Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating

Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating

Journal of Rare Earths xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Rare Earths journal homepage: http://www.journals.elsevie...

1MB Sizes 0 Downloads 3 Views

Journal of Rare Earths xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths

Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating* Fengqing Wang a, Guiping Tang a, Baoru Bian a, Lulu Yao a, Youhao Liu b, Qiang Zheng c, Juan Du a, * a

CAS Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences (CAS), Ningbo 315201, China State Key Laboratory of Rare Earth Permanent Magnetic Materials, Hefei 231500, China c School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2018 Received in revised form 20 February 2019 Accepted 22 February 2019 Available online xxx

Nanostructured anisotropic Nd-Fe-B/Fe(C) composite powders were prepared by coating Fe(C) softmagnetic nanoparticles on HDDR Nd-Fe-B hard magnetic powders using iron pentacarbonyl Fe(CO)5 as soft-phase precursor. The effect of Fe(CO)5-loading amount on soft-phase purity, coating morphology and magnetic properties of the composite powders was investigated. Dense and continuous Fe(C) softphase coatings with average particle sizes of 58e68 nm are obtained at Fe(CO)5 loading amounts of x  12 wt%, leading to enhanced remanence and improved energy product of the coated powders. Positive value in dM-plots and single-phase-like demagnetization curves are observed in the Nd-Fe-B/ Fe(C) composite powders, indicating the exchange coupling effect between the coated Fe(C) soft phases and the Nd-Fe-B hard phase. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Keywords: Permanent magnet Anisotropy Nanocomposite Coating Thermal decomposition Rare earths

1. Introduction Rare-earth (RE) permanent magnets have been widely used in our modern society especially in energy-related electric motors, generators and transformers.1e4 For the resource scarcity and large cost of RE elements, there is a powerful drive to develop strong permanent magnets containing less RE elements. The well-known two-phase nanocomposite magnets, which consist of a hard magnetic phase that provides a high coercivity and a soft magnetic phase that provides a high magnetization, are potential candidates to achieve these objectives. As predicted, nanocomposites with hard-phase contents below 60% can achieve giant energy products of 90e120 MGOe in Sm-Fe-N/Fe-Co, Nd-Fe-B/Fe and Nd-Fe-B/FeCo-N systems,4e6 compared with about 55 MGOe for the best

* Foundation item: Project supported by the National Natural Science Foundation of China (51771219, 51771220, and 51422106), the State Key Laboratory of Rare Earth Permanent Magnetic Materials Opening Foundation (SKLREPM17OF03), the China Postdoctoral Science Foundation (2016M601989), and the Natural Science Foundation of Ningbo City (2016A610249 and 2017A610030). * Corresponding author. E-mail address: [email protected] (J. Du).

commercial Nd-Fe-B permanent magnets. So far, two key challenges are most concerned for obtaining such extraordinary high magnetic properties: (1) orienting the hard phase; (2) achieving magnetic exchange coupling between the soft and hard phases by controlling the size of the soft phase (about 10 nm as theoretically predicted) and distributing them uniformly around the hard phase. By orienting the hard phase before bulking, “bottom-up” route is considered easier than “top-down” to achieve anisotropic bulk magnets. Hence, coating soft-phase nanoparticles firstly on anisotropic hard phase becomes an important issue. This issue has attracted much attention especially since anisotropic Sm-Co hardphase nanoparticles and nanoflakes were successfully fabricated by surfactant-assisted high energy ball milling.7e9 As a consequence, many efforts were focused on coating soft-phase nanoparticles on anisotropic Sm-Co hard phase for anisotropic nanocomposites with exchange coupling effect and enhanced magnetic properties. But the critical soft-phase size for exchange coupling seems changing in these nanocomposites. Wang et al. synthesized anisotropic (Sm,Pr) Co5/Co composite nanoflakes with Co soft-phase sizes of 20e50 nm, but magnetic decoupling with pronounced kink was also observed in the demagnetization curves.10 The large size of the soft phase, which is over the theoretical value of twice the domain

https://doi.org/10.1016/j.jre.2019.02.012 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012

