Cerium-based RCo5 (R = Ce, La0.35Ce0.65, and misch-metal) type nanocrystalline hard magnetic materials with high coercivity

Nanocrystalline RCo5 (R = Ce, La0.35Ce0.65, and misch-metal noted as MM) ribbons with hexagonal crystal structure and an average grain size of 5 nm have been prepared via a one-step melt-spinning technique. Coercivity as high as 13.0, 13.8, and 10.9 kOe has been obtained at 300 K for the CeCo5, La0.35Ce0.65Co5, and MMCo5 ribbons, respectively. High thermal stability is also achieved as shown by the high coercivity of 9.3 kOe, 10.2 kOe, and 8.8 kOe at 400 K for CeCo5, La0.35Ce0.65Co5, and MMCo5 ribbons, respectively. The coercivity mechanism is studied by magnetization analysis and microstructural observations. The nanocrystalline grains promote a strong exchange interaction, as indicated by the positive δM and the relatively high remanence ratio (∼0.8). In addition, the temperature dependence of coercivity of RCo5 ribbons shows the low coercivity temperature coefficient of −0.2% to −0.25%/K. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5104295., s


I. INTRODUCTION
Among various rare-earth transition-metal intermetallic compounds, RCo5 (R = rare-earth elements) compounds are particularly interesting due to their extremely high magnetocrystalline anisotropy and very high Curie temperature. 1,2 The SmCo5 compound has the highest uniaxial magnetocrystalline anisotropy (20 × 10 7 erg/cm 3 ) and is the major phase of the first generation of rare-earth permanent magnets. 2 In comparison to SmCo5, the other RCo5 compounds possess similar inherent magnetic properties, however they have not been used as permanent magnets because of technical difficulties in achieving proper microstructures necessary for developing high coercivity. For instance, even though CeCo5 exhibits quite high magnetocrystalline anisotropy (7.2 × 10 7 erg/cm 3 ), 1,3,4 the reported magnetic coercivity reaches only 3%-5% of the corresponding anisotropy field (H k ). 2,5 Recent research for the new economical alternatives to the current commercial rare-earth permanent magnets showed that despite having the mixed Ce 3+ /Ce 4+ valency problem, 6 typically adverse for the magnetocrystalline anisotropy, the Ce-substituted Nd 2 Fe 14 B magnet systems are comparable with the commercial high-flux grade magnets and also have lower material cost. 6,7 Attempts have also been made to replace Sm by Ce or La, or other low cost mixed rare-earths like MM (25-35 wt. % La, 45-55 wt. % Ce, 4-10 wt. % Pr, 14-18 wt. % Nd) in SmCo5-type magnets. 8,9 Unfortunately, the coercivity of the prepared RCo5 hard magnetic materials is low. 2,5,9 ARTICLE scitation.org/journal/apm An alternative magnet is the Sm 2 Fe 17 N 3 phase due to its high saturation magnetization (1.54-1.57 T), anisotropy field (21-26 T), and Curie temperature (∼470 ○ C). 10 More importantly, the Sm 2 Fe 17 N 3 phase has been obtained in powder form by the nitrogenation of Sm 2 Fe 17 alloy powder. However, the conventional sintering techniques cannot be applied to the production of magnets because the Sm 2 Fe 17 N 3 phase lacks stability at high temperatures and decomposes into α-Fe and SmN. It is therefore essential to develop new magnetic materials for more applications in modern products which require strong permanent magnets. In this study, we have prepared nanocrystalline RCo5 (R = Ce, La 0.35 Ce 0.65 , MM) ribbons with hexagonal phase and an average grain size of 5 nm via a one-step melt-spinning technique. The average nanocrystal size is calculated by using a statistical analysis of 70-100 nanocrystals. Magnetic properties of the ribbons are investigated systematically, and the results show high promise for the Ce-based RCo5 magnets for future applications with high performance and low cost.

II. MATERIALS AND METHODS
RCo5 (R = Ce, La 0.35 Ce 0.65 , MM) ingots were prepared by arcmelting in a high purity argon gas atmosphere. The purity of starting materials is 99% for Ce and La 0.35 Ce 0.65 and MM = 28.27 wt. % for La, 50.46 wt. % for Ce, 5.22 wt. % for Pr, 15.66 wt. % for Nd, ≤ 1 wt. % for others, and 99.95% for Co. The ingots were melted five times to ensure homogeneity, and then a part of the ingot was ground down into powders of less than 150 μm in particle size. The phase structure was determined by X-ray powder diffraction (XRD) with Cu Kα radiation. The RCo5 ribbons were prepared by induction melting the ingot in a quartz tube with a nozzle size of 0.1 mm and then ejected onto the surface of a water-cooled copper wheel with a surface velocity of 40-60 m/s. The grain size of the RCo5 ribbons was determined by transmission electron microscopy (TEM). Magnetic properties were measured by a DynaCool physical property measurement system (PPMS) with a maximum field of 90 kOe and in the temperature range of 5-500 K.

