Jay Davis Freeman

A high‐sensitivity magnetoresistive magnetometer

A high‐sensitivity, small‐size, low‐power‐consumption, broad‐bandwidth magnetometer has been built in which a magnetoresistive element is located adjacent to a gap between two thin‐film flux concentrators. The concentrators magnify the sensed component of magnetic field 20‐fold while simultaneously shielding orthogonal fields by more than a factor of 10. The sensor plus bias coil fit inside a 6‐mm‐diam probe. The intrinsic sensitivity is approximately 50 mV/(V Oe), and is flat from dc out to 50 MHz. With a sensitivity of 300 mV/Oe at 100 mW power, a 2‐Hz slot signal‐to‐noise ratio (SNR) of 30 dB at dc, and a 200‐Hz slot SNR of 40 dB at 1 MHz, has been measured when detecting a 1 gamma (10−5 Oe or 1 nT) signal field. With design enhancements and ac detection techniques to eliminate thermal drift and 1/f‐type noise, detection of low‐frequency magnetic fields of the order 1 mgamma could be achievable.

A new high density magnetoresistive tape head

An 8-track magnetic tape head for high-frequency, high-density applications is described. It includes an unshielded magnetoresistive read head and two thin-film record heads for bidirectional operation with write verification. The MR sensors are biased by specially shaped thin-film permanent magnets (PM) that provide fields along both sensor axes to linearize output and eliminate Barkhausen noise. Nearly all films in the head are deposited using dry processes. Deposition conditions for the PM have been optimized to produce a high-coercivity, high-remanence isotropic film. The final head assembly has a contour that utilizes longitudinal slots to achieve intimate contact with low head-to-tape pressure. The associated data channel uses both read and write equalization to obtain the desired output pulse shape from the unshielded head. >

Permeability resonances in permalloy stripes (abstract)

This work presents the experimental low field (Hac≊0.01 Oe) complex permeability spectra μ(f)=μ’(f)−jμ‘(f) (between 1 and 200 MHz) of L=1‐cm‐long NiFe stripes as a function of stripe width W (10 μm to 1 cm), film thickness T (0.1 to 4 μm), and the history‐dependent initial domain configuration. It will be demonstrated that occurrence of strong resonances in μ(f) is closely linked to the domain structure of the stripes, and in particular the linear density of the 180° transverse domain walls (observed by Bitter techniques) characteristic of a periodic flux‐closed domain pattern. Square films (L=W=1 cm) of all measured thicknesses that have a low linear density (<30 walls/cm) of 180° domain walls show no observable resonances, and their spectra μ(f) are instead well described by classical eddy current predictions. In contrast, highly pronounced resonance peaks (μ‘≳μdc≊2000) are observed in 0.1 μm≤T≤2 μm stripes that have been anhysteretically demagnetized with a longitudinal field, where the static domain s...