NdFeB nanocomposite alloys

Journal of Magnetism and Magnetic Materials 361 (2014) 219–223

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First-order-reversal-curve analysis of exchange-coupled SmCo/NdFeB nanocomposite alloys Mingxiang Pan, Pengyue Zhang n, Hongliang Ge, Nengjun Yu, Qiong Wu Magnetism Key Laboratory of Zhejiang Province, China Jiliang University, Hangzhou 310018, China

art ic l e i nf o

a b s t r a c t

Article history: Received 12 March 2013 Received in revised form 30 October 2013 Available online 5 March 2014

Exchange-coupled SmCo5/Nd2Fe14B nanocomposite magnets have been fabricated by ball milling of the micrometer sized SmCo5 and Nd2Fe14B powders. The influence of Nd2Fe14B content on the microstructure and magnetic properties of these hybrid alloys was investigated. The alloys that show strong intergrain exchange-coupling behavior with (BH)max ¼ 2.95 MGOe was obtained when the two hard phases are well coupled. A first-order-reversal-curve (FORC) analysis was performed for both SmCo5 single-phase magnet and SmCo5/Nd2Fe14B hybrid magnet; the FORC diagrams results show two major peaks for the hybrid magnets. In both cases, the magnetization reversal behaviors for these alloys were discussed in detail and are consistent with the results of δM plots. & 2014 Elsevier B.V. All rights reserved.

Keywords: Exchange-coupled Nanocomposite magnet First-order reversal curve Magnetization reversal

1. Introduction During the last few decades, permanent magnet development has been centered on the production of nanostructured permanent magnets, because of the coercivity enhancement at the single-domain size, approximately in the nanometer scale, while the reduced remanence is also enhanced in nanoscaled isotropic magnets due to the strong exchange coupling effect between the magnetic grains [1]. Unfortunately, each type of the nanostructured permanent magnets has its advantages and drawbacks in their properties, which limits its further application and development. For instance, NdFeB-based magnets have excellent magnetic performance at the room temperature, but poor thermal stability, owing to their low Curie temperature of  580 K. On the other hand, SmCo-based magnets have excellent thermal stability, high Curie temperature and corrosion resistance while their magnetic properties are poor [2]. More recently, in order to fill the large performance gap between the SmCo and NdFeB magnets, hybrid (Sm,Pr)Co/(Nd,Pr) FeB magnets were investigated [3,4]. However, most reported hybrid magnets had limited success so far because of two technical difficulties: (1) the different types of thermal processes applied to the hybrid magnets and (2) the interdiffusion between the two hard phases will take place at the elevated temperatures [3]. For this reason, many researchers have focused on the hybrid PrCo/ PrFeB magnets that have the same rare earth in the component,

n

Corresponding author. E-mail address: [email protected] (P. Zhang).

http://dx.doi.org/10.1016/j.jmmm.2014.02.044 0304-8853 & 2014 Elsevier B.V. All rights reserved.

which can eliminate the interdiffusion and enhance the magnetic properties. However, till date, the literature that systematically reported concerning the magnetization reversal behaviors for hybrid SmCo/NdFeB nanocomposite magnets is limited. Therefore, in the present work, we examine the effect of the NdFeB content on the magnetization reversal processing of the SmCo5/Nd2Fe14B composite system by measuring the first-order-reversal-curve (FORC). On a FORC diagram each magnetic system exhibits a characteristic pattern, which contains detailed information about the magnetic properties [5,6].

2. Experiment procedures The precursor SmCo5 alloy was obtained by melting the element Sm and Co in high argon atmosphere. The alloy ingot was crushed into powder size of  300 μm and was mixed with commercially available Nd2Fe14B powders with the particle sizes from  10 to 50 μm. The two-phase powders were prepared with varying SmCo5:Nd2Fe14B ratios from 1:0 to 1:0.5 in weight. Heptane (99.8% purity) and oleic acid (90%) were used as a solvent and surfactant, respectively, during milling. The two-phase powders were milled for 2 h under argon atmosphere using a highenergy mill of Spex 8000 with a ratio of powder to ball of 1:10. The microstructure of the as-milled powders was determined by X-ray diffraction (XRD) with Cu Kα radiation. Magnetic properties were measured with a Vibrating Sample Magnetometer (VSM) with the maximum applied field of 20 kOe. To analyze the magnetization reversal behavior of the samples after saturation, the magnetization reversal process for the as-milled samples was also measured.

