Through innovative microstructural design, dual-phase nanocrystalline composite permanent magnet materials overcome the mutual constraint between coercivity and remanence in traditional permanent magnets and are expected to become a new generation of permanent magnets.
Definition of Dual-phase Nanocrystalline Composite Permanent Magnet Materials
Dual-phase nanocrystalline composite permanent magnet materials are a new class of high-performance magnets formed by combining hard magnetic phases (such as Nd₂Fe₁₄B) and soft magnetic phases (such as α-Fe) at the nanoscale. Their core feature lies in the exchange coupling effect at the nanoscale, whereby the magnetic moments of the soft magnetic phase remain aligned with those of the hard magnetic phase under the “tight binding” effect of the hard magnetic phase, thereby simultaneously possessing the high coercivity of the hard magnetic phase and the high saturation magnetization of the soft magnetic phase.
Principle of Dual-phase Nanocrystalline Composite Permanent Magnet Materials
When the grain sizes of both the soft magnetic phase and the hard magnetic phase are reduced to the nanometer scale (typically less than 50 nm, ideally 5–10 nm), and the interfaces between the two phases are in close contact, strong ferromagnetic exchange coupling occurs between the grains of the two phases. Under the “tight binding” effect of exchange coupling, the magnetic moments of the soft magnetic phase rotate in synchronization with those of the hard magnetic phase, suppressing demagnetization of the soft magnetic phase.
The smaller the grain size, the larger the specific surface area, the stronger the exchange coupling, and the better the magnetic performance; however, if the grain size becomes too small (entering the superparamagnetic range), the coercivity will decrease. Therefore, the grain size must be strictly controlled within the single-domain size range.
Advantages of Dual-phase Nanocrystalline Composite Permanent Magnet Materials
| High remanence: At the nanoscale, under the exchange coupling of the hard magnetic phase, the magnetic moments of the soft magnetic phase remain parallel to those of the hard magnetic phase, causing the material to exhibit a single magnetization behavior and thereby producing a “remanence enhancement effect.” Its actual remanence (e.g., in NdFeB-based composite materials) is higher than that of conventional isotropic magnets.
| Low rare-earth content: Compared with traditional rare-earth permanent magnet materials, the rare-earth content of this material is significantly reduced (e.g., in NdFeB-based nanocomposite materials), effectively lowering raw material costs.
| Flexible preparation processes: It can be prepared by various processes such as melt spinning, high-energy ball milling, and chemical synthesis, meeting the requirements of different applications regarding material shape and microstructure. The flexibility of the processes also helps optimize grain size and interfacial coupling.
Disadvantages of Dual-phase Nanocrystalline Composite Permanent Magnet Materials
| Difficult to control: Grain size and distribution requirements are demanding. The prerequisite for achieving ideal exchange coupling between soft and hard magnetic phases is that the grain size is at the nanometer scale and uniformly distributed. In actual preparation, precise control of grain size is difficult, and grain coarsening or non-uniform size can weaken the exchange coupling.
| Poor mechanical properties: Nanocrystalline materials have extremely fine grains and a high proportion of grain boundary volume, resulting in high brittleness and poor plasticity. Under stress, they are prone to cracking or fracturing, making conventional mechanical processing such as stamping and bending difficult, and preventing the fabrication of magnets with complex shapes.
Applications of Dual-phase Nanocrystalline Composite Permanent Magnet Materials
| New energy vehicles: Dual-phase nanocrystalline composite permanent magnet materials can improve the energy utilization efficiency of new energy vehicles and meet the requirements for high power density and fast response, primarily used in high-performance motors of new energy vehicles.
| Power/electrical: The material’s high-frequency low-loss and high-permeability characteristics help improve the conversion efficiency of power electronic devices, enabling device miniaturization and reduced heat generation. It is mainly used in magnetic components of high-frequency electrical equipment.
| Aerospace: This material can provide a stable and reliable magnetic field source, with small size and excellent magnetic performance, helping to achieve high precision and miniaturization of guidance systems and meeting the strict requirements of aerospace equipment for weight and space.
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