Comparison on the coercivity enhancement of sintered NdFeB magnets by grain boundary diffusion with low-melting (Tb, R) 75 Cu 25 alloys (R= None, Y, La, and Ce)

A significant coercivity enhancement of the commercial NdFeB magnets with the magnetic properties of (BH) max = 48.4 MGOe and i H c = 17.5 kOe through grain boundary diffusion (GBD) with low-melting Tb 55 R 20 Cu 25 alloys is demonstrated. Adopting Tb 55 R 20 Cu 25 alloys as GBD sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively. Yet, (BH) max is slightly reduced to 46.2-48.2 MGOe. The preferential appearance of Cu at grain boundary and triple junction of the grains, and the core-shell structure occurred due to Tb at grain surface remarkably enhance the coercivity. Interestingly, higher coercivity enhancement per wt% Tb usage ( Δ i H c /wt%Tb) of 7.2 kOe/wt% for the magnet with Tb 55 Ce 25 Cu 25 than 5.9 kOe/wt% for that with Tb 75 Cu 25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La. Lower melting point (637 ○ C) for Tb 55 Ce 20 Cu 25 than Tb 75 Cu 25 (743 ○ C) leads to larger diffusion depth of Tb into the magnet and therefore contributes to higher efficiency of coercivity enhancement for the magnet with R=Ce.


I. INTRODUCTION
NdFeB sintered magnets exhibit the highest energy product ((BH)max) among all developed permanent magnets due to the outstanding magnetically intrinsic properties of Nd 2 Fe 14 B at room temperature (RT). Therefore, NdFeB sintered magnets have been applied in lots of fields, such as tablets, electric vehicles, wind generators, etc. [1][2][3][4][5][6][7][8] However, low Curie point (Tc) leads to sharp decrease of intrinsic coercivity ( i Hc) with the temperature and causes a difficulty for high temperature applications. [1][2][3][4][5][6][7][8] Traditionally, Dy or Tb is added to the Nd-Fe-B magnets to enhance i Hc for the applications at higher temperature. Since the price surge of critical RE elements in 2011 and also the trade war arises recently, how to reduce the usage of Dy or Tb in the magnets while keeping high coercivity for practical applications becomes the crude issue again. Nevertheless, the grain boundary diffusion (GBD) technique was developed to conquer this challenge. [9][10][11][12] GBD sources, which have been reported to significantly enhance coercivity, include heavy RE (HRE) compounds, HREbased, 13 Di 16 mixed TbH 2 powders and Al powders as a GBD source, and the coercivity of the NdFeB sintered magnets with thickness of 6.5 mm was increased from 13.7 kOe to 23.3 kOe after proper GBD process.
In order to reduce the usage of Tb and enhance coercivity of NdFeB sintered magnets, Tb75Cu 25 , Tb55Y 20 Cu 25 , Tb55La 20 Cu 25 and Tb55Ce 20 Cu 25 alloy powders are adopted as diffusion sources in this study. Magnetic properties and microstructure of NdFeB sintered magnets through GBD treated with Tb55R 20 Cu 25 (R = None, Y, La, and Ce) alloy powders are explored.

II. EXPERIMENT
Commercial 48H sintered NdFeB magnets with size of 7×7×5 mm 3 were used to be the original magnets. Tb55R 20 Cu 25 alloys (R = None, Y, La, and Ce), prepared by arc melting in the Ar atmosphere and then melt-spinning, were pulverized to fine powders with diameter smaller than 100 μm by ball milling. Magnets, covered with 2 wt% Tb55R 20 Cu 25 powders at both the top and the bottom surfaces perpendicular to the field alignment direction, were GBD treated at 900 ○ C for 6 hrs and subsequently annealed at 500 ○ C for 3 hrs in vacuum better than 5×10 −6 torr. Melting points of Tb55R 20 Cu 25 alloys were measured by differential thermal analysis (DTA). Magnetic properties at various temperatures were measured by a B-H tracer (NIM-2000). Microstructures and element distributions were observed by field emission electron probe microanalyzer (FE-EPMA, JEOL JXA-8500F). Figure 1 shows DTA scans of Tb55R 20 Cu 25 alloys. It is seen that the melting points of Tb75Cu 25 , Tb55Y 20 Cu 25 , Tb55La 20 Cu 25 , and Tb55Ce 20 Cu 25 alloys are 743 ○ C, 753 ○ C, 654 ○ C, and 637 ○ C, respectively. La or Ce addition could substantially reduce the melting points from 743 ○ C to 654 ○ C and 637 ○ C, respectively. According to Fick's second law, 20 lowering melting point of diffusion source could reduce the activation energy and thus promote diffusion process not only along GB but also through grain interior (GI). In other words, low-melting-point diffusion source alloy is advantageous to diffuse into the deeper interior of the magnets possibly.

