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Nanocrystals
Self-Assembled Epitaxial Core–Shell Nanocrystals with Tunable Magnetic Anisotropy Sheng-Chieh Liao, Yong-Lun Chen, Wei-Cheng Kuo, Jeffrey Cheung, Wei-Cheng Wang, Xuan Cheng, Yi-Ying Chin, Yu-Ze Chen, Heng-Jui Liu, Hong-Ji Lin, Chien-Te Chen, Jeng-Yih Juang, Yu-Lun Chueh, Valanoor Nagarajan, Ying-Hao Chu,* and Chih-Huang Lai* The interfacial interaction becomes significant as the dimension of structure shrinks to the nanoscale.[1] This interaction has triggered huge attention for decades on core–shell nanostructures, such as nanocrystals and nanowires applied for photovoltaics,[2,3] resistance random-access memory,[4] magnetic resonance imaging,[5] and spintronic devices.[6] However, several limitations of conventional core–shell nanocrystals restrict degrees of freedom to manipulate their salient properties. One limitation is the lack of control on crystal orientations among nanostructures. The random orientations of nanocrystals drop the anisotropy information that originally existed in each nanocrystal.[7] Another limitation is the confined capability of core–shell inversion by conventional chemical methods, which hinders further engineering on physical properties by altering the core–shell sequence.[8,9] These limitations impede the exploration of
S.-C. Liao, Y.-Z. Chen, Prof. Y.-L. Chueh, Prof. C.-H. Lai Department of Materials Science & Engineering National Tsing Hua University Hsinchu 30013, Taiwan E-mail: [email protected] Y.-L. Chen, Dr. H.-J. Liu, Prof. Y.-H. Chu Department of Materials Science & Engineering National Chiao Tung University Hsinchu 30010, Taiwan E-mail: [email protected] W.-C. Kuo, Prof. J.-Y. Juang Department of Electrophysics National Chiao Tung University Hsinchu 30010, Taiwan J. Cheung, X. Cheng, Prof. V. Nagarajan School of Materials Science and Engineering University of New South Wales Sydney 2052, Australia W.-C. Wang Graduate Program for Science and Technology of Accelerator Light Source National Chiao Tung University Hsinchu 30010, Taiwan Dr. Y.-Y. Chin, Dr. H.-J. Lin, Dr. C.-T. Chen National Synchrotron Radiation Research Center Hsinchu 30076, Taiwan DOI: 10.1002/smll.201500627 small 2015, 11, No. 33, 4117–4122
new functions in core–shell nanostructures. In this study, an alternative and generic approach to obtain self-assembled core–shell nanocrystals based on epitaxy is proposed to overcome these limitations. The orientation alignment between nanocrystals is enabled through epitaxial relation between nanocrystals and substrate, and the core–shell inversion is simply achieved by reversing the growth sequence. Antiferromagnetic (AFM) cobalt oxide (CoO)/ferrimagnetic (FiM) cobalt ferrite (CoFe2O4, CFO) are chosen to demonstrate the unique features of epitaxial core–shell nanocrystals, where the AFM–FiM exchange coupling existing at the CoO–CFO interface can be used for modifying magnetic properties.[10–12] We first show that the morphology and magnetic anisotropy of nanocrystals can be manipulated by varying the out-ofplane orientations. We also demonstrate that changes of the core–shell sequence and core–shell volume ratio can tailor properties of nanocrystals due to the presence of interfacial exchange coupling between core and shell. Our approach opens a new avenue to engineer the functionalities in nanocrystal systems. To demonstrate aligned CoO–CFO core–shell nanocrystals, we grow nanocrystals epitaxially on a single-crystal substrate, which provides an easy way to align the crystallographic orientation of nanocrystals.[13] In the previous work, we proposed that the addition of melted materials increased the diffusion length so that discrete nanocrystals could be grown successfully.[14,15] This method, however, is not sufficient to achieve discrete core–shell nanocrystals because part of shell materials may nucleate on the substrate and form undesirable cores. To prevent this situation, substrate with larger lattice mismatch to the shell is selected to provide larger interfacial energy, which increases the energy barrier to nucleation. As a result, the shell materials would just nucleate on the existing cores with smaller lattice mismatch and form the epitaxial shell layers of the core–shell structure, as shown in Figure 1a. In the case of CoO–CFO core–shell nanocrystals, we choose SrTiO3 (STO) as the substrate because the lattice mismatch between STO and CoO/CFO (9.2%/7.4%) is much larger than that between CoO and CFO (1.5%).[16–18] Bismuth oxide (Bi2O3, with melting point of 824 °C[19] is selected as a liquid additive to improve the diffusivity; Bi2O3 is immiscible to CoO and CFO, and it could be removed by evaporation under the vacuum environment.[15,20] The fabrication
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Figure 1. a) Selective growth of shells on cores with the aid of melted materials and higher lattice mismatch between shell and substrate. b) Detailed growth process of CoO (core)–CoFe2O4 (shell) nanocrystals.
