I have mentioned it before in other contexts, but an excellent source of
high permeability, Metglas (amorphous metal/glass) strips is in the
anti-theft tags on CD=92s and such from places such as Best Buy, Circuit
City and the like.  The Metglas strips are 0.24=94 x 1.45=94 x 0.0012=94 =
and
are excellent, low coercivity materials.  Just the ticket for sensors.
When you cut the tags apart, slit them like you would fillet a fish,
cutting off the =93humped=94 non-sticky side.  The bright, shiny, very
springy, rectangular ribbon is the Metglas.  Further down, after an
intervening plastic layer, (closer to the sticky side) is a another
piece of metal, usually with the ends cut on an angle.  This is steel
and easily bent and designed to be permanently magnetized and useless
for your purposes.  Although not a lot of material per tag, the tags are
often very readily available in the parking lot of my local Best Buy.  I
often pick up 5 or 10 on my walk to and from the store.  (Leave them
curb-side on your way in or you may be explaining why you set off alarms
on your way out!  Other excellent sources of low coercivity Permalloy
strips are most libraries, public or university.  Strips 0.14=94 x 6.45=94=
 x
approx. 0.003=94 are inserted in the books as anti-theft devices.  They
come in two forms, adhesive coated with a release strip and plastic
laminated with a long plastic =93tail=94 in order to pull them into place=
 in
a book.  Talk to your librarian, and he/she might give you some or sell
you a few.  They only cost the library something like a $0.25 apiece or
less, I believe.  Both of these can also form the basis of very
sensitive flux-gate magnetometers.  For use in sensors, the Metglas is
likely to be the better material if you want to run at higher
frequencies, as it=92s conductivity is much lower than Permalloy.
Generally the conductivity will get in the way of many sensing schemes
by ending up de-sensitizing the inductance parameter you're trying to
measure.

Charles R. Patton


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