Figure 34.1: Hysteresis in a ferromagnetic material. The vertical axis (labeledD) is the internal magnetic
field induced in the material, and the horizontal axis (labeledE) is the external magnetic field. Starting at the
origin (an unmagnetized material in no magnetic field) and following the path withEincreasing, we find the
induced field in the material increases until it reaches a saturation level, labeledDs. When the external field
Eis removed, though, the path does not return to the origin; instead a remanent field (Dr, theremanence)
remains in the material. The point labeledEcis called thecoercivityof the material, and is the external field
needed to de-magnetize the material. (Credit: Wikipedia, ©GNU-FDL, Wikimedia Commons.)
34.4 Permanent Magnets
Permanent magnetsare ferromagnetic materials that have a permanent magnetic field. They are manufactured
in a variety of materials; one of the most common isalnico, which is an alloy of aluminum, nickel, and cobalt
(hence Al-Ni-Co, or “alnico”), and has a shiny metallic appearance like steel. Many horseshoe and bar
magnets are made of this material, as well as heavy-duty handle magnets.
Ferriteorceramicmagnets are made of a brownish, brittle ceramic material mixed with ferric oxide
(Fe 2 O 3 ). They are sometimes used as components in electronic circuits.
Rare-earthmagnets are made of alloys that include rare-earth elements (basically the “lanthanides” row
of the periodic table). Two kinds of rare-earth magnets are made:samarium-cobalt(often used in stereo head-
phones and speakers), andneodymium. Neodymium magnets are made of an alloy of neodymium with iron
and boron (Nd 2 Fe 14 B), and are the most powerful permanent magnets made. Even very small neodymium
magnets are surprisingly powerful, and must be handled with care: two such magnets will attract each other
with a very strong force, and can easily shatter. Once stuck together, two neodymium magnets can be very
difficult to separate.
34.5 Curie Temperature
Once a ferromagnetic material has been magnetized by exposure to an external magnetic field, it may be
de-magnetized by heating it above a temperature called theCurie temperature. Above this temperature,
the thermal motion of the atoms is sufficient to re-scramble the magnetic dipole moments of the magnetic
domains, and the material becomes de-magnetized. The Curie temperatures of ferromagnetic elements are
shown in Table 34-1.