- Email: [email protected]
High-performance, cost-effective permanent nanomagnet: Microstructural and magnetic properties of Fe-substituted SmCo nanofiber
Applied Surface Science 471 (2019) 273–276
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
High-performance, cost-effective permanent nanomagnet: Microstructural and magnetic properties of Fe-substituted SmCo nanofiber
T
Jimin Leea, Tae-Yeon Hwangb, Min Kyu Kangc, Gyutae Leec, Hong-Baek Choa, Jongryoul Kima,c, ⁎ Yong-Ho Choaa,b, a
Department of Materials Science and Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea Department of Fusion Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea c Department of Materials Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Permanent magnets Electrospinning Samarium-cobalt-iron Reduction–diffusion process Magnetic properties
We propose the substitution of some Co atoms in SmCo nanofibers with earth-abundant Fe as a means to prepare low-cost magnets with enhanced intrinsic magnetic properties compared to those of pure SmCo. To investigate the effect of Fe substitution upon microstructural and magnetic properties, we synthesized SmCoFe nanofibers (150–200 nm diameter) having various degrees of Fe substitution (0, 5, 10, 20, and 40 at%) via electrospinning and subsequent reduction using CaH2. All Fe-substituted samples showed an enhancement in (BH)max as compared to their non-substituted counterpart. Interestingly, substituting appropriate amount of Fe for Co enabled simultaneous increase of Ms, Hci and thus (BH)max of the SmCo nanofibers, resulting from an effective exchangecoupling effect: Superior Hci (about 7375 Oe) and a (BH)max (about 13.17 MG·Oe) over 153% that of nonsubstituted SmCo nanofibers were obtained in the 10 at% Fe-substituted sample. This work describes the synthesis of SmCoFe ternary magnetic nanofibers and elucidates the phase formation mechanism and the effect of Fe substitution in optimizing magnetic performance. This understanding can be extended to the synthesis of other SmCo phases having different chemical compositions and may enable access to a path far beyond the limitations of traditional magnetic materials.
1. Introduction Rare-earth (RE) based magnet materials such as neodymium–iron–boron (NdFeB) and samarium–cobalt (SmCo) have played a crucial role in practical applications for permanent magnets (i.e., small motors in hybrid electric vehicles, data storage devices, and biomedical devices) despite their expense and inclusion of rare elements [1]. Because RE elements contribute to the enlargement of magnetocrystalline anisotropy, RE-based magnets can possess outstanding magnetic properties (approaching 30 MG·Oe of theoretically possible maximum energy product; (BH)max) compared to RE-free magnets such as barium ferrites (∼5 MG·Oe) and manganese–bismuth (∼8 MG·Oe) [2,3]. With the advent of high-efficiency miniaturized devices in modern industry, there is high demand for enhanced magnetic energy. Many attempts to enhance magnetism have focused upon microstructural modification or doping [4–7]. There have been considerable improvements in coercivity, but no exponential improvements due to an immovable physical constraint: theoretically possible (BH)max cannot
exceed a quarter of μ0 Ms2 , where Ms is the saturation magnetization of a given magnet (e.g., Sm2Co17 has Ms of 1.0 T) and μ0 is a constant (i.e., the vacuum permeability constant of 4π × 10−7 H/m) [8]. Therefore, new chemical compositions of magnets should be sought that increase both intrinsic magnetization and coercivity. To improve upon inherent magnetic properties, the substitution of earth-abundant Fe atoms into Co sites in SmCo nanomagnets is a promising method that yields a net magnetic moment increase (see Fig. S1 for magnetic materials with comparable energy capacity). To date, however, there has been no report on Fe-substituted SmCo nanoscale magnets and the effect of the Fe substitution upon the magnetic properties of SmCo is not well understood. Motivated by the prominence of synthesizing new types of magnets, herein we report on Fe-substituted SmCo one-dimensional nanomagnets having various Fe contents. A combination of electrospinning and a subsequent reduction–diffusion method was investigated as a simple approach suitable for quantity production of nanofibers with improved magnetic properties [9,10]. Based on a single-domain theory, the
⁎ Corresponding author at: Department of Materials Science and Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea. E-mail address: [email protected] (Y.-H. Choa).
https://doi.org/10.1016/j.apsusc.2018.11.217 Received 5 September 2018; Received in revised form 8 November 2018; Accepted 27 November 2018 Available online 01 December 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
outstanding maximum coercivity can be achieved in nano-scaled magnetic materials, compared to their bulk counterparts [11–14]. 2. Experimental section 2.1. Chemicals The following were utilized as raw materials: samarium(III) nitrate hexahydrate [Sm(NO3)3·6H2O, 99.9%; Sigma-Aldrich, USA], cobalt(II) nitrate hydrous [Co(NO3)2·6H2O, 99.9% up; Kojundo Chemical Laboratory Co., Ltd., Japan], and iron(III) nitrate nonahydrate [Fe (NO3)3·9H2O, 98%; Junsei Chemical Co., Ltd., Japan] as metal precursors; polyvinylpyrrolidone (PVP, Mw ≈ 1,300,000; Sigma-Aldrich, USA) as a polymer; citric acid anhydrous (99.5% up; Daejung Chemical & Metals Co., Ltd., South Korea) as an additive; and calcium hydride (CaH2, 92%; Thermo Fisher Scientific Inc., England) as a reducing agent. All chemicals were used without further purification. 2.2. Fe-substituted SmCo nanofiber synthesis To prepare the precursor solution for electrospinning, stoichiometric amounts of nitrate sources were dissolved in deionized water to give the [Sm3+]:[Co2+] ratio of 2:17. To test various degrees of Fe substitution, a series of solutions were prepared with [Fe3+] substituting [Co2+] at ratios from 0 to 40 at%. All precursor solutions contained fixed amounts of PVP (0.3 wt%) and citric acid (1.0 M) to ensure that they had viscosity and electrical conductivity within the ranges of 97–100 cP and 20.0–21.0 mS/cm, respectively. For electrospinning, the mixture was loaded into a 12 mL-plastic syringe with a 30gauge needle. The solution feeding rate was maintained at 0.3 mL/h. A voltage of 20 kV was applied, while the distance between the needle tip and the collector was set to 15 cm. Subsequent calcination process was performed at 700 °C in air to decompose all the organics in the spun fibers, as verified by TG/DTA results (data not shown) [10]. The as-calcined sample was mixed with CaH2 granules (CaH2:ascalcined powder = 2:1 (vol.)) as a reductant, and subsequently heated at 700 °C for 3 h in argon atmosphere. The reduced samples were sifted through a fine 16 mesh sieve to remove most residual reductant and byproduct, were rinsed with 0.1 M NH4Cl/methanol solution and then with methanol, and were stored in a vacuum oven. The overall procedure was slightly modified from a previous method that we have described in detail elsewhere [14,15].
