§ 4 Ferromagnetics
- substances which have the internal magnetic field in the hundreds
and thousands of times caused it, the external magnetic field.
ferromagnetic materials have a magnetization in a absence of
magnetic field. Ferromagnetism observed in crystals of transition
metals Fe, Co, Ni, and a number of alloys. Ferromagnetism is the result of the exchange forces
А > 0 - condition of ferromagnetism.
Ferromagnetic properties observed in materials at temperatures below the so-called Curie temperature - TC. At T > TC
ferromagnetic transition to the paramagnetic state. At temperatures
below the Curie point of a ferromagnet is divided into small regions of
uniform spontaneous (spontaneous) magnetization - domains. Linear dimensions of the domains: 10-5 -10-4 m.
Within each domain
material magnetized to saturation. In the absence of a magnetic field
the magnetic moments of domains oriented in space, so that the net
magnetic moment of the ferromagnet is zero. When a magnetic field is
magnetized ferromagnet, ie acquires a nonzero magnetic moment. With
increasing field the magnetization increases slowly at first (section ab
in Fig.), Then the magnetization increased tenfold (section bc).
Further growth of the magnetization again slows down (cd). This
behavior of the magnetization due to the fact that the action of the
field on the domains at different stages of the process of
magnetization - is different. At point 0, when demagnetized
ferromagnet, square domains 1,3,5 ..., the magnetic moments which makes
an acute angle with the direction
of equal to the area of domains 2,4,6, ..., in which the angle between the magnetic moment and the external field -
As the external magnetic field is initially an increase in the square
??domain 1,3,5 by reducing the area of ??the domain 2,4,8. In a
ferromagnet there is a magnetic moment, whose direction coincides with
the direction of the magnetic moment of domains 1,3,5, with the increase
of the magnetizing field this process is as long as domains with acute angles to the field (which have a lower magnetic field energy) is absorbed completely energetically favorable domains 2,4,8 - section ab
in Fig. Around the point b is merging collinear domains and
ferromagnetic goes into single-domain state. With a further increase
in the external field, the magnetic moment of the ferromagnet is
rotated in the direction of the external field (paramagnetic effect)
for as long as the direction of the match ferromagnet and (to the point b on Fig.). Section vg
in Fig. corresponds to the saturation of the ferromagnet, when the
increase of the field leads to a very small increase in the magnetic
moment of the ferromagnet at the expense of the magnetic moments, which
are due to thermal motion and other causes were randomly oriented
against the field. Magnetic hysteresis - is that the magnetization and
demagnetization of ferromagnetic describe different curves
(magnetization lags behind in the reduction of the field). When reducing
the external field from Vsat. to 0 magnetization does not change the curve - oabvg - basic magnetization curve, and in accordance with the curve of the dg. When reducing the external field to zero magnetization ferromagnet has called residual (point e).
on the site is first reorientation of the magnetic moment of the
ferromagnet partition into domains, increase of domains 2,4,6 and
1,3,5 decrease of domains due to thermal motion. Upon application of
oppositely directed field, ie the area is de further growth areas
"even" domain magnetic moments are now an acute angle with the field by
reducing the area of "odd" domains. At point e the square "even" domains are areas of "odd", the total magnetic moment of the ferromagnet is zero.
BC field, demagnetizing ferromagnetic, called the coercive force. If you change the magnetic field on the BC to -BC and back curve characterizing the magnetization forms a closed loop - hysteresis loop.
Materials with high coercivity are called hard magnetic and low -
soft. Soft magnetic materials are used to make the cores of the
electromagnets (which is important to have high values ??of the maximum
induction field and low coercivity), as cores of transformers and AC
machines (generators, motors) in the cores of the magnets of
accelerators. Hard magnetic materials are used in permanent magnets:
with high coercive force and a relatively large residual magnetization
of these magnets may long time to create strong magnetic fields.
Permanent magnets are used in magneto-electric measuring devices,
speakers, microphones, small generators, microelectromotors etc.
- each surrounded by a magnetic moment antiparallel to the magnetic
moment. Spontaneous magnetization does not arise because magnetic
moments of the atoms are mutually compensated. Lack of full
compensation of the magnetic moments of the sublattices leads to the
fact that in an antiferromagnet there is some resultant non-zero, the
Materials such as to combine the properties of ferro-and antiferromagnets. They are called ferrimagnets or ferrite