Coercivity enhancement in Ce-Fe-B based magnets by core-shell grain structuring

High performance neodymium-based permanent magnets are key components of motors in electrical and hybrid vehicles. These magnets now contain Dy, the Dy partially substituting Nd in the Nd₂Fe₁₄B phase, to have enough coercivity (H_c) so as to maintain their remanent magnetization at the motor’s maximum operating temperature of about 473K. However Dy, and to a lesser extent Nd, are considered to be critical elements due to rapidly increasing demand for permanent magnets in motors. From this point of view, Ce-based intermetallic magnets are attracting attention¹ because of the greater abundance of Ce compared to Nd and Dy. However, the intrinsic magnetic properties, namely spontaneous magnetization (J_s) and anisotropy field (H_a), of Ce₂Fe₁₄B are far inferior to those of Nd₂Fe₁₄B. The room temperature values of J_s and H_a of single crystal Ce₂Fe₁₄B are 1.18T and 2.40 MA/m, respectively, while those of single crystal Nd₂Fe₁₄B are 1.61T and 5.36 MA/m.² To use Ce, the partial substitution of another R for Ce in the rare-earth sites of Ce₂Fe₁₄B, and an optimization of the microstructure are required to improve extrinsic properties, in particular coercivity.

This study concerns Ce-based nano-structured permanent magnets which consist of core-shell R₂Fe₁₄B grains separated by a non-magnetic grain boundary phase. The Ce-based 2-14-1 cores are surrounded by a higher anisotropy shell in which Ce is partially replaced by Nd. The concept of forming core-shell grains, via grain boundary diffusion with Nd-Cu, is expected to improve coercivity due to a suppression of magnetization reversal thanks to the high anisotropy shell and improved magnetic isolation thanks to the non-magnetic Nd-Cu grain boundary phase.³ Here we report on the structural and magnetic properties of such magnets.

The precursor materials were rapidly quenched Ce_13.8-XNd_XFe_75.66Co_4.46B_5.66Ga_0.38 (X is from 0 to 10.35) melt-spun ribbons. The melt-spun ribbons were crushed to a few hundred microns and then sintered at 1020 K under a pressure of 100 MPa. The sintered bulk magnets were hot-deformed to a height reduction of about 80% to develop (001), i.e. easy magnetization axis, texture of the (Ce,Nd)₂Fe₁₄B grains. The degree of texture, defined as the ratio of remanence to saturation magnetization, was about 0.9. The hot-deformed magnets were infiltrated with liquid Nd₇₀Cu₃₀ alloy at 970 K for 165 minutes, the amount of eutectic corresponding to 10 or 40 wt% of the magnet. During infiltration, Nd-Cu diffuses between grains and forms high anisotropy shells at the grain surface by partial substitution of Nd for Ce. The as-hot-deformed magnets are hereafter referred to as “alloy magnets” while the infiltrated magnets are referred to as “core-shell magnets”.

Microstructure observations and composition analyses have been performed using scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) spectroscopy, respectively. Magnetization measurements were made on 2 × 2 × 2 mm³ samples in the temperature range 300 K to 473 K, using a vibrating sample magnetometer (VSM), following sample magnetization in a field of 10 T. First order reversal curve (FORC) measurements was made on core-shell magnets following saturation, decreasing the magnetic field from 2.4 MA/m and sweeping it back to 2.4 MA/m in steps of 0.04 MA/m.

An STEM image and EDX elemental maps of a Ce₂Fe₁₄B hot-deformed magnet following infiltration with Nd-Cu are shown in Figure 1. Comparing the Ce and Nd maps, we can clearly identify the Nd-rich shell formed on the periphery of the Ce₂Fe₁₄B grains (white circled areas in Fig. 1). The compositions of a series of core-shell magnets (samples 1-4) and alloy magnets (samples 5-7), analyzed by STEM-EDX line profiles, are listed in Table I.

Comparison of the room temperature magnetization curves (Fig. 2) of a Ce₂Fe₁₄B alloy magnet (sample 5) and a core-shell magnet made from such a magnet (sample 1) reveals that infiltration with 10 wt% Nd-Cu increases coercivity from 0.02 to 0.41 MA/m. The relationship between coercivity and the mean Nd/ (Nd+ Ce) composition of core-shell magnets (samples 1 - 3) and alloy magnets (samples 5-7) is shown in Figure 3. The composition of samples 1-3 and 5-7 were obtained from STEM-EDX line profiles. The coercivity values of all core-shell magnets are superior to those of alloy magnets with the same overall Nd content in the R₂Fe₁₄B grains. The temperature dependence of coercivity of a core-shell magnet (sample 2) is compared to that of a Nd₂Fe₁₄B hot-deformed magnet in figure 4. While the magnets have the same room temperature value of coercivity, the core-shell magnet has higher values at elevated temperatures, despite having 25at% less Nd.

We attribute the coercivity enhancement demonstrated here to the formation of a high anisotropy shell in the Ce based R₂Fe₁₄B grains,⁴ which suppresses magnetization reversal, and to an improvement in the magnetic isolation of these grains by the formation of a grain boundary phase.³ FORC measurements, which may provide information about magnetization reversal behavior (coercive field distribution, magnetic interactions),⁵ have been performed to experimentally clarify the difference in reversal behavior between alloy and core-shell samples. FORC measurements and the corresponding FORC diagrams of core-shell magnets (sample 1 and 4) made from Ce₂Fe₁₄B magnets with infiltration of 10 wt% and 40 wt% Nd-Cu, respectively, are shown in Figure 5 (coercivity of sample 1 and 4 are 0.41 MA/m and 0.67 MA/m respectively). The presence of two distributions of reversal field along the H_c axis of the sample with the lower Nd-Cu content indicates the presence of heterogeneous microstructures with both a high coercivity portion and a low coercivity portion. On the other hand, the reversal field distribution of the sample with the higher Nd-Cu content (narrow along H_c, broad along interaction field (H_u)) is typical of a high coercivity homogeneous system in which the grains have approximately the same coercive field values and the distribution of H_u is essentially equal to the distribution of demagnetizing field.^6,7 Altogether, the change in FORC behavior from low Nd-Cu content to high Nd-Cu content characterizes simply the associated increase in coercivity due to shell formation.

Structural and magnetic characterization has been carried out on Ce-based R-Fe-B based core-shell magnets fabricated by infiltration of Nd-Cu into hot-pressed magnets. The increase in coercivity achieved for a given Nd content of the R₂Fe₁₄B phase is attributed to two factors. Firstly, the formation of a shell within the R₂Fe₁₄B grains having higher magnetocrystalline anisotropy than the core, thanks to the partial substitution of Nd for Ce. Secondly, improved magnetic decoupling of R₂Fe₁₄B grains due to the formation of a non-magnetic grain-boundary phase. The homogeneous character of reversal in Nd-richer infiltrated magnets is illustrated by FORC analysis.

This paper is based on results obtained from the “Development of magnetic material technology for high-efficiency motors” program commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.