Magnetic Materials Knowledge Encyclopedia

Definition

Magnetic materials refer to a type of functional materials that have magnetism or can interact with magnetic fields, enabling the mutual conversion of magnetic energy, electrical energy, and mechanical energy. Their core feature is the ordered arrangement of internal magnetic domains, which can generate or respond to magnetic fields. They are indispensable basic materials in modern industry, electronic technology, new energy and other fields. From daily-used refrigerator magnets and headphones to new energy vehicle motors and aerospace sensors in high-end manufacturing, the application of magnetic materials runs through all aspects of production and life.

Core Characteristics

The core performance of magnetic materials is determined by their internal magnetic domain structure. The key characteristics include magnetism (remanence, coercivity, magnetic energy product, etc.), magnetic permeability (characterizing the magnetic conductivity of materials), temperature stability (Curie temperature as the core indicator), corrosion resistance and mechanical strength. These characteristics directly determine the application scenarios and service life of magnetic materials.

Hard Magnetic Materials

Hard magnetic materials, also known as permanent magnetic materials, refer to magnetic materials that can maintain magnetism for a long time after magnetization without the need for an external magnetic field to maintain it. Their core characteristics are high coercivity and large magnetic energy product, which can stably generate magnetic fields. According to the IEC 60404-8-1:2023 standard, permanent magnetic materials are divided into three categories according to metallurgical relationships and processes: Class R (hard magnetic alloys), Class S (hard magnetic ceramics) and Class U (bonded hard magnetic materials).

Rare Earth Permanent Magnetic Materials

At present, they are the permanent magnetic materials with the best comprehensive performance, with rare earth elements as the core component. Their magnetic energy product and coercivity are much higher than other types, which are divided into two mainstream types.

• Neodymium Iron Boron (NdFeB): The most widely used rare earth permanent magnetic materials, divided into sintered NdFeB, bonded NdFeB and hot-deformed NdFeB. The magnetic energy product of sintered NdFeB can reach 210-410 kJ/m³, remanence 1060-1480 mT, and coercivity 880-2800 kA/m. It has extremely strong magnetic performance, small volume and light weight, and is suitable for high-end manufacturing scenarios. Bonded NdFeB is formed by injection or compression molding and can be made into complex shapes. Among them, the magnetic energy product of anisotropic HDDR NdFeB bonded magnets can reach 75-151 kJ/m³, with outstanding cost performance. Hot-deformed NdFeB is a new type added in the IEC 60404-8-1:2023 standard, with radial or axial orientation, suitable for servo motors and other scenarios with special requirements on shape and performance. Its shortcoming is poor corrosion resistance, so it needs surface protection such as galvanizing and nickel-copper-nickel composite plating. The temperature resistance of conventional models is 80-150℃, which can be increased to 200℃ by adding dysprosium, terbium and other elements.
• Samarium Cobalt (SmCo): Divided into SmCo5 and Sm2Co17, with magnetic energy products of 120-170 kJ/m³ and 140-240 kJ/m³, remanence 800-1150 mT, and coercivity 700-2000 kA/m respectively. It has excellent temperature resistance (250-350℃), strong corrosion resistance and high magnetic stability, and is suitable for extreme environments. However, its cost is high and cobalt resources are scarce, so it is mainly used in aerospace, high-temperature motors and other special fields.
• Cerium Iron Boron (CeFeB): It is a low-cost option for rare earth permanent magnetic materials, replacing part of neodymium with cheap cerium. Its remanence is about 790 mT, magnetic energy product is about 64 kJ/m³, and coercivity is about 400 kA/m. It can balance the utilization of rare earth resources and is suitable for mid-to-low-end scenarios with low requirements on magnetic performance.

Ferrite Permanent Magnetic Materials

Belonging to Class S hard magnetic ceramics, it uses iron oxide, barium/strontium and other oxides as raw materials, with extremely low cost, good corrosion resistance and simple process, divided into isotropic and anisotropic types. The magnetic energy product of anisotropic sintered ferrite is 20-41 kJ/m³, remanence 320-470 mT, and coercivity 135-440 kA/m. Among them, the Ca-La-Co ferrite newly added in the IEC 60404-8-1:2023 standard has better magnetic performance and temperature stability than traditional varieties, with coercivity up to 440 kA/m, suitable for temperature-sensitive scenarios such as automotive electric power steering. Ferrite has medium magnetic performance and large volume, mainly used in cost-sensitive scenarios such as home appliances, toys and small motors, accounting for about 60% of the global permanent magnetic material market share.

