Magnetically hard materials. Classification, basic properties, obtaining, application.

Hard magnetic materials are magnetic materials that are magnetized to saturation and remagnetized in relatively strong magnetic fields with a strength of thousands and tens of thousands of A/m (10 2 -10 3 Oe).

Magnetically hard materials are characterized by high values of coercive force H С , residual induction Br, and magnetic energy (BH)max in the demagnetization section. After magnetization, hard magnetic materials remain

permanent magnets due to the high values of Br and H C . The large coercive force of magnetically hard materials can be due to the following reasons:

1) a delay in the displacement of domain boundaries due to the presence of outsiders

inclusions or strong deformation of the crystal lattice;

2) precipitation in a weakly magnetic matrix of small single-domain ferromagnetic particles having either strong crystalline anisotropy or shape anisotropy.

By application, hard magnetic materials are divided into materials for permanent magnets and materials for recording and long-term storage of sound, images, etc.

Magnetically hard materials are classified according to various criteria, for example, according to the physical nature of the coercive force, according to technological characteristics, and others. Of the hard magnetic materials, the most important in technology have acquired: cast and powder (non-deformable) magnetic materials of the Fe-Al-Ni-Co type; wrought alloys such as Fe-Co-Mo, Fe-Co-V, Pt-Co; ferrites (hexaferrites and cobalt ferrite). Compounds of rare earth elements (especially light ones) with cobalt are also used as hard magnetic materials; magnetoplasts and magnetoelasts from alnico powders, ferrites with a binder of plastics and rubber, materials from Fe, Fe-Co, Mn-Bi, SmCo 5 powders.

Cast high-coercivity alloys.

This group includes alloys of the systems Fe – Ni – Al – Al and Fe – Ni – Co – Al. The high-coercivity state of these alloys is due to their dispersive decomposition into two phases upon cooling to a certain temperature.

Thus, a composition is obtained from a non-magnetic matrix and single-domain magnetic inclusions.

Materials with such a structure are magnetized mainly due to the processes of rotation of the magnetic moments of the domains.

However, these alloys are practically not used without alloying elements. In addition to cobalt, common alloying additions are copper, titanium, and niobium. Additives not only improve the magnetic properties, but also provide better repeatability of the characteristics, i.e., weaken the dependence of the magnetic properties on the chemical composition, the presence of impurities, and deviations from the specified heat treatment mode.

The magnetic properties of hard magnetic materials depend on the crystallographic and magnetic textures. The magnetic texture of high-coercivity alloys is created by cooling them in a strong magnetic field. This achieves an ordered arrangement of the lamellar elements of the strongly magnetic phase, which are oriented with their axes of easy magnetization in the direction of the field.

Such magnetic texturing is effective only for alloys with a high cobalt content.

The textured material is magnetically anisotropic. Its best properties are found in the direction in which, when cooled, a magnetic field acted on it.

The crystal texture is created by the method of directed crystallization of an alloy poured into a mold using special heat removal conditions. The disadvantage is the difficulty of manufacturing products of exact dimensions due to brittleness and high hardness. Of all types of machining, only grinding is allowed.

Powder magnets.

The need to obtain small products with strictly consistent dimensions led to the use of powder metallurgy methods for the production of permanent magnets. There are ceramic-metal magnets and magnets from powder grains held together by a binder ( metal-plastic magnets).

Ceramic-metal magnets are obtained by pressing a powder consisting of finely dispersed finely dispersed hard magnetic alloys, and then sintering at high temperatures, similar to ceramic firing processes.

The process of manufacturing metal-plastic magnets is similar to the process of pressing plastic parts, only the powder contains a filler in the form of grains of crushed hard magnetic alloy, which requires high specific pressures on the material, reaching up to 500 MPa.

Metal-powder magnets are cost-effective in mass automated production, complex configurations and small sizes of magnets. The metal-plastic technology makes it possible to obtain magnets with fittings.

Magnetically hard ferrites.

The most famous barium ferrite BaO ∙ 6 Fe 2 O 3 (ferroxdur), has a hexagonal crystal lattice with uniaxial anisotropy.

Its high coercive force y is due to the small size of crystal grains and strong magnetic crystallographic anisotropy. The production technology is similar to the production of soft magnetic ferrites, only finer grinding is carried out and sintered at low temperatures (to avoid recrystallization).

To impart anisotropy of magnetic properties, the material is textured by molding a mass of creamy consistency in a strong magnetic field (with a strength of 650–800 kA/m).

They are made in the form of washers and thin disks: they are highly stable in relation to the effects of external magnetic fields and are not afraid of shaking and shock.

The disadvantages include low mechanical strength, high fragility, strong dependence of magnetic properties on temperature.

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