2006).
S. Oh, K. Cicak, J. S. Kline, M. A. Sillanpää, K. D. Osborn, J. D. Whittaker, R. W. Simmonds, and D. P. Pappas, “Elimination of two level fluctuators in superconducting quantum bits by an epitaxial tunnel barrier,” Phys. Rev. B 74, 100502R (2006).
D. P. Pappas, A. V. Nazarov, D. Stevenson , S. Voran, M. E. Read, E. M. Gormley, J. Cash, K. Marr, J. J. Ryan, “Second-harmonic magnetoresistive imaging to authenticate and recover data from magnetic storage media”.
B. L. Zink, K. D. Irwin, G. C. Hilton, J. N. Ullom, D. P. Pappas, “Erium-doped gold sensor films for magnetic microcalorimeter x-ray detectors,” J. Appl. Phys. 99, 08B303 (2006).
S. Oh, D. A. Hite, K. Cicak, K. D. Osborn, R. W. Simmonds, R. McDermott, K. B. Cooper, M. Steffen, J. M. Martinis, D. P. Pappas, “Epitaxial growth of rhenium with sputtering,” Thin Solid Films 496, 389-394 (Feb. 21, 2006).
D. P. Pappas, A. Nazarov, D. Stevenson, S. Voran, M. E. Read, E. M. Gormley, J. Cash, K. Marr, and J. Ryan, “Second Harmonic Magneto-Resistive Imaging to Authenticate and Recover Data from Magnetic Storage Media,” J. Electronic Imaging 14, 013015 (Jan.-Mar. 2005).
D. P. Pappas, C. S. Arnold, “Application of W-Re thermocouples for in situ ultrahigh vacuum use over a wide temperature range,” Rev. Sci. Instrum. 76, 016104 (Jan. 2005).
N. A. Stutzke, S. E. Russek, D. P. Pappas, M. Tondra, “Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors,” J. Appl. Phys. 97, 10Q107 (May 15, 2005).
J. M. Martinis, K. B. Cooper, R. McDermott, M. Steffen, M. Ansmann, K. D. Osborn, K. Cicak, S. Oh, D. P. Pappas, R. W. Simmonds, C. C. Yu, “Decoherence in Josephson qubits from dielectric loss,” Phys. Rev. Lett. 95, 210503 (Nov. 18, 2005).
S. Oh, K. Cicak, R/ McDermott, K. B. Cooper, K. D. Osborn, R. W. Simmonds, M. Steffen, J. M. Martinis, D. P. Pappas, “Low-leakage superconducting tunnel junctions with a single-crystal Al2O3 barrier,” Superconductor Science & Technology 18, 1396-1399 (Oct. 2005).
R. McDermott, R. W. Simmonds, M. Steffen, K. B. Cooper, K. Cicak, K. D. Osborn, S. Oh, D. P. Pappas, J. M. Martinis, “Simultaneous state measurement of coupled Josephson phase qubits,” Science 307, 1299-1302 (Feb. 25 2005).
R. W. Simmonds, K. M. Lang, D. A. Hite, S. Nam, D. P. Pappas, and J. M. Martinis, “Decoherence in Josephson Phase Qubits from Junction Resonators,” Phys. Rev. Lett.. 93, 77003/1-4 (August 2004).
K. M. Lang, D. A. Hite, R. W. Simmonds, R. McDermott, D. P. Pappas, and J. M. Martinis, “Conducting Atomic Force Microscopy for Nanoscale Tunnel Barrier Characterization,” Rev. Sci. Instrum. 75, 2726-2731 (August 2004).
K. B. Cooper, M> Steffen, R. McDermott, R. W. Simmonds, S. Oh, D. A. Hite, D. P. Pappas, J. M. Martinis, "Observation of quantum oscillations between a Josephson phase qubit and a microscopic resonator using fast readout,” Phys. Rev. Lett. 93, 180401 (Oct. 29 2004).
D. P. Pappas, F. da Silva, R. N. Clarke, J. Unguris, W. C. Uhlig, and A. Kos, “Zigzag Shaped Magnetic Sensors,” Appl. Phys. Lett. 85, 6022-602 (Dec. 13 2004).
M. G. Pini, A. Rettori, D. P. Pappas, A. V. Anisimov, and A. P. Popov, “Surface Magnetic Canting in a Nonuniform Film,” J. Magn. Magn. Mater. 272-276 1152-1153 (May 2004).
B. L. Zink, K. D. Irwin, G. C. Hilton, D. P. Pappas, J. N. Ullom, and M. E. Huber, “Lithographically Patterned Magnetic Calorimeter X-ray Detectors with Integrated SQUID Readout,” Nucl. Instrum. Methods Phys. Res. A 520, 52-55 (March 2004).
A. V. Nazarov, F. C. S. da Silva, D. P. Pappas, “Arrays of magnetoresistive sensors for nondestructive testing,” J. Vacuum Science & Technology A 22, 375-1378 (Jul.-Aug. 2004).