EEEL, Magnetic Technology Division Banner

Project Leader:
David Pappas

Staff-Years (FY 2001):
1 professional
2 research associates
1 student

Funding:
NIST (50%)
Other (50%)

Previous Reports:
2001

Magnetic Technology Division
325 Broadway
Boulder, Colorado 80305
Phone 303-497-5477
Fax 303-497-5316

magtech@boulder.nist.gov

June 28, 2002

Magnetic Recording Measurements

Goals

Magnetic Recording Measurements Staff

Magnetic Recording Measurements Staff.

This project addresses national measurement needs in magnetic data storage for industry, advanced applications, and national security. It is developing novel calibration standards, ranging from standards for the determination of the absolute magnetic moment of thin magnetic films to active inductive standards for magnetometers. Using state-of-the-art vacuum technology and spin-resolved electron spectroscopy, project members are doing research on the magnetic anisotropy of thin films used in data storage. Magnetic imaging techniques and measurements are developed for the authentication of recorded information and the recovery of data from samples of damaged magnetic recording media. The project has recently developed new techniques for imaging forensic evidence for the Federal Bureau of Investigation (FBI) and for data recovery for the National Transportation Safety Board (NTSB). It has published definitive work on the magnetic-reorientation transition of ultra-thin (two atomic layers) films of Fe on Gd as well the magnetic phase transition of the atomically clean Gd surface.

Customer Needs

Magnetic data storage has been a growing industry for almost one hundred years. With the advent of wide-spread use of computers and mass-storage media, it can be expected to continue to grow for the foreseeable future. Magnetic data storage products include analog audio and video products in various formats (standard and micro-cassettes, audio tapes, VHS), digital-media removable data storage (digital audio tapes, floppy disks, read/write compact disks), and non-removable data storage (such as hard disk drives and airline flight-data recorders). Because of this wide range of products, there are many customers for magnetic-recording metrology. The hard-disk-drive industry represents the cutting edge of technology in this area, highly competitive in terms of both scientific development and profit margins.

The requirements of the high-density storage industry for reproducible fabrication of thin magnetic films have pushed quality assurance to its limit. This extends to a wide range of magnetic properties and requires magnetometers that are calibrated over many orders of magnitude in sensitivity. We are currently working on two types of novel standards for calibrated measurements of magnetic moments of thin films.

Forensic analysts are constantly battling to keep up with the combined effects of increased usage of magnetic recording and the improved technology that allows higher densities. We address these needs by utilizing state-of-the-art magnetoresistive sensors to study relatively low-density storage media (analog audio, VHS) most encountered by the forensics investigator. In addition, the possibility of recovering digital data from recording media that were either intentionally erased or accidentally damaged is an important problem in criminal and airline-crash investigations.

Technical Strategy

A three-pronged approach is used. This includes standards development, imaging with advanced magnetoresistive heads for forensic analysis, and in-situ surface magnetometry for metrology and scientific research. In order to respond to immediate needs of the data-storage industry and the magnetic instrumentation companies that service it, two types of magnetometer reference samples are being developed in collaboration with instrument manufacturers. These will be magnetic thin-film coupons that have an integral superconducting flux-measurement loop and an inductive magnetic-flux standard.

In 1996, NIST initiated a five-year program to develop competence in the area of metrology for magnetic data storage. This program has resulted in advanced measurement techniques for imaging information stored on magnetic media with high resolution and relative ease. The nanoscale recording system (NRS) developed under this program is a general-purpose instrument that uses read-write heads similar to those in computer hard disk or tape drives to image and write data on magnetic media. The NRS can image by rastering either the head with computer controlled micrometers with 50 nanometer resolution or the storage medium with a piezoelectric x-y stage with 1 nanometer resolution. The NRS is being used as a prototype for forensic analysis of audio tapes.

It has been shown that high-speed imaging of tape samples can be accomplished, and the signatures of erase heads and write heads have been identified on test samples. In addition, reconstruction of both analog and digital data from test samples has been accomplished.

The NRS is also capable of detecting magnetic fields due to currents. This is useful for failure analysis of very-large-scale-integration (VLSI) semiconductor chips, where it is necessary to locate the location of a current drain due to a short circuit somewhere in a buried circuit. By imaging the magnetic fields and inverting Maxwell's equations within the appropriate limits, an important metrological tool is being developed for the semiconductor industry.

Finally, basic research is being conducted in the area of surface and interfacial magnetism. This area is important for development of metrology relevant to advanced devices, such as giant magnetoresistive heads, tunnel junctions, and perpendicular recording media. In our surface-science laboratory, we use spin-resolved electron spectroscopy as a magnetometer to map-out magnetic phase transitions as functions of temperature and film thickness. All three components of spin polarization are analyzed, allowing us to study any type of recording media. The electrons are sensitive to the first few atomic layers of the surface.

