71046
to cause the enlargement of domains with a magnetisation direction similar to that of the
applied field at the expense of domains in which the magnetisation is oppositely directed.
How far the domain wali boundaries move is dependent upon the strength of the applied
field, with the result that the magnetisation relations equation 5.10 and 5.14 are on a
macroscopic scalę observed. When however the applied field is madę strong enough, the
materiał becomes magnetised as a single domain in which the magnetisation has the
same direction as the applied field and has a value equal to the magnetic moment of
an individual atom multiplied by the number of atoms per unit volume. This terminal
behaviour is described by the equations | M | = Mo(5.17)
Mis directed along <Ho>(5.18)
where Mois called the saturation magnetisationof the materiał, Hois the externally
applied field, and the osymbol represents the time average value. No further increase
in the magnitude | M | of the magnetisation is then possible. This behaviour has little
resemblance to that of dielectric media.
What happens when time-varying magnetic fields of smali amplitudę are then superimposed
upon the large steady magnetic field Honormally required for saturation bears even less resemblance to the corresponding behaviour in dielectric media. To understand
what will happen, we must take notę of a coupling which is known to exist at the atomie
level between the angular momentum of the electrons in the materiał and the magnetic
moments of those electrons. This coupling is responsible for the gyromagnetic
behaviour
discussed below.
If time-varying magnetic fields are applied in a saturated ferromagnet in a direction
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