Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001
A.Magnetic field induced by electrical current
H - magnetic field strength (amperes/meter)
2.Magnetic Flux Density - indicates response of material subjected to a Magnetic Field
B - Magnetic Flux Density (teslas - webers/square meter)
m - magnetic permeability (Wb/A-m)
3.Magnetic Field Strength in a vacuum given by
where m 0 is the permeability of a vacuum (4p x10-7 H/m)
4.Relative permeability - indicates the relative ability of a material to be magnetized by an external
5.Magnetization - M - represents the magnetic field strength contributed by the magnetization of the medium
where c m is the magnetic susceptibility which is also given by c m = m r - 1
B.Material Response to a Magnetic Field
a.No permanent magnetic dipoles
b.Induced magnetic dipoles in atoms align in a direction opposite to the applied field
c.The magnetic flux density is thus slightly less than it would be in a vacuum, m r < 1
a.Permanent magnetic dipoles randomly arranged when no field applied - thus no magnetism
b.Magnetic dipoles in atoms align in the same direction as the applied field
c.The magnetic flux density is thus slightly greater than it would be in a vacuum, m r > 1
a.Strong permanent magnetic dipoles
c.Atomic dipoles tend to align over relatively large areas even without an applied field
d.Saturation magnetization (Ms) occurs when all dipoles align with external field
e.Contribution of individual atoms to magnetization sums to total Ms
a.Permanent magnetic dipoles naturally align in opposing orientations
b.No net magnetic moment results
a.Ceramics may exhibit permanent magnetization
b.Magnetization depends on crystallographic orientation of atoms in lattice
II.Temperature and Magnetization
A.Saturation magnetization decreases with increased temperature
B.Curie Temperature - Tc - temperature at which ferromagnetism ceases, 768° C for iron
III.Magnetic Domains and Hysteresis
1.Magnetic dipoles in a domain aligned
2.Dipole arrangement varies from domain to domain
3.Domains usually smaller than grain size
4.Dipole orientation transition across domain wall boundary
5.Random domain orientation gives unmagnetized material
a.Applied H field causes domains to align
b.Reducing H field to zero leaves permanent magnetization in ferromagnetic material
c.H field required to reduce B to zero is the Coercivity, Hc
d.Energy absorbed in cycling through hysteresis loop - proportional to area inside curve
e.Demagnetization by cycling hysteresis curve from large amplitude down to zero
C.Soft vs. Hard Magnetic Materials
a.High Remanence and Coercivity (large hysteresis)
c.High energy loss in cyclic field
a.Low Remanence and Coercivity (small hysteresis)
c.Low energy loss in cyclic field
d.Good for motor and solenoid cores