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One-pot synthesis of high-quality bimagnetic core/ shell nanocrystals with diverse exchange coupling Yahong Chai, Feng Feng, Qilong Li, Chanchan Yu, Xueyan Feng, Pan Lu, Xiaolin Yu, Maofa Ge, Xiuyu Wang, and Li Yao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12888 • Publication Date (Web): 09 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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One-pot synthesis of high-quality bimagnetic core/shell nanocrystals with diverse exchange coupling Yahong Chai1, 2, Feng Feng1, 2, Qilong Li1, 2, Chanchan Yu1, 2, Xueyan Feng1, 2, Pan Lu1, 2, Xiaolin Yu1, Maofa Ge1, 2, Xiuyu Wang1*, Li Yao1, 2* 1

Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,

CAS Research/Education Center for Excellence in Molecular Sciences Zhongguancun North First Street 2, Beijing 100190, China 2

University of Chinese Academy of Sciences, Beijing 100049, China E-mail: [email protected]; [email protected] Supporting Information Placeholder ABSTRACT:

Exchange coupled bimagnetic core/shell nanoparticles are promising for emerging multiferroic and spintronic technologies compared with traditional, single-phase materials, as they deliver numerous appealing effects, such as large exchange bias, tailored coercivities, and tunable blocking temperatures. However, it remains a challenge to manipulate their magnetic properties via exchange coupling due to the lack of a straightforward method that enables the general preparation of desired composites. Here we report a robust and general one-pot approach for the synthesis of different kinds of bimagnetic core/shell nanostructures (BMCS NSs). The formation of highly crystalline and monodisperse BMCS NSs adopted a self-adaptive sequential growth, circumventing the employment of complex temperature control and elaborate seeded growth techniques. As a result of large lattice misfit, the presence of interfacial imperfections as an extra source of anisotropy induced diverse exchange coupling interactions in ferro-ferrimagnetic and ferro-antiferromagnetic systems, which had great effects on the improvement of the magnetic properties of BMCS NSs. We envision that this new strategy will open up exciting opportunities toward large-scalable production of such high-quality BMCS NSs, thereby greatly potentiating the prospective applications of nanomagnetic materials.

Exchange coupling describes a magnetic interaction across the interface between two phases, such as ferri-ferromagnetic (FiM-FM) phases, ferro-antiferromagnetic (FM-AFM) phases, and ferri-antiferromagnetic (FiM-AFM) phases.1 It bestows nanomaterial with novel magnetic properties, such as large exchange bias, tailored coercivity, and tunable blocking temperature etc. Studies have demonstrated superiorities of these exchange-coupled nanomaterials to conventional single-phase

magnetic materials in various fields as magnetic shielding, magnetic hyperthermia, and magnetic resonance imaging.2-7 A requirement for effective exchange coupling is that the two interactive magnetic phases be contained to nanometer scales.2 The interface is particularly critical for electron coupling as its sharpness, lattice mismatch, and chemical gradient greatly influence electron behaviors.8 BMCS NSs are ideal candidates to build exchange-coupled magnetic materials, because the dimensions of the two magnetic phases are tuned by the core diameter and shell thickness, and the direct contact between the core and shell ensures strong interaction.9 Syntheses of FeO/Fe3O4,10 MnO/Mn3O4,11 and Co0.3Fe0.7O/Co0.6Fe2.4O412 could be achieved with one-pot methods. However, a straightforward and robust method that enables the general preparation of alloy/metallic oxide BMCS NSs has rarely been reported. During the epitaxial growth of a strained material domain onto a mismatched substrate, the lattice misfit introduces into the system an interfacial energy,13 which is a severe obstacle particularly to core/shell nanocrystals (NCs) with (quasi-) spherical cores that have highly curved surfaces.14 Moreover, for two distinct materials to grow into well-defined core/shell NSs, the kinetic and thermodynamic parameters must be carefully controlled; otherwise, the resulting NSs could be very heterogeneous in that some contain incompletely covered cores, some multiple oxide domains on the cores, while some others form large clusters.13 Herein, we report a general one-pot synthetic strategy, self-adaptive sequential growth of NSs, where exchange-coupled core/shell nanocrystals with well-defined sizes and shapes, high-quality crystallines and a broad range of compositions (e.g., [email protected], [email protected], [email protected], and [email protected]) were produced with no complex temperature control. It involves dumping all the reactants into the surfactant mixture, and spontaneous growth of the nanocrystals proceeds. Strong exchange coupling was observed at the core/shell

