e-mail cldtx1@sbcglobal.net
Abstract: The magnetic properties of matter are due to the spins of its electrons. There are three conditions, diamagnetic, paramagnetic and ferromagnetic. This paper explains how these conditions can arise.
When any material (solid, liquid or gas) is placed in the poles of a powerful magnet, a force, usually quite small, is noted [1]. If the material is repelled by the magnet, it is said to be diamagnetic [2].
If
it is attracted to the strongest part of the magnet, it is paramagnetic. If the material is, or becomes a permanent
magnet, it is said to be ferromagnetic.
ELECTRON SPIN
All
magnetism is caused by electron spin, which may be + or - (clockwise or
counterclockwise). See my paper on “Why
Spin” [3] for a speculation on the magnitude of spin. To comply with the principle of “Least
Energy”, electrons usually pair off by spins, and the majority of substances
have a net zero electron spin. Such
substances are diamagnetic, and the reaction to a strong magnet is virtually nil.
Some
compounds such as NO and NO2 have odd numbers of electrons and hence are paramagnetic. All “free radicals such as ·OH have a single,
odd electron spin. All odd atomic numbered atoms such as 19K40 are also paramagnetic.
Oxygen and Sulfur both have 2 unpaired electron spins, although they
have an even number of electrons.
All
odd and even some even atomic numbered “Transition” Elements and “Rare Earth”
elements have unpaired electron spins, and are paramagnetic. The unpaired spins are “protected” by their
positions in the Periodic Table [4]. The
unpaired electrons have minimum energy configuration. Gd, for example, a
“rare earth”, has
7 unpaired spins. Most of the “rare
earths” are strongly paramagnetic, and Gd is
ferromagnetic below 20°C.
FERROMAGNETIC MATERIALS
The
other ferromagnetic materials are the “Transition” elements Fe, Co, and Ni
(Iron, Cobalt and Nickel). See reference
[4] to visualize their “protected” position.
Note particularly that of Gd. Some compounds and alloys of these elements
are also ferromagnetic, for example, magnetite, Fe3O4. This compound has 5+ unpaired electrons.
There
are a number of alloys of these elements that are very strong permanent
magnets. Most of the alloys were
developed by empirical methods. Most
contain iron.
FERROMAGNETISM OF IRON DOMAINS
.Very
pure iron is only paramagnetic. It is
used in electromagnets, losing its magnetism when the current is turned
off. Steel contains 0.5% or more Carbon,
and can be permanently magnetized, the more strongly the more C is present (up
to a point). Steel has been found to
contain “domains” consisting of roughly 1015 atoms. Assuming a spherical particle, it would have
a diameter of about 2 x 10-3 cm. These
“domains” are randomly distributed in direction of their magnetic axes until
the steel is put into a strong magnet.
Then, they align with the magnet.
The process of alignment can be observed with a microscope [2]. When they are all aligned, the magnetism is maximum. They will
basically stay aligned until the temperature of the magnet is raised to 770° C,
at which point the domain order is broken, and the steel again becomes
paramagnetic. The whole process can be
repeated when the steel is cooled. The
temperature of the transition is called the “Curie Temperature”. (It is also the transition temperature from
the a to the b crystalline form of iron).
Adding
more Carbon to the steel increases both the ferromagnetism and the mechanical
strength of the steel. The point is
eventually reached when the domains can no longer be turned by exposure to a
strong magnet.
It
is here proposed that the domains consist of some compound of Fe and C,
possibly Fe4C3. This compound would have 5+
unpaired electrons on the Fe[5.6].
Cobalt
and Nickel have not been investigated to the same extent that Iron has, but
they are reduced from their ores by Carbon, as is the case with Iron. They probably contain compounds such as Co4C3 and Ni2C [6].
In
the case of Gd below 20°C, the atoms with their 7
unpaired electron spins line up with the magnetic
field. Above the Curie Point, the atoms have enough energy that the magnetic order
is destroyed. It is here proposed that a
phase modification must take place at 20°C,since the
Curie Point is a definite transition,
Some sort of physical transition must take place as in the case of Iron.
SUMMARY
Paramagnetism is caused by unpaired electron spins on atoms or compounds. Ferromagnetism occurs only in the case of Fe,
Co, Ni and Gd and their compounds or alloys. In the case of Fe, there are “domains” which
probably consist of Fe4C3 that has 5+ unpaired electron spins. These domains
may be aligned by exposure to a strong magnet.
This causes the steel to become a permanent magnet.
REFERENCES
1. W. Hume-Rothery,
“Atomic Theory for Students of Metallurgy”, The
institute of Metals,
2. D. Vitogran,
“Elements of Electric and Magnetic Circuits”, Rinehart, San Francisco, 1971,
p194ff
3. C. Dulaney,
“Why Spin?’
4. C. Dulaney,
"nuclear Structure-Radioactivity"
5. R. Weast,
Ed., “Handbook of Chemistry and Physics, 63rd Edition.
6. F. Cotton and G Wilkinson,
“Advanced Inorganic Chemistry”. John Wiley, 1980, p628
NOTE: All my papers may be found on my website,
http://nywebpage.netscape.com/clarencedulaney
© Clarence Dulaney