Ferromagnetism
What is ferromagnetism?
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A few elemental solids and alloys, especially those containing iron, nickel, and cobalt, exhibit extremely large positive susceptibilities and are termed ferromagnetic. Whereas paramagnetism and diamagnetism are properties of individual atoms or molecules, ferromagnetism is a property of a group of atoms or molecules in a solid crystal or lattice. All ferromagnetic substances have unpaired electron spins that are strongly entwined by a quantum mechanical force, exchange interaction. Large groups of atoms in a ferromagnetic substance form magnetic domains in which arrays of electron spins become locked together in alignment.
Exchange interaction, also called exchange coupling, is a quantum mechanical phenomenon arising from the balancing of kinetic energies and repulsive forces among outer shell electrons in various spin orientations. Because it is primarily electrostatic (rather than magnetic in nature), the exchange interaction is localized to just a few neighboring atoms.
The formation of magnetic domains accounts for the extremely large values of χ observed in ferromagnetic materials. It also gives them a "magnetic memory," or remanence, in which the actual value of χ depends on the past magnetization history of the material. Iron, for example, can retain appreciable residual magnetization even after the external magnetic field has been removed. This magnetic memory is one reason iron and iron alloys are frequently chosen for making permanent magnets.
Exchange interaction, also called exchange coupling, is a quantum mechanical phenomenon arising from the balancing of kinetic energies and repulsive forces among outer shell electrons in various spin orientations. Because it is primarily electrostatic (rather than magnetic in nature), the exchange interaction is localized to just a few neighboring atoms.
The formation of magnetic domains accounts for the extremely large values of χ observed in ferromagnetic materials. It also gives them a "magnetic memory," or remanence, in which the actual value of χ depends on the past magnetization history of the material. Iron, for example, can retain appreciable residual magnetization even after the external magnetic field has been removed. This magnetic memory is one reason iron and iron alloys are frequently chosen for making permanent magnets.
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Magnetic domains are small, randomly oriented and spontaneously magnetized regions in a ferromagnetic material whose spins are locked into orderly arrays. Application of small external fields moves the walls between them to form larger domains. Higher external fields reorient the domains to face the same direction and increase net magnetization.
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Video under Kerr microscope shows how a single domain in steel changes its magnetic orientation as the external field is increased. When fully dark in final image, it is completely aligned with the external field. (Click image for animation)
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Single domain particle caught half way in transformation. Direction of external field is from top to bottom of image. White color in domain reflects magnetization opposing field; dark color reflects magnetization aligned with the external magnetic field.
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Permanent MR magnets are purposely made of ferromagnetic materials, usually alloys of rare earth alloys and iron, such as neodymium-iron-boron (Nd-Fe-B).
References
"Ferromagnetism." Wikipedia, The Free Encyclopedia.
"Exchange Interaction." Wikipedia, The Free Encyclopedia.
"Magnetic Domain." Wikipedia, The Free Encyclopedia.
Stampfl HC. Advanced Condensed Matter Theory Lecture Notes. 2003. Chapter 8. Magnetism. (A detailed explanation of ferro- and other types of magnetism from a Fellow in Physics at the University of Sydney)
"Ferromagnetism." Wikipedia, The Free Encyclopedia.
"Exchange Interaction." Wikipedia, The Free Encyclopedia.
"Magnetic Domain." Wikipedia, The Free Encyclopedia.
Stampfl HC. Advanced Condensed Matter Theory Lecture Notes. 2003. Chapter 8. Magnetism. (A detailed explanation of ferro- and other types of magnetism from a Fellow in Physics at the University of Sydney)