Alnico Permanent Magnetic Materials

Belonging to Class R hard magnetic alloys, it is the earliest industrialized permanent magnetic material, with remanence 650-1300 mT, magnetic energy product 26-72 kJ/m³, and coercivity 48-150 kA/m. It has high mechanical strength, good remanence stability and extremely low temperature coefficient (Br temperature coefficient is only -0.02%/℃), and can work stably in extreme high temperature environment of 550℃. However, its coercivity is low, and it is easy to demagnetize under the interference of external magnetic fields. It is mainly used in traditional scenarios such as pointer instruments, magnetoelectric sensors and teaching experimental equipment.

Soft Magnetic Materials

Soft magnetic materials refer to magnetic materials whose magnetism disappears quickly when the external magnetic field disappears after magnetization. Their core characteristics are high magnetic permeability and low coercivity, which are easy to magnetize and demagnetize. They are mainly used for energy conversion and signal transmission, focusing on playing the role of magnetic conduction and magnetic concentration.

• Silicon Steel Sheet: The most commonly used soft magnetic material, divided into hot-rolled and cold-rolled. Adding silicon elements can improve magnetic permeability and reduce iron loss. It is mainly used for the iron cores of transformers, motors and generators, and is the core material of the power industry.
• Soft Ferrite Materials: Low cost and small high-frequency loss, divided into manganese-zinc ferrite (suitable for low and medium frequency) and nickel-zinc ferrite (suitable for high frequency), used in electronic components such as filters, inductors, transformers and antennas.
• Amorphous/Nanocrystalline Soft Magnetic Materials: Prepared by rapid solidification technology. Amorphous materials have disordered atomic structure, and nanocrystalline materials are developed on the basis of amorphous (such as FeCuNbSiB alloy). They have extremely high magnetic permeability, extremely low iron loss and good corrosion resistance, and are used in high-end electronic equipment such as high-frequency transformers, switching power supplies and sensors.
• Permalloy: Iron-nickel alloy with extremely high magnetic permeability, used in precision instruments, magnetic shielding, small transformers and other scenarios with high requirements on magnetic performance.

Core Magnetic Performance Parameters

• Remanence (Br): Unit mT (millitesla), refers to the magnetic flux density remaining when the magnetic material is saturated and magnetized and the external magnetic field is removed, which characterizes the ability of the magnetic material to retain magnetism.
• Coercivity (HcJ/HcB): Unit kA/m (kiloampere per meter), HcJ is the coercivity of magnetic polarization, HcB is the coercivity of magnetic flux density, usually HcJ > HcB, which characterizes the ability of magnetic materials to resist demagnetization.
• Magnetic Energy Product ((BH)max): Unit kJ/m³ (kilojoule per cubic meter), which measures the ability of magnetic materials to store magnetic energy. It is the core performance indicator of permanent magnetic materials.
• Magnetic Permeability (μ): Unit H/m (henry per meter), which characterizes the ability of magnetic materials to conduct magnetic fields. It is the core parameter of soft magnetic materials.
• Curie Temperature (Tc): Unit ℃, refers to the critical temperature at which magnetic materials lose their magnetism. Beyond this temperature, the internal magnetic domain arrangement of magnetic materials is disordered, and the magnetism disappears permanently or irreversibly attenuates. The Curie temperature of different magnetic materials varies greatly: Alnico (above 550℃) > SmCo (300-350℃) > Ferrite (above 250℃)> NdFeB (310-400℃).

Physical and Process Parameters

• Density: Unit Mg/m³, the density of different magnetic materials varies significantly, such as SmCo (8.3-8.5 Mg/m³) > Alnico (7.1-7.3 Mg/m³) > NdFeB (7.5-7.7 Mg/m³) > Ferrite (4.6-5.1 Mg/m³).
• Hardness/Toughness: Mechanical performance indicators. For example, alnico has good toughness and can be processed into complex shapes; sintered NdFeB has high hardness but high brittleness, which is easy to break and needs to avoid collision.
• Forming Process: Divided into sintering, bonding, hot deformation, etc. Sintered magnetic materials have high performance but complex forming; bonded magnetic materials can be made into complex shapes but have slightly lower performance; hot-deformed magnetic materials are suitable for special shape requirements.

Environmental Adaptability Parameters

• Operating Temperature Range: Exceeding the range will lead to magnetic attenuation. For example, the operating temperature of conventional NdFeB is 80-150℃, the high-temperature type can reach 200℃, and SmCo can reach 350℃.
• Corrosion Resistance: Some magnetic materials (such as NdFeB) are easy to oxidize and rust, and need to improve corrosion resistance through surface treatment. Common surface treatment methods include galvanizing, nickel plating, etc.; ferrite and SmCo have good corrosion resistance and do not need additional treatment.
• Relative Reversible Magnetic Permeability (μrec): Describes the reversible degree of magnetic flux recovery after the material is demagnetized. The closer the typical value is to 1, the better the linearity, which is convenient for evaluating the demagnetizing field strength.

