Investigation of fabrication of Mn-Ga-Al/Fe-Co nanocomposite hard magnetic materials

Mn-Ga-Al/Fe-Co nanocomposite hard magnetic materials were combined from hard magnetic and soft magnetic phases. The nanoparticles of hard and soft magnetic phases were fabricated by using high energy ball milling method with milling times of 8 h and 32 h to have size of about 50 nm and 70 nm, respectively. The hard and soft magnetic phases were then mixed together for 1 h with different weight fractions of soft magnetic phase from 5 to 20%. The nanocomposites show quite good exchange-spring coupling of hard and soft magnetic phases. Saturation magnetization and coercivity of the nanocomposites strongly depend on the weight fraction of the soft magnetic phase. The increases rapidly, while the decreases with increasing the fraction of the soft magnetic phase. With appropriate fabrication conditions, the Mn-Ga-Al/Fe-Co nanocomposites can possess emu g−1 and . Maximum energy products, , above 4 MGOe have been achieved for this kind of rare earth-free hard magnetic materials.

also focusing on reducing content of rare earth elements and finding new rare earth-free hard magnetic phases. Among rare earth-free magnets, the Mn-based compounds such as Mn-Bi, Mn-Ga are concentrated to study because they have high potential in practical applications [11][12][13]. The Mn-Bi system was discovered long time ago, but this system was not much concentrated because of the development of the rare earth magnets. The advantage of the Mn-Bi system is large amplitude and positive temperature coefficient of its coercivity. Recently, another Mn-based system, the Mn-Ga compounds have been found to reveal a strong hard magnetic phase, which have a relatively large coercivity of 20 kOe [14][15][16]. In terms of structure, the magnets are divided into micrometer-structure magnets and nanometer-structure magnets. The magnets with nanostructures have been more and more investigated and applied because of their interesting physical properties in nanometer scale. In 1993, the Nd 2 Fe 14 B/αFe nanocomposite material was found [17], which had superior magnetic properties compared to those of previous hard magnetic materials. According to theoretical calculations, nanocomposite hard magnetic materials might have a maximum energy product (BH) max more than 100 MGOe [18] based on a combination of strong coercivity of hard magnetic phases and high saturation magnetization of soft magnetic phases. Recently, the Mn-Ga based hard magnetic materials have been attracting research interests as a new hard magnetic phase for the nanocomposite materials [19][20][21][22][23]. By adding elements such as Cu, Fe, Al… the D0 22 -type hard magnetic phase is formed more easily and its coercivity can exceed 10 kOe, high enough for coupling with the soft magnetic phase. As for the soft magnetic phase of the nanocomposites, Fe-Co alloys are commonly selected because of their high saturation magnetization up to 245 emu g −1 [24,25]. There are several methods to fabricate the nanocomposites including melt-spinning, high energy mechanical milling, spark-plasma sintering... In this report, we present the results of studying Mn 65 Ga 20 Al 15 /Fe 65 Co 35 nanocomposites prepared by high energy mechanical milling and subsequently annealing.

Experiment
The Mn 65 Ga 20 Al 15 hard magnetic and Fe 65 Co 35 soft magnetic phases were prepared by high energy ball milling method from high-purity (99.9%) Mn, Ga, Al, Fe, and Co elements.   Before milling, the Mn 65 Ga 20 Al 15 ingots were fabricated by arc-melting method. The Mn 65 Ga 20 Al 15 and Fe 65 Co 35 alloys were milled with the same ratio of ball/powder of 4/1 but different milling times of 8 and 32 h, respectively, in pure ethanol solvent to limit oxidation and clustering of particles. After milling, the alloy powders were dried in a vacuum chamber for a time period of 12 h. The Mn 65 Ga 20 Al 15 nanoparticle powder was annealed from 500 • C to 700 • C for various times from 2 to 45 min. The Mn 65 Ga 20 Al 15 hard magnetic nanopaticles were mixed into the Fe 65 Co 35 soft magnetic ones with various weight fractions from 5 to 20%. The mixed powder was then pressed and annealed at 650 • C for 15 min to obtain nanocomposite magnets. The milling and mixing processes were carried out on a SPEX 8000D Mixer/Mill and the mixed powder was made into pellets under pressure of 3 ton cm −1 . The structure and particle size of the samples were analyzed by x-ray diffraction (XRD) and scanning electron microscope (SEM) methods. The magnetic properties of the samples were investigated by magnetization measurements on pulsed field and vibrating sample magnetometers.  and D0 22 . The number of diffraction peaks of the MnAl phase is more than that of the Mn 3 Ga phase. Besides, the intensity of the Mn 3 Ga peaks is lower than that of the MnAl phase. From hysteresis loops in figure 2(b), one can see that the Fe 65 Co 35 sample shows a soft magnetic behavior with a high saturation magnetization of 227 emu g −1 and a low coercivity of 73 Oe. As for the Mn 65 Ga 20 Al 15 sample, its saturation magnetization and coercivity are less than 5 emu g −1 and 10 Oe, respectively. Thus, the milling process does not produce a hard magnetic phase for the Mn-Ga-Al alloy.

