Ferromagnetic materials belong to strong magnetic materials, which are widely used in electrical equipment and scientific research. According to their chemical composition and performance, can be divided into metal magnetic materials and non-metallic magnetic materials (ferrite) two groups.
1. Metal magnetic materials
Metal magnetic material is a metal alloy or compounds made of magnetic material, for the most part is based on iron, nickel or cobalt, add other elements made by high temperature melting, machining and heat treatment, the magnetic material under the condition of high temperature, low frequency and high power, have a wide range of applications, but in high frequency range, its application is limited. Metal magnetic materials can also be divided into hard magnetic materials, soft magnetic materials and magneto-barometric materials. Experiments show that the hysteresis loop shapes of different ferromagnetic materials vary greatly. The figure shows the hysteresis loop of three different ferromagnetic materials. The coercivity and remanence of hard magnetic materials are larger. The hysteresis loop of ferrite material is approximately rectangular, so it is also called moment magnetic material.
Soft magnetic material is characterized by relative permeability Ur Bmax and saturated magnetic induction strength is compared commonly big, but much more than small hard magnetic coercive force Hc, surrounded by a hysteresis loop area is small, no significant hysteresis characteristics, soft magnetic material in magnetic field is easy to be magnetized, and because of its coercive force is small, so easy to demagnetization, therefore, soft magnetic material is very suitable for manufacturing electromagnets, transformer, ac motor, alternator, etc electrical core of another reason.
Hard magnetic material is also called permanent magnet material, its characteristic is remanence Br and coercive force Hc is bigger, surrounded by a hysteresis loop area is big, the hysteresis characteristics is significant, so the hard magnetic material on the external magnetic field, the magnetization in sparing the strong magnetic, and the residual magnetism is not easy to be eliminated, so the hard magnetic material is suitable for manufacturing permanent magnet. Permanent magnets are commonly used to obtain a stable magnetic field in various electric meters and other electrical equipment. On June 3, 1998, the "discovery" space shuttle carried by America, a chinese-american physicist Samuel ting, a professor at the permanent magnets used in the organization and leadership of the alpha magnetic spectrometer, is developed by the Chinese academy of sciences institute of electrical and other unit of rare earth materials ndfeb permanent magnets, the magnetic induction intensity up to 0. 14 t, the diameter of the permanent magnet is 1. 2 m high 0. 8 m, and the alpha magnetic spectrometer is used to detect the antimatter and dark matter in the universe, this is the first time that will be a large permanent magnets into space, to direct observation of charged particles in the universe, It is very likely to open up a completely new scientific field and bring a new scientific breakthrough. Piezomagnetic materials have strong magnetostrictive properties. The so-called magnetostrictive means that the shape and volume of ferromagnetic objects will change when the magnetic field changes, especially the length of the object in the direction of the magnetic field. When the alternating magnetic field effect on ferromagnetic objects, it with the increase of the magnetic field can be extended, or shortening, such as cobalt steel is elongation, and nickel is reduced, but the change of the length is very small, about the original long 1/100000, magnetostriction in technology has important applications, such as mechanical and electrical transducer is used for drilling, washing, also can be used as acoustic electric transducer used for detecting depth of the ocean, fish, etc.
2, non-metallic magnetic materials -- ferrite
Ferrite, also known as ferrigan oxygen, is a group of compounds. It is made by sintering iron oxide (Fe2O3) with powders of other metallic oxides (such as NiO, ZnO, MnO, etc.). The characteristics of the ferrite is not only has high permeability, and has a high resistivity, resistivity of it about between 104 ~ 1011 Ω · m, some 1014 Ω m., resistivity than metallic magnetic material (about 10-7 Ω m.) to most, so the ferrite eddy current loss is small, often used in high frequency technology. Figure (c) is the hysteresis loop of the moment ferrite. It can be seen from the figure that the hysteresis loop is approximately rectangular. In electronic computers, the rectangular loop characteristic of the moment ferrite is used as a memory element. In addition ferrite is widely used as the core of antenna and inductance in electronic technology.