Cause
The magnetism of a substance is related to its internal electronic structure. The electrons in the atoms of diamagnetic metals are all paired. The number of electrons in the positive and anti-spin spins is equal. The magnetic moments produced by the spins of the electrons cancel each other out. Therefore, the atomic magnetic moments are zero, so they are not attracted by the external magnetic field. In paramagnetic metal atoms, the number of electrons with positive and negative spins is not equal, and the magnetic moment of the atom is not zero. Due to the irregular thermal motion, the directions of the atomic magnetic moments are different. When placed in a magnetic field, the atomic magnetic moment is oriented along the direction of the magnetic field and slightly deflected, showing a weak magnetization. After the external magnetic field is removed, the atomic magnetic moment is chaotically distributed and the magnetization disappears.
The origin of ferromagnetism is similar to that of paramagnetism. It comes from unpaired electrons in atoms. However, there are many small local areas called "magnetic domains" in ferromagnetic materials. In these small areas, the magnetic moments of adjacent atoms are aligned and tend to be aligned with each other. The spontaneous magnetization between the magnetic domains The direction is disordered, so the macroscopic magnetic moment of the whole material is zero, and it does not show magnetism to the outside. When in a magnetic field, the magnetic moments of each magnetic domain will be aligned along the direction of the magnetic field to a certain extent. In this way, one magnetic domain aligning in sequence along the magnetic field is equivalent to the alignment of many atomic magnetic moments. Therefore, the interaction between ferromagnetic materials and the magnetic field is much greater than that of paramagnetic materials. Except for the external magnetic field, the magnetic domains still try to maintain the orientation formed when the original magnetic field exists as much as possible. At this time, the parts of the magnetic domains are arranged in order, so that the material retains residual magnetism, so the material is "permanently" magnetized. . The ferromagnetic material can be permanently magnetized by rubbing a piece of permanent iron. The magnetism of permanent magnetic materials can also be destroyed by heating or violent impact that causes the direction of the magnetic domains to become disordered.
Technical parameters
Magnetic alloys are widely used in industries such as electric power, telecommunications, machinery and electronics, instrumentation and computing technology, and are indispensable materials in various sectors of the national economy. Magnetic parameters and technical Magnetic properties are technical parameters that measure the performance of magnetic alloys, such as saturation magnetization Ms (saturation magnetic induction intensity Bs), residual magnetization Mr (residual magnetic induction intensity Br< /sub>), coercivity, various permeability, squareness ratio, hysteresis loss, etc. are all reflected in the magnetization curve and hysteresis loop of the alloy (see Figure 1). Different magnetic alloys have different magnetization curves and hysteresis loops, which are suitable for the design and application of different devices. These are the technical magnetism of magnetic alloys. It is closely related to the influence of external conditions and the change of magnetic state, involving the formation and structure of magnetic domains inside the alloy, as well as the movement and changes of magnetic domains under various conditions (such as external magnetic field, stress, temperature, etc.).
Magnetic classification
According to the processing and forming process, magnetic alloys can be divided into cold-worked and hot-worked types (most magnetic alloys belong to this category), casting type (such as Al-Ni-Co permanent magnet alloys) ) And powder metallurgy (such as rare earth permanent magnet alloys) magnetic alloys. According to the atomic configuration, it can be divided into crystalline type (traditional magnetic alloys belong to this category), amorphous type magnetic alloy, and nanocrystalline type magnetic alloy. Traditionally, magnetic alloys can be divided into soft magnetic alloys, permanent magnetic alloys and semi-hard magnetic alloys, as well as magnetostrictive alloys and magnetic recording materials, according to their magnetic properties.
