Amorphization, Crystallization, and Magnetic Properties of Melt-Spun SmCo_𝟕−𝑥(Cr_𝟑C_𝟐)_𝑥 Alloys

Abstract

Effects of Cr₃C₂ content and wheel surface speed on the amorphous formation ability and magnetic properties have been investigated for melt-spun SmCo_7−𝑥(Cr₃C₂)_𝑥 (𝑥=0.10−0.25) alloys. Ribbon melt-spun at lower wheel speed (30 m/s) has composite structure composed of mostly SmCo₇, a small amount of Sm₂Co₁₇, and residual amorphous phases. The grain size of SmCo₇ phase decreases with the increase of Cr₃C₂ content 𝑥. When melt spinning at 40 m/s, SmCo_7−𝑥(Cr₃C₂)_𝑥 alloys can be obtained in the amorphous state for 0.15≤𝑥≤0.25 with intrinsic coercive 𝐻_ci of the order of 40–70 Oe. DSC analysis reveals that SmCo₇ phase first precipitates from the amorphous matrix at 650^∘C, followed by the crystallization of Sm₂Co₁₇ phase at 770^∘C. Optimal coercivity 𝐻_ci of 7.98 kOe and remanent magnetization 𝑀_𝑟 of 55.05 emu/g have been realized in SmCo_6.8(Cr₃C₂)_0.20 magnet subjected to melt spinning at 40 m/s and annealing at 650^∘C for 5 min. The domain structure of the annealed ribbon is composed of interaction domains typically 100–400 nm in size, which indicates the presence of a strong exchange coupling between the grains.

1. Introduction

Permanent magnet materials capable of operating at elevated temperatures are needed for advanced power systems [1]. Most attention has been paid to the Sm–Co 1 : 7 magnets because of their large coercivity and high Curie temperature [2]. Powder metallurgy method has been used successfully to fabricate Sm(Co,Fe,Cu,Zr)₇ bulk magnets with a coercivity of 10 kOe at 500°C [3]. The microstructure of sintered Sm–Co 1 : 7 magnets consists of 2 : 17R phase as cells are surrounded by 1 : 5H boundary with Zr-rich platelet phases running across cells and cell boundaries. 1 : 5H phase is responsible for enhancement of coercivity by domain wall pinning mechanism.

An alternative route to fabricate nanostructure, high-temperature magnets is mechanical alloying [4, 5]. Sm–Co 1 : 7 nanophase hard magnets with high coercivity and enhanced remanent magnetization are synthesized using mechanically induced amorphization and the crystallization of nanoscale grains during the subsequent annealing processes. Optimal coercivity of 21 kOe and remanent magnetization of 73.4 emu/g have been obtained in Sm_12.5Co_85.5Zr₂ magnet [6].

Besides mechanical alloying, melt spinning has been proved to be another effective route to fabricate nanocomposite permanent magnets, especially in the Nd–Fe–B systems. To obtain nanocrystalline microstructure and high coercivity, it is necessary to make amorphous ribbons first and then crystallize it by annealing. Unfortunately, the amorphous formation ability of Sm–Co alloys is very poor [7]. Thus, the fine microstructure for high coercivity is difficult to realize in melt-spun ribbons. However, it has been shown that a small amount of carbon addition is helpful for the grain refinement in the system of Sm–Co–Hf–C [8], Sm–Co–Nb–C [9], and Sm–Co–Fe–C [7]. Recently, we focus our investigation on the effect of the addition of Cr₃C₂ on the magnetic properties and microstructure of SmCo₇ magnets. It has been found that, even melt-spun at a low wheel surface speed of 20 m/s, the grain size of Cr₃C₂-doped SmCo₇ alloys is significantly reduced from 300–600 nm to below 80 nm [10]. Therefore, in this work, the effect of Cr₃C₂ content and wheel speed on the amorphization behavior of the melt-spun SmCo_7−𝑥(Cr₃C₂)_𝑥 alloys has been further investigated.

2. Experimental Procedure

Alloys with nominal compositions of SmCo_7−𝑥(Cr₃C₂)_𝑥 (𝑥=0.10−0.25) were prepared by arc melting under argon. The ingots were melted four times for homogeneity and an excess of 7 wt.% Sm was added to compensate for the Sm loss during processing. The arc-melted ingots were cut into small pieces and then were melt-spun at 30 and 40 m/s. Thickness of this two types of ribbons was about 30 and 25 micrometer, respectively. The as-spun ribbons were sealed in quartz tube under vacuum and then annealed at 650–800°C for 5 min to crystallize and develop a fine microstructure. The crystal structure of the ribbons was identified by Bruker D8 Advance/Discover X-ray diffraction (XRD) with the Phillips diffractometer using the Co 𝐾_𝛼  radiation. The phase transformation temperatures were determined by differential scanning calorimeter (DSC) at a heating rate of 40 K/min. Hard magnetic properties at room temperature were measured by a Lake Shore 7410 vibrating sample magnetometer (VSM) with a maximum field of 23 kOe. The magnetization of the ribbons could not be saturated using VSM; therefore the maximum magnetization 𝑀^2𝑇 under 20 kOe is used to represent the saturation magnetization 𝑀_𝑠. The magnetic domain structure and corresponding atomic force microscopy (AFM) image were studied using a Digital Instruments NanoScope IIIA D-3000 magnetic force microscope (MFM) at RT.

