The Demagnetizing Field and Demagnetization Factors

Josh Marion

Presented on: 6/8/2011

Effect of Sample Geometry

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M

H

H1

H2H3

H1

H2

H3

Ferromagnetic Cylinder

Effect of Sample Geometry

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M

H

Stray field lines at gap result in free surface poles, “shearing” of M-H loop, can dramatically lower . Different shapes have different degrees of shearing. This is due to demagnetizing fields.

Magnetic field lines contained entirely within toroid, no free surface “poles”.

Demagnetizing Fields

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Jiles, D., Introduction to Magnetism and Magnetic Materials, Taylor & Francis Inc., US, 1998.O’Handley, R., Modern Magnetic Materials: Principles and Applications, Wiley-Interscience, New York, USA, 2000.

Demagnetizing Fields

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Hd

Demagnetization Factor, N

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Determining Demagnetization FactorShape Factor of Simple Geometries

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Sample Geometry Shape Factor, N (SIU, dimensionless)

1

0

0

½

⅓

Very often, we approximate sample as ellipsoid, a shape of uniform N, using sample aspect ratio

Osborne J.A., Phys. Rev., 67 (1945) 11-12. pp. 351-357.

Consequences of Hd

In perm. magnets, shape effects important for very small aspect ratio samples (large N) due to large intrinsic

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Minimize magnetostaticenergy in each domain become multidomain

Domain walls move as Hext applied, shape of domain changes

Change in domain shape = change in N = nonlinear M-H curve

M

H

http://www.gitam.edu/

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Questions?