Fabrication of Mn-Bi Nanoparticles by High Energy Ball Milling

Fabrication of Mn-Bi Nanoparticles by High Energy Ball Milling Nguyen Mau Lam1,2, Tran Minh Thi2, Pham Thi Thanh3, Nguyen Hai Yen3 and Nguyen Huy Dan3,+ 1Hanoi Pedagogical University No. 2, Xuan Hoa, 280000 Vinh Phuc, Vietnam 2Hanoi National University of Education, 136 Xuan Thuy, 100000 Hanoi, Vietnam 3Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, 100000 Hanoi, Vietnam The MnBi low temperature phase with high value and positive temperature coefficient of its coercivity has a potential for production of both the nanocomposite and hybrid permanent magnets. In this report, we present our results of investigation of fabrication of Mn55Bi45 nanoparticles by using high energy ball milling method. The Mn55Bi45 alloy was first arc-melted and then ball-milled for various time of 0.25­ 8 h in different environments of Argon, Alcohol, Petrol and Xylene. The resulted powder was subsequently annealed at temperatures of 200 and 250°C for time periods of 0.5­4 h in Ar gas. The fraction of the MnBi low temperature phase and the size of the particles strongly depend on the fabrication conditions. The desired MnBi nanoparticles with size of 25­100 nm and coercivity ®0Hc > 1T can be achieved by choosing appropriate fabrication conditions. [doi:10.2320/matertrans.MA201577] (Received March 6, 2015; Accepted May 12, 2015; Published June 19, 2015) Keywords: MnBi nanoparticles, high energy ball milling method, hard magnetic material 1. Introduction The intermetallic compound MnBi with NiAs-type hex- agonal crystalline structure has attracted much attention in recent years due to its potential for high temperature and low cost permanent magnets.1­7) The unique feature of the positive temperature coefficient of the coercivity makes the MnBi low temperature phase become a good hard magnetic phase for exchange coupling nanocomposite magnets.8) MnBi nanoparticles can be used to produce hybrid magnets such as MnBi/NdFeB leading to the high coercivity, thermal stability and large operating temperature range for the magnets.9,10) However, the formation of the single MnBi low temperature phase is very difficult, because of the segregation of Mn from the MnBi liquid at the temperature of 719K and the slow diffusion of Mn through MnBi in solid state. Therefore, the optimization of fabrication technology to create a pure MnBi low temperature phase is still concerned to study. Several methods, which have been utilized to fabricate the Mn-Bi magnetic materials, include arc-melting, spark plasma sintering, melt-spinning and high energy ball milling.11­16) Among them, the high energy ball milling method can be used to create desired microstructures for the material. In this work, we investigated influence of technological conditions such as milling environment, mill- ing time, annealing temperature+ on structure and magnetic properties of Mn55Bi45 nanoparticles prepared by high energy ball milling and subsequent annealing. 2. Experimental Procedure The Mn55Bi45 ingots were prepared from manganese and bismuth high purity (99.9%) chips by arc-melting method. Because Mn strongly evaporates during melting process, its mass was compensated by 15% before arc-melting. The Mn55Bi45 ingots were crashed into small pieces and coarsely milled before being placed into the milling-vials. To investigate the influence of the arc-melting process on structure and magnetic properties of the material, a mixture of the raw chips of Mn and Bi with their atomic ratio of 55 : 45 were done similarly to the arc-melted Mn55Bi45 ingots. Hereafter, the samples obtained from the arc-melted ingots and the raw chip mixture are referred to as ones “with arc-melting” and “without arc-melting”, respectively. The coarse powder was then milled on a SPEX 8000D high energy ball mill for various time of 0.25­8 h in different environments of Argon, Alcohol, Petrol and Xylene to obtain nanoparticles. The weight ratio of ball to powder was about 4 : 1. The nano-powder was compressed into cylinders with diameter of 3mm. After that, the samples were annealed at temperatures of 200 and 250°C for time periods of 0.5­4 h. All the arc-melting and annealing processes were performed under Ar atmosphere to avoid oxidization. The structure of the samples was examined by using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. The magnetic properties of the samples were investigated by magnetization measurements on a pulsed field magnetometer. 3. Results and Discussions Figure 1(a) shows XRD patterns of the Mn55Bi45 powder milled for 6 h in Ar gas for both the two cases with and without arc-melting. We can see that diffraction peaks of the MnBi crystalline phase on the XRD pattern of the sample with arc-melting are higher than those of the sample without arc-melting. That means the arc-melting process favored the formation of the MnBi crystalline phase in the material. This is in accordance with the results of the magnetic hysteresis measurements. Both the coercivity and the saturation mag- netization of the sample with arc-melting are higher than those of the sample without arc-melting (Fig. 1(b)). Note that, the hysteresis loops of these two samples were performed on free-powder state (not pressed). Therefore, the coercivity of the samples is reduced in comparison with+Corresponding author, E-mail: dannh@ims.vast.ac.vn Materials Transactions, Vol. 56, No. 9 (2015) pp. 1394 to 1398 Special Issue on Nanostructured Functional Materials and Their Applications ©2015 The Japan Institute of Metals and Materials that of the pressed samples (see Fig. 5). This reduction of the coercivity is due to the rotation of magnetic particles in the free-powder samples during demagnetizing process. The influence of different milling environments on structure and magnetic properties of the