Fig. 1. (a) FE-SEM micrographs of samples as a function of Fe substitution content, obtained after CaH2 reduction, with a subsequent rinsing process; (b) XRD patterns of Fe-substituted Sm2Co17 nanofibers having various degrees of Fe substitution. Right panel: enlarged view of the most intense diffraction peaks in the 2θ range of 42–46°, indicating the phase decomposition from Sm2(Co,Fe)17 to a Sm-rich compound of Sm2Co7 and soft magnetic Fe phase.
having different degrees of Fe substitution. Fig. 1b shows representative X-ray diffraction patterns of Fe-substituted SmCo nanofibers with different degrees of Fe substitution (0–40 at%). Through the oxide phases including Sm2O3 and M3O4 (M = Co, Fe), only metallic SmCoFe phase was observed in all samples [15]. For the sample having no Fe substitution, the pattern indicated a pure hexagonal Sm2Co17 phase (JCPDS card No. 65-7762). When Fe is substituted by Co, the smaller atomic radius of Co (1.67 Å) compared to Fe (1.72 Å) leads to lattice shrinkage in the unit cell; therefore, a shift in the Sm2Co17 peak position to a lower angle occurred (see Fig. S3 for the reference diffraction patterns of Sm2Co17 and Sm2Fe17 hexagonal structures) [16]. As Fe content increased up to 40 at%, crystalline Fe and a superimposed Sm-rich pattern (i.e., Sm2Co7) were observed. This is closely associated with the reactions during the reduction–diffusion process. According to the Ellingham diagram shown in Fig. S4, Sm is the last element to be reduced from SmCoFe oxides due to its highly negative reduction potential (e.g., Sm3+/Sm = −2.41 eV; compare to Co2+/Co = −0.28 eV and Fe3+/Fe = −0.037 eV) and high oxidation energy [17,18]. During diffusion of Sm into Co and Fe, SmCo phase can easily form at a lower temperature compared to SmFe phase [19,20]. Also, once the SmCo phase forms, interdiffusion between SmCo and the Fe binary phase becomes more unlikely even at higher temperature [21]. Thus, based on this stoichiometry, small amounts of foreign phases including Fe and Sm-rich phases (induced by surplus Sm) are inevitable in this nanostructure (see Fig. S5 for SmCo binary phase diagram). For comprehensive study of the phase distribution and the formation mechanism of the phases, synthesized nanofibers with 20 at% of substituted Fe were analyzed via TEM. The basic principle of SmCo alloy formation by means of reduction–diffusion can be described by
2.3. Characterization Surface morphology, corresponding microstructures and elemental compositions of the Fe-substituted Sm2Co17 nanofibers were analyzed by means of field emission scanning electron microscopy [FE-SEM; MIRA-3, Tescan, Czech Republic] and transmission electron microscopy [TEM; JEM-2100F, JEOL, Japan] with energy-dispersive X-ray spectroscopy [EDS; JEM-2100F, JEOL, Japan]. Structural characterizations of the metallic fibers were performed by means of X-ray diffractometry [XRD; D/MAX-2500/PC, Rigaku Co., Japan] using a Cu Kα radiation source (1.5406 Å). Room-temperature magnetic properties of the nanofibers were measured by means of vibrating sample magnetometry [VSM; VSM7410, LakeShore, USA] under the maximum field of 2.5 T, without additional magnetic alignment or sintering processes. 3. Results and discussion 3.1. Microstructural characteristics of Fe-substituted SmCo nanofibers The surface morphologies of Fe-substituted SmCo products are shown in Fig. 1a (also see Fig. S2). After reduction, fibers of 150 nm diameter and aspect ratio > 50 were observed in all samples. There was no distinct difference in dimension or morphology among samples 274
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
Fig. 3. (a) M-H curves of pure and Fe-substituted Sm2Co17 nanofibers of various Fe substitution content (0–40 at%), obtained under the applied field of 25 kOe (2.5 T) at room temperature; (b) Dependence of various magnetic properties upon Fe concentration: maximum magnetization (M2.5T), remanence (Mr), squareness (Mr/Ms), intrinsic coercivity (Hci), and calculated maximum energy product ((BH)max).
