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In article <BtwExB.Gto@news.cso.uiuc.edu> ritley@uiucmrl.bitnet () writes:
>
>Anyone know what PERMALLOY is commonly used for?  I was lead to believe
>it was used for masks on CRT screens, but I have recently been informed that
>INVAR is more commonly used for this purpose.
PERMALLOY is an old permanent magnet alloy (CRC Handbook has 45
permalloy, 78 permalloy, 4-79 permalloy and 2-81 permalloy's listed).
Basically iron-nickel alloys with high permeability (the number
indicates nickel level, the number in front of the hyphen is Mo).
Invar is very similar, in that it is also iron nickel, but it is the
36% nickel alloy at the TCE minimum.  Fro this reason it is often used
for sealing to glass or ceramics, as there is little or no thermal
coefficient of expansion (TCE) mismatch.  I would have thought that
the ferromagnetic nature of any of these alloys would make them dicey
to use for CRT masks, but the TCE match may force the issue and they
are cheaper than Molybdenum.  My impression is that they have been
largely supplanted by ceramic magnets for permanent magnet
applications.
Steve Axdal
Computing Devices International
app@shamash.cdc.com

app@shamash.cdc.com ( GSD Advanced Packing/Processes) writes:
>In article <BtwExB.Gto@news.cso.uiuc.edu> ritley@uiucmrl.bitnet () writes:
>>Anyone know what PERMALLOY is commonly used for?
>PERMALLOY is an old permanent magnet alloy   
>My impression is that they have been
>largely supplanted by ceramic magnets for permanent magnet
>applications.
>Steve Axdal
>Computing Devices International
>app@shamash.cdc.com

Yes, permalloy is a permanent magnet, but its applications are quite
distinct from those of most ceramic magnets, so it has not been
supplanted by ceramic magnets for most of its applications.

Some ceramic magnets such as Nd2Fe14B and hexagonal ferrites and
rare-earth magnets like SmCo belong to the class of HARD magnets.
These magnets will only reverse or lose their magnetization in a large
external applied field, so they are good for harse EMI environments
where power losses are not critical.  Example: holding notes on the
refrigerator, creating pulses in a pickup coil for a rotation sensor,
computer hard disk media.

Permalloy and other transition-metal alloy magnets are more typically
SOFT magnets.  These magnets are typically used as transducers/sensors
in systems where you want a large magnetization (or magnetoresistance)
change for a small external field change.  These magnets are used for
example in magnetoresistive read heads in hard drives.  Some soft
magnets are used for magnetic shielding.  The famous mu metal is a
small variant of permalloy.

On the other hand, many cubic ferrites are soft magnets.  Soft
ferrites are ubiquitous in transformer cores.

Very often hard magnets and soft magnets are used in pairs in position sensor
systems.    The hard magnet will be glued to the moving part and the
soft magnet will be used to sense its position.

More info: _Physics of Magnetism_ by Chikazumi and Charap


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