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interfaces in FM/FiM and FM/AFM systems, providing an extra source of anisotropy for magnetization stability and modulation.

Figure 1. Characterization of [email protected] core/shell nanostructures. (a) TEM image, (b, c) STEM-ADF images, (d-g) EDS elemental mapping, (h) Individual particle line scan EDS analysis. Figure 1 highlights the synthesis results of typical cubic [email protected] core/shell NSs. The nanocrystals show good alignment in short-range (Figure 1a, σ=0.066). The high-angle annular dark-field scanning TEM (HAADF STEM) images present a definite core/shell structure (Figure 1b) and high-quality crystallinity across both the small core and thick shell structure (Figure 1c). The averages of edge length and core size are 8.9±1.1 nm and 2.2±0.3 nm, respectively (Figure S1). The fcc phase of FePt core and the cubic spinel structure of magnetite (Fe-oxide) shell were revealed with power X-ray diffraction (XRD) (Figure S2) and X-ray photoelectron spectrum (XPS) (Figure S3).15 To analyze the elemental composition, energy-dispersive X-ray spectroscopy (EDS) elemental mapping was employed. Figures 1d-g are mapping images of [email protected] nanoparticles. O, Fe, and Pt are colored in blue, red and green, respectively. Fe is distributed across the nanoparticles, but Pt is present only in the core region of each nanoparticle (Figure 1e-f, 1h). A composite image (Figure 1g) reveals that FePt cores were well shrouded by Fe-oxide shell.

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Fe3O4, respectively. A progressive misalignment between (200)FePt planes and (400)Fe3O4 planes was observed (Figure 2b), which, indicative of the lattice mismatch as large as 8%, ultimately leads to edge dislocations.16 It demonstrates a heteroepitaxial orientation relationship between the core and shell.17 Moreover, point defects such as substitutions, vacancies and impurities observed in the [email protected] interface regions (Figure 2c) accommodate the large lattice mismatch. Another interesting structural feature revealed in Figure 2c is the juxtaposition of iron atoms of (110)FePt planes and (220)Fe3O4 planes. It is imaginable, for the differences of chemical nature between the core and shell significantly impact the heteroepitaxial growth.16 The presence of these high densities of point defects, lattice mismatch and dislocation in the interface provides an extra source of anisotropy, which significantly affects the exchange coupling interactions.18 The well-defined core/shell NSs adopted a self-adaptive sequential growth pattern (Scheme S1, Figure S4). During the heating process, Pt-surfactant and Fe-surfactant are formed in the solution with the fast ligand exchange.16 Pt2+ is easily reduced to Pt owning to their high redox potential.19 Pt-rich nuclei formed due to a lower nucleation temperature threshold.16 With the catalysis of electron transfer between oleylamine and Fe ions by Pt,20,21 the reduction of iron (Fe3+→Fe) and formation of FePt NCs were achieved.22 Following Pt depletion in the mixture, the FePt core absorption of iron ions and ligands triggers subsequent heteroepitaxial growth of the Fe-oxide shell.21 It turns out that the homogeneous nucleation of the separate Fe3O4 nanoparticles does not occur, for the solution supersaturation is soon relieved.16 Final refluxing of the mixture leaded to the growth of well-define cubic Fe-oxide shell on FePt core. Excessive iron precursors provide a kinetic advantage in our system.21 As Fe:Pt ratio increases from 1:1 to 6:1, the morphology of NCs transformed from simple fcc FePt NCs to uniform [email protected] core-shell heterostructures (Figure S5).