process of CoO (core)–CFO (shell) nanocrystals is shown in Figure 1b. CoO–Bi2O3 and BiFeO3 targets are used to generate CoO–Bi2O3 and Fe3O4–Bi2O3 mixtures by pulsed laser ablation. The deposition is carried out at 850 °C under high oxygen pressure (35 mTorr for CoO and 15 mTorr for Fe3O4). Each deposition stage is followed by 10 min evacuation of chamber (down to 10−6 Torr) to remove Bi2O3 by evaporation, and thus discrete nanocrystals are formed.[21] Note that Fe3O4 would further react with CoO to form CoxFe3-xO4 during deposition.[22] Consequently, CoO cores and CFO shells are formed. The growth process is monitored by in situ reflective high-energy electron diffraction (RHEED) and results are shown in Figure S1, Supporting Information. Examinations of structure and morphology of CoO (core)–CFO (shell) nanocrystals on STO (001) substrates are shown in Figure 2. The X-ray reciprocal space mapping (RSM)[23] in Figure 2a exhibits only the phases of CoO (rocksalt) and CoxFe3-xO4 (spinel). The value of x is determined as 1 by X-ray magnetic circular dichroism shown in Figure S2, Supporting Information. That is, CoFe2O4 is formed due to the reaction between CoO and Fe3O4 at the growth temperature (850 °C). The epitaxy of these two phases is confirmed by X-ray φ-scans (Figure 2b) with an epitaxial relationship of CFO (001)[100]//CoO (001)[100]//STO (001)[100]. This single epitaxial relationship ensures the orientation alignment of nanocrystals, as shown in Figure 2c: Each nanocrystal possesses a pyramid shape with all edges aligned along 〈110〉STO directions. Facets of these pyramid-shaped nanocrystals are
identified as {111}CFO by measuring the angle between facets of nanocrystals and surface of substrate with an atomic force microscope. The surface element mapping by Auger electron spectroscopy (AES) in Figure 2d confirms that each nanocrystal contains both Fe and Co, and the spacing between nanocrystals, representing the bare substrate, contains only Ti. No bismuth signal is observed in the AES spectrum, suggesting that bismuth was completely evaporated. The cross-sectional element mapping shown in Figure S3, Supporting Information, further reveals that Fe mainly lies near the surface while Co lies in the core. Figure 2e shows the image of cross-sectional high-resolution transmission electron microscopy. Core–shell interfaces (indicated by yellow arrows) are observed parallel to the facets, suggesting that the core–shell structure possesses {111} interfaces. {111} planes are also the facets of CoO coreonly nanocrystals (see Figure S4a, Supporting Information). Although {111} planes are not ones with the lowest surface energy in CoO crystals, the existence of melted Bi2O3 on CoO cores during growth may alter the surface energy.[24] Consequently, orientation of core–shell interface is controlled by interfacial energy between CoO cores and melted Bi2O3 rather than surface energy of bare CoO nanocrystals during the growth of cores. Due to the epitaxial growth of core–shell nanocrystals on STO substrates, we can change the STO orientation to explore varied properties in this core–shell system. Figure 3a shows the surface morphology of CoO (core)–CFO (shell)
Figure 2. Structure and morphology of epitaxial CoO (core)–CFO (shell) nanocrystals on STO (001) substrate. a) X-ray reciprocal space mapping. b) X-ray φ-scan. c) An image of plane-view scanning electron microscopy. d) Auger element mapping. e) An image of cross-sectional high-resolution transmission electron microscopy. The arrows indicate the core–shell interface.
