Magnetic microstructure and magnetic properties of spark plasma sintered NdFeB magnets

Magnetic microstructure and magnetic properties of spark plasma sintered NdFeB magnets

Journal of Magnetism and Magnetic Materials 399 (2016) 175–178 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials...

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Journal of Magnetism and Magnetic Materials 399 (2016) 175–178

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Magnetic microstructure and magnetic properties of spark plasma sintered NdFeB magnets Y.L. Huang a,n, Y. Wang a, Y.H. Hou a, Y.L. Wang a, Y. Wu a, S.C. Ma a, Z.W. Liu b, D.C. Zeng b, Y. Tian c, W.X. Xia c, Z.C. Zhong a a

School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, PR China c Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 July 2015 Received in revised form 21 September 2015 Accepted 25 September 2015 Available online 28 September 2015

Nanocrystalline NdFeB magnets were prepared by spark plasma sintering (SPS) technique using meltspun ribbons as starting materials. A distinct two-zone structure with coarse grain zone and fine grain zone was formed in the SPSed magnets. Multi-domain particle in coarse grain zone and exchange interaction domain for fine grain zone were observed. Intergranular non-magnetic phase was favorable to improve the coercivity due to the enhancement of domain wall pinning effects and increased exchangedecouple. The remanent polarization of 0.83 T, coercivity of 1516 kA/m, and maximum energy product of 118 kJ/m3 are obtained for an isotropic magnet. & 2015 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Microstructure Magnetic domain NdFeB Spark plasma sintering

1. Introduction

2. Experimental

NdFeB magnets are critical components for numerous devices ranging from electric motors to disk drives to traction motors to wind generators [1–4]. Nanocrystalline NdFeB magnets have attracted much attention not only because of their good magnetic properties but also their exceptional fracture toughness and thermal stability comparing to the conventional microcrystalline sintered magnets. As a rapid sintering technique, the advantages of the spark plasma sintering (SPS) technique make it suitable for the preparation of nanocrystalline NdFeB magnets [5]. For nanocrystalline magnets, previous studies have focused on the microstructure, magnetic properties, and corrosion resistance [6–11]. The studies on the magnetic microstructure and their relation to the magnetic properties are still lacking. The knowledge of magnetic structure is not only of fundamental interest, but also of technological significance. The understanding the magnetic microstructure is an indispensable step toward the realization of high performance magnets. In this work, a detailed understanding in the magnetic microstructure and their relation to the magnetic properties is investigated in details.

Commercial melt spun ribbons with nominal compositions of Nd13.5Co6.7Ga0.5Fe73.5B5.6 were put into the cylindrical graphite die for SPS in vacuum using the facility of SPS-825 (Sojitz Machinery Co.). The unscreened initial ribbons (o400 μm) and o45 μm size powder were used, respectively. The SPS sintering temperature Tsps, Pressure Psps and holding time tsps are in the ranges of 600– 800 °C, 30–50 MPa and 0–5 min, respectively. Microstructure for the magnets was characterized by scanning electronic microscope (FEI Quanta FEG 250). TEM specimens were prepared by mechanical polishing of thin sections of the material followed by ion milling. Magnetic domain ns microstructure was investigated by TEM (JEOL2100F). Magnetic properties were tested by physical properties measurement system (PPMS-9, Quantum Design, USA) equipped with a 9 T vibrating sample magnetometer (VSM). Magnetic domains were imaged by means of atomic force microscopy with a magnetic force microscopy (MFM) (Cypher, Asylum Research).

n

Corresponding author. E-mail addresses: [email protected] (Y.L. Huang), [email protected] (Z.C. Zhong). http://dx.doi.org/10.1016/j.jmmm.2015.09.079 0304-8853/& 2015 Elsevier B.V. All rights reserved.

