Exchange-Coupling Interaction in Zero- and One-Dimensional

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Functional Nanostructured Materials (including low-D carbon)

Exchange-coupling Interaction in Zero- and Onedimensional SmCo /FeCo Core-Shell Nanomagnets 2

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Jimin Lee, Jiwon Kim, Danbi Kim, Gyutae Lee, Yeong-Been Oh, Tae-Yeon Hwang, Jae-Hong Lim, Hong-Baek Cho, Jongryoul Kim, and Yong-Ho Choa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 26, 2019

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ACS Applied Materials & Interfaces

Exchange-coupling Interaction in Zero- and Onedimensional Sm2Co17/FeCo Core-Shell Nanomagnets Jimin Lee,a Jiwon Kim,b Danbi Kim,c Gyutae Lee,d Yeong-Been Oh,a Tae-Yeon Hwang,e JaeHong Lim, f Hong-Baek Cho,a Jongryoul Kim,a and Yong-Ho Choa*a aDepartment

of Materials Science and Chemical Engineering, Hanyang University, 55,

Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do, 15588, Korea bAdvanced

Materials & Processing Center, Institute for advanced engineering, 175-28, Goan-ro

51beon-gil, Baegam-myeon, Cheoin-gu, Yongin-si, Gyeonggi-do, 17180, Korea cDepartment

of Physics, Pukyong National University, 45 Yongsoro, Namgu, Busan, 48513,

Korea dDepartment

of Materials Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-

gu, Ansan-si, Gyeonggi-do, 15588, Korea eCenter

for Quantum Information, Korea Institute of Science and Technology (KIST), 5,

Hwarang-ro 14-gil. Seongbuk-gu, Seoul 02792, Korea fDepartment

of Materials Science and Engineering, Gachon University, 1342 Seongnamdaero,

Sujeong-gu, Seongnam-si, Gyeonggi-do, 13120, Korea.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS

permanent

magnet,

Sm2Co17/FeCo

nanocomposite

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magnets,

core-shell

nanomagnet, exchange-coupling effect, shape tuning, electroless plating

ABSTRACT Rare-earth-based core-shell spring nanomagnets have been intensively studied in the permanent magnet industry. However, the inherent agglomeration characteristics of zerodimensional (0-D) magnetic nanoparticles is an issue in practical fabrication of magnetic nanocomposites due to deterioration in exchange-coupling interactions, resulting in inferior magnetic performance. Here, with an aim to overcome the structural limitations, we report a new type of SmCo/FeCo core-shell nanomagnet with a well-dispersed one-dimensional (1-D) structure prepared by a combination of electrospinning and electroless plating processes. An FeCo layer with a tailored thickness on nano-scale SmCo was produced to achieve a sufficient exchange-coupling effect. The influence of electroless plating time on the microstructure of fibers was discussed, and comparisons were made as a function of magnet shape. A 1-D SmCo/FeCo spring nanomagnet having a core diameter ranging from 150 to 200 nm and a shell thickness of 15-20 nm showed a potent exchange-coupling effect compared to its 0-D counterpart. It effectively reduced self-aggregation and further showed a remarkable enhancement in (BH)max (above 45.7%). We think that this novel structure marks a new era in the exchange-spring magnet industry and may overcome the limitations of traditional core-shell nanomagnets.


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Introduction With the advent of a new magnetic structure serving as an “exchange-coupled magnet” by Kneller and Hawig, astonishing progress in nanostructured magnetic composites having enticed much interest in the fields of permanent magnets, biomedical applications, superconducting materials, and turbomachinery was witnessed in the 21st century.1,2 Theoretically, an exchangespring magnet consisting of a rare-earth (RE)-based hard magnet and soft magnetic phase can exhibit superior magnetic properties, i.e., over 100 MGOe of maximum energy products ((BH)max), compared to their single-phase counterparts.3,4 To realize the potential of a hard-soft magnetic nanocomposite in terms of effective exchange coupling, the exchange-spring magnet needs to be well designed. In particular, well-controlled interphase homogeneity and well-defined microstructural parameters, such as hard-soft phase distributions, are crucial.5,6 Nandwana et al. reported that exchange-coupling interactions in core-shell nanostructures are substantially stronger than in heterodimer structures due to better interphase homogeneity.7 Since then, many different RE-based spring magnets with soft magnetic sheaths have been investigated. For example, SmCo5/FeNi, (Sm,Pr)Co5/Fe, and NdFeB/Fe were synthesized through an electroless plating process that easily forms a continuous nanoscale coating layer on a raw RE magnet.8-15 However, particle aggregation is inevitable in zero-dimensional (0-D) structures. In addition to attractive van der Waals forces, internal magnetic attraction produces a strong aggregation tendency in 0-D magnetic nanoparticles. The aggregated particles are usually larger than the exchange length; thus, the powders cannot be sufficiently exchange-coupled.16 Considering the