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No correction was made for the demagnetization field effect. Morphology of the samples was investigated using a scanning electron microscope (SEM, ZEISS ∑IGMA) and a transmission electron microscope (TEM, FEI Tecnai G2 F30 S-Twin). The first-order-reversal-curve (FORC) technique has been used to study more details of the magnetization reversal for SmCo5/ Nd2Fe14B nanocomposite alloys. A FORC measurement begins by saturating the sample with a large positive applied field, after which it is decreased to a reversal field named Ha. The FORC is the curve that starts at Ha and proceeds back to positive saturation. A family of FORCs is measured at different Ha [7–9]. The FORC distribution is defined as a mixed second order derivative: ρðH a ; H b Þ ¼ 

1 ∂2 MðH a ; H b Þ 2 ∂H a ∂H b

where M(Ha, Hb) is the magnetization of the sample at the applied field Hb on the FORC with reversal field Ha. Alternatively, ρ can be seen as a function of the local coercivity Hc and bias field Hu after a coordinate transformation: Hu ¼ (Ha þHb)/2 and Hc ¼(Hb  Ha)/2 [8]. Both coordinate systems are discussed in this paper.

3. Experimental results and discussion

Intensity (a.u.)

Fig. 1 shows the XRD patterns of the SmCo5/Nd2Fe14B nanocomposite powders as a function of different Nd2Fe14B contents after the ball milling. The XRD pattern of the ball milled SmCo5 powders used as starting component of the SmCo5/Nd2Fe14B hybrid magnets is also used for comparison. It can be seen (Fig. 1(a)) that the dominant phase in the as-milled SmCo5 powders is CaCu5-type (space group P6/mmm) SmCo5 phase with no oxide phase found in the alloys. With the addition of Nd2Fe14B powders, Fig. 1(b) and (c) shows that only 1:5 and 2:14:1 phases were obtained, which means that the addition of Nd2Fe14B powders has no influence on the single-crystal SmCo5 alloy with the same milling time. The average grain size of the as-milled samples with different Nd2Fe14B contents was close to 20 nm by applying Scherrer's formula. To reveal the microstructure of the hybrid SmCo5/Nd2Fe14B powders by the high-energy mill, the SEM and TEM investigations were conducted on the sample with 20 wt% Nd2Fe14B content (see Fig. 2). The SEM image (Fig. 2(a)) shows that the as-milled sample has the nanoflakes microstructure; the thickness of nanoflakes is in the range of 10–100 nm while their length is 0.6–8 μm. The aspect ratio of these nanoflakes reached as high as 102–103. Fig. 2(b) shows TEM micrographs of the as-milled sample with a grain size in the range of 15–25 nm, which is consistent with the results of XRD measurements.

20

30

40

50

60

70

80

2θ ( degree ) Fig. 1. XRD patterns of the SmCo5/Nd2Fe14B nanocomposite powders as a function of different Nd2Fe14B contents.

Fig. 2. SEM (a) and TEM (b) images of the SmCo5/Nd2Fe14B nanocomposite powders with 20 wt% Nd2Fe14B content.

The XRD and morphology of the SEM and TEM results show that the desired nanoscale hybrid SmCo5/Nd2Fe14B magnets have been achieved by directly ball milling. In order to investigate the resulting magnetic properties VSM measurements were performed. Fig. 3(a) shows the demagnetization curves for the SmCo5/Nd2Fe14B nanocomposite powders with different Nd2Fe14B contents, demonstrating how the Nd2Fe14B content affects the magnetic properties of the hybrid magnets. Meanwhile, Fig. 3 (b) presents magnetic properties of the hybrid SmCo5/Nd2Fe14B magnets as a function of different Nd2Fe14B contents. For a comparison, the magnetic properties of ball milled SmCo5 and Nd2Fe14B single-phase magnets are also shown in Table 1. As one can see, the magnetic parameters of SmCo5/Nd2Fe14B magnets with different Nd2Fe14B contents are lower than single-phase SmCo5, indicating that the magnetic properties of the hybrid magnets are sensitive to the ratio of the two hard phases and confirming the existence of a decoupled two-phase exchangecoupling interaction in the hybrid magnets. Fig. 3 shows further information about the magnetic properties of different Nd2Fe14B contents in the hybrid SmCo5/Nd2Fe14B magnets. It shows that the coercivity (Hcj) decreased monotonically with increasing Nd2Fe14B content, similar to the results observed by Wang et al. [10] for their SmCo5/Nd2Fe14B hybrid magnets. Additionally, the maximum energy product (BH)max and remanence (Mr) show the same tendency, that is they increase first and then decrease with increasing Nd2Fe14B content. The largest (BH)max value of 2.95 MGOe was obtained for the Nd2Fe14B content at 20 wt% in terms of the combined effect of Hcj and Mr. A more detailed

