A layer of ruthenium just a few atoms thick can be used to fine-tune the sensitivity and enhance the reliability of magnetic sensors, tests at the National Institute of Standards and Technology (NIST) show.* The nonmagnetic metal acts as a buffer between active layers of sensor materials, offering a simple means of customizing field instruments such as compasses, and stabilizing the magnetization in a given direction in devices such as computer hard-disk readers.
In the NIST sensor design, ruthenium modulates interactions between a ferromagnetic film (in which electron "spins" all point in the same direction) and an antiferromagnetic film (in which different layers of electrons point in opposite directions to stabilize the device). In the presence of a magnetic field, the electron spins in the ferromagnetic film rotate, changing the sensor's resistance and producing a voltage output. The antiferromagnetic film, which feels no force because it has no net magnetization, acts like a very stiff spring that resists the rotation and stabilizes the sensor. The ruthenium layer (see graphic) is added to weaken the spring, effectively making the device more sensitive. This makes it easier to rotate the electron spins, and still pulls them back to their original direction when the field is removed.
NIST tests showed that thicker buffers of ruthenium (up to 2 nanometers) make it easier to rotate the magnetization of the ferromagnetic film, resulting in a more sensitive device. Thinner buffers result in a device that is less sensitive but responds to a wider range of external fields. Ruthenium layers thicker than 2 nm prevent any coupling between the two active films. All buffer thicknesses from 0 to 2 nm maintain sensor magnetization (even resetting it if necessary) without a boost from an external electrical current or magnetic field. This easily prevents demagnetization and failure of a sensor.
The mass-producible test sensors, made in the NIST clean room in Boulder, Colo., consist of three basic layers of material deposited on silicon wafers: The bottom antiferromagnetic layer is 8 nm of an iridium/manganese alloy, followed by the ruthenium buffer, and topped with 25 nm of a nickel/iron alloy. The design requires no extra lithography steps for the magnetic layers and could be implemented in existing mass-production processes. By contrast, the conventional method of modulating magnetoresistive sensors-capping the ends of sensors with magnetic materials-adds fabrication steps and does not allow fine-tuning of sensitivity. The new sensor design was key to NIST's recent development of a high-resolution forensic tape analysis system for the Federal Bureau of Investigation
Magnetic Tape Analysis "Sees" Tampering in Detail
The National Institute of Standards and Technology (NIST) has developed an improved version of a real-time magnetic microscopy system that converts evidence of tampering on magnetic audio and video tapes-erasing, overdubbing and other alterations-into images with four times the resolution previously available. This system is much faster than conventional manual analysis and offers the additional benefit of reduced risk of contaminating the tapes with magnetic powder. NIST recently delivered these new capabilities to the Federal Bureau of Investigation (FBI) for validation as a forensic tool.
Earlier versions of this system made images with a resolution of about 400 dots per inch (dpi). www.nist.gov/public_affairs/releases/tape_analysis.htm. The new system uses four times as many magnetic sensors, 256, embedded on a NIST-made silicon chip that serves as a read head in a modified cassette tape deck. The NIST read head operates adjacent to a standard read head, enabling investigators to listen to a tape while simultaneously viewing the magnetic patterns on a computer monitor. Each sensor in the customized read head changes electrical resistance in response to magnetic field patterns detected on the tape. NIST developed the mechanical system for extracting a tape from its housing and transporting it over the read heads, the electronics interface, and software that convert maps of sensor resistance measures into digital images.
The upgrade included quadrupling the image resolution to 1600 dpi, the capability to scan both video and audio tapes, complete computer control of tape handling, and the capability to digitize the audio directly from the acquired image. The software displays the audio magnetic track pattern from the tape to identify tiny features, from over-recording marks to high-intensity signals from gunshots. The system is designed to analyze analog tapes but could be converted to work with digital tapes, according to project leader David Pappas.
The new nanoscale magnetic microscope also has been used experimentally for non-destructive evaluation of integrated circuits. By mapping tiny changes in magnetic fields across an integrated circuit, the device can build up an image of current flow and densities much faster and in greater detail than the single-sensor scanners currently used by the chip industry, says Pappas.
The FBI's Forensic Audio, Video and Image Analysis Unit receives hundred of audio tapes for analysis annually, representing evidence from crimes such as terrorism, homicide and fraud. The FBI provided partial funding for development of the NIST tape imaging systems.