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structural constraints in terms of uncontrollable forces, development of non-agglomerated structures would be a significant breakthrough. We may be able to achieve enhanced magnetic properties by modifying the shape of the nanomagnet into a one-dimensional (1-D) structure. It is well known that 1-D nanostructured materials (e.g., nanofibers, nanowires) possess a high aspect ratio and could prevent aggregation due to their unique shape characteristics.17 Moreover, Kim reported that theoretical (BH)max of a ferromagnetic material can be determined as a function of demagnetization factor (N), and a highest value can be achieved in a cylindrical structure with N=0.5 rather than in a spherical material (i.e., N=0.33).18 Therefore, a 1-D RE nanomagnet that is free from aggregation becomes an ideal building block for fabricating REbased spring magnets, as shown in our latest works.19-21 Magnetic hard-soft phase distribution is another key parameter in spring magnet design. To obtain a large value of (BH)max, the exchange-spring magnet needs to have the highest possible volume fraction of the high magnetization soft phase.22 A thin, soft magnetic layer within twice the domain-wall width in the hard magnetic phase is theoretically required for achieving fine exchange coupling. The domain-wall widths of Sm2Co17, SmCo5, and Nd2Fe14B are around 10.0 nm, 5.1 nm, and 5.2 nm, respectively.23,24 Therefore, when the soft magnetic sheath is thicker than 20 nm, the plated layer can be magnetically exchange coupled with Sm2Co17. For SmCo5 and Nd2Fe14B, only a partial layer (~10 nm in thickness) can be exchange coupled with each hard magnet due to the limitation in exchange coupling length. The remainder behaves as an independent soft phase, resulting in magnetic decoupling, which is no longer fit for an ideal exchange-coupled magnet. Nevertheless, the majority of electroless plating studies is limited to Nd2Fe14B and SmCo5 powders.8-10,12,13,15


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Herein, with a view to overcoming the structural and material limitations, we chose magnetically hard Sm2Co17 and soft Fe11Co5, which has the highest saturation magnetization (i.e., 240 emu/g) among soft magnetic materials, to synthesize 1-D and 0-D hard-soft core-shell nanomaterials and assess their potential as exchange-spring structures for achieving maximum magnetic properties (Figure 1). We verified the optimal microstructure for the exchange-coupled nanomagnet by comparing the morphological and magnetic characteristics of each structure.

Experimental Section The hard-soft bimagnetic nanostructures (Sm2Co17 as a hard core and Fe11Co5 as a soft shell) were prepared by synthesis of hard magnets with different shapes (i.e., particle and fiber forms) and a subsequent electroless plating process. To synthesize fairly fine and homogenous 0-D Sm2Co17 nanoparticles (300 ± 56 nm of diameter), a glycine-nitrate process (GNP) incorporating a sol-gel self-combustion was employed. A detailed description of the GNP for synthesis of 0-D Sm2Co17 nanoparticles is given in the Supplementary Information Method S1 with representative data (Fig. S1). Single-phase 1-D Sm2Co17 nanofiber (150–200 nm diameter and over 30 μm length) synthesis was performed using a procedure modified from our previous work that involved electrospinning and calciothermic reduction (see Supplementary Information Method S2 and Fig. S2 for details of the synthetic process).19,20 Prior to soft magnet plating, the raw hard nanomagnet was treated with sensitizing and activating solutions for catalytic activation, followed by FeCo deposition. Detailed descriptions of the experimental process and conditions are given in Method S3 and Table S1, respectively. We conducted FeCo electroless plating with durations of 5, 10, 15, and 20 minutes to understand


