Interaction and Magnetic Properties of NdCeFeB Melt-Spun Ribbons

Abstract

The alloy ingots with nominal compositions of (Nd_1-xCe_x)_2.4Fe₁₄B (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by induction melting and then melt-spun to form nanocrystalline ribbons. Phase composition, magnetic properties and microstructure were investigated. XRD results show that all melt-spun ribbons exhibit the tetragonal structure (Nd,Ce)₂Fe₁₄B phase. When Ce substitution content is more than x=0.6, CeFe₂ phase appears and CeFe₂ content increases with the increase in Ce substitution content. Remanence, remanence ratio (M_r/M_s) and lattice constant decrease while increasing Ce substitution content. A coercivity of 1.31×10⁶ A/m and the maximum energy product of 103 kJ/m³ for (Nd_0.8Ce_0.2)_2.4Fe₁₄B melt-spun ribbon are achieved. The coercivity mechanism and intergrain exchange coupling were studied. The positive δM value was observed in every sample, confirming the existence of exchange coupling interaction. The δM maximum value reaches 0.76 at the Ce substitution content x=0.2, indicating that the intergranular exchange coupling effect is the strongest, which is consistent with the varying remanence ratio. SEM observation reveals that increasing Ce substitution deteriorates the columnar crystal structure of ribbons.

Keywords

NdCeFeB; melt-spun ribbons; microstructure; interaction; magnetic properties

NdFeB magnets are the most widely used permanent magnets due to the outstanding magnetic performance, which strongly rely on the rare earth elements such as Nd, Dy, Pr and Tb^[1–3]. Recently, partly substitution Ce for Nd has attracted more attention because Ce is the most abundant and cheap rare earth element, it cannot only reduce the cost of NdFeB magnets but also balance the utilization of rare earth resources^[4–5]. It has been proved that Ce can restrain grain growth and modify microstructure, which can effectively inhibit the deterioration of magnetic properties when Ce substitution is less than 30wt% of total amount of rare earth^[6–7]. According to Ref.[8–11], the coercivity of Nd-Ce-Fe-B melt-spun ribbons abnormally increases when Ce substitution is around 20wt%, leading to a structural instability according to simulation result^[12–13]; the scattering lattice constant and the change of the Ce valence state are also the direct reason for the abnormal increase^[14]. It has been shown that when increasing the Ce substitution content, the magnetic properties of Nd_12-xCe_xFe₈₂B₆ melt-spun ribbons decrease monotonously^[15]. Yang^[14] tested the recoil loops of (Nd_1-xCe_x)_31.5Fe_67.5B₁ melt-spun ribbons, and the results showed that the recoil loop is fully closed at x=0.2 and the ribbon has a better resistance to reverse magnetic field, suggesting that the magnetic properties are better than those of others. The above results all discussed the effects of Ce substitution on the microstructures and magnetic properties of nanocrystalline Nd-Ce-Fe-B ribbons. However, there are relatively few reports on the coercivity mechanism of nanocrystalline Nd-Ce-Fe-B ribbons. In this work, nanocrystalline (Nd_1-xCe_x)_2.4Fe₁₄B melt-spun ribbons were prepared and investigated in order to explore the influence of Ce content on magnetic properties and coercivity mechanism of Nd-Ce-Fe-B melt spun ribbons.

1 Experiment

The alloy ingots with nominal compositions of (Nd_1-xCe_x)_2.4-Fe₁₄B (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by induction melting. Excess Nd and Ce of 5wt% were added to compensate the mass loss due to evaporation. Each ingot was melt for three times to ensure the homogeneity. The ribbons were obtained directly by induction melting the precursor ingot in a quartz tube, and then ejected onto a surface of a copper wheel with a speed of 10‒40 m/s. Then the ribbons were ground into powder and then examined by X-ray diffraction (XRD) with Cu Kα radiation to determine the crystal structure. The magnetic hysteresis loops at room temperature were measured by vibrating sample magneto-meter (VSM) with a maximum magnetic field of 2.39×10⁶ A/m. The applied field is parallel to the plane of ribbons and no demagnetization correction for the geometry of the sample was made.

