Improvement of the Magnetic Properties of Nanocrystalline Nd 12 . 3 ( FeZrNbCu ) 81 . 7 B 6 . 0 Alloys with Dy Substitutions

Nd 12.3−x Dy x Fe 81.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 (x = 0–2.5) ribbons have been prepared by melt-spun at 30m/s and subsequent annealing. The influence of addition of Dy on the crystallization behavior, magnetic properties, and microstructure were investigated. Differential scanning calorimeter (DSC) and X-ray diffraction (XRD) revealed a single-phase material. Microstructure studies using transmission electron microscopy (TEM) had shown a significant microstructure refinement with Dy addition. Wohlfarth’s analysis showed that the exchange coupling interactions increased first with Dy content x increasing, reached the maximum value at x = 0.5, and then slightly decreased with x further increasing. Optimal magnetic properties with J r = 1.09T,Hci = 1048 kA/m, and (BH)max = 169.5 kJ/m 3 are achieved by annealing the melt-spun ribbons with x = 0.5 at% at 700C for 10min.


Introduction
Nd-Fe-B magnets markets have been growing [1] and nanocrystalline Nd-Fe-B has been developed as an essential permanent magnet material for industrial applications [2].The nanostructure and the composition of the alloy have an influence on the magnetic properties of the isotropic nanocrystalline Nd-Fe-B permanent magnets; extensive efforts have been made via some special methods [3,4] or adjusting composition to improve the magnetic properties.At present, elemental substitution has been found to be one of the most effective methods for excellent performance.Pr, Zr, Nb, Cu, and Dy substitutions have been widely used in Nd-Fe-B magnets.Studies indicate that Zr or Nd addition can inhibit grain growth, leading to much finer microstructure and higher magnetic properties [5][6][7].Cu and Nb addition also can extend the temperature range of the heat-treatment for optimizing the magnetic properties [8].As we know that the magnetocrystalline anisotropy field (  ) of Dy 2 Fe 14 B (  = 12576.8kA/m) is much higher than the one of Nd 2 Fe 14 B (  = 5572 kA/m) [9], Dy substitutes Nd can improve the magnetocrystalline anisotropy field (  ) and optimize the microstructure of the Nd 2 Fe 14 B matrix and decrease the grain size [10].The researchers in the past have been concentrating on nanocomposite Fe 3 B/Nd 2 Fe 14 B or Nd 2 Fe 14 B/-Fe [11,12].However, the report of the single addition of Dy was rare.In this work, we present our research about the crystallization behavior, microstructure, magnetic properties, and exchange coupling of single phase Nd 12.3−x Dy x Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 ( = 0-2.5)ribbons.

Experiment
The Nd 12.3−x Dy x Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 ( = 0-2.5)ribbons were obtained by applying the melt-spinning technique at a wheel speed of 30 m/s.The ribbon samples with a width of 2-3 mm and a thickness of 40-50 m were isothermally annealed at 600-800 ∘ C for 10 min in vacuum-sealed quartz tubes to crystallize.Phase analysis of the samples was characterized by X-ray diffractometer with Cu-K radiation.temperature was employed using a LDJ9600 vibrating sample magnetometer (VSM).The length direction of the ribbons was parallel to the applied field in order to minimize demagnetization effect.The microstructures were observed by high-resolution transmission electron microscopy (HR-TEM JEM2010) with thin foils prepared by ion-beam thinning.

Results and Discussion
3.1.Phase Analysis.Figure 1 shows the XRD patterns of Nd 10.8 Dy 1.5 Fe 79.7 Zr 0.8-Nb 0.8 Cu 0.4 B 6.0 at different condition.Figure 1(a) shows the alloy has partial crystallization and highlights the weak characteristic peaks of Nd 2 Fe 14 B. This indicates that those ribbons still have many amorphous phases and the grain size of the crystalline phase is small.Figure 1(b) shows XRD pattern of as-cast Nd 10.8 Dy 1.5 Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 alloy annealed at 1050 ∘ C for 10 h.The only crystalline phase in the matrix is 2 : 14 : 1 phase at both the annealed condition and as-spun condition.After the annealing treatment, the Nd 2 Fe 14 B grains were completely formed, and there is no new phase appearing in the grain boundary.This result provides the possibility of the magnetic exchange coupling interaction between the adjacent grains.
Figure 2 shows the differential scanning calorimetry (DSC) curves of the as-spun ribbons with different Dy content at a heating rate of 10 ∘ C/min from room temperature to 800 ∘ C and the thermal properties determined from these DSC thermal scans are summarized in Table 1.Samples with  = 1.5 at% and without other elements of Nd 12.3 Fe 81.7 B 6.0 show one exothermic peak corresponding to the transformation from amorphous phase to 2 : 14 : 1 phase.It is in accordance with the XRD result in Figure 1.Compared with the curve of Nd 12.3 Fe 81.7 B 6.0 , the ribbon with substitution of 1.5 at% Table 1 and Figure 1 show that Dy and other additional elements retarded the onset temperature of crystallization and peak temperature of crystallization.When the temperature reaches 580 ∘ C, the alloy with additional elements has no crystallization.The contrary crystallization phenomenon of improving crystallization tendency of the Mn substitution in Nd 2 Fe 14 B/-Fe nanocomposites was also observed by Xie et al. [13].

