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Fundamentals and Applications of Magnetic Materials$
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Kannan M. Krishnan

Print publication date: 2016

Print ISBN-13: 9780199570447

Published to Oxford Scholarship Online: December 2016

DOI: 10.1093/acprof:oso/9780199570447.001.0001

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Other Ordered Magnetic Materials: Antiferromagnetism and Ferrimagnetism

Other Ordered Magnetic Materials: Antiferromagnetism and Ferrimagnetism

(p.115) 4 Other Ordered Magnetic Materials: Antiferromagnetism and Ferrimagnetism
Fundamentals and Applications of Magnetic Materials

Kannan M. Krishnan

Oxford University Press

Antiferromagnets and ferrimagnets show ordered arrangements of magnetic moments in a crystal structure involving two (or more) sub-lattices with antiparallel spin alignments and stabilized by indirect exchange mechanisms, often involving intervening oxygen ions. Antiferromagnets, where the contributions from the two sub-lattices are equal in magnitude but opposite in direction, show a characteristic Néel temperature proportional to the strength of their antiparallel coupling; above the Néel temperature they behave as a paramagnet. In ferrimagnets, the contributions from the two sub-lattices are unequal, with a net spontaneous magnetization that sometimes also shows unique variations with temperature, including the possibility of a compensation point where the direction of magnetization abruptly reverses itself. Similar to ferromagnets, ferrimagnets are also characterized by a Curie temperature, above which they show paramagnetism. Further, local symmetry and the crystallographic environment, including distortions, play an important role in the properties of these two classes of magnetic materials. Here, the magnetic ions cannot be considered to be completely isolated and free, for their electronic structure is altered by the specifics of their environment and these crystal field effects, in turn, influence the nature of the indirect exchange interactions that determine their long-range order. A good way to determine the magnetic arrangements and crystallographic details of these materials is to use neutron scattering, exploiting the intrinsic angular momentum of the neutrons. The magnitude, direction, and spatial ordering of the magnetic moments can be determined by measuring the intensities of magnetic scattering using different neutron scattering geometries. Antiferromagnets and ferrimagnets, especially inorganic oxides, are widely used in technology.

Keywords:   antiferromagnets, Néel temperature, crystal-field, indirect exchange, neutron scattering, ferrimagnets, compensation point, spinels, garnets, perovskites

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