2 Results and Discussion

For ribbons prepared by melt-spinning, magnetic properties are strongly dependent on the wheel velocity. Fig.1 shows the hysteresis loops (Fig.1a) and magnetic properties (Fig.1b) of (Nd_1-xCe_x)_2.4Fe₁₄B (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) ribbons prepared at optimum wheel speed. The single hard magnetic phase behavior is shown in melt-spun ribbons; the squareness of hysteresis loop becomes poorer when Ce substitution exceeds x=0.2. With increasing Ce substitution content, the coercivity decreases monotonically. Since the magnetocrystalline anisotropy of Ce₂Fe₁₄B is lower than that of Nd₂Fe₁₄B^[16], the substitution by Ce decreases the magnetocrystalline anisotropy of (Nd_1-xCe_x)_2.4Fe₁₄B, which should be responsible for the decrease in coercivity. Moreover, the coercivity values are consistent with the previous study from Hono^[17] and the typical coercivity values reported for Nd-Fe-B-based melt-spun ribbons are 0.8‒2.3 T. The change in remanence of the ribbons is fluctuant, and at the substitution content of x=0.4, the remanence is a little higher than that at x=0.2. Besides, the maximum energy product of ribbons slightly increases first when the substitution content is no more than x=0.4, and significantly decreases with increase in Ce substitution content. When Ce substitution content is x=0.2, the ribbon reaches the optimal performance: H_ci=1.31×10⁶ A/m, (BH)_max=103 kJ/m³. The value of remanence ratio M_r/M_s of an isotropic magnet composed of uniaxial anisotropy single-domain grains without intergrain interaction is 0.5, as seen in Fig.1b, due to the nanoscale grain size, the values of remanence ratio M_r/M_s are all over 0.5 and peaks appear at Ce substittion of x=0.2, which indicates that the intergranular exchange coupling interaction is strong in (Nd_1-xCe_x)_2.4Fe₁₄B melt spun ribbons. The change of remanence ratio affects the change of the maximum energy product.

Fig.1  Hysteresis loops (a) and magnetic properties (b) of

(Nd_1-xCe_x)_2.4Fe₁₄B ribbons

Fig.2 shows the XRD patterns of prepared (Nd_1-xCe_x)_2.4Fe₁₄B ribbon powders. Obviously, only (Nd,Ce)₂Fe₁₄B phase and CeFe₂ phase emerge in the XRD patterns, no other phases are present, and no Nd-rich phase and α-Fe are found. When the ribbon does not contain Ce (x=0) and Ce substitution is small (x=0.2, 0.4), XRD patterns confirm that (Nd,Ce)₂Fe₁₄B phase is the dominant phase; diffraction patterns of CeFe₂ phase appear when the Ce substitution content is higher (x=0.6, 0.8, 1.0) and the intensity increases with increasing Ce substitution content, which means that the content of CeFe₂ phase increases. The CeFe₂ contents at x=0.8 and x=1.0 are 3.6% and 9.6%, respectively. The CeFe₂ phase is paramagnetic at room temperature, and has a smaller magnetic moment compared to (Nd,Ce)₂Fe₁₄B phase. CeFe₂ content increases with increasing Ce substitution content, which reduces the magnetic properties of the ribbons as shown in Fig.1. Previous studies have shown that the CeFe₂ phase in the magnet is inevitable when the cerium content is high which is generally considered to be harmful to the performance of magnets^[18]. The appearance of CeFe₂ phase and the increasing content CeFe₂ phase with Ce substitution are another main reason for the change of magnetic properties. In addition, as the Ce substitution content increases, the XRD peaks shift to higher degree because the Ce atomic radius is smaller than Nd atomic radius, which indicates that the lattice constants decrease. The lattice constants calculated from XRD data are summarized in Fig.3. It can be seen that the lattice constants and cell volume decrease while increasing the substitution content. The grain size of melt-spun ribbons with different Ce substitution contents was calculated based on Scherrer formula, and the average grain sizes are 49, 43, 46, 47, 49 and 54 nm at x=0, 0.2, 0.4, 0.6, 0.8, 1.0, respectively, as shown in Fig.3. The average grain size varies with the change in Ce content, it is the lowest at x=0.2, and then increases with the increase in Ce substitution content. Zha^[5] found the same grain size variation trend in the (Nd_1-xCe_x)_12.2Fe_81.6B_6.2 melt-spun ribbons.

