Classification of Magnetic Materials

1.1.1 Classification by Internal Structure and Magnetic Properties in External Magnetic Fields

According to the strength of the magnetic properties exhibited by substances in external magnetic fields, they can be classified into diamagnetic substances, paramagnetic substances, ferromagnetic substances, antiferromagnetic substances, and ferrimagnetic substances. Most materials are diamagnetic and paramagnetic, showing weak responses to external magnetic fields; ferromagnetic and ferrimagnetic substances are strong magnetic materials, and the term "magnetic materials" usually refers to strong magnetic materials.

Ferromagnetic materials

Ferromagnetic materials refer to materials that exhibit significant magnetization intensity and direction under the influence of an external magnetic field. Ferromagnetic materials include metals such as iron, cobalt, and nickel, as well as artificial synthetic materials like ferrites and neodymium iron boron. Ferromagnetic materials have strong magnetism and good stability, but they are prone to magnetization and have large hysteresis losses.

(2) Diamagnetic materials

Diamagnetic materials refer to those that do not exhibit significant magnetization intensity or direction under the influence of an external magnetic field. Diamagnetic materials include metals such as copper, silver, and gold, as well as alloys like aluminum, iron, and magnesium. Diamagnetic materials have weak magnetic properties and good stability, but they are not easily magnetized.

(3) Paramagnetic materials

Paramagnetic materials refer to substances that exhibit significant magnetization and a definite magnetic direction under the influence of an external magnetic field. Such materials include metals like iron, nickel, and chromium, as well as synthetic materials such as iron oxide and chromium oxide. Paramagnetic materials have strong magnetic properties but poor stability, being susceptible to changes in temperature and magnetic field.

1.1.2 Classification by Magnetic Properties

According to magnetic properties, materials can be classified into soft magnetic materials, hard magnetic materials and rectangular magnetic materials.

Hard magnetic materials

Hard magnetic materials, also known as permanent magnetic materials, refer to materials that can retain a strong residual magnetism after being magnetized by an external magnetic field and the external magnetic field is removed. They have strong coercive force, high residual magnetization intensity, large magnetic energy product, wide hysteresis loop, high residual flux density and high stability. Hard magnetic materials mainly include rare earth hard magnetic materials, metallic hard magnetic materials and ferrite hard magnetic materials. Currently, the most widely used ones are neodymium iron boron strong magnets and ferrite magnets. Neodymium iron boron magnetic materials are alloys of neodymium, iron oxide, etc., also known as magnetic steel, and are the latest development of rare earth permanent magnetic materials. Neodymium iron boron has extremely high magnetic energy product, coercive force and high energy density. Ferrite magnets are produced by powder metallurgy methods, have low residual magnetism, low recovery magnetic permeability, high coercive force and strong anti-demagnetization ability.

(2) Soft magnetic materials

The main functions of soft magnetic materials are to conduct magnetic fields, convert and transmit electromagnetic energy. They are required to have high magnetic permeability, high magnetic induction intensity, narrow hysteresis loops and low magnetic loss. Compared with hard magnetic materials, they have smaller coercive force and narrower hysteresis loops. Soft magnetic materials mainly include pure iron and carbon steel, nickel-iron alloys, magnetic ceramic materials, amorphous alloys, nanocrystalline soft magnetic materials. Currently, the most widely used ones are magnetic powder cores and non-wound iron cores. Among them, magnetic powder cores include iron powder cores, iron-silicon-aluminum powder cores, high flux powder cores, permalloy powder cores and ferrite cores; wound iron cores mainly include silicon steel sheets, permalloy, amorphous and nanocrystalline alloys.

1.1.3 Introduction to the Classification of Magnetic Powder Cores

Magnetic powder cores are a type of soft magnetic material made by mixing ferromagnetic powder particles with insulating media and then pressing them. Due to the small size of the ferromagnetic particles and their separation by non-magnetic electrically insulating film substances, on the one hand, eddy currents can be isolated, making the material suitable for higher frequencies; on the other hand, the gap effect between the particles results in low permeability and constant permeability characteristics of the material. Additionally, because the particle size is small, skin effect is basically non-existent, and the permeability changes relatively stably with frequency. The magnetic and electrical properties of magnetic powder cores mainly depend on the permeability of the powder material, the size and shape of the powder particles, the filling factor, the content of the insulating medium, the forming pressure, and the heat treatment process. Commonly used magnetic powder cores include iron powder cores, permalloy powder cores, and iron-silicon-aluminum powder cores.

Iron powder core

Common iron powder cores are composed of carbon-based ferromagnetic powder and resin carbon-based ferromagnetic powder, and they are the cheapest among powder cores. The saturation magnetic induction intensity is around 1.4T, and the magnetic permeability range is from 22 to 100. The initial magnetic permeability has good stability with frequency changes. They have good DC superposition performance, but the loss is large at high frequencies.

(2) Permalloy powder core

The main types of permalloy powder cores are aluminum permalloy powder cores (MPP) and high flux powder cores. The saturation magnetic induction value of MPP is around 7500 GS; its permeability range is 14-550, and it has the lowest loss among powder cores with excellent temperature stability. The saturation magnetic induction value of high flux powder cores is around 15000 GS; its permeability range is 14-160, and it has the highest magnetic induction intensity and the highest DC bias capability among powder cores, with a small core volume.

(3) Iron-silicon-aluminum powder core

Iron-silicon-aluminum powder cores mainly replace iron powder cores, with a loss 80% lower than that of iron powder cores. They can be used at frequencies above 8KHz, with a saturation magnetic flux density around 1.05T, a permeability range of 26-125, and a magnetostriction coefficient close to zero. They produce no noise when operating at different frequencies. Compared with MPP, they have a higher DC bias capability and offer the best cost performance.