Quantum coherent materials have become important in magnetic sensors as well as information processing technology. For magnetic sensors, spin-based transport in metals and across tunnel barriers is important because it can increase the signal to noise and reduce the power. These devices have applications in a wide variety of fields, ranging from sensing to imaging. We have developed large (256 element) arrays and systems to make forensic copies of magnetic tapes for data migration and authenticity analysis. In the area of quantum computing, we are developing crystalline tunnel junctions that will improve quantum state visibility and measurements. We are also working to improve the coherence of these devices by developing low-loss dielectrics.
NIST has become a world leader in the area of high sensitivity magnetic field sensors. To achieve this, we have explored spintronic devices, including anisotropic magneto-resistive (AMR) and tunneling magneto-resistive (TMR) devices. With the AMR devices, we have developed tape read heads that have many sensing elements. These devices are designed to operate at DC, and are used to image magnetic tapes. The system that we built was delivered to the FBI and is currently in the process of being validated for use on analog audio tape evidence. The TMR devices are targeted towards low noise field sensors and are integrated with a flux concentrator. We have reached noise levels better than one millionth of the Earth’s magnetic field, with a dynamic range allowing us to run them in an un-shielded environment. These devices are very low power and are targeted towards geological and exploratory work.
For the quantum computing work, we have been developing superconducting devices using a novel approach to making a tunnel barrier. Traditionally these tunnel junctions use convenient amorphous, thermal oxide barriers because the thickness is easily controllable and they tend to be pin-hole free. However, it has been shown that they have energy-absorbing defects at random frequencies due to their amorphous nature. Therefore, we developed a process to grow epitaxial tunnel junctions. This is challenging because the tunnel current depends exponentially on the barrier thickness, and epitaxial barriers require high temperatures to crystallize. We worked around these issues using precision, ultra-high vacuum molecular beam epitaxy (MBE) growth and lattice matched, superconducting rhenium films on sapphire substrates. We are the first to be able to fabricate, characterize, and successfully integrate them into devices. In addition, the performance of the devices improved significantly, enabling us to use optical lithography rather than e-beam.
One second of audio in a cassette tape imaged using the 256-sensor array.
Lead Organizational Unit:pml
David Pappas, Project Leader
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