3. Results and discussion Fig. 1 shows the microstructure of SPSed magnets prepared using unscreened powders (o400 μm). Two distinguished zones with different grain sizes which form layers perpendicular to the

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Fig. 1. Microstructure of SPSed NdFeB magnets, SEM images (a) and (b), TEM images (c) and (d), Inset shows the SAED pattern.

pressing direction are noticed. These two zones are referred as coarse grain zones and fine grain zones. The formation of coarse grain zone and fine grain zone are attributed to the sintering mechanism of SPS. Local high temperature zone was generated by the pulsed energy existed between the particle contacting surfaces. The temperature at the center of the particle was dramatically lower than that in the particle boundaries. Song et al. reported that the temperature in the particle contacting surface is nearly 3000 K higher than that at the particle center during SPS [12]. The huge difference in the temperature between the particle contacting surface and the center is the main reason of two distinguished zones formation. From the TEM images of coarse grain zone and fine grain zone,

demonstrated in the Fig. 1(c) and (d), respectively, it is found that the coarse grain zones consist of equiaxed grains, while the fine grain zones are mainly composed of elongated grains with various aspect ratios. This suggests their anisotropic growth due to the non-equilibrium process of SPS. Unfortunately, due to the small size, their crystallographic orientation was varied at this stage. As a whole, the spark plasma sintered magnet is isotropic. A selected area diffraction pattern can also evidence the isotropic structure of SPSed magnets, shown in the Fig. 1(c) inset. The optimum magnetic properties with Jr ¼ 0.83 T, jHc ¼ 1516 kA/m and (BH)max ¼118 kJ/m3 are obtained for a nanocrystalline SPSed magnet at Tsps ¼ 700 °C, Psps ¼50 MPa and tSPS ¼5 min, demonstrated in the Fig. 2(a).

Fig. 2. Magnetic hysteresis curves (a) and differential susceptibility curves (b) for SPSed NdFeB magnets prepared using various particle size powders.

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Fig. 3. Low magnification (a) and high magnification (b) SEM images of SPSed NdFeB magnets prepared by less than 45 μm size powder.

Normally, the fine and uniform grain size is beneficial in improving coercivity. Therefore, the disappearance or reduction of coarse grain zone should be favorable to improve magnetic properties. By using NdFeB magnetic powder with less than 45 μm particle size, the SPSed magnets with only a very small amount of coarse grain zone were prepared, as indicated by the white arrow shown in the Fig. 3(a). The mean grain size for this sample is almost similar to that in the fine grain zones for the SPSed magnet prepared using unscreened powder (o 400 μm). For SPSed billet prepared by less than 45 μm size powder, the volume fraction of particle boundary is higher than that in the SPSed magnets produced with o400 μm size powder during sintering process, resulting in weak electric current intensity in the former. However, spark phenomenon only occur at large electric current intensity [13]. Therefore, no or little spark phenomenon occurred in the contacting surface is the main reason of coarse grain zone disappearance for SPSed magnets produced with o 45 μm size powder. Confusingly, magnetic properties could not be improved, although a fine and relative uniform microstructure is observed for this sample. As demonstrated in the Fig. 2, the optimum magnetic properties are obtained at Tsps ¼ 700 °C, Psps ¼ 50 MPa and tsps ¼5 min, including Jr ¼0.82 T, jHc ¼1466 kA/m and (BH)max ¼115 kJ/m3. Comparing to the SPSed magnet prepared with unscreened powder ( o400 μm), the coercivity decreases by  3%. Also, the density reduces from 7.56 g/cm3 to 7.46 g/cm3. Low sintering efficiency, aroused by the deficiency of spark phenomenon, is the main reason of low density of SPSed magnets prepared using o 45 μm size powder. It seems to be necessary to obtain non-uniform microstructure in order to prepare nanocrystalline NdFeB magnets with optimal magnetic properties. Fig. 4(a) shows a Lorentz microscope image under an over-focus condition for the coarse grain zone. Two types of magnetic domain patterns, maze-like domain and stripe-shaped domain with 180 ° domain wall structure can be noted in the multi-