(1) Soft magnetic alloy. The magnetic alloy with coercivity Hc<1kA/m is characterized by low coercivity, high permeability and low core loss. It is very easy to magnetize under the action of an external magnetic field; when the external magnetic field is removed After that, the magnetism disappears. Such alloys are widely used in various transformers, motors, relays, electromagnets, magnetic recording, magnetic shielding and communication engineering, telemetry and remote sensing systems, and as magnetic components in instruments and meters. Due to the different requirements for magnetic alloys in applications, a variety of alloys have been developed. According to the chemical composition, it can be divided into industrial pure iron, silicon steel, iron-nickel alloy, iron-cobalt alloy, iron-aluminum and iron-silicon-aluminum alloy. According to the characteristics of use, it can be divided into high initial permeability alloys, high magnetic induction alloys, high permeability alloys, high squareness ratio alloys, constant permeability alloys, corrosion resistant soft magnetic alloys and temperature compensation alloys. In addition, according to the crystalline state, it can be divided into crystalline and amorphous soft magnetic alloys. Due to the variety of properties of soft magnetic alloys, the variety of factors affecting its properties is determined. The main influencing factors are: chemical composition, impurities, stress and its distribution, structure, crystal orientation, orderly transformation, magnetic annealing, etc. For example, the alloy's saturation magnetization, Curie temperature, magnetostriction coefficient, resistivity and corrosion resistance are closely related to the chemical composition. The impurities in the alloy, especially the elements that form interstitial solid solutions such as carbon, nitrogen, oxygen and hydrogen, have significant damage to the soft magnetic properties, because when interstitial atoms are formed, the microscopic stress distribution caused by lattice distortion directly pin the domain wall Displacement, which significantly deteriorates the coercivity and permeability, should be removed as much as possible. But under certain conditions, certain impurities will also play a beneficial role. For example, a small amount of interstitial impurity elements can improve the resistivity and mechanical properties of the alloy. Another example is the fine AlN, MnS and trace amounts of oxygen, which are beneficial to the development of the secondary recrystallization of silicon steel and the control of the crystallization of iron-nickel alloys.
(2) Permanent magnet alloys. Magnetic alloys with coercivity Hc>20kA/m include Al-Ni-Co alloys, Fe-Cr-Co alloys, deformable permanent magnet alloys, platinum-cobalt alloys and rare earth permanent magnet alloys. The characteristics of this type of alloy are high coercive force, high saturation magnetic induction and remanence induction, and the hysteresis loop is wide and approximately square to ensure a high maximum magnetic energy product (BH) max. After magnetization, the magnetization state of this kind of alloy remains basically unchanged when the magnetization field is removed, that is, it is not easy to demagnetize and has a certain degree of magnetic "hardness", so it is also called hard magnetic alloy. This type of alloy is widely used in electromagnetic instruments, oscilloscopes, speakers, traveling wave tubes, gyroscopes, relays, circuit breakers, magnetic separators, magnetic bearings, magnetic couplers, nuclear magnetic resonance imaging, audio-visual and communication equipment, and magnetized energy-saving equipment Etc.
(3) Semi-hard magnetic alloy. Including hysteresis alloys, reed switch alloys, relay cores and memory component materials. The coercivity of this type of alloy is not high, which is between soft magnetic alloys and permanent magnetic alloys. They work under the condition that the magnetism changes with the external magnetic field, and the alloy is required to have high Br and the largest possible hysteresis loop area under a certain magnetic field, that is, a large hysteresis loss. This type of alloy is mainly used to make hysteresis motor rotors, relay cores, reed switch components and memory components.
(4) Magnetostrictive alloy. Magnetic alloys with large magnetostriction coefficients include pure nickel, iron-cobalt alloys, iron-aluminum alloys and rare earth-iron alloys. The coercivity of this type of alloy is not high, but it has a high saturation magnetostriction value. It is mainly used for ultrasonic transmission and reception, sonar systems, electromechanical filters, precision control systems, various valves, drivers, etc.
(5) Magnetic recording materials. Materials used in computers for recording, storing and reproducing information, including magnetic head alloys and magnetic recording media materials. Magnetic alloys used for magnetic heads include iron-nickel-based permalloy, iron-aluminum alloy, sendust alloy and cobalt-based amorphous alloy. Used as a magnetic recording medium are alloy powder coatings of iron, cobalt, and nickel, and magnetic alloy films such as cobalt-nickel, cobalt-chromium, etc., made by electroplating, chemical or evaporation methods.
Practical application
The main elements that make up permanent magnet materials in metals are Fe, Co, Ni and some rare earth elements. The permanent magnet alloys used include rare earth-cobalt, iron-chromium-cobalt and manganese-aluminum-carbon alloys. Among them, the rare earth series has experienced three generations. The first-generation permanent magnet material is represented by RECo5 (RE stands for rare earth element), and SmCo5 has the best performance; later, the second-generation permanent magnet material Sm2Co17, which reduces the amount of rare earth, will appear; Nd-Fe-B neodymium, which was successfully developed in the 1980s Iron-boron is the third generation, the main component of which is iron (about 2/3), the cost is significantly reduced, and the performance is better. The magnetic properties of NdFeB alloy produced in my country are in a leading position in the world. Magnetic alloys are increasingly widely used in emerging technologies such as electric power, electronics, computers, automatic control, and electro-optics.