Controllably Manipulating Three-Dimensional Hybrid Nanostructures

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Controllably Manipulating Three-Dimensional Hybrid Nanostructures for Bulk Nanocomposites with Large Energy Products Xiaohong Li, Li Lou, Wenpeng Song, Qian Zhang, Guangwei Huang, Yingxin Hua, Hai-Tian Zhang, Jianwei Xiao, Bin Wen, and Xiangyi Zhang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00264 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Controllably

Manipulating

Three-Dimensional

Hybrid

Nanostructures for Bulk Nanocomposites with Large Energy Products Xiaohong Li,†‡ Li Lou,† Wenpeng Song,† Qian Zhang,† Guangwei Huang,† Yingxin Hua,† HaiTian Zhang,§ Jianwei Xiao,† Bin Wen,† and Xiangyi Zhang*† †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, China ‡

College of Science, Yanshan University, Qinhuangdao 066004, China

§

Department of Materials Science and Engineering, The Pennsylvania State University,

University Park, PA, 16802, USA

ABSTRACT: Hybrid nanostructures that comprise two or more nanoscale functional components are fascinating for applications in electronics, energy conversion devices, and biotechnologies. Their performances are strongly dependent on the characteristics of the individual components including the size, morphology, orientation, and distribution. However, it remains challenging to simultaneously control these structural properties in a three-dimensional (3D) hybrid nanocstructure. Here, we introduce a robust strategy for concurrently manipulating these characteristics in a bulk SmCo/Fe(Co) nanocomposite. This method can tune nanocrystals in size (down to sub-10 nm), morphology (sphere, rod or disc), and crystallographic orientation (isotropic or anisotropic). We have therefore achieved the desired nanostructures: oriented hard magnetic SmCo grains and homogeneously distributed soft magnetic Fe(Co) grains with high

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fractions (~26 wt.%) and small sizes (~12.5 nm). The resulting anisotropic nanocomposite exhibits an energy product which is approximately 50% greater than that of its corresponding pure SmCo magnet and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Our findings pave a new way to manipulating 3D hybrid nanostructures in a controllable manner. KEYWORDS: Nanostructuring, hybrid nanostructures, heterostructures, nanocomposite magnets, bulk nanomaterials

Hybrid nanocomposites have attracted considerable interest for advanced applications in electronics,1 energy conversion and storage devices,2,3 and biotechnologies4 due to their enhanced or novel functional properties. The performances of the nanocomposites are strongly dependent on the characteristics of their individual components including the size, morphology, orientation, and distribution.2,5-7 Precise control over these properties is therefore crucial for their fascinating applications. This has inspired researchers to explore various approaches to synthesize hybrid nanostructures with well-controlled characteristics.6-10 To date many types of hybrid nanostructures have been produced with a variety of chemical approaches, including metal reduction and thermal decomposition,11-13 colloidal synthesis,14 template-assisted method,9,15 nanoparticle self-assembly,16,17 and chemical bath deposition.18 The size, morphology, orientation and distribution of nanocrystals within the materials can be precisely controlled through rationally tuning their nucleation and growth processes, but these methods limit the materials to nanoparticles,8 one-dimensional (1D) or 2D nanomaterials9,10 and thus are not appropriate for making practical 3D bulk materials such as permanent magnets and thermoelectric materials. Physical approaches can efficiently produce bulk nanomaterials,19,20 but


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it remains a challenge to fabricate 3D hybrid nanocomposites with controllable nanocrystal size, orientation, morphology, and distribution, as the properties required for individual components are often conflicting in a bulk material.2,5 For example, it is challenging to align sub-20-nm grains along a certain crystallographic direction in a 3D hybrid nanocomposite, owing to the failure of the usual aligning strategy for 1D or 2D nanomaterials in bulk materials which applies epitaxial relation with substrate to direct the growth of nanocrystals;9,10 moreover, to maximize the interaction of the individual components, they need to be uniformly distributed in the composite, which is particularly challenging when they have high fractions and small sizes.2 Some new approaches have recently been proposed for controlling 3D hybrid nanostructures including bottom-up approach,21,22 self-assembly,23,24 and severe plastic deformation.25,26 Although these methods can efficiently control the characteristics of the single constituent phase, they cannot manipulate all the properties of different constituent phases simultaneously. The fabrication of structure-controllable 3D hybrid nanocomposites has hitherto remained a challenge. To tackle this problem, we have devised a novel strategy to concurrently manipulate nanocrystals in size, orientation, morphology and distribution in a 3D hybrid nanostructure. We illustrate the power of this strategy by producing a bulk nanocomposite consisting of magnetically hard and soft phases, where simultaneous control over the size, orientation, morphology and distribution of the two constituent phases is needed to yield a large energy productthe figure of merit to evaluate the performance of a permanent magnet.2,27 However, it is challenging to realize the theoretically required nanostructures:2 aligned hard-phase grains and homogeneously distributed soft-phase grains with high fractions (> 20%) and small sizes (~10 nm). This constitutes the formidable task for fabricating a bulk nanocomposite with larger energy products than its corresponding single-phase hard magnetic material.2,27-29