Fig.2  XRD patterns of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons powder

Fig.3  Lattice constants and grain size of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons powder

As Ce-substituted ribbons have shown significant effects on structure and magnetic properties, their microstructure was analyzed by SEM (Fig.4). It can be clearly seen that the free side of the ribbons is composed of columnar grains; there is a significant difference for these samples. With Ce substitution for Nd by 20%, the columnar grains are finer and uniform; but for other composite samples, columnar grains vary, and some grains are coarse, the microscopic morphology is deteriorated.

Fig.4  SEM images of free-side of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons:

(a) x=0, (b) x=0.2, (c) x=0.8, and (d) x=1.0

Minor hysteresis loops were used to investigate the coercivity mechanism of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons. Fig.5 reveals the dependence of normalized coercivity and remanence in minor loops on the applied field. The inset is the original minor loops measured by VSM. Asymmetric minor loops are observed when the maximum applied field for certain minor loops is lower than the intrinsic coercivity. This phenomenon may be resulted from the pinning effect of domain wall movements during the magnetization process. The normalized coercivity and remanence increase slowly when the applied magnetic field is lower than the coercivity of ribbon. The normalized coercivity increases faster than remanence, which is another evidence for the dominant role of pinning mechanism on the coercivity of ribbons.

Fig.5  Dependence of H_c and B_r on applied field in minor loops of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons: (a) x=0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8, and (f) x=1.0

To investigate the exchange coupling between the grains, Henkel plot was measured for all samples, which can be defined as δM=[2M_r(H)+M_d(H)]/M_r-1^[19], where M_r(H) is the remanence obtained after the application and subsequent elimination of the applied field H on the thermally demagnetized samples. M_d(H) is obtained after saturation in one direction and a subsequent application and removal of a field H in the reverse direction, and M_r is the remanence for the sample magnetized to saturation. When the value of δM is positive, the exchange coupling interaction plays a main role, which indicates that grain interaction promotes magnetization state. When the value of δM is negative, the long-range mag-netostatic interaction is relatively stronger, which indicates that grain interaction enhances the demagnetization effect. According to the Wohlfarth's analysis, the large amplitude of δM peak reflects the stronger intergranular action among grains. The fields corresponding to the peak of positive δM are comparable to the H_c of samples. Fig.6 displays the δM values of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons. Different maximum values show the difference in intergrain interaction among samples, and with increasing the Ce substitution content, the positive and negative δM value gradually decreases. Positive maximum values of δM are observed for all ribbons at the field around coercivity, indicating that the exchange coupling is very strong in all these nanoscale ribbons, which should be partially responsible for the high remanence ratio for these samples. The values of δM for Nd-Ce-Fe-B ribbons are higher than that for Ce-Fe-B ribbon, and lower than that for Nd-Fe-B ribbon, and δM maximum value reaches 0.76 at x=0.2. The variation of δM is consistent with the variation of grain size, and the fine grain sizes promote the exchange coupling between grains. As the substitution content increases, the grain size gradually increases, the exchange coupling decreases and the long-range magnetostatic interaction becomes stronger and the dipolar interactions appears. The interaction for Nd-Ce-Fe-B alloy will be further investigated.

Fig.6  Henkel plots of (Nd_1-xCe_x)_2.4Fe₁₄B ribbons

3 Conclusions

Phase composition and magnetization reversal behavior of nanocrystalline (Nd_1-xCe_x)_2.4Fe₁₄B (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) melt-spun ribbons are investigated. The ribbons can crystallize in a tetragonal 2:14:1 structure. The high remanence ratio (greater than 0.5) indicates the strong exchange coupling interaction. The magnetization reversal behavior (δM plots) is taken into consideration. The positive δM values are observed in every sample, which confirms the existence of exchange coupling interaction. Evidently, a coercivity of 1.31×10⁶ A/m, and the maximum energy product of 103 kJ/m³ for (Nd_0.8Ce_0.2)_2.4Fe₁₄B melt-spun ribbon are achieved.