3. Experimental Results and Discussion

The progress of the amorphization process by melt spinning can be seen through the measurement of relative intensity of the XRD patterns of SmCo₇-type phase. The XRD patterns for SmCo_7−𝑥(Cr₃C₂)_𝑥 (𝑥=0.10−0.25) ribbons melt-spun at 30 m/s are shown in Figures 1(a)–1(d). It is found that only the SmCo₇ phase exists for the ribbon with 𝑥=0.10. Two phases, including SmCo₇ and Sm₂Co₁₇, are detected for a higher Cr₃C₂ substitution. With the increase of Cr₃C₂ content 𝑥, the XRD peaks become significantly lower and accompanied with a broad increase in backgrounds, indicating a considerable decrease in the grain size of the SmCo₇ phase. Figures 1(e)–1(h) show the XRD patterns of SmCo_7−𝑥(Cr₃C₂)_𝑥 ribbons melt-spun at 40 m/s as a function of Cr₃C₂ content. It can be seen that the peaks are found to be broadened and the intensities become significantly lower with the increase of wheel surface speed to 40 m/s, indicating that the alloy is driven towards amorphous structure. For the alloys with 𝑥≥0.15, the crystalline structure disappears completely and an amorphous-type phase is developed progressively in the alloys.

Figure 1 

XRD patterns for SmCo7−𝑥(Cr3C2)𝑥 (𝑥=0.10−0.25) ribbons melt-spun at 30 and 40 m/s.

In addition, it can be seen from Figure 1 that the intensity of diffraction (002) for the SmCo₇ phase is gradually strengthened when Cr₃C₂ content 𝑥 increases from 0.10 to 0.25. This is similar to that observed in SmCo₇Ti and SmCo₅ alloys melt-spun at much lower wheel surface speed of 10–15 m/s. In the SmCo₇- or SmCo₅-type magnets, the intensity of (002) plane is considered as measure of texture. For the ribbon with 𝑥=0.20, the intensity ratio 𝐼₍₀₀₂₎/𝐼₍₁₁₁₎ is 4.04 which is much higher than 3.2 for SmCo₇Ti and 2.9 for SmCo₅ magnets [11]. This indicates that the addition of Cr₃C₂ may favor the alignment of SmCo₇ crystalline grains during melt spinning. Further investigations are needed to understand this point.

Hysteresis loops of the alloys melt-spun at 30 m/s are shown in Figure 2. A systematic change in the shape of the loop with the addition of Cr₃C₂ can be seen and the magnetic properties evaluated from these loops are shown in Table 1. With increasing Cr₃C₂ content 𝑥, the remanence 𝑀_𝑟 of the alloys increases up to a maximum value of 56.78 emu/g at 𝑥=0.20, beyond which it then decreases to 14.52 emu/g at 𝑥=0.25. Meanwhile, the remanence ratio 𝑀_𝑟/𝑀^2𝑇 of the alloys increases from 0.67 at 𝑥=0.10 to 0.76 at 𝑥=0.20 and then decreases to 0.34 at 𝑥=0.25. The 𝑀_𝑟 increases with increasing Cr₃C₂ content which is likely attributed to the stronger intergrain exchange coupling between SmCo₇ phase due to the finer grain size as observed in the broadened XRD patterns.

Table 1 

Magnetic properties of SmCo_7−𝑥(Cr₃C₂)_𝑥 (𝑥=0.10−0.25) melt-spun at 30 m/s.

Figure 2 

Hysteresis loops of SmCo7−𝑥(Cr3C2)𝑥 (𝑥=0.10−0.25) melt-spun at 30 m/s.

On the other hand, the coercivity 𝐻_ci initially increases from 4.2 kOe at 𝑥=0.10 to 5.04 kOe at 𝑥=0.15 and thereafter decreases to 0.25 kOe at 𝑥=0.25. This behavior is attributed to size effect of coercivity in fine grain sizes, that is, from multidomain configuration to superparamagnetic state through single domain size [12]. Another reason for the decrease of coercivity may be ascribed to the formation of minor amorphous phase [13]. In magnetization reversal, the amorphous phase can act as reverse domain wall nucleation site and will decrease the coercivity.