2

F. Wang et al. / Journal of Rare Earths xxx (xxxx) xxx

wall width of the hard phase (~7 nm for SmCo5 and PrCo5), is considered as the main reason for the decoupling. Similar decoupling and kinks were also observed by Poudyal when coating 20e50 nm Fe-Co soft phase on SmCo5 nanoflakes of 1e5 mm in length and 20e150 nm in thickness.11,12 By controlling the nucleation and growth of the coated soft phase, our previous works have confined the Co soft phase in range of 8e20 nm on 0.5e20 mm (Sm,Pr)1Co5 and SmCo5 hard phases.13,14 Though the obtained Co soft phase was still larger than the theoretically required value, exchange coupling with smooth demagnetization curves was observed in the coated nanocomposites. The works reported by Marinescu et al.15 and Zeng et al.16 also found smooth demagnetization curves with enhanced magnetic properties when coating 50e100 nm soft-phase particles on the 21e50 mm Sm(CoFeZrCu)7.4e7.5 hard phases. These results indicate that the critical size of soft phase for exchange coupling may be not as strict as that of theoretical prediction and may be relaxed to larger sizes of above 20 nm. The factors such as the size, shape and composition of the hard phase, or even the coating method, are possible reasons for the changes of the critical size. Besides, the energy product of these Sm-Co based nanocomposite powders is up to now below 20 MGOe, which is much lower than the theoretical value. In order to achieve high energy product of the two-phase nanocomposite powders and to further study the changes of critical soft-phase size under alternative fabrication route and hard phases, this study was exercised to coat Fe-based soft-phase nanoparticles on anisotropic Nd-Fe-B hard phase. The Nd-Fe-B hard-phase powders were fabricated by hydrogenationdisproportionation-desorption-recombination (HDDR) process for its advantage in producing high-coercivity anisotropic Nd-Fe-B powders in large scale.17 A simple and low-cost thermal-decomposition method was chosen to achieve the soft-phase coatings instead of the conventional wet-chemistry method, in which the soft-phase nanoparticles obtained sonochemically from alkaline solutions may tend to be air-unstable and meanwhile the usual flowing inert gas and vigorous agitation make the coating process relatively complex and expensive.18,19 Iron pentacarbonyl, Fe(CO)5 (at%), was used as soft-phase precursor for its relatively low decomposition temperature. As a result, high purity Fe(C) soft phases with average particle sizes of 58e68 nm has been coated uniformly on the anisotropic Nd-Fe-B hard phase. The coated softphase particles are exchange-coupled with the hard phase, leading to enhanced positive value in dM plots, smooth demagnetization curves, and enhanced magnetic properties of the Nd-Fe-B/Fe(C) nanocomposites. The effect of Fe(CO)5-loading amount (quantified by the weight of Fe elements in as-loaded Fe(CO)5 precursor and the weight of Nd-Fe-B hard phase) on the soft-phase purity, coating morphology and magnetic properties of the coated composites was also revealed.

Afterwards, the autoclave reactor was opened in the glove box and the products were collected for further characterizations. The Fe(CO)5-loading amount, which is quantified by the ratio between the weight of Fe elements in as-loaded Fe(CO)5 precursor and the weight of Nd-Fe-B hard phase, was set to be x ¼ 0, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt% and 20 wt%. The structure and morphology of coated powders were characterized by Bruker AXS D8-Advanced X-ray diffraction (XRD, with Cu Ka radiation) and scanning electron microscopy (SEM, HitachiS4800, equipped with EDS). To characterize the powder anisotropy, the products were aligned and immobilized in epoxy resin under a magnetic field of 2.7T (27 kOe). Magnetic properties of aligned powders were measured at room temperature by a vibrating sample magnetometer (VSM, LakeShore 7410) with a maximum magnetic field of 2.1T (21 kOe). The density of all magnetic materials was assumed to be 7.6 g/cm3. To study intergrain or interlayer interaction between the coated soft phase and the NdFe-B hard phase, the powder products were firstly immobilized in epoxy resin without magnetic alignment and then subjected to recoil-loop measurements using a superconducting quantum interference device magnetometer (SQUID, Quantum Design, Inc.) with a maximum applied field of 4.0 T. Typical characterizations on uncoated Nd-Fe-B powders were also conducted for comparison. 3. Results and discussion Fig. 1 shows XRD patterns of the non-aligned powders before and after the coating process (x ¼ 0, 10 wt%, 20 wt%). The inset gives an enlarged comparison for samples with x ¼ 0 and 10 wt%. It can be seen that Nd-Fe-B/Fe(C) crystallized nanocomposites were yielded after the coating process, and the Fe(C) soft phases tend to be contaminated by impurity phases when the Fe(CO)5-loading amount is too high. For x  12 wt%, the Fe(C) soft phases were basically pure and crystallized as Fe (JCPDS No. 85e1410 and