Effect of Dy Content on Magnetic Properties of Ribbons.
Since the amorphous phase is undesired, in order to achieve the best magnetic properties for the samples with different Dy content, a thermal treatment at 600-800 ∘ C for 10 min is employed individually to quenched ribbons.Figure 3 summarizes variation of  ci ,   , and (BH) max with Dy content.The intrinsic coercivity  ci significantly increases with increasing Dy content.That is because the magnetocrystalline anisotropy field (  ) of the Dy 2 Fe 14 B (12600 kA/m) is larger than that of the Nd 2 Fe 14 B (5600 kA/m) and they are totally mutual soluble [9].The remanence   of Dy from 0 at% to 0.5 at% doped samples shows little change.Then, the   decreases with increasing Dy content.On the one hand, the saturation magnetization (  ) of Dy 2 Fe 14 B (0.71 T) is smaller than that of the Nd 2 Fe 14 B (1.60 T) [14].As we all know that Nd and Dy are both rare elements and chemical properties, the atomic radius of Dy is larger than that of Nd.When Dy atoms substitute the site of Nd atoms, the volume and the local atomic structure of Nd will be influenced.With  further Dy addition, the effect of Dy on reducing   and   becomes stronger and the remanence decreases.Magnetic energy product (BH) max increases initially and then decreases and reaches the peak value when the content of Dy is 0.5 at%.
When the content of Dy is more than 1.0 at%, (BH) max begins to greatly reduce, due to the deviation of demagnetization curve square degree.The optimal magnetic properties were indicated in Table 2.As can be seen, the addition of Dy can improve significantly the magnetic properties.A small amount of Dy can refine the grains and optimize the microstructure.After the optimum annealing treatment, the coercivity of Nd 10.8 Dy 1.5 Fe 79.7 Zr 0.8-Nb 0.8 Cu 0.4 B 6.0 is enhanced from 775.5 kA/m for  = 0 to 1440 kA/m for  = 2.0.However, with the increase of the Dy content, the remanence is increased first from 0.98 T for  = 0 to 1.09 T  = 0.5 and then decreased to 0.98 T for  = 2.0.The maximum energy product is increased firstly and then decreased.All the maximum energy products of the annealed sample with Dy substitution are larger than 130 kJ/m 3 .Optimum magnetic   properties with   = 1.09T,  ci = 1048 kA/m and (BH) max = 169.5 kJ/m 3 are achieved by annealing the melt-spun ribbons with  = 0.5 at% at 700 ∘ C for 10 min.Compared to the Dyfree sample,  ci ,   , and (BH) max of alloy with  = 0.5 at% are increased by 40%, 11%, and 27%.

Effect of Dy Content on
Microstructure. Figure 4 shows TEM images of the Dy-free and Dy-containing ribbons annealed at 700 ∘ C for 10 min.The average grain size is estimated to be 40 to 60 nm for the Dy-free ribbons and the grain size of the Dy-doped ribbons is significantly finer.The average grain size for the samples with  = 0.5 is estimated to be 30 to 50 nm, and the grains become more uniformly distributed.It implies that the slight amount of Dy is effective to uniform the grains and suppress the growth of the grains during the heat treatment.The fine and homogeneous microstructure is favored for exchange coupling interaction.the crystallization is perfectly complete.No boundary phase is found between the grains.The grain boundaries are totally crystallographically coherent.This is different from the structure of the sintered NdFeB magnets, whose coercivity dramatically depends on the Nd-rich phase between and around the grains.In order to further analyze the large grains sandwiched small grains inside the grains and on the grain boundaries, the Fourier filtered images corresponding to different sections of Figure 5 are shown in Figure 6.As can be seen, either inside the grains or on the grain boundaries, the lattice regular arrangement and the grain boundaries are partly coherent.No second phase or grain boundary phase is found.It also proves that the coercivity mechanism in this series of materials is not similar to the traditional one in the sintered magnets.The magnetic coercivity is probably realized by the effect of the magnetic exchange-coupling interaction between grains.The exchange coupling interaction is often evaluated using  plots [15], which can be defined as () =   () − (1 − 2  ()), in which   is reduced magnetization and   is reduced remanence with respect to the applied magnetic field, .According to Wohlfarth's analysis [16], higher positive () peaks indicate stronger exchange coupling interactions among grains.
Figure 7 shows plots of () for the annealing Nd 12.3−x Dy x Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 ( = 0-2.0)samples.All samples are composed of one magnetically hard phase, and the remanence ratio   /  is larger than 0.5, which indicates the existence of the exchange coupling between grains.It is clear that the magnitude of  peaks increases first with Dy content increasing and reaches maximum value at  = 0.5 and then decreases with further increase of Dy content, indicating that the intergranular exchange coupling effect appears to strengthen with the increase of  up to 0.5 and then diminish with further increase in .The plots of () for Nd 10.3 Dy 2 Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 ( = 2.0) sample has still a high peak.According to Wohlfarth's analysis, higher positive () peaks indicate stronger exchange coupling interactions among grains.The XRD pattern in Figure 1(b) shows that there is only one crystalline phase (Nd 2 Fe 14 B).It

Table 1 :
Summary of thermal properties of the melt-spun ribbons alloys determined from the DSC curves.
: onset temperature of crystallization;   : peak temperature of crystallization.

Table 2 :
Optimal magnetic properties of Nd 12.3− Dy  Fe 79.7 Zr 0.8 Nb 0.8 Cu 0.4 B 6.0 at different experiment conditions.