The Latest Research Progress of Rare Earth Permanent Magnet Materials-Sintered NdFeB

At present, rare earth permanent magnet neodymium iron boron (Nd-Fe-B) has been widely used in various aspects of life, such as maglev trains, electric vehicles, wind power, sound and so on. However, the coercivity of sintered NdFeB products is only 20-30% of the theoretical value (Stoner-Walfart limit) (commonly known as Brownian paradox), which severely limits the application of NdFeB. According to the existing theory, the coercive force of sintered NdFeB is mainly determined by the nucleation field required for the reverse magnetic domain generated near the grain boundary during demagnetization. Therefore, it is especially important to carry out three-dimensional quantitative analysis on how the grain boundary affects the coercive force. This not only can deepen the understanding of the coercivity mechanism of rare earth permanent magnets, but also has guiding significance for practical production.

Recently, Associate Professor Zheng Rongkun (corresponding author) and the first author of the University of Sydney, Dr. Chen Hanjun, and team members reported using microscopy simulation techniques based on backscatter diffraction, atomic-scale three-dimensional atom probe technology, and experimental results for simulation parameters. In the sintered NdFeB, the coercive force is further reduced due to the uneven composition of the crystal at the nanometer scale, and the grain boundary composition and the coercive force are three-dimensional quantitatively analyzed. Studies have shown that ferromagnetic elements (iron and cobalt) in the grain boundaries of sintered NdFeB are from 67 at 70 nm. % reduced to 10 at. %. The nucleation field required to produce a reverse magnetic domain near the grain boundaries of such a component is 27% smaller than the nucleation field required to produce a reverse magnetic domain with a uniform grain boundary containing the same ferromagnetic element content. This achievement is not only useful for industrial production, such as controlling the grain boundary composition at the nanometer scale, but also the analytical method used in this paper can be applied to the study of the relationship between the composition and magnetic properties of other magnetic materials. The research results were published in Physical Review Materials under the title "Coercivity degradation caused by inhomogeneous grain boundaries in sintered Nd-Fe-B permanent magnets".

[Graphic introduction]

Figure 1: Hysteresis loops of sintered NdFeB at different temperatures (280, 300, 320, 340 and 360 K).

Figure 2: Microstructure of sintered NdFeB at the micrometer scale.

(a) a secondary electron map;

(b) backscattered electron maps;

(c) Phase diagrams of the different phases of the calibration (2:14:1 main phase is calibrated in red, rare earth phase is calibrated in blue, white is uncalibrated, black lines depict large angular boundaries).

Scale is 20 microns

Figure 3: Three-dimensional atomic probe results of heterogeneous grain boundaries at the atomic scale, showing the main phase grains (MG1 and MG2) and grain boundaries of iron, lanthanum, cerium, boron, cobalt, copper, gallium and aluminum, respectively. Distribution under (GB).

The boundary scale is ~70 nm × ～70 nm × ～190 nm

Figure 4: Quantitative analysis of the results of three-dimensional atom probes at the atomic scale of heterogeneous grain boundaries. It was found that the ferromagnetic elements (iron and cobalt) in the inhomogeneous grain boundaries in the sintered NdFeB ranged from 67 at 70 nm. % reduced to 10 at. %.

(a) the three-dimensional distribution of iron atoms and the equivalent surface of iron (74.8 at.%);

(b) the distribution of composition of iron, bismuth and bismuth in the direction of the arrow in the red and blue cubes;

(c) The compositional distribution of iron, bismuth, bismuth and cobalt in the direction of the arrow in the green cube.

The boundary scale is ~70 nm × ～70 nm × ～190 nm

Supplementary material Figure 2: Micromagnetic model for simulation.

(a) Schematic diagram of a NdFeB sandwich model (grain boundary of primary phase grain 1 - 10 nm wide - main phase grain 2), dimension 100 nm × 100 nm × 100 nm;

(b) Schematic diagram of the compositional variation (x-z plane) of the inhomogeneous grain boundary based on the three-dimensional atom probe, with a size of 100 nm × 100 nm.

Figure 5: Micromagnetic simulation experiments based on the results of three-dimensional atom probes. It was found that the nucleation field required to generate the reverse magnetic domain near the grain boundary (pink curve) of such a component is more than the uniform grain boundary (blue curve) containing the same ferromagnetic element content to generate the reverse magnetic domain. The required nucleation field is 27% smaller.

(a) The demagnetization curve of the micro-magnetic simulation based on the NdFeB sandwich model (grain boundary of the main phase grain 1 - 10 nm wide - the main phase grain 2), the green, blue, pink and yellow curves respectively represent crystal The magnetic element of the boundary iron is 0, 40 at. %, 38.7 at. % (uneven) and 67 at%;

(b) In the case of demagnetization, the magnetic moment inversion of a sandwich model with a grain boundary ferromagnetic element composition of 0 (willing to form a reverse magnetic domain from the boundary);

(c) In the case of demagnetization, the magnetic moment inversion of the grain boundary ferromagnetic element composition is 67 at% of the sandwich model (willing to form a reverse magnetic domain from the interface between the grain boundary and the main phase grain).

(d) In the case of demagnetization, the grain boundary ferromagnetic element composition is 38.7 at. The magnetic moment reversal of the % (non-uniform) sandwich model (willing to form a reverse magnetic domain from the interface between the grain boundary region where the ferromagnetic element content is high and the main phase grain).

Figure 6: Magnetization, exchange field, magnetocrystalline anisotropy field, demagnetizing field simulated for uniform grain boundaries and uneven grain boundaries. It was found that in the demagnetization process, the reverse magnetic domain is more likely to be generated from a region of low magnetocrystalline anisotropy, high exchange field, and high demagnetization field.

(a) The grain boundary is 40 at. The magnetization, exchange field, magnetocrystalline anisotropy field, and demagnetizing field simulated by a sandwich model of % Fe (even);
The magnetization, exchange field, magnetocrystalline anisotropy field, and demagnetizing field are 1.17 × 106, 2.29 × 104, 5.51 × 106, and 3.35 × 105 A/m, respectively.

(b) The grain boundary is 38.7 at. The % (uneven) sandwich model simulates the magnetization, exchange field, magnetocrystalline anisotropy field, and demagnetizing field.
The range of magnetization, exchange field, magnetocrystalline anisotropy field, and demagnetizing field are 1.31 × 106, 2.15 × 104, 5.51 × 106, and 1.59 × 106 A/m, respectively.

In this paper, three-dimensional atom probe technology is used to quantitatively study the compositional changes of elements along grain boundaries and through grain boundaries. Based on the experimental data, the saturation magnetization, magnetocrystalline anisotropy constant and exchange constant of the grain boundaries of different compositions were fitted, and the influence of compositional changes on the coercivity at the grain boundary nanometer scale was quantitatively analyzed.

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