M. Pan et al. / Journal of Magnetism and Magnetic Materials 361 (2014) 219–223

221

0.3 10% 15% 20% 30% 40% 50%

20

0.2

δΜ

Moment (emu/g)

40

free 10% 20% 30%

0.1

0

0.0 -20 -15

-10

-5

-0.1

0

Applied field (kOe)

0

10

20

30

40

H(kOe) 9

3.0

Hcj

Fig. 4. The variation of δm with applied magnetic field of the SmCo5/Nd2Fe14B nanocomposite powders for the Nd2Fe14B content at free, 10 wt%, 20 wt%, and 30 wt%.

40

(BH)max 2.9

2.8 7 2.7

38

36

Mr (emu/g)

H cj (kOe )

8

(BH)max (MGOe)

Mr

6 10

20

30

40

50

2.6

34

NdFeB content (wt%) Fig. 3. (a) Demagnetization curves for the SmCo5/Nd2Fe14B nanocomposite powders with different Nd2Fe14B contents. (b) Variations in intrinsic coercivity (Hcj), maximum energy product (BH)max, and remanence (Mr) on the SmCo5/Nd2Fe14B nanocomposite powders as a function of different Nd2Fe14B contents.

Table 1 Remanence (Mr), intrinsic coercivity (Hcj), and maximum energy product (BH)max of the single-phase SmCo5 and Nd2Fe14B magnets. Magnets

Mr (emu/g)

Hcj (kOe)

(BH)max (MGOe)

SmCo5 Nd2Fe14B

36.53 60.79

9599.21 3479.26

2.97 3.73

analysis about the mechanism of the exchange-coupled SmCo5/ Nd2Fe14B hybrid nanocomposite alloys is discussed as follows. To gain a further insight into different magnetic behaviors of these samples, the interaction between the grains was examined by the use of the so-called δM plot, which can be defined as δMðHÞ ¼ ½M d ðHÞ  M r þ 2M r ðHÞ=M r , where Mr(H) is acquired after the application and subsequent removal of a direct field H, Md(H) is measured after magnetizing at –H on the demagnetization curve and Mr is the saturation remanence [11,12]. According to Wohlfarth's analysis [13], the interaction can be classified as a shortrange intergrain exchange or long-range magnetostatic one, while the positive values of δMðHÞ are due to the intergrain exchange coupling effect and the negative values of δMðHÞ represent the dipolar interaction [14,15]. With the decrease of intergrain exchange coupling, the maximum value of δM drops [11,12]. The δM–H curves of SmCo5/Nd2Fe14B nanocomposite powders for the different Nd2Fe14B contents are shown in Fig. 4. It can be seen that SmCo5 single-phase magnet possesses the higher positive peak height and lower negative value, which indicates the existence of the strong exchange-coupling interactions. With the addition of the Nd2Fe14B in hybrid magnets, it can be seen that the positive and negative peak height values of δM plots show the tendency of increase first and then decrease. More important behavior is that

