June 8, 1:00 pm - 2:00 pm
ADVANCES AND RETREATS IN CRYSTALLINE PERMALLOY DEVELOPMENTS FOR LOW NOISE FLUXGATE MAGNETOMETRY
Barry Narod, University of British Columbia
Nickel-Iron alloys, a.k.a. Permalloys, began their development in the 1920?s, primarily for ocean-bottom communications cables, beginning with 4%Mo alloys. Geophysical magnetometry piggybacked on this effort, starting with the needs of WWII. However the history of soft magnetic alloys actually began 40 years earlier with the first documentation of magnetic hysteresis (Ewing, 1881).
Developed in the 1960s for use in high-performance ring-core fluxgate sensors, 6?81.3 Mo permalloy became the state of the art for permalloy-cored fluxgate magnetometers. The magnetic properties of 6?81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction, are all optimum in the Fe?Ni?Mo system.
In such polycrystalline permalloy fluxgate sensors, a single phenomenon may cause both fluxgate noise and magnetic hysteresis; explain Barkhausen jumps, remanence and coercivity; and avoid domain denucleation. This phenomenon, domain wall reconnection, is presented as part of a theoretical model. In the unmagnetized state a coarse-grain high-quality permalloy foil ideally forms stripe domains. Driven to saturation, domain walls reconnect via Barkhausen jumps to form a new domain configuration that I have called "channel domains", which are attached to surfaces. The approach to saturation now continues as reversible channel domain compression. Returning from saturation the channel domain structure will survive through zero H, thus explaining remanence. The Barkhausen jumps, being irreversible exothermic events, are sources of fluxgate noise powered by the energy available from domain wall reconnection.
A simplified domain energy model can then provide a predictive relation between ring-core magnetic properties and fluxgate sensor noise power. Four properties are predicted to affect noise power, two of which are well known: saturation total magnetic flux density and magnetic anisotropy. The two additional properties are easy axes alignment and foil thickness/grain size. Flux density and magnetic anisotropy are primary magnetic properties determined by an alloy's chemistry and crystalline lattice properties. Easy axes alignment and foil thickness/grain size are secondary, geometrical properties related to an alloy's polycrystalline fabric and manufacture. Improvements to fluxgate noise performance can in principle be achieved by optimizing any of these four properties in such a way as to minimize magnetostatic energy.
Fluxgate signal power is proportional to B - H loop curvature [d2B/dH2]. The degree to which Barkhausen jumps coincide with loop curvature is a measure of noise that accompanies the fluxgate signal. B - H loops with significant curvature beyond the open hysteresis loop may be used to advantage to acquire the fluxgate signal with reduced noise.
Permalloys fortune has gone through cycles as major uses arrived or disappeared. Today their fortune is clearly down as ferrites, semiconductors, LCDs and amorphous alloys have taken over most end-uses. Geophysics and space physics which have traditionally piggy-backed on other users must now stand alone and reinvent their favorite materials, this time with better understanding.