Fig. 2. (a, b) TEM micrographs with cross-sectional compositional line profile and whole area mapping image for 20 at% Fe-substituted SmCo nanofiber: (a) Sm2O3–Co(Fe) (insufficiently reduced) and (b) Sm2Co17–Sm2Co7–Fe. (c) Schematic of the phase formation mechanism in the 20 at% Fe-substituted SmCo nanofiber.
structure, and to understand the change in Fe distribution inside the final SmCoFe nanomagnets.
the following reactions [15]:
2SmCoO3 + 3Ca → Sm2 O3 + 2Co + 3CaO
(1)
Co3 O4 + 4Ca → 3Co + 4CaO
(2)
Sm2 O3 + 17Co + 3Ca → Sm2 Co17 + 3CaO
(3)
3.2. Magnetic performance of Fe-substituted SmCo nanofibers To elucidate the effect of Fe substitution upon magnetic properties, VSM analysis was carried out. M-H curves of samples of various Fe contents are shown in Fig. 3a (full version in Fig. S6), and Table S1 lists the corresponding numerical data. All curves showed single-phase-like magnetization behavior even though some samples consisted of two or more phases (i.e., soft magnetic Fe; hard magnetic Sm2(Co,Fe)17, and Sm2Co7). The magnetic parameters determined from Fig. S6, including the maximum magnetization under the maximum applied field (M2.5T), remanence (Mr), squareness (Mr/Ms), intrinsic coercivity, (Hci) and calculated maximum energy product ((BH)max), are plotted in Fig. 3b. As the Fe content was increased, the M2.5T and Mr of the nanofibers increased, which can be correlated to an increase in the formation of SmCoFe phase and/or soft magnetic Fe phase, each of which have high magnetic moment. On the other hand, the Hci initially increased, reaching a maximum value of 7375 Oe, and then showed a declining tendency with further increases in Fe content. A similar phenomenon is seen in (BH)max. For the sake of simple comparison, (BH)max values were calculated from each M-H curve collected from samples not subjected to magnetic alignment or sintering processes. With increasing Fe substitution content, the nanofibers showed an increase in calculated (BH)max from 8.61 MG·Oe (no
Due to the difference in oxidation energy, Co oxides and Fe oxides were reduced to their respective metallic forms and Sm2O3 reduction ensued (see Fig. S4 for the standard Gibbs energy changes for some elements versus temperature). A Co(Fe) core and Sm2O3 shell structure was observed to form during the reactions, corresponding to Eqs. (1) and (2) (Fig. 2a). When the oxides were fully reduced, SmCo metallic phase was formed (Eq. (3)), and interestingly, the surplus Fe no longer remained as a core (Fig. 2b). As verified by the line mapping and whole area mapping analysis, it was observed that the Fe intergranular phase was homogeneously distributed inside the nanofibers. D. Gengfeng et al. carried out the reduction–diffusion process on a Sm2O3/Fe bulk specimen, finding that the Fe nucleus still exists in the granular center even when the reduction–diffusion process is nearly over [19]. In this regard, the different Fe phase distribution presently observed inside the nanofibers is an important discovery that overturns the previous report. The illustration of Fig. 2c may help to better understand the flow of the phase transition, starting from oxides and ending in a reduced metal 275
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
Appendix A. Supplementary material
substitution) to 13.17 MG·Oe at 10 at% Fe substitution, then gradually decreased to 9.36 MG·Oe with further increases in Fe content. It is worth noting that the (BH)max values of all Fe-substituted samples surpassed that of pure SmCo nanofibers. This improvement is obviously a consequence not only of improved net magnetic moment but also of the exchange-coupling interaction between the intergranular soft magnetic Fe and the hard magnetic SmCo(Fe) phase. A hard magnetic matrix having a soft magnetic inclusion is one of the possible structures for exchange-spring magnets [8]. Also, such a smooth M-H curve with no kink, squareness of over 50% (Mr/M2.5T), and drastic (BH)max increase are the clear manifestations of an effective exchange-coupling effect [22]. When the Fe content is higher than the optimal value (i.e., 10 at%), the magnetic interaction would be weakened owing to a predominant dipolar interaction [23]. Thus, these results demonstrated that exchange-coupling occurs in the SmCoFe nanofiber.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.11.217. References [1] P. Campbell, Permanent Magnet Materials and Their Application, Cambridge University Press, 1996. [2] N. Poudyal, C. Rong, V.V. Nguyen, J. Ping Liu, Hard-phase engineering in hard/soft nanocomposite magnets, Mater. Res. Express 1 (2014) 016103. [3] J.M.D. Coey, Hard magnetic materials: a perspective, IEEE Trans. Magn. 47 (2011) 4671–4681. [4] S. Yu, X. Zhao, S. Wu, M.C. Nguyen, Z.-Z. Zhu, C.-Z. Wang, K.-M. Ho, New structures of Fe3S for rare-earth-free permanent magnets, J. Phys. D Appl. Phys. 51 (2018) 075001. [5] T.N. Lamichhane, M.T. Onyszczak, O. Palasyuk, S. Sharikadze, T.