Figure 2. Interface characterizations of [email protected] core/shell nanostructures. (a) Lattice relationships between planes of individual lattices. (b) Edge dislocations. (c) Atom site occupations and point defects (oxygen, iron atoms of Fe3O4 were circled in green and red, platinum and iron atoms of FePt in white and yellow, point defects in blue). To elucidate the interfacial structures of [email protected] core/shell NSs, high-resolution HAADF analyses were carried out. As shown in Figure 2a, the interplanar spacings of 2.23 Å and 1.48 Å correspond to (111) planes of FePt and (440) planes of

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Figure 3. (a) Schematic illustration of one-pot syntheses of various BMCS NSs (b-g) Typical TEM images (b, c, d), and HRTEM images (e, f, g) of [email protected], [email protected] and [email protected] BMCS NSs, respectively. Most importantly, our one-pot strategy is quite robust and can be extended to produce various BMCS NSs (Figure 3a). Magnetic nanoparticles of 3d transition metal (e.g., Fe, Mn, Co and Ni) oxides were used to develop varied exchange anisotropy through coupling to the ferromagnetic FePt spin system. Considering that these metal oxides are common products of metal acetylacetonates decomposition, we separately introduced three M-element (M= Mn, Co and Ni) acetylacetonates into our system. TEM images show that the particles have distinct configuration of FePt core and metal oxide shell (Figure 3b-d). High-resolution TEM (HRTEM) analysis and XPS measurements revealed the core/shell NSs of [email protected], [email protected] and [email protected] (Figure 3e-g, Figure S6a-c), where take [email protected] for example (Figure 3e), most displayed facets show lattice fringes with an interplanar spacing of 2.58 Å, corresponding to the (111)MnO planes. The XPS of Mn 2p1/2 and 2p3/2 reveal that the signals are located at 653.4 and 641.4 eV, respectively (Figure S6a), in accordance with literature values of Mn2p in bulk MnO.23 Put together, these results demonstrate a general and robust synthetic approach to produce various BMCS NSs.

loop of [email protected] exhibits a smooth change of the magnetization as a function of applied field, indicative of an effective coupling between the two phases (Figure 4a, Figure S9a).16 Comparison of coercivities (Hc) of FePt, Fe3O4 and [email protected] reflects internal FM/FiM exchange coupling interaction within the composite (Figure 4b).4 Interestingly, the saturation magnetization (MS) of the composite is much higher than that of each component alone (Figure 4a), potentially caused by the many interface defects between FePt core and Fe-oxide shell.18 Such strong coupling interaction was further confirmed by the remanence (Mr) measurements.24 In contrast to individual components, composite [email protected] evidently has enhanced remanence (Figure 4c and Figure S9b-c).25 We found the blocking temperature (TB) shifted to a higher value (Figure 4d). This attributes to exchange-coupling induced suppression of thermal fluctuation of magnetic spins.26 The magnetic properties of annealed FePt and [email protected] NCs are shown in Figure S10. Taken together, these results demonstrate strong exchange coupling interactions within NSs, shedding lights on approaches of obtaining large effective additional anisotropies. Strong exchange interactions were also found in coupled FM/AFM systems of [email protected], [email protected] and [email protected], where smooth hysteresis loops and enhanced TB were observed (Figure 4e, f and Table S1). As expected, [email protected] and [email protected] exhibit Hc enhancement and hysteresis loop shift (i.e., exchange bias HE) (Figure 4f, g), typical characteristics of FM/AFM exchange coupling interactions.6 Since magnetic systems containing NiO are low-crystalline anisotropy AFM materials, [email protected] shows decreased HC and no hysteresis loop shift.1 Altogether, BMCS NSs demonstrate the availability to separately tune the thermal stability (e.g., TB) and magnetic stability (e.g., Hc) of magnetic NCs by controlling the type and magnitude of exchange coupling through varying the shell components.5 These exotic magnetic properties arise from interactions among lattice, spin, charge, and orbital degrees of freedom coupled with defect and strain across the interface between two phases.18, 26