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the phase distribution, interphase homogeneity, and exchange coupling interactions of the FeCo plated Sm2Co17 1-D nanostructure. The as-prepared Sm2Co17 nanoparticles were also electrolessly plated for comparison with the plated 1-D nanostructure. The surface morphology, corresponding microstructures, and elemental composition of the FeCo-deposited Sm2Co17 nanomaterials were analyzed by means of field emission scanning electron microscopy [FE-SEM, MIRA-3, Tescan] and transmission electron microscopy [TEM, JEM-2100F, JEOL] with energy dispersive X-ray spectroscopy [EDS, JEM-2100F, JEOL]. Structural characterization of the mixed-phase nanostructures was performed using an X-ray diffractometer [XRD, D/MAX-2500/PC, Rigaku] with Cu Kα radiation (1.5406 Å). Room temperature magnetic properties of the bimagnetic nanocomposites were measured using a vibrating sample magnetometer [VSM, VSM7410, LakeShore] in a maximum field of 25 kOe, without additional magnetic alignment or sintering processes.

Results and Discussion Figure 2(a) shows the morphology of the Sm2Co17/FeCo nanofibers as a function of electroless plating time (0 to 20 min). The as-synthesized pristine Sm2Co17 fibers have diameters ranging from 150 to 200 nm and appear to possess superior dispersibility. After electroless plating, there was no considerable difference in size or dispersibility, but the fiber surface appeared fluffy, suggesting formation of a continuous FeCo layer on the Sm2Co17 surface.10 Extended plating times (~20 min) resulted in FeCo growth and independent FeCo crystal aggregation, indicated by a red arrow in Fig. 2(a).


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Two distinct regions (i.e., a cylindrical core surrounded by a rippled FeCo layer and a shell region) were analyzed in detail by TEM (Fig. 2(b)). Interfaces between hard and soft magnets were clearly observed, confirming that they are suitable for exchange coupling interactions.25 In the 5-min-plated sample, an amorphous FeCo layer with a thickness of 10-15 nm was firmly attached to the hard magnetic core. As the plating time increased to 20 min, the produced FeCo layer gradually thickened and grew, reaching about 40 nm, and separated into individual nanograins. The high-resolution TEM (HRTEM) image of the fluffy shell region of the 20-minplated sample showed fringe spacing of about 0.2142 nm, corresponding to the (220) planes of the Fe11Co5 crystal. The selected area electron diffraction (SAED) pattern revealed sharp diffraction spots of that soft magnetic nanocrystal. Figure 2(c) shows the powder X-ray diffraction patterns of Sm2Co17-based magnetic nanofibers with different FeCo-plating durations. A clear hexagonal Sm2Co17 (JCPDS No.657762) pattern was observed without any secondary phase or byproduct in all the samples. This implies that no damage occurred to the resulting nanofibers irrespective of pH and temperature of the plating precursor solution during the sensitizing, activating, and electroless plating processes. Even for a relatively long plating duration of ~15 min, a tiny FeCo (JCPDS No.757978) peak appeared at 2θ=44.830° and was superimposed with that of Sm2Co17 (see XRD patterns for their single-phase counterparts in Fig. S3). The magnetic hysteresis loops of the nanofibers as a function of electroless plating duration are depicted in Fig. 2(d). The corresponding saturation magnetization at 25 kOe (M25kOe), remanence (Mr), intrinsic coercivity (Hci), squareness (Mr/M25kOe), and calculated maximum energy product ((BH)max) are given in Table S2.


