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§ 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 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 |