2. Experimental HDDR processed Nd-Fe-B powders with sizes of 0.2e150 mm in Nd30Co5.28Al0.6Zr0.14Ga0.53Fe62.39B1.05 composition were used as a starting hard-phase material. The HD process was held at 880  C for 1 h and the DR process was performed in vacuum. The thermal decomposition of Fe(CO)5 was employed to coat Fe(C) soft phase on the HDDR Nd-Fe-B hard-phase powders. Before coating, the Nd-FeB powders were firstly pretreated ultrasonically in HCl solution (0.5 mmol HCl and 40 mL ethanol) under nitrogen/argon gas protection, and dried in a vacuum chamber after washing in ethanol for several times. The dried Nd-Fe-B powders and Fe(CO)5 were then transferred to an autoclave reactor in glove box under high purity argon atmosphere (>99.999%). The decomposition of Fe(CO)5 and the coating process was then held in an oven at 473 K for 2 h.

Fig. 1. XRD patterns of non-aligned powders with Fe(CO)5-loading amounts of x ¼ 0, 10 wt% and 20 wt%.

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012

F. Wang et al. / Journal of Rare Earths xxx (xxxx) xxx

87e0721, marked at 44.3 and 44.6 , respectively) and Fe100emCm (2.75 wt%  m  7 at%, JCPDS No. 44-(1289e1293)). With the Fe(CO)5-loading amount elevated to x ¼ 15 wt% and 20 wt%, the Fe(C) soft phases was contaminated by relatively larger amount of Fe3(CO)12 phase (JCPDS No.71e1602) and slight Fe0.942O phase (JCPDS No.73e2144). The formation of Fe and Fe-C phases by Fe(CO)5 decomposition has been reported by many studies.19e21 The existence of Fe3(CO)12 phase was caused by the increasing pressure of Fe(CO)5-vapor and CO gases which impedes the Fe(CO)5 decomposition process.22,23 Apart from the changes of crystallization behavior of the deposited soft phase, it is also observed that the overall intensity of the composite powders declines monotonously as more Fe(CO)5 precursor was loaded. Such an intensity decline, which was also observed by Marinescu et al.15 and Zeng et al.,16 probably relates to a coating structure and an increasing coating thickness of the composite products. To evaluate the amount of the coating Fe element, which may reflect the amount of the coated soft phase especially for x  12 wt% samples, it is noticed that the strawcolored liquid Fe(CO)5 precursor basically disappeared after the coating process, indicating that the amount of Fe element involved in the coating phase could possibly be similar with that of as-loaded Fe(CO)5 materials. In order to clarify the distribution, size, and shape of the soft phase, SEM measurements were carried out on the composite products. The left and right columns of Fig. 2 present respectively the typical low- and high-magnification SEM images for samples with Fe(CO)5-loading amounts of x ¼ 0, 5 wt%, 12 wt% and 15 wt%. As shown in Fig. 2(a) and (b), the Nd-Fe-B powder surfaces (x ¼ 0) are generally clear and smooth, though minor fine particles may

Fig. 2. SEM images of powders with Fe(CO)5-loading amounts of (a, b) x ¼ 0; (c, d) x ¼ 5 wt%; (e, f) x ¼ 12 wt%; and (g, h) x ¼ 15 wt%. The right column shows the enlarged images for typical areas in the adjacent left panels.

