From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: How Can I Cut A Magnet ?
Date: Wed, 06 Jan 1999 16:55:37 GMT

On 5 Jan 1999 01:04:52 GMT, fmgst+@pitt.edu (Filip M. Gieszczykiewicz)
wrote:

>In Article <76gcqo$1e2$1@nnrp1.dejanews.com>, through puissant locution, jpikarsk@my-dejanews.com soliloquized:
>[snip]
>>You will not affect the magnetization.  If you heat a magnet it will lose
>>it's magnetization at a temperature characteristic to the material, usually
>>over 1000 F.  This can be used to test the temperature of ferrous materials.
>>The magnetization returns when the material cools, with some loss.
>                                                    ^^^^^^^^^^^^^^

Some magnets can lose their magnetism far below 1000 F.  The curie
point for gadolinium  is 16 degrees C. Dysprosium has an even lower
curie point. I believe magnetism is usually lost when a magnet is
heated above the curie point unless it is recreated by applying a
magnetic field.

>[snip]
>
>Why would a super-strong (strong enough not allow pulling off a metal surface
>by a stong guy with a good grip on it :-) lose _all_ its magnetism when heated
>to cherry red? It's downright pathetic right now... the little advertising,
>pliable magnetic strips are stronger than it is....
>
>In this case, "some loss" seems to have been 99.999999999999999999999999% :-)


ALL ferro-magnetic materials have a definite temperature of transition
at which the phenomena of fero-magnetism disappears and the material
becomes paramagnetic. Many materials will lose essentially all of
their magnetism after being heated above the curie point and then
cooled. Some can be returned to the status of a permanent magnet just
by placing them in a strong magnetic field. Others require heat
treatment in a strong field.

A simplified explanation is that a material consists of dipoles (tiny
magnetic domains) If you cut a magnet in half you end up with two
magnets. Keep cutting and you keep getting smaller magnets each with a
north-south pole until theoretically you reach the size of a dipole.

In a mass of material those dipoles are pointed in random directions
but have some (if limited) movement. If you place the material in a
strong magnetic field you can force those dipoles to line up so in
effect you have them N-s,n-s,n-s,n-S where the capitals are at the end
of the piece. If you now remove the field some materials will keep the
dipoles lined up and you now have a permanent magnet with a north and
south pole. Some don't remain lined up and are considered soft
magnetic materials.

If you hammer the hell out of a magnet or heat it and then cool it
without a magnetic field the dipoles may randomize again because they
had the freedom to move and you lose the magnetism.

Some hard magnetic materials need to be heated (to allow the dipoles
to wiggle around) and then cooled in a strong magnetic field to attain
maximum magnetization. The heat treatment program can be fairly
complex. The Alnico magnets are in this category. As cast they are
lousy magnets.

Permanent magnets are materials which can lock dipoles in position
just like some crystal structures can be locked in place. Just as a
particular heat treatment can create different degrees of hardness
e.g. some special heat treatments can produce different magnetism in a
material. If you heat it again you can lose the magnetism just as you
can lose hardness.

Some soft magnetic materials lose their magnetism as soon as the
magnetic field is removed. This is important in applications such as
laminations in a power transformer. In this case you don't want the
magnetism to remain when the field is reversed.

The alloy of choice here has been silicon-iron. In addition it was
found that magnetic properties were not only dependent on the
composition but also on the orientation of the cubic crystal with
reference to the shape of the lamination. A well guarded secret was
how to roll this steel so that the cube-on-edge structure would be
present in the final sheet.

That hum you hear coming from a power transformer does not create joy
in the hearts of a transformer designer. They of course would also
like to build them without the cooling tubes.

There is an alloy (Mn 25%, Al 14%, Cu 61%) which is magnetic even
though  the components are considered only slightly magnetic. Liquid
oxygen is definitely magnetic and if you happen to have some bismuth
metal around suspend it from a thread and bring a permanent magnet up
close.

From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: How Can I Cut A Magnet ?
Date: Fri, 08 Jan 1999 13:29:39 GMT

On 7 Jan 1999 16:13:27 GMT, "Don Foreman"
<foreman_don@htc.honeywell.com> wrote:

>
>
>Mike Graham <mike@headwaters.com> wrote in article
><369ad5fd.26669791@news.headwaters.com>...
>>   Does this mean that if you heat the magnet back up to cherry and let
>> it cool beside another magnet it will get its magnetism back?
>
>Not all of it.  You need to saturate it magnetically for full magnetism and
>that won't happen just sitting next to another magnet.

You won't necessarily get it all back even if you saturate it
magnetically. Some of these exotic magnets are created by a heat
treatment which is a complex schedule of cooling and holding at
various temperatures which create a particular crystal structure which
is then "frozen" into the magnetic domain structure.

When you heat these magnets too hot you not only destroy the magnetic
properties but also change the crystal structure. You can apply a
strong magnetic field but if you don't apply the proper heat treatment
to recreate the crystal structure you won't get back to where you
were.