MS-12-P-1511 Determination of magnetic flux density of grain boundary phase in Nd-Fe-B permanent magnets

Nanostructure optimization is crucial for enhancing the coercivity of Nd-Fe-B permanent magnets, which allow a significant degree of miniaturization of electric parts because of the large energy product. Regarding the materials science/engineering of Nd-Fe-B magnets, an essential problem is to understand the magnetism of the ultrathin grain boundary (GB) phase which envelopes the individual crystal grains of Nd₂Fe₁₄B [1]. However, revealing the magnetism in the GB region (~3 nm in width) remains challenging. Here, we used electron holography to determine the magnetic flux density in the GB phase using a sintered magnet subjected to optimal annealing.

As schematically shown in Fig. 1(a), which represents the cross section of a thin-foil specimen, the phase shift of electrons was measured in the line connecting the points R and S. The thin GB phase was tilted away from the direction of incident electrons: i.e., the trace of GB phase (W_GB) was approximately 110 nm in this experimental setup. The magnetic flux density in the GB phase (B_GB) can be determined from Δφ, which represents the deviation of the observed phase shift (in the GB area) from the extrapolated curve assuming the absence of the GB phase [Fig. 1(b)].

Figure 1(c) shows a transmission electron microscope image of the thin-foil specimen, which contained five Nd₂Fe₁₄B grains, A-E. Using the reconstructed phase image such as shown in Fig. 1(d), we accurately measured the phase shift in the R-S line which crossed the GB (shown in yellow) between Nd₂Fe₁₄B grains A and B. The observations are plotted with light-blue dots in Fig. 2(a). Split-illumination electron holography [2] achieved the sufficient precision for measuring the phase shift of ±0.08 rad. To obtain the phase information due to the GB phase, curve fitting was carried out for area A (containing only grain A); refer to the red curve in Fig. 2(a). The deviation Δφ was plotted as a function of position along the R-S line, as shown in Fig. 2(b). The value of Δφ continued to decrease over the GB area, reaching a minimum of −0.34 rad at the border of this area. Following the simulations of Δφ, the value of B_GB that explains well the observation (i.e., −0.34 rad, at the border of GB area) is ~1.0 T [Fig. 2(c)]. The result explicitly indicates that the GB phase is ferromagnetic, contrary to the traditional understanding. Our observation implies significant exchange coupling between Nd₂Fe₁₄B grains, which explains the avalanche-propagation of magnetization reversal observed in sintered magnets.

References:

[1] H. Sepehri-Amin et al., Acta Mater. 60 (2012) 819.

[2] T. Tanigaki et al., Appl. Phys. Lett. 101 (2012) 043101.