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# 12: Properties of Magnetic Materials

• 12.1: Introduction
We have defined the magnetic fields $$B$$ and $$H$$. To define $$B$$, we noted that an electric current situated in a magnetic field experiences a force at right angles to the current, the magnitude and direction of this force depending on the direction of the current. We accordingly defined $$B$$ as being equal to the maximum force per unit length experienced per unit current,
• 12.2: Magnetic Circuits and Ohm's Law
Some people find it helpful to see an analogy between a system of solenoids and various magnetic materials and a simple electrical circuit. They see it as a "magnetic circuit".
• 12.3: Magnetization and Susceptibility
The $$H$$-field inside a long solenoid is $$nI$$. If there is a vacuum inside the solenoid, the B-field is $$\mu_o H = \mu_o nI$$. If we now place an iron rod of permeability $$\mu$$ inside the solenoid, this doesn't change $$H$$, which remains $$nI$$. The B-field, however, is now $$B=\mu H$$. This is greater than $$\mu_oH$$, and we can write $B = \mu_o(H+M)$
• 12.4: Diamagnetism
Diamagnetic materials have a very weak negative susceptibility. All materials are diamagnetic, even if their diamagnetism is hidden by their greater para- or ferromagnetism.
• 12.5: Paramagnetism
Diamagnetism makes itself evident in atoms and molecules that have no permanent magnetic moment. Some atoms or molecules, however, do have a permanent magnetic moment, and such materials are paramagnetic. They must still be diamagnetic, but often the paramagnetism will outweigh the diamagnetism. The magnetic moment of an atom of a molecule is typically if order of a Bohr magneton.
• 12.6: Ferromagnetism
What we normally think of as magnetic materials are technically ferromagnetic. The susceptibilities of ferromagnetic materials are typically of order $$+10^3$$ or $$10^4$$ or even greater. However, the ferromagnetic susceptibility of a material is quite temperature sensitive, and, above a temperature known as the Curie temperature, the material ceases to become ferromagnetic, and it becomes merely paramagnetic.
• 12.7: Antiferromagnetism
The susceptibility of an antiferromagnetic material starts at zero, and its transformation to a paramagnetic material results in an increase (albeit a small increase) in its susceptibility.
• 12.8: Ferrimagnetism
Ferrimagnetics have domain structure with alternate magnetic moments that are pointing in opposite directions. But this does not result is complete cancellation of the magnetization of a domain.

Thumbnail: A piece of ferromagnetic material which is not magnetized, where the domain poles are not aligned. Image used with permission (http://www.itacanet.org).