After about 30 years of development, the magnetic-energy product of NdFeB permanent - magnetic materials has approached its theoretical limit, making significant further improvement quite challenging. Scientists have been actively seeking magnetic compounds with a higher magnetic - energy product, yet as of now, none have been discovered. Thus, researchers turned to the development of nanocomposite permanent - magnetic materials to fully exploit the inherent magnetic properties of existing magnetic materials.
The first three generations of rare - earth permanent - magnetic materials are classified mainly according to their maximum magnetic - energy product. Evidently, the magnetic - energy product can serve as a key criterion for evaluating the magnetic properties of materials. It is a well - established fact that the magnitude of the magnetic - energy product is, to a certain degree, determined by the saturation magnetization and the magnetic anisotropy field of the material. However, in the first three generations of rare - earth permanent - magnetic materials, there has always been a trade - off between saturation magnetization and the magnetic anisotropy field; it has been difficult to optimize both simultaneously. Nanocomposite permanent - magnetic materials, a new class of permanent - magnetic materials emerging in this context, hold the potential to optimize both of these intrinsic properties.
High coercivity and remanence are fundamental requirements for permanent - magnetic materials. While hard - magnetic materials typically have high coercivity, their saturation magnetization is relatively low. In nanocomposite permanent - magnetic materials, the exchange - coupling interaction between the two phases can enhance the magnetic properties of the permanent - magnetic materials. This not only improves the performance of the materials but also potentially reduces the cost of the alloy.
In 1988, Dutch researcher Coehoom et al. subjected Nd4Fe77.5B18.5 amorphous ribbons to crystallization heat - treatment at different temperatures and obtained isotropic magnetic powder. They discovered that the magnetic powder with a lower Nd content exhibited a notable remanence enhancement effect. Structural analysis of this powder revealed that it consisted of a hard - magnetic Nd2Fe14B phase and a soft - magnetic Fe3B phase. Subsequent studies indicated that the exchange - coupling between grains was responsible for the remanence enhancement effect in these magnetic particles.
In 1991, Kneller et al. from Germany theoretically explained that the exchange - coupling effect between two - phase grains could increase the magnetic - energy product of materials. In 1993, Skomski and Coey et al. theoretically predicted that the anisotropic nanocomposite magnet Sm2Fe17N/α - (Fe, Co) could have a magnetic - energy product of 1 MJ/m³.
In experimental terms, in 2005, J Zhang et al. inserted a Cu partition layer into the prepared SmCo/Fe films. This was done to prevent contact and diffusion between the Fe layer and the SmCo layer during the annealing heat - treatment process, thereby maintaining a better multilayer - film structure. As a result, the magnetic - energy product reached 32 MGOe (255 kJ/m³), which is higher than the theoretical magnetic - energy product of single - phase SmCo5.
In 2011, S Sawatzki et al. hot - deposited an epitaxial SmCo5/Fe/SmCo5 three - layer film on an MgO (110) substrate at high temperature. The maximum magnetic - energy product achieved was 312 kJ/mol, which is 73% higher than the theoretical limit of 230 kJ/mol for the SmCo5 hard - magnetic phase.
Based on this, in 2012, S Sawatzki et al. prepared epitaxial [SmCo5/Fe]nSmCo5 multilayers without altering the total thickness of the hard and soft magnetic layers. When n = 2, the maximum magnetic - energy product exceeded 400 kJ/m³. In the same year, Wei BinCui et al., while studying the NdFeB monolayer film, found that after the Nd - rich phase diffused into the NdFeB magnetic layer, the contact between NdFeB grains was disrupted, leading to a significant increase in coercivity. Subsequently, they inserted a non - magnetic Ta layer between the anisotropic NdFeB and FeCo. This prevented the mutual diffusion of FeCo and the Nd - rich phase while preserving a better microstructure, and they obtained a nanocomposite permanent - magnetic material with a maximum magnetic - energy product of up to 486 kJ/m³.