Figure 4. Exchange-coupled properties of BMCS NSs investigated with SQUID. (a-d) Magnetism characterization of FePt, [email protected], and Fe3O4 NCs: M−H curves at 300 K (a), Hc (b), Mr (c), and ZFC magnetization curves (d). (e, f, g) Magnetism characterization of [email protected], [email protected] and [email protected] NCs: ZFC-FC magnetization curves (e), M−H curves at 5 K (f, inset in the low field region) and Hc (HE) values (g). Superconducting quantum interference device (SQUID) characterization of these BMCS NSs allows for the analysis of diverse exchange anisotropies including FM/FiM and FM/AFM exchange coupling. [email protected] was used to study the FM/FiM coupling effect. For the convenience of comparison, we conducted control experiments using FePt NCs (~2.3 nm, Figure S7) and cubic Fe3O4 NCs (~10.2 nm, Figure S8). The hysteresis

The one-pot method is easy to carry out and can be extended to produce different kinds of magnetic core/shell NSs (Figure S11). It saves the taxing stepwise administering of reactants and meticulous controlling of temperature, which are necessary to the previously reported seed-mediated17 or temperature-regulated methods16. It can be rationalized that the process of self-adaptive sequential growth of core/shell NSs depends on high Fe:Pt molar ratio, which controls the kinetics of particle growth,21 and the crystallographic defects at the interface efficiently compensate the interfacial strain.17 Hyeon and co-workers reported that coincidence lattices and chemical affinity can work in a complementary way to lead to the formation of low-energy interfaces in the systhesis of multicomponent NSs.27 Owning presumably to the small atoms/ions size mismatch,28 the selective growth of the shell component follows after the formation of FePt alloy (Table S2, Figure S12 and S13). While the size, shape and interface control of BMCS NSs is not sophisticated as other methods, and many aspects remain unclear, it does have unique

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advantage. Further work is necessary to understand the fundamental mechanism of this process. In summary, a robust and general one-pot methodology for the synthesis of well-defined alloy (FePt)-metallic (M= Fe, Co, Mn, Ni) oxide BMCS NSs is reported for the first time. The formation of highly crystalline and monodisperse core/shell NSs adopted a self-adaptive sequential growth. These unique BMCS NSs with large lattice misfit and high-density interfacial imperfections provide an intriguing playground, allowing for the manipulation of magnetic properties via diverse exchange coupling. We envision that this new strategy provides a very promising approach to synthesize large-scale high-quality core/shell NSs and widen the range of possible material combinations for the design of heterogeneous magnetic nanomaterials, which have great potential in advancing sensitive diagnostics, therapeutics, and high-density magnetic data recording, etc.

ASSOCIATED CONTENT Supporting Information Experimental section and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by grants from the National Key Research and Development Program of China (2018YFA0208800), National Natural Science Foundation of China (21778055, 21573250), Beijing Natural Science Foundation (L172048), the Chinese Academy of Sciences (QYZDB-SSW-SLH024). We are grateful to Dr. Song Hong (Analytical Test Center, Beijing University of Chemical Technology, China) for performing the STEM measurements.