Results and discussions
In order to improve the Mn 3 Ga hard magnetic phase, the Mn 65 Ga 20 Al 15 powder was annealed at different temperatures. Figure 3 shows the XRD patterns of the Mn 65 Ga 20 Al 15 powder annealed at different temperatures for 15 min.
From the XRD patterns, one can see that, the two crystalline phases of Mn 3 Ga and Mn 3 Al 2 were formed in the samples with the annealing temperatures of 600 • C − 675 • C. Intensity of XRD peak of the Mn 3 Ga hard magnetic phase is higher than that of the Mn 3 Al 2 phase and that of the peak at 2θ = 41.5 • dramatically increases. Thus, the annealing process improved  the D0 22 -Mn 3 Ga hard magnetic phase. When the annealing temperature increases to 700 • C, the number and intensity of the D0 22 -Mn 3 Ga peaks decrease. Along with the reduction of the D0 22 -Mn 3 Ga phase, there appear some XRD peaks corresponding to MnAl phase. From the XRD patterns, we can realize that the optimal annealing temperature for the formation of the hard magnetic phase is from 650 • C to 675 • C.
Hysteresis loops at room temperature of the Mn 65 Ga 20 Al 15 powder annealed in various conditions is presented in figure 4. Most of the hysteresis loops have quite large squareness and coercivity, indicating that the magnetocrystalline anisotropy in the samples is high. The results showed that the optimal annealing temperature for the best magnetic properties is 650 • C. The results are consistent with those obtained on the XRD patterns (figure 3). Figure 4(b) shows the hysteresis loops of the samples annealed at 650 • C for various times. The results of our hard magnetic properties obtained on these Mn-Ga-Al samples are similar to those of other research groups on the same material [20,23,26]. From the obtained results, we selected the annealing temperature of 650 • C and To produce the Mn-Ga-Al/Fe-Co nanocomposite magnets, the Mn 65 Ga 20 Al 15 powder was mixed with the soft magnetic Fe 65 Co 35 powder with various weight fractions. After pressing into pellets, the mixed powder was annealed at 650 • C for 15 min. Figure 5(a) shows hysteresis loops of the nanocomposite samples with 5, 10, 15 and 20 wt% of the Fe 65 Co 35 soft magnetic phase. One can see that the saturation magnetization M s of the nanocomposites increases with increasing the weight fraction of the soft magnetic phases while their coercivity H c rapidly decreases. The hysteresis loops of the nanocomposites with high fraction of the soft magnetic phase are sunken. The sunken level of the hysteresis loops increases with increasing the fraction of the soft magnetic phase. This can be explained as follows. According to theoretical models, Skomski-Coey and Kneller-Hawig [17,18], for nanocomposite materials, to have good interactions between the two hard/soft magnetic phases, the optimal particle size is about 10 nm. In this work, the particle size of the samples prepared by the high energy ball milling method is rather larger (50-70 nm). Thus, the particle size of the hard and soft magnetic phases is not optimal. Therefore, a part of the crystalline particles of the hard magnetic and soft magnetic phases has no interaction between them but exists independently. Figure 6 presents the maximum energy product (BH) max of the nanocomposite samples with various weight fractions of the soft magnetic phase. Table 1 shows saturation magnetization, M s , coercivity, H c , and maximum energy product, (BH) max , of the nanocomposites with various fractions of the soft magnetic phase. One can see that the saturation magnetization monotonously increases from 32 to 54 emu g −1 , while the coercivity fast decreases from 12 to 1.7 kOe with increasing the weight fraction of the soft magnetic phase of Fe 65 Co 35 . As for the maximum energy product, its value reaches a maximum of 4.3 MGOe at 15 wt% of the soft magnetic phase. To enhance (BH) max , the optimal size of the magnetic nanoparticles should be found to make the best exchange-spring interaction for the material.
In comparison with other rare earth-free hard magnetic materials, our Mn-Ga-Al/Fe-Co nanocomposite magnets have moderate magnetic parameters. For examples, the Co-C based hard magnetic phases (Co 2 C, Co 3 C, …) have H c of about 3 kOe [27,28], the Hf-Co based ones (HfCo 7 , Hf 2 Co 11 B, …) have H c of about 4.5 kOe [29,30]. The maximum energy product of these rare earth-free hard magnetic materials is commonly less than 5 MGOe.

Conclusions
We have successfully fabricated the Mn 65 Ga 20 Al 15 hard magnetic nanoparticles with coercivity of 12 kOe and the Fe 65 Co 35 soft magnetic nanoparticles with a high saturation magnetization of 227 emu g −1 by high energy mechanical milling method. The magnetic properties of the Mn 65 Ga 20 Al 15 /Fe 65 Co 35 nanocomposites were investigated with variation of the fraction of the soft magnetic phase from 5 to 20 wt%. The maximum energy product of 4.3 MGOe has been achieved for these rare earth-free nanocomposites.