Declination

Angle between magnetic, geographic

pole

Latitude

Can be estimated with a dip needle

28.6 - Ferromagnetism

Ferromagnetism: A strong

magnetic effect exhibited by

the atoms of certain elements,

notably iron. It is the cause of

the magnetic field of

commonly used magnets.

Ferromagnetism is the basis of the most familiar type
of magnetic devices. When you see a magnet affixed
to a refrigerator door, you are witnessing the results
of ferromagnetism. “Ferro” comes from the Latin word for iron, since iron is the most
common element to exhibit this property. Ferromagnetism is present in the magnets
that can be purchased in toy or hardware stores.

Nickel and cobalt are two other common elements with atoms that exhibit
ferromagnetism, though to a lesser extent. Gadolinium and dysprosium are two exotic
elements that can be used to make strong magnets that perform well at low
temperatures.

In order to explain the source of ferromagnetism, we have to pay a quick visit to a
concept developed in quantum theory. The classic model of atoms í electrons revolving
about a central nucleus í does not suffice. Ferromagnetism is caused by a property of
electrons called spin. For the purposes of this discussion, we say that spin is used to
describe the angular momentum of an electron due its rotation, akin to the angular
momentum of the Earth due to its rotation about its axis. (This is only an analogy. A
more detailed, much less mechanistic description requires the context of quantum
theory.) This is one form of angular momentum possessed by the electrons.

They also possess orbital angular momentum due to their motion about the nucleus.
Both contribute to the magnetic moment of the electron, but only spin is relevant to the discussion of ferromagnetism. The magnetic moment
vector helps quantify how much torque a magnetic dipole will experience in a magnetic field; the larger the moment, the greater the torque. The
moment points from the south pole to the north pole of the dipole.

Spin means that each electron has its own magnetic field. It can be considered as acting like a bar magnet, a dipole with north and south poles.

Color image: magnetic domains in an amorphous ferromagnet.

Inset: magnetic domains in heat-treated carbon steel.

Inside a domain

Electron spins align due to exchange

coupling

·Result is magnetic domains

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