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Here, we describe the fabrication of a bulk SmCo/Fe(Co) nanocomposite with the desired nanostructures: oriented SmCo hard-phase grains and high fractions (~26 wt.%) of homogeneously distributed Fe(Co) soft-phase grains with ultrafine sizes (~12.5 nm). Our nanocomposite exhibits a large energy product which is approximately 50% greater than that of its corresponding pure hard magnetic material and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Contrary to conventional methods, we made the nanostructures through a combined multifield coupling deformation that enables separate control over the properties of the soft and hard phases, where the stress, strain and temperature-gradient fields are used to control the preferential growth of hard-phase nanocrystals in an amorphous matrix. This new strategy could also be extended to other material systems such as thermoelectric and multiferroic materials for their nanostructuring. Our findings make an important step toward controllable synthesis of 3D hybrid nanostructures. We chose SmCo hard-magnetic material and Fe(Co) soft-magnetic material as functional components to fabricate bulk SmCo/Fe(Co) hybrid nanostructures, as the SmCo material has high magnetocrystalline anisotropy and high Curie temperature,30 making the composite suitable for high-temperature applications, and the Fe(Co) material has large magnetization which contributes to high energy products.5 The characteristics of the soft and hard phases were sequentially controlled with a combined multifield coupling deformation (Figure 1). First, we produced homogeneously distributed α-Fe(Co) grains with high fractions (~31 wt.%) and small sizes (~5−6 nm) in SmCo-based amorphous precursors (Figure S1, Supporting Information) through mechanically milling SmCo raw materials and commercial pure Fe powders (see Figure 1a). Then, we performed temperature-gradient deformation on the amorphous-nanocrystalline precursors at high stress (~1 GPa) and large strain (~80%) (Figure 1b). This step allows us to


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tune SmCo hard-phase nanocrystal in size, orientation and morphology through control over its growth process in amorphous matrix. We expected this strategy to generate bulk SmCo/Fe(Co) nanocomposites with the desired structures (Figure 1c): aligned hard-phase grains and homogeneously distributed soft-phase grains with high fractions and small sizes. To check the power of our approach, we characterized the microstructures of the produced bulk materials. X-ray diffraction (XRD) studies show that the materials are mainly composed of α−Fe(Co) and TbCu7-type SmCo7 (1:7) crystalline phases (see Figure 2a), and that most SmCo7 grains are aligned along its easy magnetization axis (00l) parallel to the pressure direction. This conclusion is derived from the high (002) reflection peak of the SmCo7 phase, comparable to the highest (111) peak of isotropic SmCo7 crystal, on the face perpendicular to the pressure direction (Figure 2b), while a small (002) peak on the face parallel to the pressure direction (Figure 2c). The aligned SmCo7 nanograins are further confirmed by transmission electron microscope (TEM) observations (see below). High weight fraction (~26 wt.%), small grain size (~12.5 nm) and lattice constant a = 2.861 Å were determined for the α−Fe(Co) phase by analyzing the XRD pattern in Figure 2a using the Rietveld refinement procedure (Supporting Information). From the measured lattice constant, the Co content (~ 15 wt.%) in the α−Fe(Co) solid solution was calculated according to the previously reported relationship between the Co concentration and the lattice constant of Fe−Co alloy;31 naturally, owing to atomic interdiffusion, we surmise that the 1:7 hard phase is Sm(Co,Fe)7 which is named SmCo7 below. To study the 3D architecture of the synthesized SmCo/Fe(Co) nanocomposite, we performed TEM observations on the two faces of the material: parallel (longitude section) and perpendicular (cross section) to the pressure direction, which is a routine method for characterizing the morphology of nanocrystals in a bulk material.32 The longitude-sectional TEM