-H. Kim, M.J. Kramer, R. MacCallum, A.L. Wysocki, M.C. Nguyen, V.P. Antropov, Single Crystal Permanent Magnet: Extraordinary Magnetic Behavior in the ta, Cu and Fe Substituted CeCo5 System, arXiv preprint arXiv:1805.06545, 2018. [6] P. Tozman, H. Sepehri-Amin, Y. Takahashi, S. Hirosawa, K. Hono, Intrinsic magnetic properties of Sm (Fe1-xCox) 11Ti and Zr-substituted Sm1-yZry (Fe0. 8Co0. 2) 11.5 Ti0. 5 compounds with ThMn12 structure toward the development of permanent magnets, Acta Materialia 153 (2018) 354–363. [7] F. Meng, R.P. Chaudhary, K. Gandha, I. Nlebedim, A. Palasyuk, E. Simsek, M.J. Kramer, R.T. Ott, Rapid assessment of the Ce-Co-Fe-Cu system for permanent magnetic applications, JOM (2018) 1–7. [8] D. Li, D. Pan, S. Li, Z. Zhang, Recent developments of rare-earth-free hard-magnetic materials, Sci. China Phys. Mech. Astronomy 59 (2015). [9] D.L. Schodek, P. Ferreira, M.F. Ashby, Nanomaterials, Nanotechnologies and Design: An Introduction for Engineers and Architects, Butterworth-Heinemann, 2009. [10] O. Frank Agyemang, A.S. Faheem, A.-N. Richard, Y. Xinsheng, K. Hern, A simple method of electrospun tungsten trioxide nanofibers with enhanced visible-light photocatalytic activity, Nano-Micro Lett. 7 (2015). [11] N.A.M. Barakat, B. Kim, H.Y. Kim, Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties, J. Phys. Chem. C 113 (2009) 531–536. [12] G.C. Hadjipanayis, G.A. Prinz, Science and Technology of Nanostructured Magnetic Materials, Springer US, 2013. [13] K.H.J. Buschow, Concise Encyclopedia of Magnetic and Superconducting Materials, Elsevier Science, 2005. [14] J. Lee, T.-Y. Hwang, H.-B. Cho, J. Kim, Y.-H. Choa, Near theoretical ultra-high magnetic performance of rare-earth nanomagnets via the synergetic combination of calcium-reduction and chemoselective dissolution, Sci. Rep. 8 (2018) 15656. [15] J. Lee, T.-Y. Hwang, M.K. Kang, H.-B. Cho, J. Kim, N.V. Myung, Y.-H. Choa, Synthesis of Samarium-cobalt sub-micron fibers and their excellent hard magnetic properties, Front. Chem. 6 (2018) 18. [16] T.-Y. Hwang, J. Lee, H.-R. Lim, S.-J. Jeong, G.-H. An, J. Kim, Y.-H. Choa, Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method, Ceram. Int. 43 (2017) 3879–3884. [17] M. Saito, H. Fujiwara, J. Mizuno, T. Homma, Preparation of nano-structured CoCu films by electrodeposition, in: Meeting Abstracts, The Electrochemical Society, 2006, pp. 1278–1278. [18] S. Talukdar, J.-M. Fang, Reduction and coupling reactions of carbonyl compounds using samarium metal in aqueous media, J. Organ. Chem. 66 (2001) 330–333. [19] D. Gengfeng, J. Qingxiu, W. Xiuhong, H. Guirong, Y. Xinyu, Synthesis mechanism of Sm2Fe17 alloy produced in reduction-diffusion process, J. Rare Earths 28 (2010) 420–424. [20] G. Qi, M. Hino, A. Yazawa, Experimental study on the reduction-diffusion process to produce Fe–Nd, Fe–Sm, Co–Nd and Co–Sm alloys, Mater. Trans. JIM 31 (1990) 463–470. [21] R. Horikawa, H. Fukunaga, M. Nakano, T. Yanai, Magnetic properties of isotropic and anisotropic SmCo5/α-Fe nanocomposite magnets with a layered structure simulated by micromagnetic theory, J. Appl. Phys. 115 (2014) 17A707–717A707. [22] L.Q. Yu, Y.P. Zhang, Z. Yang, J.D. He, K.T. Dong, Y. Hou, Chemical synthesis of Nd2Fe14B/Fe3B nanocomposites, Nanoscale 8 (2016) 12879–12882. [23] F. Yi, Magnetic properties of hard (CoFe2O4)–soft (Fe3O4) composite ceramics, Ceram. Int. 40 (2014) 7837–7840.
4. Conclusion The novel synthesis of SmCo nanofibers including the earth-abundant element Fe as a Co substituent was successfully demonstrated via electrospinning and subsequent heat treatment. Interestingly, the substitution achieved simultaneous enhancements in magnetic performance and cost-effectiveness: 10 at% Fe substitution yielded the largest (BH)max of 13.17 MG·Oe compared to that of pure Sm2Co17 nanofibers (about 8.61 MG·Oe). This can be ascribed to the critical role of Fe in enhancing the net magnetic moment, and the importance of optimizing the Fe content. Moreover, based on kinetics and microstructural analysis, we conclude that the homogeneous distribution of a small amount of Fe grains, a Sm-rich phase (induced by surplus Sm), and a Fe-substituted SmCoFe phase leads to an exchange-coupling interaction. The present work provides an innovative approach to producing SmCoFe one-dimensional nanomagnets having various Fe substitution contents as a means to understand the mechanism of phase formation, particularly the reduction–diffusion reaction occurring in this ternary system. This novel nanostructure can be extended to further enhance magnetic properties by modifying microstructural characteristics or to develop exchange-coupled nanomagnets, and may open a path to improved materials far beyond the limitations of traditional magnetic materials. Acknowledgments 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 (NRF2016M3D1A1027836). This research was also financially supported by the Ministry of Trade, Industry & Energy (MOTIE), and the Korea Institute for Advancement of Technology (KIAT) through the Encouragement Program for Regional Industry Nurturing Program (Project No. R0004915). Notes The authors declare no competing financial interest.