Mn55Bi45 powder with milling time of 8 h was investigated. From Fig. 2(a), we can realize that the diffraction peaks appeared on the XRD patterns correspond to the crystalline phases of MnBi, Bi and Mn. The diffraction peaks characterizing for the MnBi phase are observed for the samples which were milled in the environments of Argon, Alcohol and Petrol. While this crystalline phase was hardly formed in the Xylence environ- ment. It should be noted that, the volume fraction of the MnBi phase in Ar gas is highest. This result of structure analysis is in good agreement with that of magnetization measurements for the samples. The highest value for both the saturation magnetization and coercivity was obtained on the sample with the milling environment of Ar gas (Fig. 2(b)). Therefore, Ar gas was chosen as milling environment for the later investigations. Figure 3(a) and 3(b) show SEM images of the Mn55Bi45 powder with the milling time of 0.5 h and 4 h. The SEM images reveal that with the milling time of 0.5 h the average grain size is larger than 50 nm. After milling for 4 h, the grain size is smaller than 50 nm. The results of XRD measurements in Fig. 4(a) show that, the intensity of MnBi peaks decreases when increasing milling time from 0.5 to 8 h. While, their full width of half maximum peak increases with increasing the milling time. The size of the grains with various milling time for the Mn55Bi45 powder was calculated from the XRD data (Fig. 4(b)). It can be seen that the grain size quickly decreases when increasing milling time from 0.5 to 2 h. The smallest grain size of ³25 nm was obtained with milling time of 4 h. With further increasing the milling time (longer than 4 h), the grains size does not continuously decrease but gradually increases. This can be explained that with long milling time (> 4 h), cold welding process might happen leading to the increase of the grain size of the MnBi grains. We can also realize that the values of the grain size determined from the XRD data are smaller than those observed on the SEM images. This probably is due to the clustered characteristic of the fine grains and the SEM technique could not distinguish the individual grains. The clustered characteristic of the grains does not infuluence on the formation of the diffraction peaks, i.e. the grain size determined from the XRD data. Figure 5 exhibits hysteresis loops of the Mn55Bi45 powder with various milling time. We can see that all the samples reveal hard magnetic behavior with quite large coercivity. However, squareness of the hysteresis loops of all the samples is still bad. The dependence of the coercivity ®Hc at room temperature and the saturation magnetization Ms of the Mn55Bi45 powder (a) (b) Fig. 1 XRD patterns (a) and hysteresis loops (b) of Mn55Bi45 powder milled for 6 h in Ar gas for two cases with and without arc-melting. (a) (b) Fig. 2 XRD patterns (a) and hysteresis loops (b) of Mn55Bi45 powder with milling time of 8 h in various environments. Fabrication of Mn-Bi Nanoparticles by High Energy Ball Milling 1395 on milling time tM is shown in Fig. 6. From Fig. 6(a), we can realize that ®0Hc quickly increases from 0.7 to 1.6 T when increasing milling time from 0.5 to 2 h. The highest ®0Hc of 1.7 T was obtained for the sample with milling time of 4 h. The coercivity slightly decreases when increasing milling time from 4 to 8 h. This is in accordance with the results obtained from the SEM and XRD measurements. The grain size of the Mn55Bi45 powder becomes minimum with the milling time of 4 h. As known, in order to get the highest coercivity for a hard magnetic material, it is necessary to pulverize it into fine particles approaching to a single domain size.17) Thus, the milling time of 4 h may be optimal for the highest coercivity and it can be selected to fabricate the MnBi particles. The saturation magnetization shows an increasing tendency with increasing milling time (Fig. 6(b)). The opposite variation tendencies of the coercivity and the saturation magnetization can be explained as the following. When the volume fraction of the MnBi ferromagnetic phase increases, the non-ferromagnetic phases (Mn, Bi) decrease leading to the increase of the saturation magnetization. It means that the density of the ferromagnetic MnBi particles is higher and the exchange interaction between the ferromagnetic particles is stronger leading to the decrease of the coercivity. When the volume fraction of the MnBi ferromagnetic phase decreases, the saturation magnetization consequently decreases. On the other hand, the density of the ferromagnetic particles decreases and these particles are isolated by the non-ferromagnetic phases leading to the increase of the coercivity. The influnence of annealing process on magnetic proper- ties of the material was also investigated. Hysteresis loops of the annealed Mn55Bi45 samples with variation of milling time Fig. 3 SEM images with milling time of 0.5 h (a) and 4 h (b) of Mn55Bi45 powder. Fig. 5 Hysteresis loops of Mn55Bi45 powder with various milling time tM from 5min to 8 h. (a) (b) Fig. 4 XRD pattern (a) and grain size (b) of Mn55Bi45 powder with various milling time. N. M. Lam, T. M. Thi, P. T. Thanh, N. H. Yen and N. H. Dan1396 tM, annealing time ta and annealing temperature Ta are shown in Fig. 7. In general, squareness of hysteresis loop of all the samples becomes better after annealing. For some samples, their coercivity slightly decreases but their saturation mag- netization quite strongly increases after annealing. The highest value of the saturation magnetization Ms and the coercivity ®0Hc achieved on the annealed samples is about 50Am2/kg and 1.1 T, respectively. To simultaneously increase saturation magnetization and coercivity of the material needs further studies. 4. 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