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images show numerous rod-shaped nanograins (indicated with the arrows) approximately along the pressure direction (Figure 2d), which have a boundary spacing (namely the diameter) of ~14 nm and an aspect ratio >2 (Figure S2a,b; Supporting Information). These rod-shaped nanograins are unambiguously identified as the SmCo7 phase by selection area electron diffraction (SAED) (Figure 2e), dark-field TEM (Figure 2f) and high-resolution TEM (HRTEM) (Figure 2g,h) studies. The strong (002) diffraction spots in the SAED pattern (Figure 2e) indicate the (00l) crystal texture of the SmCo7 grains; and the HRTEM images directly show that the [00l] direction of the SmCo7 rod-shaped grains is approximately along the pressure direction (Figure 2g), which is in agreement with the XRD studies (Figure 2a,b). The cross-sectional TEM images show approximately sphere-shaped nanograins (Figure 2i), which have a size distribution similar to that seen in Figure 2d (Figure S2c, Supporting Information). This result suggests that the α−Fe(Co) grains have an equiaxed shape with an average diameter of ~14 nm as supported by HRTEM observations (Figure 2j), which is similar to the value of ~12.5 nm calculated from the XRD measurement. Elemental mapping is an alternative approach for characterizing the morphology of nanocrystals;26 this method combined with TEM sectional observation can be used to further characterize our nanostructure, which will be studied in the future. These results demonstrate that our approach can simultaneously control various characteristics of hybrid nanostructures in a bulk material, thus producing the desired bulk nanostructures: aligned hardphase grains and high fractions (~26 wt.%) of homogeneously distributed soft-phase grains with small sizes (~12.5 nm). We further show that the size, morphology and crystallographic orientation of the bulk hybrid nanostructures can be tuned by changing deformation temperature. We produced the nanostructures at lower (Td = 550 °C) and higher (Td = 700 °C) deformation temperatures. We


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found that the SmCo/Fe(Co) nanostructures yielded at the lower Td = 550 °C exhibited ultrafine equiaxed grains with an average diameter of ~ 9 nm (Figure 3a,b); the continuous diffraction rings in the SAED pattern indicate an isotropic nanostructure (Figure 3c), which is supported by XRD studies (Figure S3a,b; Supporting Information). To study the 3D nanostructures made at the higher Td = 700 °C, we also performed TEM observations on the longitude and cross sections of the bulk material. The longitude-sectional TEM images (along the pressure direction) show a dual-morphology grain structure that consists of equiaxed (25-35 nm in diameter) and lathshaped (50-80 nm in long axis; indicated with the arrows) grains (Figure 3d; Figure S4a, Supporting Information). The strong SmCo7 (002) diffraction spots in the SAED pattern indicate a (00l) crystal texture for the SmCo7 grains (Figure 3e,f), which is confirmed by XRD studies (Figure S3c, Supporting Information). Interestingly, the cross-sectional TEM images (perpendicular to the pressure direction) exhibit two kinds of approximately sphere-shaped grains with small or large size (Figure 3g); these grains have a size distribution similar to that seen in Figure 3d (Figure S4b). These results unambiguously demonstrate that the achieved nanostructures comprise small equiaxed grains (25-35 nm in diameter) and large disc-shaped grains (50-80 nm in diameter). The longitude-sectional HRTEM studies show that the large discshaped grains are the SmCo7 phase with the [00l] axis approximately along the pressure direction (Figure 3h) and the equiaxed grains are the Fe(Co) phase (Figure 3i), which is further supported by the cross-sectional HRTEM studies (Figure 3j). These results show that the SmCo nanocrystals in the bulk nanostructures can be tuned in size (down to sub-10 nm), orientation (isotropic or anisotropic), and morphology (sphere, rod or disc) with our approach. Molecular dynamic calculations suggest that the deformation-induced strain energy anisotropy and surface