276
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
High-performance, cost-effective permanent nanomagnet: Microstructural and magnetic properties of Fe-substituted SmCo nanofiber
T
Jimin Leea, Tae-Yeon Hwangb, Min Kyu Kangc, Gyutae Leec, Hong-Baek Choa, Jongryoul Kima,c, ⁎ Yong-Ho Choaa,b, a
Department of Materials Science and Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea Department of Fusion Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea c Department of Materials Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Permanent magnets Electrospinning Samarium-cobalt-iron Reduction–diffusion process Magnetic properties
We propose the substitution of some Co atoms in SmCo nanofibers with earth-abundant Fe as a means to prepare low-cost magnets with enhanced intrinsic magnetic properties compared to those of pure SmCo. To investigate the effect of Fe substitution upon microstructural and magnetic properties, we synthesized SmCoFe nanofibers (150–200 nm diameter) having various degrees of Fe substitution (0, 5, 10, 20, and 40 at%) via electrospinning and subsequent reduction using CaH2. All Fe-substituted samples showed an enhancement in (BH)max as compared to their non-substituted counterpart. Interestingly, substituting appropriate amount of Fe for Co enabled simultaneous increase of Ms, Hci and thus (BH)max of the SmCo nanofibers, resulting from an effective exchangecoupling effect: Superior Hci (about 7375 Oe) and a (BH)max (about 13.17 MG·Oe) over 153% that of nonsubstituted SmCo nanofibers were obtained in the 10 at% Fe-substituted sample. This work describes the synthesis of SmCoFe ternary magnetic nanofibers and elucidates the phase formation mechanism and the effect of Fe substitution in optimizing magnetic performance. This understanding can be extended to the synthesis of other SmCo phases having different chemical compositions and may enable access to a path far beyond the limitations of traditional magnetic materials.
1. Introduction Rare-earth (RE) based magnet materials such as neodymium–iron–boron (NdFeB) and samarium–cobalt (SmCo) have played a crucial role in practical applications for permanent magnets (i.e., small motors in hybrid electric vehicles, data storage devices, and biomedical devices) despite their expense and inclusion of rare elements [1]. Because RE elements contribute to the enlargement of magnetocrystalline anisotropy, RE-based magnets can possess outstanding magnetic properties (approaching 30 MG·Oe of theoretically possible maximum energy product; (BH)max) compared to RE-free magnets such as barium ferrites (∼5 MG·Oe) and manganese–bismuth (∼8 MG·Oe) [2,3]. With the advent of high-efficiency miniaturized devices in modern industry, there is high demand for enhanced magnetic energy. Many attempts to enhance magnetism have focused upon microstructural modification or doping [4–7]. There have been considerable improvements in coercivity, but no exponential improvements due to an immovable physical constraint: theoretically possible (BH)max cannot
exceed a quarter of μ0 Ms2 , where Ms is the saturation magnetization of a given magnet (e.g., Sm2Co17 has Ms of 1.0 T) and μ0 is a constant (i.e., the vacuum permeability constant of 4π × 10−7 H/m) [8]. Therefore, new chemical compositions of magnets should be sought that increase both intrinsic magnetization and coercivity. To improve upon inherent magnetic properties, the substitution of earth-abundant Fe atoms into Co sites in SmCo nanomagnets is a promising method that yields a net magnetic moment increase (see Fig. S1 for magnetic materials with comparable energy capacity). To date, however, there has been no report on Fe-substituted SmCo nanoscale magnets and the effect of the Fe substitution upon the magnetic properties of SmCo is not well understood. Motivated by the prominence of synthesizing new types of magnets, herein we report on Fe-substituted SmCo one-dimensional nanomagnets having various Fe contents. A combination of electrospinning and a subsequent reduction–diffusion method was investigated as a simple approach suitable for quantity production of nanofibers with improved magnetic properties [9,10]. Based on a single-domain theory, the
⁎ Corresponding author at: Department of Materials Science and Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do 15588, Republic of Korea. E-mail address: [email protected] (Y.-H. Choa).
https://doi.org/10.1016/j.apsusc.2018.11.217 Received 5 September 2018; Received in revised form 8 November 2018; Accepted 27 November 2018 Available online 01 December 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
outstanding maximum coercivity can be achieved in nano-scaled magnetic materials, compared to their bulk counterparts [11–14]. 2. Experimental section 2.1. Chemicals The following were utilized as raw materials: samarium(III) nitrate hexahydrate [Sm(NO3)3·6H2O, 99.9%; Sigma-Aldrich, USA], cobalt(II) nitrate hydrous [Co(NO3)2·6H2O, 99.9% up; Kojundo Chemical Laboratory Co., Ltd., Japan], and iron(III) nitrate nonahydrate [Fe (NO3)3·9H2O, 98%; Junsei Chemical Co., Ltd., Japan] as metal precursors; polyvinylpyrrolidone (PVP, Mw ≈ 1,300,000; Sigma-Aldrich, USA) as a polymer; citric acid anhydrous (99.5% up; Daejung Chemical & Metals Co., Ltd., South Korea) as an additive; and calcium hydride (CaH2, 92%; Thermo Fisher Scientific Inc., England) as a reducing agent. All chemicals were used without further purification. 2.2. Fe-substituted SmCo nanofiber synthesis To prepare the precursor solution for electrospinning, stoichiometric amounts of nitrate sources were dissolved in deionized water to give the [Sm3+]:[Co2+] ratio of 2:17. To test various degrees of Fe substitution, a series of solutions were prepared with [Fe3+] substituting [Co2+] at ratios from 0 to 40 at%. All precursor solutions contained fixed amounts of PVP (0.3 wt%) and citric acid (1.0 M) to ensure that they had viscosity and electrical conductivity within the ranges of 97–100 cP and 20.0–21.0 mS/cm, respectively. For electrospinning, the mixture was loaded into a 12 mL-plastic syringe with a 30gauge needle. The solution feeding rate was maintained at 0.3 mL/h. A voltage of 20 kV was applied, while the distance between the needle tip and the collector was set to 15 cm. Subsequent calcination process was performed at 700 °C in air to decompose all the organics in the spun fibers, as verified by TG/DTA results (data not shown) [10]. The as-calcined sample was mixed with CaH2 granules (CaH2:ascalcined powder = 2:1 (vol.)) as a reductant, and subsequently heated at 700 °C for 3 h in argon atmosphere. The reduced samples were sifted through a fine 16 mesh sieve to remove most residual reductant and byproduct, were rinsed with 0.1 M NH4Cl/methanol solution and then with methanol, and were stored in a vacuum oven. The overall procedure was slightly modified from a previous method that we have described in detail elsewhere [14,15].
