l/Magnetic core memory.
m/A hysteresis curve.
Ferromagnetism
The very last magnetic phenomenon we’ll discuss is probably the
very first experience you ever had of magnetism. Ferromagnetism is
a phenomenon in which a material tends to organize itself so that it
has a nonvanishing magnetic field. It is exhibited strongly by iron
and nickel, which explains the origin of the name.k/A model of ferromagnetism.Figure k/1 is a simple one-dimensional model of ferromagnetism.
Each magnetic compass needle represents an atom. The compasses
in the chain are stable when aligned with one another, because each
one’s north end is attracted to its neighbor’s south end. The chain
can be turned around, k/2, without disrupting its organization, and
the compasses do not realign themselves with the Earth’s field, be-
cause their torques on one another are stronger than the Earth’s
torques on them. The system has a memory. For example, if I want
to remind myself that my friend’s address is 137 Coupling Ct., I can
align the chain at an angle of 137 degrees. The model fails, how-
ever, as an explanation of real ferromagnetism, because in two or
more dimensions, the most stable arrangement of a set of interacting
magnetic dipoles is something more like k/3, in which alternating
rows point in opposite directions. In this two-dimensional pattern,
every compass is aligned in the most stable way with all four of its
neighbors. This shows that ferromagnetism, like diamagnetism, has
no purely classical explanation; a full explanation requires quantum
mechanics.
Because ferromagnetic substances “remember” the history of
how they were prepared, they are commonly used to store infor-
mation in computers. Figure l shows 16 bits from an ancient (ca.
1970) 4-kilobyte random-access memory, in which each doughnut-
shaped iron “core” can be magnetized in one of two possible direc-
tions, so that it stores one bit of information. Today, RAM is made
of transistors rather than magnetic cores, but a remnant of the old
technology remains in the term “core dump,” meaning “memory
dump,” as in “my girlfriend gave me a total core dump about her
mom’s divorce.” Most computer hard drives today do store their in-
formation on rotating magnetic platters, but the platter technology742 Chapter 11 Electromagnetism