domain grain A and grain B, respectively. The differences in the domain morphology are attributed to the variation of c-axis orientation. Normally, in order to minimize the internal stray field energy due to the demagnetization field, a maze-like domain structure can be observed, while c-axis is perpendicular to the observed plane. If c-axis is parallel to the observed plane, stripedshaped domain structure would be presented due to the large magnetostatic interaction among the grains. This suggests that caxis orientation of grain A and grain B is in-plane and out of plane, respectively. Demonstrated in the Fig. 4(b) and (c) are magnetic domain structure of coarse grain zone. Stripe-shaped magnetic domain with 180 ° domain wall can be found in the grain C and D, respectively. Interestingly, the whole magnetic domain in the grain C seems to move a distance of  45 nm comparing to that in the grain D along the grain boundary. The domain walls are discontinuous across the grain boundary, which should be attributed to the existence of non-magnetic phase resulted from non-equilibrium SPS process of super high temperature in the coarse grain zone. The initial magnetization curves show an obvious stepwise shape, as presented in the Fig. 2(a). For the SPSed magnet prepared with o45 μm size powder, the characteristics of stepwise shape degenerate comparing to that of the SPSed magnet with unscreened powders. Fig. 2(b) shows the differential susceptibility curves of initial magnetization curves. Obviously, one peak can be observed on each curve, corresponding to the stepwise characteristics of the initial magnetization curve. It is believed that magnetic domain wall pinning mechanism should be responsible for such behavior [14]. The domain walls are pinned where they encounter the intergranular phase. The variation of domain wall pinning effects should be the main reason of stepwise shape degeneration for initial magnetization curves. For SPSed magnet prepared using unscreened powder, non-magnetic intergranular phase in the coarse grain zone can be acted as effective domain

Fig. 4. Magnetic microstructure of coarse grain zone for SPSed NdFeB magnet, Lortentz micrograph for demagnetized state, acquired in an over-focus (a), just-focus (b), and under-focus condition (c).

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Fig. 5. MFM images of SPSed NdFeB magnet prepared by less than 45 μm size powder (a) and magnetic microstructure of fine grain zone for SPSed magnet prepared using unscreened powder (b).

wall pinning position, and a higher intensity of peak can be observed on the differential susceptibility curve. An improving coercivity of 1516 kA/m can be obtained due to the role of intergranular non-magnetic phase in elevating pinning effects and reducing exchange coupling interaction despite the presence of coarse grain zone. For SPSed magnets prepared by less than 45 μm size powder, due to various c-axis orientation and nanostructured grains, it is difficult in obtaining a comprehensive observation of magnetic domain by Lortentz microscopy. A more realistic description of magnetic domain observed by magnetic force microscopy (MFM) is shown in the Fig. 5(a). Note that the magnetic domain width varies from ∼185 nm to ∼1.75 μm, suggesting that the grains with a size below the single-domain particle should participate within the interaction domain [15]. The variation of interaction domain size is attributed to the various grain morphology and non-uniform grain size. The formation of exchange interaction domain indicates that there existed strong exchange coupling interaction between neighboring Nd2Fe14B grains. Also, the observed sharp boundary of magnetic domain evidences strong exchange coupling interaction between neighboring grains, as demonstrated in the Fig. 5(a) [16]. For the SPSed magnets prepared by large size powder, magnetic domain morphology and width in the fine grain zone are almost similar to that of the SPSed magnet produced using less than 45 μm size powder, as shown in the Fig. 5(b). It is concluded that strong intergranular exchange coupling and low densification contribute to the low coercivity of this sample, although a relative fine and uniform grain can be obtained. Therefore, the optimal magnetic properties should be obtained by the suitable ratio of coarse grain zone and fine grain zone. These observations will provide useful information in nanocrystalline NdFeB magnets to enhance coercivity.

4. Conclusion In summary, nanocrystalline isotropic NdFeB magnets were prepared by spark plasma sintering technique. A distinct two-zone structure with coarse grain zone and fine grain zone was obtained in the SPSed magnets. For coarse grain zone, multi-domain particle with maze-like domain and stripe-shaped domain were observed. The enhancement of domain wall pinning effects and increased exchange-decouple induced by non-magnetic intergranular phase, resulted in high coercivity of SPSed magnets prepared by unscreened powder. For the fine grain zone, exchange interaction domain was noted. Strong exchange interaction and low densification contributed to the low coercivity of SPSed magnet prepared using o45 μm size powder.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 51401103, 11304146, and 51564037), the Natural Science Foundation of Jiangxi Province (Grant nos. 20151BAB216003 and 20151BAB212005), the Aeronautical Science Foundation of China (Grant no. 2014ZF56017), the Science and Technology Support Project of Jiangxi Province (Grant nos. 20121BBE50001 and 20132BBG70034).

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