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energy minimization of the nanocrystals are responsible for the microstructure changes (see below). Having fabricated the bulk SmCo7/Fe(Co) hybrid nanocomposites with controlled characteristics, we examined their magnetic properties with a vibrating sample magnetometer (VSM). We found that the material consisting of oriented SmCo7 rod-shaped grains and equiaxed Fe(Co) grains with the high fraction (~26 wt.%) and small size (~12.5 nm) exhibited superior magnetic properties. A higher remanence ratio of Mr /Ms = 0.9 (where Mr and Ms are the remanent and saturation magnetizations, respectively) and larger coercivity of Hci = 5.1 kOe are achieved parallel to the pressure direction than perpendicular to it (Figure 4a). This yields a large energy product of (BH)max = 26 MGOe for the bulk SmCo7/Fe(Co) hybrid nanostructure (Figure 4b), which is 47 % greater than that of the best-performing single-phase oriented SmCo7 magnets (17.7 MGOe; ref. 30) and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems26,33-36 (Figure 4c), 19.2 MGOe for the SmCo5/FeCo magnet.26 These results suggest that a bulk nanocomposite magnet stronger than its corresponding single-phase rare-earth magnet can be achieved by producing the theoretically required nanostructures. To understand the reasons for obtaining the large energy product in our nanostructure, we examined both the exchange coupling between the hard and soft grains using a Henkel plot37 and the coercivity mechanism. We found that our nanocomposite exhibited a value of δm =1.1 in the Henkel plot (Figure 5a), which indicates the exchange-coupling strength of magnetic grains37 (Supporting Information). The achieved δm value is much larger than the values (δm = 0.2−0.6) reported for the nanocomposite magnets with high soft-phase fractions (> 20 wt.%).23,38-41 This result suggests that our nanostructure has a stronger exchange coupling between the hard and soft grains at high soft-phase fractions (~26 wt.%), which is reflected in the µm-sized exchange-


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coupled domains (see the inset in Figure 5a), much larger than those (150−300 nm) in previously reported nanocomposite magnets.41-43 We suggest that both the strong exchange coupling (at high soft-phase fractions) and the aligned hard-phase grains contribute to the high remanence ratio (Mr/Ms) and large remanence Br (Figure 4a,b). The coercivity mechanism of the material was studied by measuring its initialmagnetization curve (Figure 5b). The material showed a typical step-type magnetization process with increasing the applied field along the pressure direction, indicating a coercivity mechanism that involves domain-wall pinning, as reported in previous studies.23,38,41,44 A large pinning field Hp = 4.2 kOe contributes to the large coercivity (Hci = 5.1 kOe) with the high soft-phase fraction of ~ 26 wt.%.45 A gradual increase in the magnetization with increasing the applied field perpendicular to the pressure direction (Figure 5b) leads to a small coercivity in this direction. We therefore conclude that the high coercivity Hci and high remanence Br obtained along the pressure direction yield the large energy product (BH)max = 26 MGOe for our nanocomposite. Previous studies have demonstrated that severe plastic deformation, including high-pressure torsion deformation and ball milling, is a promising approach for tuning the size, morphology, and distribution of the soft-phase grains in bulk Nd2Fe14B/Fe (ref. 25) and SmCo5/FeCo (ref. 46) nanocomposites, but the resulting nanostructures exhibit isotropic hard-phase grains, thus yielding low energy products (< 20 MGOe). This problem has now been overcome with our multifield coupling deformation strategy that enables the oriented growth of hard-phase nanocrystals in amorphous matrix, producing anisotropic bulk nanocomposites. Furthermore, combining the multifield coupling deformation with a magnetic field could be a powerful method for the alignment of hard-phase grains.47


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Although our bulk nanostructure exhibits a larger energy product than previously reported isotropic bulk nanocomposites in SmCo/FeCo systems (Figure 4c), the achieved energy product (26 MGOe) is still smaller than the theoretical value of (BH)max = 65 MGOe for anisotropic SmCo/FeCo systems48 and the experimental value (39 MGOe) for anisotropic SmCo5/Fe/SmCo5 thin-film model materials.49 This implies that our nanocomposite still shows big potential for further enhancing the energy product, which can be realized through engineering strain-energy anisotropy and temperature gradient during the deformation process that are crucial for sufficiently aligning the hard-phase grains (see below); moreover, the small coercivity of our nanocomposite could also be enhanced by introducing a multiphase hybrid nanostructure44 or by engineering interfacial structure41,48 to increase domain-wall pinning strength.45 We expect that high-performance bulk nanocomposite magnets will be obtained once these issues are tackled in future studies. To study their thermal stability, we annealed the hybrid nanostructures at a high temperature of Ta = 400 °C for 500 hours. The nanostructures showed no degradation in the (BH)max, Hci and Br after annealing (Figure 5c), thus demonstrating an excellent long-term thermal stability. Manipulating the SmCo/Fe(Co) nanostructure with deformation temperature (see above) leads to a significant change of magnetic properties (see Figure 5d and Figure S5). The nanocomposite made at lower (Td = 550 °C) or higher (Td = 700 °C) deformation temperature exhibited a lower (BH)max and smaller Hci, while the one produced at the optimum Td = 630 °C showed the highest (BH)max with the largest Hci. We believe that the variation of the magnetic properties is associated with the dissimilar nanostructures yielded at different Td. Our results suggest that besides its orientation and size, the morphology of the hard-phase grain also has a significant effect on magnetic properties, and that the rod-shaped oriented grains are particularly