III. RESULTS AND DISCUSSION
The demagnetization curves of the original and diffused magnets measured at RT are shown in Fig. 2. The magnetic properties of those magnets are summarized in Table I. After GBD with Tb75Cu 25 , i Hc of the magnet is significantly increased from 17.5 kOe to 29 kOe, but Br is slightly decreased from 14.0 kG to 13.6 kG. When replacing 20 at% Tb with R in Tb55R 20 Cu 25 as a GBD source, an obvious coercivity enhancement is also found. The coercivity increment (Δ i Hc) is comparable for the sample with R = Ce, but slightly inferior for those with R = Y and La to the magnet with Tb75Cu 25 .  Figure 3 depicts the temperature-dependent coercivity for the original and diffused magnets. The decrease of i Hc with temperature (T) results from the reduction of magnetocrystalline anisotropy field (Ha) for 2:14:1 phase with T. Larger decrement of i Hc with T is found for the 48H magnet than GBD-treated magnets. The i Hc and temperature coefficient of coercivity (β) at 180 ○ C of the 48H magnet are 3.2 kOe and −0.52%/ ○ C, and they are improved to 7.8-8.9 kOe and β in the range from −0.41 to −0.45%/ ○ C by GBD with

ARTICLE scitation.org/journal/adv
Tb55R 20 Cu 25 , respectively. Large i Hc and β for GBD treated magnets reveals better thermal stability and thus suitable applications at high temperature of 180 ○ C. Figure 4 shows EPMA images of (Tb, R)75Cu 25 (R = None, Y, and Ce) diffused magnets at depth of 100 μm from diffusion surface. As shown in (a1)-(d1) of Fig. 4, the core-shell structure is found for Tb75Cu 25 and Tb55R 20 Cu 25 (R = Y, La, Ce) diffused magnets. The element mapping analysis indicates that the shells mainly consist of (Nd, Tb) 2 Fe 14 B phase. Because of high Ha for Tb 2 Fe 14 B, the core-shell structure can effectively improve the coercivity of NdFeB magnet with a slight Br reduction. As shown in (a2)-(d2) of Fig. 4, Tb75Cu 25 diffused magnet has thicker shells than Tb55R 20 Cu 25 (R = Y, La, Ce) diffused magnets. For the former magnet, according to Fick's second law, 20 Tb can diffuse into deeper region and form core-shell structure in the magnet interior due to higher Tb concentration. This explains why higher i Hc obtained for Tb75Cu 25 diffused magnet. On the other hand, the elemental mapping results show that a thinner network distribution of Tb is found for Tb55La 20 Cu 25 and Tb55Ce 20 Cu 25 diffused magnet. Nevertheless, thinner (Nd, Tb) 2 Fe 14 B shell could also substantially curb the formation of reversed domain yet with lower decrement in Br than that in Tb75Cu 25 diffused magnet.
As shown in (a3)-(d3) of Fig. 4, it is found that Cu prefers to appear at the triple junction and GB for all the studied magnets. On the other hand, Y, La and Ce almost distribute at GB for Tb55Y 20 Cu 25 , Tb55La 20 Cu 25 and Tb55Ce 20 Cu 25 diffused magnets, respectively, shown in (b4)-(d4) of Fig. 4. Even though the anisotropy field of the core-shell phase may be reduced by the distribution of Y, La and Ce at GB and the triple junction, lower melting point of the GB phase for Tb55Y 20 Cu 25 , Tb55La 20 Cu 25 and Tb55Ce 20 Cu 25 diffused magnets could smoothen GB and inhibit the nucleation of reversed domain. In addition, the magnetization of GB phase is reduced which may reduce the coupling effect between grains, and accordingly contribute to the coercivity enhancement. Part of Y is observed to enter into the 2:14:1 grains. The entrance of Y into 2:14:1 phase could lead to lower i Hc for the sample with R = Y but improve thermal stability of the magnet, since Ha of Y 2 Fe 14 B phase is slightly increased with temperature in the range of 300-420 K. 3 For Tb55La 20 Cu 25 diffused magnet, the amount of La distribution at GI and GB in the magnet is in between Y for the sample with Tb55Y 20 Cu 25 (more Y in GI) and Ce for that with Tb55Ce 20 Cu 25 (prefer at GB), shown in (b4)-(d4) of Fig. 4, which results in intermediate coercivity possibly related to the in-between melting point of Tb55La 20 Cu 25 .
To enhance the coercivity of GBD NdFeB magnet, increasing Ha of the core-shell of 2:14:1 grains and reducing the coupling effect between grains are critical. In this study, not only the core-shell structure with thinner (Nd, Tb) 2 Fe 14 B shell but also continuous Ce distribution at GB makes the magnets GBD with Tb55Ce 20 Cu 25 exhibit better magnetic properties than that diffused with Tb55Y 20 Cu 25 . The former effect could effectively inhibit the nucleation of reversed domain, and the latter is helpful to reduce the magnetization coupling effect between grains. Accordingly, the magnet through GBD with Tb55Ce 20 Cu 25 is much cost-effective in enhancing i Hc and persisting high Br and (BH)max, related to lower melting point of GBD source.

IV. CONCLUSIONS
Coercivity enhancement of sintered NdFeB magnets by GBD with various low-melting (Tb, R)75Cu 25 alloys (R = None, Y, La, and Ce) are compared. Ce addition into TbCu alloy as a GBD source could promote the formation of core-shell structure with thinner (Nd, Tb) 2 Fe 14 B shell in the magnet deeper interior possibly related to its lower melting point. Furthermore, Ce prefers to distribute over GB after GBD process and could reduce the ferromagnetic properties of the GB phase and thus reduce exchange coupling effect between the 2:14:1 grains. Both effects could effectively enhance coercivity of NdFeB magnet. As compared to the magnet GBD with Tb75Cu 25 , the magnets GBD with Tb55R 20 Cu 25 (R = Y, La, and Ce) show better (BH)max, and importantly, that with Tb55Ce 20 Cu 25 exhibits higher Δ i Hc/wt.% Tb, even slightly lower coercivity increment. Besides, the thermal stability of the magnets are also improved by GBD with either Tb75Cu 25 or Tb55R 20 Cu 25 (R = Y, La, and Ce).