3

appear occasionally. From the low-magnification images in Fig. 2(c), (e) and (g), it can be seen that the powder surfaces after the coating process show many nanoscale particles, indicating that the Fe(C) phases shown in Fig. 1 have been self-assembled into softphase coatings on the Nd-Fe-B hard phase. The dependence of coating morphology on the Fe(CO)5-loading amount was identified more clearly from the enlarged images on right column. At x ¼ 5 wt %, the coated soft-phase particles have nanosize dimension with cuboid-like shape and scattered distribution. The gaps between adjacent soft-phase particles are often of tens to hundreds of nanometers. When the Fe (CO)5-loading amount rises to x ¼ 10 wt% and 12 wt%, the size and shape of the coated soft-phase particles were similar to those at x ¼ 5 wt%, but the coating layers become denser and the gaps between adjacent soft-phase particles were narrowed quickly, leading to continuous soft-phase coating layers on the Nd-Fe-B hard phase at x ¼ 12 wt%. The increasing coating density and high flatness of the coating layers indicate monolayer structure of the soft-phase coatings for x  12 wt%. Besides the nanosize particles, it is noticed that large particles with sizes over 100 nm may occasionally appear at x ¼ 12 wt% (see Fig. 2(e)). The number of these large particles rises sharply when the Fe(CO)5loading amount increases to x ¼ 15 wt% and 20 wt%, forming additional large-particle layers on the first ones. The monolayers at x  12 wt% are mainly composed of Fe(C) soft phases as analyzed in Fig. 1(b). The additional large-particle coating layers at x ¼ 15 wt% and 20 wt% are highly correlated to impurity phases of Fe3(CO)12 as shown in Fig. 1(c). Fig. 3 gives the size of coated soft-phase particles as a function of the Fe(CO)5-loading amount. As is shown, the size of the soft-phase particles is in range of 58e68 nm for x ¼ 5 wt%e 12 wt%, generally unchanged before the coated particles can form continuous coating layers at x ¼ 12 wt%. Thus, the increasing coating density below 12 wt% results from the particle nucleation of the coated Fe(C) soft phases. Magnetic anisotropy is one of the key issues for permanent magnetic materials. In order to study whether the coated composites are magnetically anisotropic, magnetic hysteresis loops of aligned powders were measured parallel (//) and perpendicular (⊥) to the aligning direction. As a result, significant difference between the two direction loops, which means strong magnetic anisotropy, was observed in all the coated composite powders. Typical twodirection demagnetization curves for x ¼ 5 wt% samples are shown in Fig. 4. The strong magnetic anisotropy arises from the caxis grain alignment of the nanocomposite particles, in similar case with that of our Sm-Pr-Co/Co composite coating system.13 Fig. 5 gives the parallel demagnetization curves for powders with and without soft-phase coatings. It is surprising that all powders with soft-phase coatings show smooth demagnetization curves although the coating particles are quite larger than 20 nm. The smooth demagnetization curves represent a single-phase-like demagnetization behavior, suggesting magnetic coupling between the Nd-Fe-B hard and Fe(C) soft phases. The effect of the Fe(CO)5-loading amount on magnetic properties (i.e., magnetization, remanence, and intrinsic coercivity) of the Nd-Fe-B/Fe(C) products is summarized in Fig. 6. It should be noted that the coated powders may not be magnetized to saturation at a limited magnetic measurement field of 2.1 T. Thus, we used 4pM2.1T to replace the saturation magnetization 4pMs. As is shown, with the Fe(CO)5-loading amount rising from x ¼ 5 wt% to x ¼ 20 wt%, the magnetization 4pM2.1T and remanence 4pMr first increase and then decrease, both peaking at x ¼ 12 wt%. The peaking magnetization and remanence are 14.3 and 13.6 kG, respectively, comparing with those of 13.5 and 13.0 kG for uncoated Nd-Fe-B powders. On the other hand, the powder coercivity shows overall a decrease tendency, but also maintains a high value of 13 kOe at x ¼ 12 wt%. As a result, the maximum energy product reaches a peak value of

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012

4

F. Wang et al. / Journal of Rare Earths xxx (xxxx) xxx

Fig. 3. Size of the coated soft-phase particles as a function of the Fe(CO)5-loading amount.