Deliverables

High-Sensitivity Magnetoresistive Sensors

In FY 2002, we will develop two types of magnetoresistor arrays. Linear arrays will be used for imaging magnetic field distributions, and staggered, high-density, planar arrays will be used for imaging analog magnetic storage media.

In FY 2002, we will develop a single-element magnetoresistive probe that has a low profile and wide bandwidth (up to 3 gigahertz).

Layout of the low-magnetic-moment reference standard, consisting of a superconducting Nb coil wound around a thin Ni-Fe magnetic film. The middle row shows the four different layers: bottom Nb loop (100 micrometers wide), magnetic layer (4 millimeters x 4 millimeters x 2 nanometers), vias, and top Nb loop. the top figure shows the superposition of the four layers. The bottom figure is a perspective view (not to scale)

Standards for Thin Film Magnetometers

In FY 2002, we will test our fluxmetric superconducting/magnetic thin-film standard at low temperature. We will measure the magnetic flux in the sample using the magnetic flux quantum as a reference.

In FY 2002, we will produce a prototype planar solenoid for cross-calibrating induction-field (B-H) loopers using a known current through the solenoid.

Magnetic Surface Science

In FY 2002, we will test a compact spin-polarized electron gun and conduct scattering experiments to determine the magnetic moment and spin-dependent reflectivity of the topmost atomic layers of magnetic films.

In FY 2002, we will grow Fe films on GaAs using an ultra-thin Ag buffer layer. We will characterize the growth of both the Ag and Fe films using low-energy electron diffraction and Auger electron spectroscopy. In addition, the emission and reflection of spin-dependent electrons from these surfaces will be measured.

Accomplishments

Magnetic Calibration Standards

Prototype Fluxmetric Standards Fabricated - A prototype wafer of fluxmetric superconducting/magnetic thin-film standards has been fabricated. The wafer includes large dies that are useful for the calibration of B-H loopers as well as small dies that are well suited for vibrating-sample magnetometers (VSMs), magnetometers based on superconducting quantum interference devices (SQUIDs), and alternating-gradient force magnetometers. The magnetic properties of the samples have been measured in a B-H looper and a SQUID magnetometer to verify that the superconducting loop is continuous and functional.

Mask Set for Planar Solenoids - A complete mask set has been developed for patterning planar solenoids on 7.6 centimeter silicon wafers. The solenoid will be used to calibrate B-H loopers.

Scanned Magnetoresistive Imaging

Array of magnetoresistive sensors. Arrow points to the linear array of GMR sensors.

Scanned Magnetoresistive Recording Head Used to Map Currents in Integrated Circuits - We measured the magnetic fields at the backs of VLSI "flip-chip" circuits. The currents in the chips were then calculated from the fields by inverting Maxwell's equations. Two devices mapped were a random-access memory (RAM) chip and a central processing unit (CPU). The locations of the calculated currents compared well with those expected.

Multiplexed Magnetoresistive Elements - We developed a new method for multiplexing arrays of magnetoresistive elements. This scheme is based on using the virtual ground of an operational amplifier to eliminate the effects of multiple ground paths in the separate rows of the array. Additionally, a multiple-frequency method of reading columns was devised. This technology will be applied to magnetoresistive imaging arrays for the forensics imaging tool.

Surface Magnetometry

Theory for Ferromagnetic Surface Phases - We developed a complete theory that describes the phase diagram of the surface magnetization of a semi-infinite ferromagnet The phase diagram for the orientation of the surface region was calculated in the parameter space defined by the surface and bulk anisotropy. Surface magnetic canting always occurs when the magnitude of the surface anisotropy is comparable with the interlayer exchange interaction. Increasing the thickness of a thin film supported on a hard magnetic substrate induces a spin-reorientation transition from the uniform, in-plane magnetic structure to a canted state. The inverse spin-reorientation transition from the canted state to the uniform, in-plane magnetic structure with thickness was demonstrated for a thin film supported on a nonmagnetic substrate. We considered the 1.5 atomic-layer system of Fe on Gd and found that it is a good physical realization of the model.

Low-magnetic-moment reference standard layout

Phase diagram for surface magnetic canting in a semi-infinite ferromagnet in coordinates of the surface and bulk magnetic energies, k_S and k_B

Workshop

The Magnetic Technology Division hosted a Tape Roadmap Workshop sponsored by the National Storage Industry Consortium (NSIC). Technical leaders in the recording tape industry met at NIST laboratories in Boulder, Colorado, to outline directions for tape technology during the next decade in the areas of read and write channels, heads, media, and tape transport. Projected trends included increases in data-transfer rate, bit density, track density, tape speed, and cartridge capacity; imporvements in tape strength, dimensional stability, and smoothness; the use of adaptive transport systems; and continued decrease in cost. Part of the dirscussion centered on needed metrics and measurement standards.