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(5) Song, Q.; Zhang, Z. J. Controlled synthesis and magnetic properties of bimagnetic spinel ferrite CoFe2O4 and MnFe2O4 nanocrystals with core-shell architecture. J. Am. Chem. Soc. 2012, 134, 10182. (6) Estrader, M.; López-Ortega, A.; Estradé, S.; Golosovsky, I. V.; Salazar-Alvarez, G.; Vasilakaki, M.; Trohidou, K. N.; Varela, M.; Stanley, D. C.; Sinko, M.; pechan, M. J.; Keavney, D. J.; Peiró, F.; Suriñach, S.; Baró, M. D.; Nogués, J. Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles. Nat. Commun. 2013, 4, 2960. (7) Yang, M. D.; Ho, C. H.; Ruta, S.; Chantrell, R.; Krycka, K.; Hovorka, O.; Chen, F. R.; Lai, P. S.; Lai, C. H. Magnetic interaction of multifunctional core-shell nanoparticles for highly effective theranostics. Adv. Mater. 2018, 1802444. (8) Li, J.; Zeng, H.; Sun, S.; Liu, J. P.; Wang, Z. L. Analyzing the structure of CoFe-Fe3O4 core−shell nanoparticles by electron imaging and diffraction. J. Phys. Chem. B 2004, 108, 14005. (9) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. Bimagnetic core/shell FePt/Fe3O4 nanoparticles. Nano Lett. 2015, 4, 187. (10) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mater. 2004, 3, 891. (11) Alberto, L. O.; Dina, T.; Elin, W.; Golosovsky, I. V.; German, S. A.; Sònia, E.; Marta, E.; Jordi, S.; Miguel Angel, G.; Santiago, S.; Jordi, A.; Francesca, P.; Roberto, D. Z.; Maria, D. B.; Josep, N. Size-dependent passivation shell and magnetic properties in antiferromagnetic/ferrimagnetic core/shell MnO nanoparticles. J. Am. Chem. Soc. 2010, 132, 9398. (12) Lottini, E.; López-Ortega, A.; Bertoni, G.; Turner, S.; Meledina, M.; Van Tendeloo, G.; Fernández, C. D. J.; Sangregorio, C. Strongly exchange coupled core|shell nanoparticles with high magnetic anisotropy: a strategy toward rare-earth-free permanent magnets. Chem. Mater. 2016, 28, 4214. (13) Sun, H.; He, J.; Wang, J.; Zhang, S. Y.; Liu, C.; Sritharan, T.; Mhaisalkar, S.; Han, M. Y.; Wang, D.; Chen, H. Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation. J. Am. Chem. Soc. 2013, 135, 9099. (14) Zhang, J.; Tang, Y.; Lee, K.; Ouyang, M. Nonepitaxial growth of hybrid core-shell nanostructures with large lattice mismatches. Science 2010, 327, 1634. (15) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441. (16) Figuerola, A.; Fiore, A.; Corato, R. D.; Falqui, A.; Giannini, C.; Micotti, E.; Lascialfari, A.; Corti, M.; Cingolani, R.; Pellegrino, T.; Cozzoli, P. D.; Manna, L. One-pot synthesis and characterization of size-controlled bimagnetic FePt-iron oxide heterodimer nanocrystals. J. Am. Chem. Soc. 2008, 130, 1477. (17) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Curri, M. L.; Innocenti, C.; Sangregorio, C.; Achterhold, K.; Parak, F. G.; Agostiano, A.; Cozzoli, P. D. Seeded growth of asymmetric binary nanocrystals made of a semiconductor TiO2 rodlike section and a magnetic γ-Fe2O3 spherical domain. J. Am. Chem. Soc. 2006, 128, 16953. (18) Chen, A.; Su, Q.; Han, H.; Enriquez, E.; Jia, Q. Metal oxide nanocomposites: a perspective from strain, defect, and interface. Adv. Mater. 2018, 1803241.