(BH)max ¼ 36.5 MGOe at x ¼ 12 wt%. The slight coercivity increase for x ¼ 5 wt% is likely resulting from the fact that the Fe content of the hard-phase particle surface may be slightly lower than the nominal composition of 2:14:1, while the reductive atmosphere of Fe(CO)5 or CO provides the compensation. Similar coercivity increase was also observed in the SmCo5/Fe system by Hou et al.24 The magnetization increase with x12 wt% is mainly caused by the high saturation magnetization of Fe (~21 kG25) and Fe100-mCm (~20 kG26) phases as compared to 13.5 kG for the Nd-Fe-B hard phase. The magnetization decrease above x ¼ 12 wt% is mainly caused by the formation of the Fe3(CO)12 and Fe0.942O impurity phases, which have none or low saturation magnetizations (see Fig. 1). The remanence enhancement for x  12 wt% cannot result from the texture variation because composites after coating usually have weaker texture than the uncoated raw materials.13 The enhanced remanence, therefore, is largely caused by the magnetic interaction between the Nd-Fe-B hard phase and the coated Fe(C) soft phases. To further certify the type of magnetic interaction between the Nd-Fe-B hard phase and coated Fe(C) soft phases, recoil loop measurements and dM(H) ¼ Md(H)e[1e2Mr(H)] plot analysis were carried out on non-aligned samples with x ¼ 0 and 10 wt% (see Fig. 7). As is known, magnetostatic interaction tends to have

Fig. 4. Demagnetization curves measured parallel and perpendicular to the aligning direction for coated composite powders with x ¼ 5 wt%.

Fig. 5. Demagnetization curves measured parallel to the aligning direction for coated composite powders with different Fe(CO)5-loading amounts.

negative dM value while exchange-coupling interaction would have positive dM value.27e29 Hence, the negative dM values for x ¼ 0 samples suggest overall magnetostatic interaction of adjacent Nd2Fe14B grains. The coating process does not seem to affect the interior structure and intergrain interactions of the Nd-Fe-B particles. Therefore, the transition of positive dM values in Nd-Fe-B/ Fe(C) composite samples demonstrates exchange-coupling interaction between the coated Fe(C) soft phases and the Nd-Fe-B hard phase. This result consists with the smooth demagnetization curves and enhanced remanence obtained in Figs. 5 and 6. The formation

Fig. 6. Magnetization (4pM2.1 T), remanence (4pMr) and intrinsic coercivity (Hci) of the coated composite powders as a function of the Fe(CO)5-loading amount. The magnetic measurements were conducted parallel to the aligning direction.

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012

F. Wang et al. / Journal of Rare Earths xxx (xxxx) xxx

5

Fig. 7. Recoil loops for powders with Fe(CO)5-loading amounts of x ¼ 0 (a) and 10 wt% (b); (c) The corresponding dM plots from (a) and (b).