Fig. 1. (a) FE-SEM micrographs of samples as a function of Fe substitution content, obtained after CaH2 reduction, with a subsequent rinsing process; (b) XRD patterns of Fe-substituted Sm2Co17 nanofibers having various degrees of Fe substitution. Right panel: enlarged view of the most intense diffraction peaks in the 2θ range of 42–46°, indicating the phase decomposition from Sm2(Co,Fe)17 to a Sm-rich compound of Sm2Co7 and soft magnetic Fe phase.
having different degrees of Fe substitution. Fig. 1b shows representative X-ray diffraction patterns of Fe-substituted SmCo nanofibers with different degrees of Fe substitution (0–40 at%). Through the oxide phases including Sm2O3 and M3O4 (M = Co, Fe), only metallic SmCoFe phase was observed in all samples [15]. For the sample having no Fe substitution, the pattern indicated a pure hexagonal Sm2Co17 phase (JCPDS card No. 65-7762). When Fe is substituted by Co, the smaller atomic radius of Co (1.67 Å) compared to Fe (1.72 Å) leads to lattice shrinkage in the unit cell; therefore, a shift in the Sm2Co17 peak position to a lower angle occurred (see Fig. S3 for the reference diffraction patterns of Sm2Co17 and Sm2Fe17 hexagonal structures) [16]. As Fe content increased up to 40 at%, crystalline Fe and a superimposed Sm-rich pattern (i.e., Sm2Co7) were observed. This is closely associated with the reactions during the reduction–diffusion process. According to the Ellingham diagram shown in Fig. S4, Sm is the last element to be reduced from SmCoFe oxides due to its highly negative reduction potential (e.g., Sm3+/Sm = −2.41 eV; compare to Co2+/Co = −0.28 eV and Fe3+/Fe = −0.037 eV) and high oxidation energy [17,18]. During diffusion of Sm into Co and Fe, SmCo phase can easily form at a lower temperature compared to SmFe phase [19,20]. Also, once the SmCo phase forms, interdiffusion between SmCo and the Fe binary phase becomes more unlikely even at higher temperature [21]. Thus, based on this stoichiometry, small amounts of foreign phases including Fe and Sm-rich phases (induced by surplus Sm) are inevitable in this nanostructure (see Fig. S5 for SmCo binary phase diagram). For comprehensive study of the phase distribution and the formation mechanism of the phases, synthesized nanofibers with 20 at% of substituted Fe were analyzed via TEM. The basic principle of SmCo alloy formation by means of reduction–diffusion can be described by
2.3. Characterization Surface morphology, corresponding microstructures and elemental compositions of the Fe-substituted Sm2Co17 nanofibers were analyzed by means of field emission scanning electron microscopy [FE-SEM; MIRA-3, Tescan, Czech Republic] and transmission electron microscopy [TEM; JEM-2100F, JEOL, Japan] with energy-dispersive X-ray spectroscopy [EDS; JEM-2100F, JEOL, Japan]. Structural characterizations of the metallic fibers were performed by means of X-ray diffractometry [XRD; D/MAX-2500/PC, Rigaku Co., Japan] using a Cu Kα radiation source (1.5406 Å). Room-temperature magnetic properties of the nanofibers were measured by means of vibrating sample magnetometry [VSM; VSM7410, LakeShore, USA] under the maximum field of 2.5 T, without additional magnetic alignment or sintering processes. 3. Results and discussion 3.1. Microstructural characteristics of Fe-substituted SmCo nanofibers The surface morphologies of Fe-substituted SmCo products are shown in Fig. 1a (also see Fig. S2). After reduction, fibers of 150 nm diameter and aspect ratio > 50 were observed in all samples. There was no distinct difference in dimension or morphology among samples 274
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
Fig. 3. (a) M-H curves of pure and Fe-substituted Sm2Co17 nanofibers of various Fe substitution content (0–40 at%), obtained under the applied field of 25 kOe (2.5 T) at room temperature; (b) Dependence of various magnetic properties upon Fe concentration: maximum magnetization (M2.5T), remanence (Mr), squareness (Mr/Ms), intrinsic coercivity (Hci), and calculated maximum energy product ((BH)max).