Figure 3 corresponds to the hysteresis loops of the alloys melt-spun at 40 m/s. Those alloys show soft magnetic behavior with narrow hysteresis loops. The coercivity of the as-spun ribbons with 𝑥≥0.15 is found to be very low, ranging from 40 Oe to 70 Oe, and decreases with the increase of 𝑥. A reduction of the amount of 1 : 7 phase and the increase of the amorphous phase are responsible for this low coercivity. Figure 4 presents the DSC curves for crystallization of amorphous SmCo_6.75(Cr₃C₂)_0.25 and SmCo_6.80(Cr₃C₂)_0.20 ribbons. There are two exothermic peaks in both crystallization curves. The first exothermic peak (650°C) can be attributed to the formation of SmCo₇ phase initially from the amorphous phase, and the second one (770°C) is related to the formation of Sm₂Co₁₇ phase. Therefore, the crystallization behavior of this SmCo₇ alloy doped with Cr₃C₂ is that SmCo₇ phase first precipitates from the amorphous matrix at 650°C, followed by the crystallization of Sm₂Co₁₇ phase at 770°C. It should also be noticed that the crystallization behavior of the two alloys with different Cr₃C₂ content is distinctly similar.

Figure 3 

Hysteresis loops of SmCo7−𝑥(Cr3C2)𝑥 (𝑥=0.10−0.25) melt-spun at 40 m/s.

Figure 4 

DSC curves of melt-spun SmCo7−𝑥(Cr3C2)𝑥 (𝑥=0.20,0.25) glassy alloy ribbons.

The crystallization of SmCo_6.80(Cr₃C₂)_0.20 spun at 40 m/s is further studied. Figure 5 shows the typical hysteresis loop of this ribbon heat-treated at 650°C for 5 min. A kink is noted in the demagnetization curve. This indicates that the specimen consists of two magnetic phases with different coercivities. The presence of a small amount of Sm₂Co₁₇ phase may give rise to the observed kink according to the DSC analysis. However, crystallization of the amorphous ribbon does result in an enhancement of magnetization and coercivity with 𝑀_𝑟 of 55.05 emu/g, 𝑀^2𝑇 of 70.99 emu/g, 𝑀_𝑟/𝑀^2𝑇 of 0.82, and 𝐻_ci  of 7.98 kOe. It is well known that carbon is one of the most effective elements for getting amorphous phases. Therefore, carbon addition gives a high amorphous forming ability for this SmCo-based magnet. The enhenced 𝐻_ci and 𝑀_𝑟 obtained by crystallization of the amorphous SmCo_6.80(Cr₃C₂)_0.20 alloys may be due to the appropriate inter-grain exchange interaction between high magnetic anisotropy grains with an optimum grain size and optimum size distributions. Figure 6(a) shows an MFM image of the domain structure of this annealed ribbon, while the corresponding AFM topographic data is presented in Figure 6(b). The AFM image reveals a uniform distribution of the grains respectively, and the average grain size is about 50 nm. The magnetic structure consists of irregular domains typically 100–400 nm in size, displayed as dark and bright areas in the MFM image. This magnetic structure is called multigrain domains or interaction domains which are considered as the result of exchange interactions between the spins of adjacent grains [14]. The occurrence of exchange coupling would result in the structure of large interaction domains and a significant enhancement of the reduced remanence [15]. It also can be found that the domain sizes are considerably larger than the grain size of the specimen. Naturally, the interaction domains are clusters composed of many exchange-coupled grains. Therefore, the grain boundaries would act as domain-wall pinning centers. Considering the magnetic behavior, it could be found that the magnetization variation is irreversible (Figure 5). In such cases domain-wall displacement is much difficult and pinning of the domain walls on the grains boundary plays a dominant role. As a consequence, enhanced coercivity is obtained in this annealed ribbon.

Figure 5 

Hysteresis loops of amorphous SmCo6.8(Cr3C2)0.2 ribbon after annealing at 650°C for 5 min.

(a)

(b)

Figure 6 

MFM image of SmCo6.8(Cr3C2)0.2 ribbon annealed at 650°C for 5 min in the thermally demagnetized state (a) and the corresponding AFM image (b).

4. Conclusions

The amorphous formation ability, crystallization behavior, and magnetic properties of melt-spun and heat-treated SmCo₇ magnets have been studied using X-ray diffraction (XRD), differential scanning calorimeter (DSC), and magnetic measurements. The ribbons melt-spun at 40 m/s exhibit amorphous structure in the range of 0.15≤𝑥≤0.25 for SmCo_7−𝑥(Cr₃C₂)_𝑥 alloys. In the amorphous state, these alloys are soft magnetic with intrinsic coercive 𝐻_ci of the order of 40–70 Oe. DSC analysis reveals that SmCo₇ phase first precipitates from the amorphous matrix at 650°C, followed by the crystallization of Sm₂Co₁₇ phase at 770°C. After optimal thermal treatment, the alloy shows enhanced magnetic properties with 𝐻_ci of 7.98 kOe, and 𝑀_𝑟 of 55.05 emu/g. It also can be drawn that the addition of Cr₃C₂ favors the high degree of alignment of SmCo₇ crystalline grains during melt spinning.

Acknowledgments

This work was supported by the Postdoctoral Science Foundation of China (no. 20080430693) and the Science Foundation of Zhejiang Province (no. Y4080082).