of impurity phases such as Fe3(CO)12 would be somewhat detrimental to the exchange coupling of the hard-soft phases since a drop of the maximum positive dM value was observed in x ¼ 20 wt% samples (not shown here). From theoretical work for exchange coupling, the size of the soft particles should be smaller than the twice of the domain wall thickness (about 10 nm for Nd2Fe14B30). In this work, the critical size of the soft phase for effective exchange coupling is likely to be relaxed to 58e68 nm, which is nearly 5 times larger than the expected size. The larger size of the anisotropic HDDR processed Nd-Fe-B hard phase may be the main reason for the exchange-coupling with such larger size of the soft phases, since the increased size of hard phase seems to tolerate bigger size of soft phase to achieve best exchange coupling and optimum energy product.31 A detailed study on this issue is currently being undertaken. The thermal decomposition coating method, which allows better adhesion strength of coated phases than in wet chemical deposition method, may also contribute to the relaxed critical size of the soft phases. In addition, changing the Fe(C) soft phases to pure Fe or Fe-Co nanoparticles by tailoring alternative coating conditions may further enhance the magnetic properties of the nanocomposite powders. 4. Conclusions Anisotropic Nd-Fe-B/Fe(C) nanocomposites were successfully prepared by coating Fe(C) soft-phase nanoparticles on the HDDR processed Nd-Fe-B hard-phase powders via thermal decomposition of Fe(CO)5 precursor. The Fe(CO)5-loading amount has strong effect on the soft-phase purity and their coating morphologies. With the Fe(CO)5-loading amount increasing from x ¼ 5 wt% to x ¼ 12 wt%, dense and continuous soft-phase coating layers with particle size averaging from 58 to 68 nm are yielded on surfaces of the hard phase. The coated Nd-Fe-B/Fe(C) nanocomposite phases show enhanced magnetic properties as compared with the single hard phase, achieving high remanence of 13.6 kG and high energy product of 36.5 MGOe in optimum condition. The enhanced remanence and energy product result from the exchange coupling effect between the hard and soft phases, which is also demonstrated by the positive value of dM-plot. References 1. Jones N. The pull of stronger magnets. Nature. 2011;472:22. 2. Gutfleisch O, Willard MA, Bruck E, Chen CH, Sankar SG, Liu JP. Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient. Adv Mater. 2011;23(7):821. 3. Liu F, Hou YL, Gao S. Exchange-coupled nanocomposites: chemical synthesis, characterization and applications. Chem Soc Rev. 2014;43(23):8098. 4. Yue M, Zhang XY, Liu JP. Fabrication of bulk nanostructured permanent magnets with high energy density: challenges and approaches. Nanoscale. 2017;9(11):3674.