Fig. 2. (a, b) TEM micrographs with cross-sectional compositional line profile and whole area mapping image for 20 at% Fe-substituted SmCo nanofiber: (a) Sm2O3–Co(Fe) (insufficiently reduced) and (b) Sm2Co17–Sm2Co7–Fe. (c) Schematic of the phase formation mechanism in the 20 at% Fe-substituted SmCo nanofiber.
structure, and to understand the change in Fe distribution inside the final SmCoFe nanomagnets.
the following reactions [15]:
2SmCoO3 + 3Ca → Sm2 O3 + 2Co + 3CaO
(1)
Co3 O4 + 4Ca → 3Co + 4CaO
(2)
Sm2 O3 + 17Co + 3Ca → Sm2 Co17 + 3CaO
(3)
3.2. Magnetic performance of Fe-substituted SmCo nanofibers To elucidate the effect of Fe substitution upon magnetic properties, VSM analysis was carried out. M-H curves of samples of various Fe contents are shown in Fig. 3a (full version in Fig. S6), and Table S1 lists the corresponding numerical data. All curves showed single-phase-like magnetization behavior even though some samples consisted of two or more phases (i.e., soft magnetic Fe; hard magnetic Sm2(Co,Fe)17, and Sm2Co7). The magnetic parameters determined from Fig. S6, including the maximum magnetization under the maximum applied field (M2.5T), remanence (Mr), squareness (Mr/Ms), intrinsic coercivity, (Hci) and calculated maximum energy product ((BH)max), are plotted in Fig. 3b. As the Fe content was increased, the M2.5T and Mr of the nanofibers increased, which can be correlated to an increase in the formation of SmCoFe phase and/or soft magnetic Fe phase, each of which have high magnetic moment. On the other hand, the Hci initially increased, reaching a maximum value of 7375 Oe, and then showed a declining tendency with further increases in Fe content. A similar phenomenon is seen in (BH)max. For the sake of simple comparison, (BH)max values were calculated from each M-H curve collected from samples not subjected to magnetic alignment or sintering processes. With increasing Fe substitution content, the nanofibers showed an increase in calculated (BH)max from 8.61 MG·Oe (no
Due to the difference in oxidation energy, Co oxides and Fe oxides were reduced to their respective metallic forms and Sm2O3 reduction ensued (see Fig. S4 for the standard Gibbs energy changes for some elements versus temperature). A Co(Fe) core and Sm2O3 shell structure was observed to form during the reactions, corresponding to Eqs. (1) and (2) (Fig. 2a). When the oxides were fully reduced, SmCo metallic phase was formed (Eq. (3)), and interestingly, the surplus Fe no longer remained as a core (Fig. 2b). As verified by the line mapping and whole area mapping analysis, it was observed that the Fe intergranular phase was homogeneously distributed inside the nanofibers. D. Gengfeng et al. carried out the reduction–diffusion process on a Sm2O3/Fe bulk specimen, finding that the Fe nucleus still exists in the granular center even when the reduction–diffusion process is nearly over [19]. In this regard, the different Fe phase distribution presently observed inside the nanofibers is an important discovery that overturns the previous report. The illustration of Fig. 2c may help to better understand the flow of the phase transition, starting from oxides and ending in a reduced metal 275
Applied Surface Science 471 (2019) 273–276
J. Lee et al.
Appendix A. Supplementary material
substitution) to 13.17 MG·Oe at 10 at% Fe substitution, then gradually decreased to 9.36 MG·Oe with further increases in Fe content. It is worth noting that the (BH)max values of all Fe-substituted samples surpassed that of pure SmCo nanofibers. This improvement is obviously a consequence not only of improved net magnetic moment but also of the exchange-coupling interaction between the intergranular soft magnetic Fe and the hard magnetic SmCo(Fe) phase. A hard magnetic matrix having a soft magnetic inclusion is one of the possible structures for exchange-spring magnets [8]. Also, such a smooth M-H curve with no kink, squareness of over 50% (Mr/M2.5T), and drastic (BH)max increase are the clear manifestations of an effective exchange-coupling effect [22]. When the Fe content is higher than the optimal value (i.e., 10 at%), the magnetic interaction would be weakened owing to a predominant dipolar interaction [23]. Thus, these results demonstrated that exchange-coupling occurs in the SmCoFe nanofiber.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.11.217. References [1] P. Campbell, Permanent Magnet Materials and Their Application, Cambridge University Press, 1996. [2] N. Poudyal, C. Rong, V.V. Nguyen, J. Ping Liu, Hard-phase engineering in hard/soft nanocomposite magnets, Mater. Res. Express 1 (2014) 016103. [3] J.M.D. Coey, Hard magnetic materials: a perspective, IEEE Trans. Magn. 47 (2011) 4671–4681. [4] S. Yu, X. Zhao, S. Wu, M.C. Nguyen, Z.-Z. Zhu, C.-Z. Wang, K.-M. Ho, New structures of Fe3S for rare-earth-free permanent magnets, J. Phys. D Appl. Phys. 51 (2018) 075001. [5] T.N. Lamichhane, M.T. Onyszczak, O. Palasyuk, S. Sharikadze, T.