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favorable for the enhanced coercivity (see the insets in Figure 5d), as demonstrated in recent studies of Sm−Co thin films.50 It should be emphasized that the higher soft-phase fraction in the nanostructure made at the lower Td and the disappearance of SmCo3 phase in the one yielded at the higher Td (Figure S3) may also lead to a small coercivity for the materials, since the SmCo3 phase and its stacking faults can contribute to a large coercivity as demonstrated in Sm−Co thin films.44 This also deteriorates the performance of the nanocomposite magnet made at Td = 550 °C or 700 °C. The coupled application of stress, strain and temperature-gradient fields is the key to aligning the SmCo7 nanocrystal and tuning its morphology in our materials. On the basis of surface energy minimization51 and strain energy anisotropy,52 we discuss SmCo7 grain alignment and morphology evolution below. Although the (00l) face of the SmCo7 crystal has a lower surface energy Es (Table S1, Supporting Information), the surface energy minimization for grain alignment was reasonably excluded by annealing SmCo-based amorphous-nanocrystalline precursors under normal pressure, where the produced SmCo7 nanograins were isotropic (Figure S6, Supporting Information). The strain-energy density Wε of the SmCo7 crystal was calculated with the molecular dynamic (MD) method (Supporting Information). The calculated Wε values for different crystal faces show an anisotropy which increases with the strain εe (Figure 6a). If the applied stress on the SmCo7 crystals produces strain along the soft direction that has smaller Wε, the total strain energy of the system will be minimized, which in turn facilitates c-axis (00l) alignment of the SmCo7 nanocrystals that grow from amorphous matrix, as the c axis has a fixed angle with the aligned soft direction. We therefore propose that the increased strain-energy anisotropy at large strain facilitates the alignment of the (00l) axis of the SmCo7 grains. This hypothesis is supported by our experimental results, where the larger the deformation amount,


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the stronger the texture of the SmCo7 nanocrystals, as indicated with the enhanced magnetic anisotropy (Figure S7, Supporting Information). Therefore, the larger deformation amount (7881%) achieved at higher Td = 630 and 700 °C (Figure 6b), coupled with the high stress of ~1 GPa, facilitates the preferential growth of SmCo7 nanocrystals in the amorphous matrix. The morphology of an oriented nanocrystal (along the normal direction of its low-surfaceenergy faces) is strongly associated with its total surface energy E (ref. 51): the rod-shaped nanocrystal has higher E, while the disc-shaped nanocrystal has lower E (Figure 6c). The lower E leads to the growth of the (00l) axis-aligned SmCo7 crystal into a disc-shaped nanocrystal as observed in the nanostructure made at Td = 700 °C, which has also been reported in previous studies.53,54 Strikingly, a large temperature gradient (ᐃT = 170 °C / mm) obtained at Td = 630 °C (Figure 6b) is likely to facilitate the growth of the oriented crystals into the rod-shaped nanocrystals, as the large temperature gradient could overcome the energy barrier of surface energy increase for crystal growth, leading to a directional growth of the oriented nanocrystals (along the temperature gradient direction) and their texture development simultaneously. The temperature-gradient-induced texture growth has been reported in previous directional solidification and annealing studies.55,56 These results suggest that the morphology of the oriented grains can be rationally tuned with the temperature gradient, which strongly correlates with the deformation temperature, deformation amount, and cooling rate in the deformation process (Figure 6b; Supporting Information), yielding a value of 170 °C / mm at Td = 630 °C. This temperature gradient (170 °C / mm) is likely to match well with the directional growth of oriented SmCo crystals, leading to the rod-shaped SmCo nanograins with a strong c-axis texture at Td = 630 °C.