5. Ding HW, Cui CX, Yang W, Sun JB. Effects of cobalt addition on microstructure and magnetic properties of PrNdFeB/Fe7Co3 nanocomposite. J Rare Earths. 2017;35:468. 6. Fan JP, Zhang XY, JiangYN, Liang RY, Sun J, Bai YH, et al. Optimization of energy product and reversal process for Nd2Fe14B/a00 -(FeCo)16N2/Nd2Fe14B exchangespring trilayer films. J Magn Magn Mater. 2017;441:43. 7. Chakka VM, Altuncevahir B, Jin ZQ, Li Y, Liu JP. Magnetic nanoparticles produced by surfactant-assisted ball milling. J Appl Phys. 2006;99(8): 08E912. 8. Akdogan NG, Hadjipanayis GC, Sellmyer DJ. Anisotropic Sm-(Co,Fe) nanoparticles by surfactant-assisted ball milling. J Appl Phys. 2009;105(7): 07A710. 9. Wang FQ, Hu XJ, Li XH, Huang GW, Zhang YM, Zhang XY. Anisotropic (Sm,Pr)1Co5 based nanoflakes fabricated by high-energy ball milling in polar milling medium. J Alloys Compd. 2014;589:283. 10. Wang XL, He HL, Wang FQ, Chen Y, Xu L, Li XH, et al. Preparation and magnetic properties of anisotropic (Sm,Pr)Co5/Co composite particles. J Magn Magn Mater. 2012;324(5):889. 11. Poudyal N, Elkins K, Gandha K, Liu JP. FeCo coating on SmCo5 nanochips by a sonochemical method. IEEE Tran Magn. 2015;51:2104704. 12. Poudyal N, Gandha K, Elkins K, Liu JP. Anisotropic SmCo5/FeCo core/shell nanocomposite chips prepared via electroless coating. AIMS Mater Sci. 2015;2(3):294. 13. Wang FQ, Hu XJ, Huang GW, Hou FC, Zhang XY. Facile synthesis of anisotropic nanostructured SmePreCo/Co magnet composites with dense coatings of fine cobalt nanoparticles. J Alloys Compd. 2015;626:212. 14. Lv L, Wang FQ, Zheng Q, Du J, Dong XL, Cui P, et al. Preparation and magnetic properties of anisotropic SmCo5/Co composite particles. Acta Metall Sin (Eng Lett). 2017;31(2):143. 15. Marinescu M, Liu JF, Bonder MJ, Hadjipanayis GC. Fe-nanoparticle coated anisotropic magnet powders for composite permanent magnets with enhanced properties. J Appl Phys. 2008;103(7):07E120. 16. Zeng Q, Zhang Y, Bonder MJ, Hadjipanayis GC. Fabrication of SmeCo/Co (Fe) composites by electroless Co and CoeFe plating. J Appl Phys. 2003;93(10):6498. 17. Sepehri-Amin H, Li WF, Ohkubo T, Nishiuchi T, Hirosawa S, Hono K. Effect of Ga addition on the microstructure and magnetic properties of hydrogenationdisproportionation-desorption-recombination processed Nd-Fe-B powder. Acta Mater. 2010;58(4):1309. 18. Nikitenko SI, Koltypin Y, Palchik O, Felner I, Xu XN, Gedanken A. Synthesis of highly magnetic, air-stable Iron-Iron carbide nanocrystalline particles by using power ultrasound. Angew Chem Int Ed. 2001;40(23):4447. 19. Kura H, Takahashi M, Ogawa T. Synthesis of monodisperse iron nanoparticles with a High saturation magnetization using an Fe(CO)x-Oleylamine reacted precursor. J Phys Chem C. 2010;114:5835. 20. Welther A, Jacobi von Wangelin A. Iron(0) nanoparticle catalysts in organic synthesis. Curr Org Chem. 2013;17:326. 21. Wang ZH, Zhang ZD, Choi CJ, Kim BK. Structure and magnetic properties of Fe(C) and Co(C) nanocapsules prepared by chemical vapor condensation. J Alloys Compd. 2003;361(1e2):289. 22. Schultz RH, Crellin KC, Armentrout PB. Sequential bond energies of Fe(CO)xþ (x ¼ 1e5): systematic effects on collision-induced dissociation measurements. J Am Chem Soc. 1991;113:8590. 23. Wen JZ, Goldsmith CF, Ashcraft RW, Green WH. Detailed kinetic modeling of iron nanoparticle synthesis from the decomposition of Fe(CO)5. J Phys Chem C. 2007;111:5677. 24. Hou YL, Sun SH, Rong CB, Liu JP. SmCo5/Fe nanocomposites synthesized from reductive annealing of oxide nanoparticles. Appl Phys Lett. 2007;91(15): 153117. €rvinen E, K€ 25. Kallio M, Lindroos T, Aalto S, Ja arn€ a T, Meinander T. Dynamic compression testing of a tunable spring element consisting of a magnet or heological elastomer. Smart Mater Struct. 2007;16(2):506. 26. Takahashi M, Takahashi Y, Sunaga K, Shoji H. New soft magnetic material of a'FeeC with high Bs. J Magn Magn Mater. 2002;239:479.

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012

6

F. Wang et al. / Journal of Rare Earths xxx (xxxx) xxx

27. Cheng Z, Kronmuller H, Shen B. Microstructure refinement and improvements of magnetic properties of two-phase exchange-coupled Sm2Fe15Ga2Cx/a-Fe nanocomposites by additional Zr. Appl Phys Lett. 1998;73:1586. 28. Zhang HW, Rong CB, Du XB, Zhang J, Zhang SY, Shen BG. Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot. Appl Phys Lett. 2003;82(23):4098. 29. Rong CB, Nandwana V, Poudyal N, Liu JP, Kozlov ME, Baughman RH, et al. Bulk FePt-based nanocomposite magnets with enhanced exchange coupling. J Appl Phys. 2007;102(2):023908.

30. Cui WB, Takahashi YK, Hono K. Nd2Fe14B/FeCo anisotropic nanocomposite films with a large maximum energy product. Adv Mater. 2012;24(48): 6530. 31. Guo NL, Bo N, Wang XH, Li M, Sun P. Simplified calculation of the maximum energy product for the hard/soft/hard trilayers. J Supercond Nov Magn. 2017;30(10):2835.

Please cite this article as: Wang F et al., Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.02.012