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Generally, a saturation magnetization (Ms) increases as the amount of soft magnet (e.g., Fe, Co, FeCo) increases in a hard-soft magnetic system.19,26 Such a tendency was evident in our study. As plating time increased, the FeCo layer became thicker, leading to an increase in M25kOe. However, the intrinsic magnetic properties might be even higher since the maximum applied field of 25 kOe in the VSM system cannot fully saturate the magnetic moment. (Note the obvious difference in the two magnetic hysteresis loops of the pure Sm2Co17 nanofibers measured by VSM (~25 kOe) and a PPMS (Physical Property Measurement System; ~90 kOe) in Fig. S4). According to previous studies, the magnetic characteristics for exchange-coupling must fulfill the following conditions: (i) a smooth single-phase hysteresis curve without kinks or shoulders, (ii) Mr should be much higher than that for the individual hard phase, (iii) Mr⁄Ms > 0.5 for a randomly oriented easy axis following Stoner-Wohlfarth theory, and (iv) (BH)max enhancement compared to the single hard phase.27-29 Up to 15 min of plating time, all obtained samples meet the conditions for exchange-coupling: their hysteresis curves showed single-phase-like magnetization behavior. Simultaneous enhancements in Mr and in (BH)max were achieved as well. M25kOe likewise increased, and the squareness remained around 0.70. In contrast, the 20min-plated sample exhibited a small collapse in the second quadrant loop, implying interphase decoupling attributed to overgrowth of FeCo crystal.12 A prerequisite for effective coupling interaction is that FeCo should be less than twice the domain wall thickness of the Sm2Co17 phase (i.e., 20 nm).30 As verified from the above TEM data, FeCo forms continuous crystalline layers on the hard magnet, and the overall thickness of the FeCo layer in the 20-min-plated sample exceeds the critical length of 20 nm. Also, insufficient proximity of the hard magnetic core and independent FeCo crystal results in a kink in the hysteresis loop.


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However, the enhancements in Mr and calculated (BH)max are not enough to represent a concrete conclusion for exchange-coupling interaction without a large reversible demagnetization curve as a direct criterion for the presence of exchange coupling interaction.31 The resulting switching field distribution (dM/dH) curves, which are sensitive to interphase interaction and directly manifest the interphase exchange-coupling interaction, are presented in the inset of Fig. 2(d).32 Additional evidence is provided in the Henkel plots (δM plots) in Fig. S5. The recoil susceptibility curves of the pure- and FeCo-plated Sm2Co17 for 5-15 min showed only a single clear peak. We inferred that stronger exchange-coupling between soft FeCo and hard Sm2Co17 grains was achieved, leading to single-phase-like behavior.11 On the other hand, twostep magnetization reversal behavior was observed in the 20-min-deposited sample. The two distinct sharp peaks, which were close to the coercive field for each single hard- and softmagnetic phases, were mainly attributed to insufficient interphase contact and floating aggregated FeCo crystals, which led to dominance of long range dipolar interactions over the exchange-coupling force.15 These results are in good agreement with the TEM data and hysteresis loops. We prepared 0-D core-shell nanomagnets via the same as electroless plating process on 1-D magnets to elucidate the morphological effect of the magnet on the magnetic characteristics. The Sm2Co17 particles densely wrapped by an ultra-thin FeCo layer agglomerated as depicted in the right panel, while the morphologies of nanofibers showed no distinct change after electroless plating (Figure 3(a), see FE-SEM data of nanoparticles as a function of plating time in Figs. S6(a-e)).


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The different shapes could be pronouncedly causative of the different magnetic behaviors (Figures 3(b) and (c)). As FeCo-coating time increased, M25kOe increased substantially regardless of magnet dimensions or magnetic coupling interaction, as expressed in Eq. (1):33 𝑀 = 𝑀𝑠𝑜𝑓𝑡 𝑓𝑠𝑜𝑓𝑡 + 𝑀ℎ𝑎𝑟𝑑 (1 ― 𝑓𝑠𝑜𝑓𝑡)

(1)

where Mhard is magnetization of the hard magnet (i.e., SmCo), Msoft is magnetization of the soft phase (i.e., FeCo), and fsoft is the volume fraction of the soft magnet. However, a distinct decrement in M25kOe exists (~8 emu/g) due to an ineluctable 0-D particle agglomeration during electroless plating process, as shown in Fig. S7. The magnetic coupling behavior in 0-D nanostructures hardly ever persists, resulting in no increase in Mr, a drastic decrease in (BH)max, and thus a serious kink in the M-H loop. The enhancement in (BH)max was only about 1.7% in a 0-D core-shell magnet, while the 1-D magnet showed an increase greater than 36.4%, which is an all-time high. This can be ascribed to the non-ideal magnetic coupling between aggregated bimagnetic nanoparticles despite the tailored thickness of the FeCo layer on the Sm2Co17 particle (see TEM micrograph and FeCo thickness data of the 0-D core-shell nanoparticles in Fig. S6(f)). Thus, we concluded that a prominent exchange-coupling effect can be fully achieved in 1-D nanomagnets having a homogeneous soft magnet coating.