III. RESULTS AND DISCUSSION
The art of making high coercivity magnetic materials is to control the microstructure at the nanoscale. 11,12 The HC value will increase with decreasing grain size until it reaches the level of the ferromagnetic domain wall width. 11 Thus, to reduce the grain size is of paramount importance for obtaining high performance in hard magnetic materials. Rapid quenching via melt-spinning is an effective way to prepare ultra-small-grain materials. However, the grain size of RCo5 ribbons prepared via the regular melt-spinning technique is in the range of 50-100 nm. 13 In order to refine the grain size down to few nanometers, the melt-spinning speed is increased to 50 m/s. 14 Because the speed 50 m/s is almost the limitation of the melt-spinning facility we used, we have reduced the nozzle size of the quartz tube from the traditional size (∼0.5 to 1 mm) to 0.1 mm, which resulted in a faster cooling rate and much smaller grains. fast cooling process that impeded the formation of impurity phases. 15 In addition, all the ribbons give broader diffraction peaks, which is expected and is attributed to the grain refinement. 5,15 Transmission electron microscopy (TEM) was performed in order to investigate the grain size and the uniformity of the as-prepared RCo5 ribbons. The TEM image and the corresponding nanocrystal size distribution histogram [shown in Figs. 2(a) and 2(b), respectively] demonstrate that the CeCo5 ribbons consist of nanocrystalline grains of average size around 5 nm. Figure 2(c) shows the selected area diffraction (SAED) pattern of CeCo5 ribbons. The SAED pattern indicates a set of rings corresponding to the reflection from the crystal planes of the CeCo5 phase. The detailed structure of the ribbons is further analyzed using the HRTEM image. coherent rotation of the magnetization which has been observed in the prepared RCo5 ribbons. Figure 3(a) shows the temperature dependence magnetization loops of the CeCo5 ribbons. The saturation magnetization (MS) of 62 emu/g and coercivity (HC) of 13 kOe at 300 K are obtained in the CeCo5 ribbons. It is worthwhile to mention that the achieved HC value is 1.5-2 times higher than that of the CeCo5 prepared by traditional methods. 2,5,16 The high coercivity in the CeCo5 ribbons can be ascribed to the existence of noncoupled CeCo5 nanocrystals, which is caused by the amorphous Ce-Co phase formed at the grain boundaries. 17 In order to understand the coercivity mechanism, the temperature dependence of HC and the dependence of δM on the reverse field (Henkel plot) are investigated. According to the magnetic hardening mechanism by Kronmüller et al., the intrinsic coercivity strongly depends on the microstructure of the magnet, which can be phenomenologically expressed by the following equation: 18,19 where K 1 , N eff , and MS are the first-order magnetic anisotropy constant, the magnetostatic interaction parameter, and the saturation magnetization, respectively. The coefficient α k reflects the local reduction of crystal anisotropy of inhomogeneous surface regions and generally incorporates the effect of the sample microstructure.
The αex describes the adverse effect of exchange coupling between neighboring grains on the coercivity. The microstructural parameter α k αex has not been analyzed separately in nanocrystalline alloys, but several researchers have shown that the α k αex ≥ 0.80 for a nonexchange coupled magnet with nearly perfect grain surface. 20 However, it decreases with an increase in the intergranular exchange coupling. 19 These parameters can be determined by linear fitting Fig. 3(b)]. The temperature dependent values of K 1 and MS are taken from the previous work on the CeCo5 single crystal. 3 The values of α k αex and N eff are obtained by using a linear fit, and they are 0.24 and 0.19, respectively. As expected for nanocrystalline melt-spun alloy ribbons, the microstructural