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(19) Bu, L.; Ding, J.; Guo, S.; Zhang, X.; Su, D.; Zhu, X.; Yao, J.; Guo, J.; Lu, G.; Huang, X. A general method for multimetallic platinum alloy nanowires as highly active and stable oxygen reduction catalysts. Adv. Mater. 2016, 27, 7204. (20) Beck, W.; Souza, C. G. S.; Silva, T. L.; Jafelicci, M.; Varanda, L. C. Formation mechanism via a heterocoagulation approach of FePt nanoparticles using the modified polyol process. J. Phys. Chem. C 2011, 115, 10475. (21) Liang, W.; Zhang, X.; Zan, Y.; Pan, M.; Czarnik, C.; Bustillo, K. C.; Xu, J.; Chu, Y.; Zheng, H. In situ study of Fe3Pt– Fe2O3 core–shell nanoparticle formation. J. Am. Chem. Soc. 2015, 137, 14850. (22) Chen, M.; Liu, J. P.; Sun, S. One-step synthesis of FePt nanoparticles with tunable size. J. Am. Chem. Soc. 2004, 126, 8394. (23) Du, Y. P.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. Self-assembled ferromagnetic monodisperse manganese oxide nanoplates synthesized by a modified nonhydrolytic approach. J. Phys. Chem. C 2009, 113, 6521. (24) Wang, X.; Hou, Y.; Yao, L.; Gao, M.; Ge, M. Generation, characterization, and application of hierarchically structured self-assembly induced by the combined effect of

self-emulsification and phase separation. J. Am. Chem. Soc. 2016, 138, 2090. (25) Daffé, N.; Sikora, M.; Rovezzi, M.; Bouldi, N.; Gavrilov, V.; Neveu, S.; Choueikani, F.; Ohresser, P.; Dupuis, V.; Taverna, D.; Gloter, A.; Arrio, M.-A.; Sainctavit, P.; Juhin, A. Nanoscale distribution of magnetic anisotropies in bimagnetic soft core–hard shell [email protected] nanoparticles. Adv. Mater. Interfaces 2017, 4, 1700599. (26) Fan, Y.; Smith, K. J.; Lüpke, G.; Hanbicki, A.T.; Goswami, R.; Li, C. H.; Zhao, H. B.; Jonker, B. T. Exchange bias of the interface spin system at the Fe/MgO interface. Nat. Nanotechnol. 2013, 8, 438. (27) Lee, J.; Yang, J.; Kwon, S. G.; Hyeon, T. Nonclassical nucleation and growth of inorganic nanoparticles. Nat. Rev. Mater. 2017, 1, 16034. (28) Lee, W.; Han, J. W.; Chen, Y.; Cai, Z.; Yildiz, B. Cation size mismatch and charge interactions drive Dopant segregation at the surfaces of manganite perovskites. J. Am. Chem. Soc. 2013, 135, 7909.

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Figure 1. Characterization of [email protected] core/shell nanostructures. (a) TEM image, (b, c) STEM-ADF images, (d-g) EDS elemental mapping, (h) Individual particle line scan EDS analysis. 468x241mm (300 x 300 DPI)

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Figure 2. Interface characterizations of [email protected] core/shell nanostructures. (a) Lattice relationships between planes of individual lattices. (b) Edge dislocations. (c) Atom site occupations and point defects (oxygen, iron atoms of Fe3O4 were circled in green and red, platinum and iron atoms of FePt in white and yellow, point defects in blue). 493x163mm (300 x 300 DPI)

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Figure 3. (a) Schematic illustration of one-pot syntheses of various BMCS NSs (b-g) Typical TEM images (b, c, d), and HRTEM images (e, f, g) of [email protected], [email protected] and [email protected] BMCS NSs, respectively. 451x455mm (300 x 300 DPI)

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Figure 4. Exchange-coupled properties of BMCS NSs investigated with SQUID. (a-d) Magnetism characterization of FePt, [email protected], and Fe3O4 NCs: M−H curves at 300 K (a), Hc (b), Mr (c), and ZFC magnetization curves (d). (e, f, g) Magnetism characterization of [email protected], [email protected] and [email protected] NCs: ZFC-FC magnetization curves (e), M−H curves at 5 K (f, inset in the low field region) and Hc (HE) values (g). 430x295mm (300 x 300 DPI)

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