Conclusion In summary, we synthesized Sm2Co17/Fe11Co5 core-shell nanomagnets with different shapes (spheres and fibers) via a chemical method (i.e., glycine-nitrate process (GNP) and electrospinning) followed by electroless plating to investigate the structural effects on the exchange-coupling interaction in spring nanomagnets. As plating progresses, the FeCo forms


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conformal coatings (a few nanometers thick) on each Sm2Co17 nanofiber. This leads to effective exchange-coupling behavior, while particle agglomeration (and thus decoupling interactions) predominantly occurred on the surface of a spherical Sm2Co17 nanomagnet. Interestingly, a record-high enhancement in (BH)max by about 45.7% was achieved in the 1-D magnetic nanocomposite compared to the spherical spring magnet. We then investigated the magnetic interactions in the 1-D nanocomposites with plating duration changes (0-20 min) using M-H loops and switching field distribution (dM/dH) curves. The strongest magnetic exchange coupling was achieved after 15 min of plating time, and the continued reaction weakened the magnetic exchange coupling, inducing FeCo overgrowth and even FeCo crystal aggregation. The well-dispersed 1-D/core-shell nanostructure was unique for resolving the fundamental problem of particle aggregation. We present the rare-earth-based, 1-D core-shell nanocomposite as a promising magnetic building block for near theoretical high-strength nanomagnets with a prominent exchange-coupling effect.


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Figures

Figure 1. (a) Scheme of the new concept for achieving effective exchange-coupling interactions and enhancement in magnetic properties and (b) expected magnetic performance in a uniform, one-dimensional, hard-soft magnetic core-shell nanocomposite and its counterpart.


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Figure 2. (a) FE-SEM micrographs of the magnetic nanofibers as a function of electroless plating time (0-20 min); (b) TEM micrographs with cross-sectional EDS line scan profiles from electroless plated nanofibers formed over 5 min and 20 min, respectively; (c) XRD patterns of FeCo-plated Sm2Co17 nanofibers with different FeCo-plating duration. Right panel shows an enlarged view of the most intense diffraction peaks in the 2θ range of 42–46°; (d) M–H curves of pure and FeCo-plated Sm2Co17 nanofibers of various FeCo plating times (5–20 min), obtained under an applied field of 25 kOe (2.5 T) at room temperature. Inset shows the corresponding switching field distribution (dM/dH; χrev) curves for Sm2Co17-based nanomagnets.


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Figure 3. (a) FE-SEM micrographs and schemes of the samples obtained after electroless FeCoplating on (top) SmCo nanoparticles and (bottom) SmCo nanofibers, respectively, for 15 min; (b) Corresponding M-H curves and enlarged view of the demagnetization curves; (c) Dependence of various magnetic properties upon electroless plating time: maximum magnetization (M25kOe), remanence (Mr), squareness (Mr/M25kOe), intrinsic coercivity (Hci), and calculated maximum energy product ((BH)max).


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ASSOCIATED CONTENT Supporting Information. Experimental and supplementary characterization details. Method S1. Synthesis of Sm2Co17 nanoparticles by a glycine-nitrate process Method S2. Synthesis of Sm2Co17 nanofibers by an electrospinning process Method S3. Electroless plating process Figure S1. Morphology of Sm2Co17 nanoparticles. Figure S2. Morphology of Sm2Co17 nanofibers. Figure S3. X-ray diffraction (XRD) patterns of pure Sm2Co17, 20-min-deposited Sm2Co17/FeCo nanocomposite, and FeCo film on a copper substrate. Figure S4. Magnetic hysteresis loops of pure Sm2Co17 nanofibers with different maximum applied fields measured by VSM (~25 kOe) and PPMS (Physical Property Measurement System; ~90 kOe). Figure S5. Henkel plots (δM plots) of Sm2Co17 nanofibers as a function of electroless plating time (0, 5, 10, 15, 20 min). Figure S6. FE-SEM micrographs of the Sm2Co17 nanoparticles as a function of electroless plating time (0, 5, 10, 15, 20 min) and corresponding TEM micrograph. Figure S7. The mechanism of SmCo/FeCo core/shell nanocomposites formation through electroless plating.