all the hybrid magnets show the higher positive peak height and lower negative value even when the coercivity is low, which means that the exchange-coupling interaction prevails over the magnetostatic interaction. It is interesting to note that two positive peaks of δM are observed in the hybrid magnets, marked by the arrow in Fig. 4. The first peak may represent Nd2Fe14B grains– intergrain interaction in the low applied magnetic field and the second may represent the SmCo5 grains when the applied field approaches the coercivity of the hybrid magnets. This indicates that these hybrid magnets show an inhomogeneous magnetization reversal behavior, which leads to a deterioration of magnetic properties. With the increase in Nd2Fe14B content, the peak in the low applied magnetic field becomes weak, indicating the effective exchange coupling. The above results show that the different Nd2Fe14B contents lead to quite different coercivity values and magnetic properties, but the enhanced maximum energy product (BH)max was obtained for the hybrid magnets with the Nd2Fe14B content at 20 wt%. This reflects that the SmCo5/ Nd2Fe14B hybrid nanocomposite with the Nd2Fe14B content at 20 wt% shows a higher and stronger exchange coupling strength. To double check the magnetic exchange interactions, a set of approximately 100 FORCs with Hmax ¼20 kOe was measured for the milled alloys. To calculate the ρ(Ha, Hb), we choose the FORC diagrams in this paper which were evaluated with smoothing factor (SF) ¼ 3 [16]. Fig. 5 shows the 2D and 3D FORC diagrams of the SmCo5/Nd2Fe14B nanocomposite powders for the Nd2Fe14B content at free, 10 wt%, 20 wt%, and 30 wt%. As one can see in the 3D FORC diagrams that there is only one major peak around the coercivity for the SmCo5 single-phase magnets. Accordingly, we may expect similar two separate major peaks with the two-hardphase hybrid magnets. In accordance with the above hypothesis, as shown in Fig. 5(b) with the Nd2Fe14B content at 20 wt%, the FORC distribution has distinct two separated peaks nearly along the bias axis, Hu ¼0, one located at Hc E 2 kOe and one located at Hc E 11 kOe, which represents the magnetization reversals of the Nd2Fe14B and SmCo5 phase, respectively. The separated peaks indicate inhomogeneous magnetization reversal behavior between the two hard phases. Meanwhile, an obvious ridge and uneven surface are observed in the 3D FORC diagrams, which can be concluded by the magnetostatic interaction between the magnetic Nd2Fe14B and SmCo5 phases. However, with increasing the Nd2Fe14B content at 20 wt% in Fig. 5(c), the amplitude of the Nd2Fe14B phase contribution in the SmCo5/Nd2Fe14B hybrid nanocomposite magnets is not as great as in the lower Nd2Fe14B content, and the maximum ρ for Nd2Fe14B phase contribution is also smaller. This indicates little irreversible switching for

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Fig. 5. The 2D and 3D FORC diagrams of the SmCo5/Nd2Fe14B nanocomposite powders for the Nd2Fe14B content at free, 10 wt%, 20 wt%, and 30 wt%. Left column is the colored FORC curves with the FORC distribution plotted inside the hysteresis loop. The middle column is the two-dimensional FORC diagrams and right column is threedimensional FORC diagrams.

M. Pan et al. / Journal of Magnetism and Magnetic Materials 361 (2014) 219–223

Nd2Fe14B phase, and thus exchange interaction starts to dominate the magnetization behavior. With the higher Nd2Fe14B content, at 30 wt%, as one can see in Fig. 5(d) that although two separate major peaks can still be observed, the weaker ridge and the even surface have been obtained, implying the weak magnetostatic interaction but strong inter-phase exchange coupling. These results are slightly different from the result of δM plots, which shows that the Nd2Fe14B content at 20 wt% possesses the highest exchange coupling behavior and results in having the enhanced maximum energy product (BH)max. The variance of these two method results can be attributed to the δM plots which only give the interactions as a whole and FORC diagrams can separate the mean field interaction effect. 4. Conclusion In summary, nanoscale hybrid SmCo5/Nd2Fe14B nanocomposite magnets have been fabricated by ball milling. The influence of Nd2Fe14B content on the magnetic properties and magnetization reversal of these hybrid alloys was investigated. The alloys that show strong intergrain exchange-coupling behavior with (BH)max ¼2.95 MGOe was obtained when the two hard phases are well coupled. From their FORC diagrams, we detected two major peaks for the hybrid magnets; FORC analysis provides more information on the magnetostatic as well as the exchange interactions. In both cases, the magnetization reversal behaviors for these alloys were discussed in detail and are consistent with the results of δM plots. Acknowledgments This work was supported by Project of the Zhejiang Province Innovative Research Team (No. 2010R50016), the Provincial Major Science and Technology Project (No. 2009C21010), the Provincial Natural Science Foundation (Y6100640), the National Natural Science Foundation of China (Nos. 51001092, 51271172, 51371163 and 51301158) and the National Public Interest Research Special (No. 201210107).

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