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In conclusion, we have demonstrated a robust strategy for synthesizing 3D hybrid nanocomposites that can simultaneously control the size, alignment, morphology, and distribution of nanocrystals. This approach enabled the fabrication of bulk nanocomposites with larger energy products than their corresponding single-phase oriented rare-earth counterpart. Our findings make an important step toward fabricating structure-controllable 3D hybrid nanocomposites. We anticipate that our strategy can be applicable to other material systems for their nanostructuring including thermoelectric and multiferroic materials, where precise control of nanostructures is crucial for determining their physical and chemical properties. Beyond its immediate utility, our strategy could inspire the development of methods that allow manipulating even more complex nanostructures in 3D hybrid material systems for exploring novel multifunctional properties.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details for the materials and sample preparation, characterization (XRD, TEM, HRTEM, and MFM), magnetic property measurement, thermal stability study, and theoretical calculations, along with additional supporting data (Figures S1−S8 and Table S1). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 51371074, 51471144 and 51471145). We thank Miss Yunyun Feng for grain size statistics.


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Figure Captions Figure 1. Schematic illustration of the synthetic route and microstructure of bulk SmCo/Fe(Co) hybrid nanocomposites. (a) Mechanically milling of SmCo raw materials and commercial Fe powders. (b) The milled powders that comprise Fe(Co) nanocrystals and SmCo amorphous matrix were compacted into a bulk sample and then suffered from a temperature-gradient (ᐃ T) deformation at high stress (σ ~1 GPa) and large strain (ε ~80%). (c) Synthesized bulk nanocomposite consisting of oriented SmCo rod-shaped grains and Fe(Co) equiaxed grains with a high fraction (> 20 wt.%) and small size (~ 12 nm). The Fe(Co) nanograins have a homogeneous distribution in the resulting nanocomposite. Figure 2. Characterization of the bulk SmCo/Fe(Co) nanocomposite made at the optimum deformation temperature of Td = 630 ºC. (a) XRD pattern of the nanocomposite measured perpendicular to the pressure direction. The inset is the schematic of the XRD measurement. (b) Zoomed view of the XRD pattern (shown in panel a) and the separated XRD peaks from the pattern. (c) Zoomed view of the XRD pattern measured parallel to the pressure direction and the separated XRD peaks from the pattern. (d-f) Longitude-sectional bright-field TEM image (d), the corresponding SAED pattern (e), and dark-field TEM image (f) of nanocrystals in the material (parallel to the pressure direction). The nanocrystals in panel d show a dual-morphology grain structure consisting of rod-shaped (indicated with the arrows) and equiaxed grains. (g,h) A longitude-sectional HRTEM image and its fast Fourier transformation (FFT) of the rod-shaped SmCo7 nanograin in the material, which has a [00l] crystallographic direction approximately along the pressure direction. (i) A cross-sectional bright-field TEM image of nanocrystals in the material (perpendicular to the pressure direction). (j,k) A typical HRTEM image and its FFT of the equiaxed α−Fe nanograin in panel d.


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Figure 3. Characterization of bulk SmCo/Fe(Co) nanocomposites made at a lower (Td = 550 ºC) and higher (Td = 700 ºC) deformation temperature. (a-c) Longitude-sectional bright-field TEM image (a), statistical grain size distribution (b), and the corresponding SAED pattern (c) of nanocrystals in the material made at Td = 550 ºC. (d-f) Longitude-sectional bright-field TEM image (d), the corresponding SAED pattern (e), and dark-field TEM image (f) of nanocrystals in the material made at Td = 700 ºC. The nanocrystals (in panel d) exhibit a dual-morphology grain structure consisting of lath-shaped (indicated with the arrows) and equiaxed grains. (g) A crosssectional bright-field TEM image of nanocrystals in the material made at Td = 700 ºC. (h,i) HRTEM images of the disc-shaped (SmCo7) (h) and equiaxed (α−Fe phase) (i) nanograins in panel d. The disc-shaped SmCo7 nanograin has a [00l] crystallographic direction approximately along the pressure direction. (j) A HRTEM image of the disc-shaped nanograin in panel g. Figure 4. Magnetic properties of the synthesized anisotropic bulk SmCo/Fe(Co) nanocomposite and the comparison of energy product for reported bulk SmCo/FeCo systems. (a) Magnetichysteresis loops of the nanocomposite made at the optimum deformation temperature (Td = 630 ºC). The loops were measured parallel and perpendicular to the pressure direction at ambient temperature. (b) Energy products (BHin curve) along the pressure direction for the nanocomposite discussed in panel a, where B = Hin + 4πM and Hin = H – NMs are the magnetic field induction and the internal magnetic field, respectively. N is the demagnetization factor and determined by experiment (Supporting Information; Figure S8). (c) Representative energy products for bulk nanocomposite magnets in SmCo/FeCo systems:26,33-36 Sm2Co17/Co (ref. 33), SmCo3/Fe (ref. 34), SmCo7/Co (ref. 35), Sm2Co7/FeCo (ref. 36), and SmCo5/FeCo (ref. 26). Previously reported bulk SmCo/FeCo nanocomposite magnets are isotropic. The data for anisotropic bulk SmCo7/FeCo nanocomposites are from the present work.


