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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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Magnetism in Metals and Alloys

Magnetism in Metals and Alloys

(p.157) 5 Magnetism in Metals and Alloys
Fundamentals and Applications of Magnetic Materials

Kannan M. Krishnan

Oxford University Press

Delocalized conduction electrons described by the free electron model can explain the magnetic properties of metals. The spin-split density of states function describes the distribution of available energy states as a function of energy, and when an external magnetic field is applied, the energies of all electrons in a metal, depending on their spin, are either raised or lowered. This gives rise to an overall, temperature-independent, Pauli paramagnetic susceptibility, which is much smaller than the paramagnetic susceptibility of insulators described by the Curie law. In ferromagnets, the internal “molecular” field, spontaneously splits these spin-up and spin-down bands and if the product of the exchange energy and the density of states at the Fermi level is greater than 1, the metal satisfies the Stoner criterion for ferromagnetism. A magnon or a quantized spin wave, is the elementary excitation in a ferromagnet. By exciting such spin waves in the delocalized electrons or conduction band of a metal it is possible to introduce instabilities and dissipate magnetic order well below the Curie temperature. The exchange interaction between a magnetic impurity and the delocalized conduction electrons in a metal has three important physical consequences: (i) oscillatory coupling, (ii) frustrations and spin-glass behaviour, and (iii) the Kondo effect. Finally, a rigid band model can be generalized (the Slater–Pauling curve) to give the average magnetic moment per atom in the binary alloys of the Fe group as a function of electron concentration. In practice, there is a wide range of magnetic alloys of technological interest including those that are amorphous in structure; they include the family Ni–Fe permalloys, rare earth transition metal compounds, platinum and palladium based ordered intermetallics, Mn-based ternary Heusler alloys, and Co–Cr-based magnetic recording media.

Keywords:   density of states, spin-split bands, Pauli paramagnetism, Stoner criterion, magnons, spin glass, Kondo effect, Slater–Pauling curve, amorphous magnets, Heusler alloys

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