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Table S1 Composition of plating bath and experimental conditions. Table S2 Room temperature magnetic parameters of the prepared Sm2Co17/FeCo nanofibers as a function of electroless plating time.


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AUTHOR INFORMATION Corresponding Author * Prof. Yong-Ho Choa, #420, 5th Engineering Building, 55 Hanyangdaehak-ro, Ansan, Gyeonggi-do 15588, Korea, [email protected] Author Contributions Jimin Lee conceived of the experiments and prepared the manuscript. Jiwon Kim, Danbi Kim, and Gyutae Lee helped perform the analysis and discussed the results. Yeong-Been Oh and TaeYeon Hwang helped perform the experiments. Jae-Hong Lim and Hong-Baek Cho contributed to editing. Jongryoul Kim and Yong-Ho Choa contributed to the manuscript, accepted responsibility for the conducted research, and provided final approval. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M3D1A1027836). ABBREVIATIONS SmCo, samarium-cobalt; FeCo, iron-cobalt; Ms, saturation magnetization; Mr, remanence; Hci, coercivity; (BH)max, maximum energy products; 1-D, one-dimensional; 0-D, zero-dimensional; GNP, glycine-nitrate process


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REFERENCES 1.

Kneller, E. F.; Hawig, R., The Exchange-spring Magnet: A New Material Principle for

Permanent magnets. IEEE Transactions on Magnetics 1991, 27 (4), 3588-3560. 2.

López-Ortega, A.; Estrader, M.; Salazar-Alvarez, G.; Roca, A. G.; Nogués, J.,

Applications of Exchange Coupled Bi-magnetic Hard/Soft and Soft/Hard Magnetic Core/Shell Nanoparticles. Physics Reports 2015, 553, 1-32. 3.

Skomski, R.; Coey, J. M. D., Giant Energy Product in Nanostructured Two-phase

Magnets. Physical Review B 1993, 48 (21), 15812-15816. 4.

Yu-Qing, L.; Ming, Y.; Yi, P.; Hong-Guo, Z., Micromagnetism Simulation on Effects of

Soft Phase Size on Nd 2 Fe 14 B/ α–Fe Nanocomposite Magnet with Soft Phase Imbedded in Hard Phase. Chinese Physics B 2018, 27 (8), 087502. 5.

Yu, Y.; Sun, K.; Tian, Y.; Li, X. Z.; Kramer, M. J.; Sellmyer, D. J.; Shield, J. E.; Sun, S.,

One-pot Synthesis of Urchin-like FePd-Fe3O4 and their Conversion into Exchange-coupled L1(0)-FePd-Fe Nanocomposite Magnets. Nano Lett 2013, 13 (10), 4975-4979. 6.

Shen, B.; Mendoza-Garcia, A.; Baker, S. E.; McCall, S. K.; Yu, C.; Wu, L.; Sun, S.,

Stabilizing Fe Nanoparticles in the SmCo5 Matrix. Nano Lett 2017, 17 (9), 5695-5698. 7.

Nandwana, V.; Chaubey, G. S.; Yano, K.; Rong, C.-b.; Liu, J. P., Bimagnetic

Nanoparticles with Enhanced Exchange Coupling and Energy Products. Journal of Applied Physics 2009, 105 (1), 014303.


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Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8.

Lamichanne, M.; Rai, B.; Mishra, S.; Nguyen, V.; Liu, J., Magnetic Properties Hard-Soft

SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating. Open Journal of Composite Materials 2012, 2, 119-124. 9.