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Figure 5. Magnetization behavior and thermal stability of the synthesized bulk SmCo/Fe(Co) nanocomposites. (a) Henkel plot (δm−H curve) of the nanocomposite made at the optimum deformation temperature Td = 630 ºC. The inset shows a magnetic force microscope (MFM) image of the material; and the white circle indicates the perimeter of an exchange-coupled magnetic domain. (b) Initial-magnetization curves measured parallel and perpendicular to the pressure direction for the nanocomposite discussed in panel a. The material exhibits a typical step-type magnetization process along the pressure direction, indicating a coercivity mechanism involving domain-wall pinning. (c) Long-term stability assessment of the bulk SmCo/Fe(Co) nanostructure (produced at Td = 630 ºC). The thermal stability was assessed by measuring the magnetic properties along the pressure direction after long-term annealing at Ta = 400 °C. (d) Effect of deformation temperature on magnetic properties. The insets are schematic illustration of the nanostructure along the pressure direction. The error bars were determined by measuring three samples for each deformation temperature. Figure 6. Grain alignment and morphology evolution of the SmCo hard phase under multifield coupling deformation. (a) Calculated strain-energy density Wε of various crystallographic directions of SmCo7 crystal as a function of elastic strain εe. The difference of Wε between various crystallographic directions increases with increasing εe, which facilitates the preferential growth of SmCo7 grains in amorphous matrix at large strain. (b) Deformation-temperature dependence of temperature gradient

ᐃT

and deformation amount ε. A larger

ᐃT

= 170 ºC/mm

was achieved for the sample deformed at Td = 630 ºC. (c) Schematic of the total surface energy per volume of an oriented nanocrystal (along the normal direction of low-surface-energy faces) as a function of crystal diameter. The total surface energy E is strongly associated with the morphology of the oriented crystal: the disc-shaped nanocrystal has a lower E and the rod-shaped


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nanocrystal has a higher E. The lower E facilitates the growth of the disc-shaped SmCo7 nanograins, while the larger temperature gradient at Td = 630 ºC leads to a directional growth of the SmCo7 nanocrystals, yielding the rod-shaped SmCo7 nanograins.


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a

b

c

Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Table of Contents Graphic and Synopsis Keyword: Nanostructuring, hybrid nanostructures, heterostructures, nanocomposite magnets, bulk nanomaterials Xiaohong Li,†‡ Li Lou,† Wenpeng Song,† Qian Zhang,† Guangwei Huang,† Yingxin Hua,† HaiTian Zhang,§ Jianwei Xiao,† Bin Wen,† and Xiangyi Zhang*†

Controllably

Manipulating

Three-Dimensional

Hybrid

Nanostructures

for

Bulk

Nanocomposites with Large Energy Products We report a robust strategy for simultaneously manipulating nanocrystals in size, orientation, morphology and distribution in a three-dimensional (3D) hybrid nanostructure. With this strategy we have fabricated the bulk SmCo/Fe(Co) nanocomposite with the desired structures. Our nanocomposite exhibits a large energy product of 26 MGOe, approximately 50% greater than that of the corresponding pure hard magnetic material and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Our findings pave a new way to fabricating structurecontrollable 3D hybrid nanocomposites.


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Controllably Manipulating Three-Dimensional Hybrid Nanostructures

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