To the list of lectures

§ 4 Ferromagnetics

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 -

obtuse. 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).

gd 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.

Antiferromagnets - 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 spontaneous magnetization.

Materials such as to combine the properties of ferro-and antiferromagnets. They are called ferrimagnets or ferrite

To the list of lectures