Wang, S.; Ma, J.-M.; Chen, Y., Preparation and Magnetic Properties of Anisotropic

(Sm,Pr)Co5/Fe Nanocomposites Particles via Electroless Plating. Journal of Chemistry 2014, 2014, 5. 10. Poudyal, N.; Gandha, K.; Elkins, K.; Liu, J. P., Anisotropic SmCo 5/FeCo Core/Shell Nanocomposite Chips Prepared via Electroless Coating. AIMS Materials Science 2015, 2 (3), 294-302. 11. Wang, F.; Hu, X.; Huang, G.; Hou, F.; Zhang, X., Facile Synthesis of Anisotropic Nanostructured Sm–Pr–Co/Co Magnet Composites with Dense Coatings of Fine Cobalt Nanoparticles. Journal of Alloys and Compounds 2015, 626, 212-216. 12. Fan, X. D.; Tian, N.; You, C. Y., Electroless Fabrication of Nd-Fe-B/α-Fe Heterostructured Composite Magnets and their Magnetic Properties. Materials Science Forum 2015, 809-810, 426-432. 13. Yanjie, W.; Cheng, Z.; Zhonglei, L., Morphology and Properties Electroless Plating at Moderate Temperature on the Surface of the Permanent Magnet NdFeB. Journal of North China Institute of Aerospace Engineering 2017, 1, 002. 14. Hou, Y.; Sellmyer, D. J., Magnetic Nanomaterials: Fundamentals, Synthesis and Applications. Wiley: 2017.


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15. Lv, L.; Wang, F.-Q.; Zheng, Q.; Du, J.; Dong, X.-L.; Cui, P.; Liu, J. P., Preparation and Magnetic Properties of Anisotropic SmCo5/Co Composite Particles. Acta Metallurgica Sinica (English Letters) 2018, 31 (2), 143-147. 16. Song, F.; Shen, X.; Liu, M.; Xiang, J., Microstructure, Magnetic Properties and Exchange–coupling Interactions for One-dimensional Hard/Soft Ferrite Nanofibers. Journal of Solid State Chemistry 2012, 185, 31-36. 17. Shen, X.; Song, F.; Xiang, J.; Liu, M.; Zhu, Y.; Wang, Y., Shape Anisotropy, ExchangeCoupling Interaction and Microwave Absorption of Hard/Soft Nanocomposite Ferrite Microfibers. Journal of the American Ceramic Society 2012, 95 (12), 3863-3870. 18. Kim, N.; Han, H.-S.; Lee, K.-S., A Limit to Predict Maximum Energy Product (BHmax) from the Magnetization Hysteresis Loop. Journal of the Korean Magnetics Society 2018, 28, 205-211. 19. Lee, J.; Hwang, T.-Y.; Kang, M. K.; Cho, H.-B.; Kim, J.; Myung, N. V.; Choa, Y.-H., Synthesis of Samarium-Cobalt Sub-micron Fibers and their Excellent Hard Magnetic Properties. Frontiers in chemistry 2018, 6, 18. 20. Lee, J.; Hwang, T.-Y.; Cho, H.-B.; Kim, J.; Choa, Y.-H., Near Theoretical Ultra-high Magnetic Performance of Rare-earth Nanomagnets via the Synergetic Combination of Calciumreduction and Chemoselective Dissolution. Scientific Reports 2018, 8 (1), 15656. 21. Lee, J.; Hwang, T.-Y.; Kang, M. K.; Lee, G.; Cho, H.-B.; Kim, J.; Choa, Y.-H., Highperformance, Cost-effective Permanent Nanomagnet: Microstructural and Magnetic Properties of Fe-substituted SmCo Nanofiber. Applied Surface Science 2019, 471, 273-276.