Typical Magnetic Material Parameter Comparison Table

Application Scenarios of Permanent Magnetic Materials

• New Energy Vehicle Field: Core applications include drive motors, electric power steering systems, air conditioning compressors, etc. Sintered NdFeB is preferred for drive motors, Ca-La-Co ferrite can be used for electric power steering systems, and ferrite is used for low-cost scenarios such as window lifting motors.
• Aerospace and Special Fields: For aerospace sensors, high-temperature motors, oil exploration equipment, nuclear industry testing instruments, etc., SmCo magnets are preferred, and alnico magnets can also be used for high-temperature components in spacecraft.
• Home Appliance Field: For air conditioners, refrigerators, washing machines, electric fans, etc., ferrite is mainly used, while bonded NdFeB can be used for high-end home appliances to achieve miniaturization and energy saving.
• Electronic Equipment Field: For smart terminals such as mobile phones, headphones, watches and hard disk drives, NdFeB and bonded NdFeB are used because of small volume and strong magnetism.
• Traditional Industry and People's Livelihood Fields: Alnico is used for pointer instruments; ferrite is used for toy motors and refrigerator magnets; sintered NdFeB is used for wind turbines to improve power generation efficiency.

Application Scenarios of Soft Magnetic Materials

• Power Industry: For transformers, generators, mutual inductors, etc., silicon steel sheets are mainly used to reduce iron loss and improve power conversion efficiency; amorphous/nanocrystalline soft magnetic materials can be used for high-voltage transformers to further reduce energy consumption.
• Electronic Circuit Field: For filters, inductors, chokes, antennas, etc., nickel-zinc ferrite is used for high-frequency scenarios, and manganese-zinc ferrite is used for low-frequency scenarios; amorphous/ nanocrystalline soft magnetic materials are used for switching power supplies and high-frequency transformers.
• Precision Instruments and Magnetic Shielding: Permalloy is used for precision sensors and medical equipment to achieve accurate signal transmission and magnetic shielding by virtue of high magnetic permeability.

Common Misunderstandings in Selection and Use

• Misunderstanding 1: The stronger the magnetism, the better.In fact, it is necessary to combine scene requirements. For example, home appliance motors do not need strong magnetism, and ferrite can be used. Blindly selecting NdFeB will increase costs.
• Misunderstanding 2: Non-standard demagnetization test methods.80% of users have errors in high-temperature demagnetization tests, such as bare magnet baking, chaotic magnet placement, and testing without cooling to room temperature. The correct method is to adopt closed-circuit test, fix the placement position and bearing material, and test after cooling to 20-25℃.
• Misunderstanding 3: Ignoring the impact of temperature on magnetic materials.Using conventional NdFeB in high-temperature environments leads to irreversible magnetic attenuation; failing to select magnetic materials with suitable Curie temperature according to the ambient temperature can also lead to mismatch.
• Misunderstanding 4: Confusing the use of soft magnetic and permanent magnetic materials.Using permanent magnetic materials in energy conversion scenarios such as transformers leads to excessive energy loss; using soft magnetic materials in scenarios requiring stable magnetic fields cannot achieve functional requirements.

Maintenance Points

• Permanent Magnetic Materials: Avoid high-temperature exposure and collision; regularly check the surface coating of NdFeB to avoid oxidation and rust caused by coating damage; avoid alnico approaching strong external magnetic fields to prevent demagnetization.
• Soft Magnetic Materials: Avoid long-term exposure to high-temperature and humid environments to prevent iron core oxidation; avoid severe collision to prevent magnetic permeability reduction; regularly clean surface dust and oil stains to ensure energy conversion efficiency.
• General Points: Discarded magnetic materials should be recycled by classification, especially rare earth permanent magnetic materials, to avoid resource waste and environmental pollution; when testing magnetic materials, strictly follow the IEC 60404-5 standard to ensure the test sample size and test method are standardized.

• High Performance: Permanent magnetic materials are developing towards higher magnetic energy product, higher coercivity and higher temperature stability. Soft magnetic materials are developing towards low iron loss, high magnetic permeability and high frequency to meet the needs of high-end electronic equipment.
• Environmental Protection and Resource Rationalization: Rare earth permanent magnetic materials are developing towards rare-earth reduction, such as CeFeB and hot-pressed NdFeB, to reduce dependence on heavy rare earths and balance the utilization of rare earth resources.
• Miniaturization and Integration: To adapt to the development needs of smart terminals and small motors, the application of bonded magnetic materials and hot-deformed magnetic materials is becoming more and more extensive, realizing integrated magnetic-material and device integration while reducing product volume and improving performance.

In the future, magnetic materials will continue to be the core supporting materials for high-end manufacturing. Technological innovation will focus on performance improvement, cost reduction and sustainable use of resources, promoting the technological iteration and upgrading of various fields.