ARTICLE
scitation.org/journal/apm parameters obtained are somewhat smaller values, which is mainly due to the refined grain size. The uniform nanocrystalline grains lead to enhanced exchange interaction and consequently decrease the local stray fields. In fact, for the noninteracting nanocrystalline systems, the remanence ratio, MR/Ms = 0.5. However, in our case, the MR/Ms ratio is 0.8, which indicates the exchange coupling between the nanograins and amorphous Fe-Co alloys within the CeCo5 ribbons. It is worth mentioning that the coercivity mechanism of the nanocrystalline exchange-coupled magnets cannot be completely determined by the above approach because of intergranular interactions among the fine grains. Furthermore, since the average grain size of the prepared CeCo5 ribbons is 5 nm, smaller than a typical domain size, therefore, both "nucleation" and "pinning" mechanisms are not applicable. Although a direct measurement is not possible under the current condition, it is likely that the coercivity is resulted from the magnetization reversal as single domain structure in isolated magnetic nanocrystals. On the other hand, we realized that mechanisms of coercivity in nanostructured ferromagnetic materials are not well understood yet. Some related phenomena like interaction domains cannot be directly characterized in a quantitative way. It is therefore necessary to carry more investigations on this topic in the future.
To further confirm the exchange interactions between the CeCo5 nanograins, Henkel plots (δM-H) were measured. The expression for δM is defined as δM =M d (H) − (1 − 2MR(H)), 11,19,24 where M d is the reduced initial demagnetization remanence and MR is the reduced initial remanence. The δM curve with a positive peak indicates magnetic exchange coupling between the grains which supports the magnetized state, whereas a negative peak indicates magnetostatic interactions that tend to demagnetize the system. In our case, the δM value for CeCo5 ribbons [ Fig. 3(c)] is initially positive and reached 0.3 when the reversal field increased to 10 kOe. With a further increase of the reversal field, the δM becomes slightly negative due to the dominating contributions from the magnetostatic interactions. The positive δM value indicates the existence of the exchange type of interaction among the grains. Moreover, for the prepared CeCo5 ribbons, the average grain size is around 5 nm, which is larger than the domain wall width π √ A/ K 1 ≈ 3.7 nm. As in our case, the average grain size is slightly larger than the domain wall width and the exchange coupling impedes the magnetization to follow the local anisotropy axis and hence results in a high MR/Ms ratio. 12,21 It should be mentioned that the high MR/MS ratio could also result from the strong exchange interaction between the CoCo5 grains and amorphous boundary like the hard/soft exchanged coupling magnets. Moreover, the amorphous Ce-Co phase possesses an MS value comparable to that of CeCo5 phase; 22 hence, the exchanged coupling could be beneficial for high remanence.
The temperature dependent coercivity analysis for CeCo5 and La 0.35 Ce 0.65 Co5 ribbons shows a linear increase in coercivity with the decrease in temperature from 500 K to 50 K [see in Fig. 3(d)]. However, the coercivity values for MMCo5 ribbons show a completely different temperature dependence as compared to that of Ce-based RCo5 ribbons. Especially, when the temperature is lower than 200 K, the coercivity values even show a slight drop, which is mainly due to the easy-plane anisotropy of NdCo5 constituent. 3 The corresponding temperature coefficient of coercivity is calculated by the following formula: 23,24 The calculated result shows that the β for CeCo5, La 0.35 Ce 0.65 -Co5, and MMCo5 ribbons are ∼−0.25%, −0.26%, and −0.2%/K between 300 K and 500 K, which are lower than that of the commercial Nd-Fe-B sintered magnets (−0.6%/K) and the high-coercivity hot-deformed Nd-Fe-B permanent magnets (−0.45%/K). 24 At a high temperature (450 K), the coercivity of RCo5 ribbons exceed 7 kOe (Table I), which is close to the coercivity of 8% Dy containing Nd-Fe-B magnets (NMX-36). 24 Especially, the coercivity shows a point of deviation from the linear increase at 5 K. Unfortunately, the low temperature behavior has not been studied in detail, and most of the reports about the magnetocrystalline anisotropy field of CeCo5 are larger than 77 K. 3 In order to explore this phenomenon, we measured the temperature dependence of ac susceptibility χac of RCo5 ribbons from 5 K to 160 K with ac amplitude 5 Oe and frequency 4997 Hz, which is shown in the inset of Fig. 3(d). It can be seen that a broad peak appears at the temperature of 32 K. It is known that the position of the peak of χac represents the weakest magnetocrystalline anisotropy at that temperature. Thus, it is often used for detection of the change temperature of easy-magnetization direction (also called spin reorientation). Thus, the deviation of HC from the linear trend at 5 K is mainly due to the changes of magnetocrystalline anisotropy field from spin reorientation. Meanwhile, it demonstrates that the spin reorientation temperature (Ts) of CeCo5 and La 0.35 Ce 0.65 Co5 is about 32 K, which is lower than that of Nd-Fe-B and hence beneficial for the low temperature applications. The high coercivity, low coercivity temperature coefficient, and low Ts of the prepared Cebased RCo5 ribbons imply a great potential of the application for abundant rare-earth based permanent magnets. Although the prepared RCo5 ribbons showed a larger coercivity compared to the traditional preparing methods, the HC value is still smaller than 20% of the corresponding anisotropy field (H k ). A precise control of nanocrystals size and shape can be utilized to improve the coercivity further.

IV. CONCLUSION
The ultrafine granular RCo5 (R = Ce, La 0.35 Ce 0.65 , MM) ribbons with high coercivity have been prepared by one-step melt-spinning technology. The strong exchange-interaction among the nanograins of 5 nm in average size leads to the enhanced MR/Ms ratio. The room temperature coercivity of CeCo5 and La 0.35 Ce 0.65 Co5 is 1.5-2 times higher than that of these magnets prepared by the traditional methods. More importantly, the coercivity values exceeding 7 kOe at the temperature 450 K and coercivity temperature coefficients of −0.2% to −0.25%/K are achieved, which are comparable or superior to the 8% Dy containing Nd-Fe-B magnets. The high coercivity and low coercivity temperature coefficients imply the great potential for developing the abundant rare-earth based permanent magnets.