A research and development effort now in progress is focused on nano-structured ferromagnetic- wire/insulator composite materials to be used as the magnetic-core materials of sensors for measuring weak magnetic fields. Figure 1 schematically depicts an example of such a sensor — a flux-gate magnetometer that resembles a traditional orthogonal flux-gate magnetometer except that, instead of a single cylindrical ferromagnetic core, there are multiple parallel ferromagnetic wire cores packed together with insulating material between them. An overriding consideration in the design, fabrication, and operation of such a magnetometer is that high effective magnetic permeability of the core is necessary as one of the prerequisites for obtaining high sensitivity.

Figure 1. This Orthogonal Flux-Gate Magnetometer has a multiple-ferromagnetic-wire core instead of a solid single core of ferromagnetic material.
The present effort was prompted in part by experimental observations that showed that the sensitivity and the effective permeability increase approximately exponentially with the number of ferromagnetic wires in the core, regardless of the width of the individual wires. The experimental observations also revealed that the sensitivity is maximized when the frequency of the AC excitation equals the frequency of a resonance that depends on the number of wires. This effect was subsequently attributed to dynamic unification of magnetic domains in the ferromagnetic wires. Also, it was noted that theoretically, the effective core volume available for magnetization (and thus the effective permeability) can also be increased by making the wire diameters comparable to, or less than, the electromagnetic skin depth at the excitation frequency so that the magnetic field can penetrate the wires more nearly completely.

Figure 2. An Array of Nanoscale Ni80Fe20 Stripes is fabricated in a process that includes the main steps here shown schematically.
Taking all of the foregoing considerations into account, it was concluded that the development of high- effective- magnetic- permeability composite materials should be guided by the following principles:

  • To maximize the effective volume of a given amount of ferromagnetic wire material, the number of wires should be as large as possible — in other words, the wires should be as narrow as possible — leading to the requirement of nanoscale wires.
  • For maximum dynamic domain unification, the ferromagnetic wires should be intimately close to each other with insulation between them — leading to the requirement of a nanostructured array.
  • In operation, all of the wires should be excited by a current of sufficient amplitude at the dynamic-domain-unification frequency in order to maximize the effective magnetic permeability.

The development effort has also included design, fabrication, and characterization of specimens, containing small numbers of nanoscale ferromagnetic stripe, pillar, and wire structures, as prototypes for assessing feasibility of fabricating ferromagnetic/insulator cores containing large numbers of wires in nanostructured arrays. The ferromagnetic material that has been used in this effort is permalloy (Ni80Fe20). Arrays of nanoscale Ni80Fe20 stripes have been synthesized by use of a combination of lithography and sputtering (see Figure 2). Template-assisted electrodeposition has been demonstrated to be feasible as a means of fabricating arrays of microscale Ni80Fe20 wires (the nanoscale has not yet been reached because of limitations on the means available for drilling narrow holes in templates). It was shown to be possible to fabricate ferromagnetic wires by electrodeposition of Ni80Fe20 onto Cu wires held at the desired lateral distances in a custom fixture. (It is envisioned that the wires would later be potted in epoxy, and the resulting workpiece would be machined to the final desired core size and shape).

In view of the limitations and difficulties of the fabrication approaches tested thus far, an alternative approach has been conceived for future testing: A large number of insulator/Ni80Fe20/Cu rods would be packed together and subjected to repeated cold drawing or cold rolling steps until each rod was reduced to the desired microscale or nanoscale width.

This work was done by Xiaoping Li of the National University of Singapore.