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22. Jiang, J. S.; Bader, S. D., Rational Design of the Exchange-spring Permanent Magnet. Journal of Physics: Condensed Matter 2014, 26 (6), 064214. 23. Schrefl, T.; Fidler, J.; Kronmüller, H., Remanence and Coercivity in Isotropic Nanocrystalline Permanent Magnets. Physical Review B: Condens Matter 1994, 49 (9), 61006110. 24. Jiles, D., Introduction to Magnetism and Magnetic Materials. CRC Press: 2015. 25. Kirkeminde, A.; Ren, S., Interdiffusion Induced Exchange Coupling of L10-FePd/alphaFe Magnetic Nanocomposites. Nano Lett 2014, 14 (8), 4493–4498. 26. Ma, C., Magnetic Properties of Exchange Coupled SmCo5/FeCo Composite Particles Synthesized by Magnetic Self-assembly. Chemical Physics Letters 2018, 696, 31-35. 27. Volodchenkov, A. D.; Kodera, Y.; Garay, J. E., Synthesis of Strontium Ferrite/Iron Oxide Exchange Coupled Nano-powders with Improved Energy Product for Rare Earth Free Permanent Magnet Applications. Journal of Materials Chemistry C 2016, 4 (24), 5593-5601. 28. Xu, X.; Hong, Y.-K.; Park, J.; Lee, W.; Lane, A. M., Ex Situ Synthesis of Magnetically Exchange Coupled SrFe12O19/Fe-Co Composites. AIP Advances 2016, 6 (5), 056026. 29. Parmar, H.; Xiao, T.; Chaudhary, V.; Zhong, Y.; Ramanujan, R. V., High Energy Product Chemically Synthesized Exchange Coupled Nd2Fe14B/alpha-Fe Magnetic Powders. Nanoscale 2017, 9 (37), 13956-13966. 30. Yu, L. Q.; Zhang, Y. P.; Yang, Z.; He, J. D.; Dong, K. T.; Hou, Y., Chemical Synthesis of Nd2Fe14B/Fe3B Nanocomposites. Nanoscale 2016, 8 (26), 12879-12882.


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31. Rama Rao, N. V.; Saravanan, P.; Gopalan, R.; Manivel Raja, M.; Sreedhara Rao, D. V.; Sivaprahasam, D.; Ranganathan, R.; Chandrasekaran, V., Microstructure, Magnetic and Mössbauer Studies on Spark-plasma Sintered Sm–Co–Fe/Fe(Co) Nanocomposite Magnets. Journal of Physics D: Applied Physics 2008, 41 (6), 065001. 32. Liu, F.; Hou, Y.; Gao, S., Exchange-coupled Nanocomposites: Chemical Synthesis, Characterization and Applications. Chem Soc Rev 2014, 43 (23), 8098-8113. 33. Shen, X.; Song, F.; Yang, X.; Wang, Z.; Jing, M.; Wang, Y., Hexaferrite/α-iron Composite Nanowires: Microstructure, Exchange-coupling Interaction and Microwave Absorption. Journal of Alloys and Compounds 2015, 621, 146-153.


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Figure 1. (a) Scheme of the new concept for achieving effective exchange-coupling interactions and enhancement in magnetic properties and (b) expected magnetic performance in a uniform, one-dimensional, hard-soft magnetic core-shell nanocomposite and its counterpart. 80x70mm (300 x 300 DPI)


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Figure 2. (a) FE-SEM micrographs of the magnetic nanofibers as a function of electroless plating time (0-20 min); (b) TEM micrographs with cross-sectional EDS line scan profiles from electroless plated nanofibers formed over 5 min and 20 min, respectively; (c) XRD patterns of FeCo-plated Sm2Co17 nanofibers with different FeCo-plating duration. Right panel shows an enlarged view of the most intense diffraction peaks in the 2θ range of 42–46°; (d) M–H curves of pure and FeCo-plated Sm2Co17 nanofibers of various FeCo plating times (5–20 min), obtained under an applied field of 25 kOe (2.5 T) at room temperature. Inset shows the corresponding switching field distribution (dM/dH; χrev) curves for Sm2Co17-based nanomagnets. 176x150mm (300 x 300 DPI)



Figure 3. (a) FE-SEM micrographs and schemes of the samples obtained after electroless FeCo-plating on (top) SmCo nanoparticles and (bottom) SmCo nanofibers, respectively, for 15 min; (b) Corresponding M-H curves and enlarged view of the demagnetization curves; (c) Dependence of various magnetic properties upon electroless plating time: maximum magnetization (M25kOe), remanence (Mr), squareness (Mr/M25kOe), intrinsic coercivity (Hci), and calculated maximum energy product ((BH)max). 176x57mm (300 x 300 DPI)


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Exchange-Coupling Interaction in Zero- and One-Dimensional

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