-H. Kim, M.J. Kramer, R. MacCallum, A.L. Wysocki, M.C. Nguyen, V.P. Antropov, Single Crystal Permanent Magnet: Extraordinary Magnetic Behavior in the ta, Cu and Fe Substituted CeCo5 System, arXiv preprint arXiv:1805.06545, 2018. [6] P. Tozman, H. Sepehri-Amin, Y. Takahashi, S. Hirosawa, K. Hono, Intrinsic magnetic properties of Sm (Fe1-xCox) 11Ti and Zr-substituted Sm1-yZry (Fe0. 8Co0. 2) 11.5 Ti0. 5 compounds with ThMn12 structure toward the development of permanent magnets, Acta Materialia 153 (2018) 354–363. [7] F. Meng, R.P. Chaudhary, K. Gandha, I. Nlebedim, A. Palasyuk, E. Simsek, M.J. Kramer, R.T. Ott, Rapid assessment of the Ce-Co-Fe-Cu system for permanent magnetic applications, JOM (2018) 1–7. [8] D. Li, D. Pan, S. Li, Z. Zhang, Recent developments of rare-earth-free hard-magnetic materials, Sci. China Phys. Mech. Astronomy 59 (2015). [9] D.L. Schodek, P. Ferreira, M.F. Ashby, Nanomaterials, Nanotechnologies and Design: An Introduction for Engineers and Architects, Butterworth-Heinemann, 2009. [10] O. Frank Agyemang, A.S. Faheem, A.-N. Richard, Y. Xinsheng, K. Hern, A simple method of electrospun tungsten trioxide nanofibers with enhanced visible-light photocatalytic activity, Nano-Micro Lett. 7 (2015). [11] N.A.M. Barakat, B. Kim, H.Y. Kim, Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties, J. Phys. Chem. C 113 (2009) 531–536. [12] G.C. Hadjipanayis, G.A. Prinz, Science and Technology of Nanostructured Magnetic Materials, Springer US, 2013. [13] K.H.J. Buschow, Concise Encyclopedia of Magnetic and Superconducting Materials, Elsevier Science, 2005. [14] J. Lee, T.-Y. Hwang, H.-B. Cho, J. Kim, Y.-H. Choa, Near theoretical ultra-high magnetic performance of rare-earth nanomagnets via the synergetic combination of calcium-reduction and chemoselective dissolution, Sci. Rep. 8 (2018) 15656. [15] J. Lee, T.-Y. Hwang, M.K. Kang, H.-B. Cho, J. Kim, N.V. Myung, Y.-H. Choa, Synthesis of Samarium-cobalt sub-micron fibers and their excellent hard magnetic properties, Front. Chem. 6 (2018) 18. [16] T.-Y. Hwang, J. Lee, H.-R. Lim, S.-J. Jeong, G.-H. An, J. Kim, Y.-H. Choa, Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method, Ceram. Int. 43 (2017) 3879–3884. [17] M. Saito, H. Fujiwara, J. Mizuno, T. Homma, Preparation of nano-structured CoCu films by electrodeposition, in: Meeting Abstracts, The Electrochemical Society, 2006, pp. 1278–1278. [18] S. Talukdar, J.-M. Fang, Reduction and coupling reactions of carbonyl compounds using samarium metal in aqueous media, J. Organ. Chem. 66 (2001) 330–333. [19] D. Gengfeng, J. Qingxiu, W. Xiuhong, H. Guirong, Y. Xinyu, Synthesis mechanism of Sm2Fe17 alloy produced in reduction-diffusion process, J. Rare Earths 28 (2010) 420–424. [20] G. Qi, M. Hino, A. Yazawa, Experimental study on the reduction-diffusion process to produce Fe–Nd, Fe–Sm, Co–Nd and Co–Sm alloys, Mater. Trans. JIM 31 (1990) 463–470. [21] R. Horikawa, H. Fukunaga, M. Nakano, T. Yanai, Magnetic properties of isotropic and anisotropic SmCo5/α-Fe nanocomposite magnets with a layered structure simulated by micromagnetic theory, J. Appl. Phys. 115 (2014) 17A707–717A707. [22] L.Q. Yu, Y.P. Zhang, Z. Yang, J.D. He, K.T. Dong, Y. Hou, Chemical synthesis of Nd2Fe14B/Fe3B nanocomposites, Nanoscale 8 (2016) 12879–12882. [23] F. Yi, Magnetic properties of hard (CoFe2O4)–soft (Fe3O4) composite ceramics, Ceram. Int. 40 (2014) 7837–7840.
4. Conclusion The novel synthesis of SmCo nanofibers including the earth-abundant element Fe as a Co substituent was successfully demonstrated via electrospinning and subsequent heat treatment. Interestingly, the substitution achieved simultaneous enhancements in magnetic performance and cost-effectiveness: 10 at% Fe substitution yielded the largest (BH)max of 13.17 MG·Oe compared to that of pure Sm2Co17 nanofibers (about 8.61 MG·Oe). This can be ascribed to the critical role of Fe in enhancing the net magnetic moment, and the importance of optimizing the Fe content. Moreover, based on kinetics and microstructural analysis, we conclude that the homogeneous distribution of a small amount of Fe grains, a Sm-rich phase (induced by surplus Sm), and a Fe-substituted SmCoFe phase leads to an exchange-coupling interaction. The present work provides an innovative approach to producing SmCoFe one-dimensional nanomagnets having various Fe substitution contents as a means to understand the mechanism of phase formation, particularly the reduction–diffusion reaction occurring in this ternary system. This novel nanostructure can be extended to further enhance magnetic properties by modifying microstructural characteristics or to develop exchange-coupled nanomagnets, and may open a path to improved materials far beyond the limitations of traditional magnetic materials. Acknowledgments 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 (NRF2016M3D1A1027836). This research was also financially supported by the Ministry of Trade, Industry & Energy (MOTIE), and the Korea Institute for Advancement of Technology (KIAT) through the Encouragement Program for Regional Industry Nurturing Program (Project No. R0004915). Notes The authors declare no competing financial interest.
276