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plane (containing [100] and [010] axis) becomes the hard plane. The anisotropy of the (001)-oriented core–shell nanocrystals will be further discussed later. In addition to out-of-plane orientation, core–shell sequence can be another tuning knob to further manipulate magnetic properties. In conventional chemical synthesis, reversed core–shell sequence may not be easily achieved. However, in our fabrication process, core–shell sequence can be simply switched by reversing the deposition order. We prepare two types of core–shell nanocrystals: CoO (core)– CFO (shell) “type I” and CFO (core)– CoO (shell) “type II” nanocrystals on STO (001) substrate. The fabrication process of “type II” nanocrystals is shown in Figure S5, Supporting Information. The morphology and phase constituent of “type II” nanocrystals are found to be the same as those of “type I” nanocrystals (see Figure S6, Supporting Information). The schematic diagram of morphology and hysteresis loops of “type II” nanocrystals are shown in Figure 4. The easy axis lies in (001) plane, which is perpendicular to the Figure 3. Orientation-controlled morphology and magnetic anisotropy. a) Observed easy axis of “type I” nanocrystals. morphology, b) schematic diagrams, and c) magnetic hysteresis loops of CoO (core)–CFO We further take advantage of the (shell) nanocrystals with different out-of-plane orientations. The magnetic easy axes lie in core–shell structure to modify the magthe [100] axis, [010] axis, and (111) plane for (001), (101), and (111)-oriented core–shell netic properties of “type I” and “type II” nanocrystals and they are depicted asarrows in Figure 3b. nanocrystals. We vary the volume ratio of core and shell, which can be achieved by nanocrystals grown on differently oriented STO substrates. changing the deposited amount of core and shell. A clear The shapes of core–shell nanocrystals become stripes and tri- dependence of anisotropy on the core/shell volume ratio angular/polygonal plates as they grow on STO (101) and STO (Vcore/Vshell) is found, as shown in Figure 5. The anisotropy (111) substrates, respectively. The CoO core-only nanocrys- can be determined by comparing the squareness (S = Mr/Ms, tals on STO (101) and STO (111) substrates (Figure S4b,c, ratio of remnant to saturation magnetization) of hysteresis Supporting Information) exhibit the same shapes as the CoO loops in different directions. The “type I” core–shell nanocrys(core)–CFO (shell) nanocrystals. The observed morphology tals (Figure 5a) show that the out-of-plane squareness (SOOP) of core-only and core–shell nanocrystals suggests that both exceeds the in-plane one (SIP) with increasing Vcore/Vshell, facets and interfaces of these core–shell nanocrystals are indicating that the easy axis changes from in-plane to out-of{111} planes as depicted in Figure 3b. Neither the {111} plane. On the other hand, the “type II” core–shell nanocrysfacets nor {111} interfaces are changed by the orientation of tals (Figure 5b) show a weak dependence of anisotropy on Vcore/Vshell, and all samples show in-plane easy axes. In other substrates. The magnetic hysteresis loops of CoO (core)–CFO (shell) words, the out-of-plane (i.e., [001] direction) easy axis is only nanocrystals with various out-of-plane orientations, shown in Figure 3c, are measured by a vibrating sample magnetometer. Due to the aligned orientations in epitaxial nanocrystals, we can measure the hysteresis loops along various crystallographic directions of nanocrystals. Clear magnetic anisotropy is found in all kinds of nanocrystals, which does not exist in conventional nanocrystal systems. The observed easy axes are [001] axis, [010] axis, and on (111) plane for (001), (101), and (111)-oriented core–shell nanocrystals, respectively. It is clear that (001)-oriented core–shell nanocrystals break the symmetry of three equivalent 〈001〉 easy axes that are typically Figure 4. a) A schematic diagram and b) hysteresis loops of CFO (core)– observed in bulk single-crystalline CFO.[25] The out-of-plane CoO (shell) “type II” nanocrystals grown on STO (001) substrates. [001] axis is observed as an easy axis while the in-plane (001) Arrows in Figure 4a represent the (001) magnetic easy plane. small 2015, 11, No. 33, 4117–4122
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Vcore/Vshell (see the upper scale in Figure 5a).[26] Increased Ainter/VFiM results that the interfacial anisotropy becomes dominant, turning the easy axis from inplane (001) to out-of-plane [001] direction. The experimental observation can be well fitted in micromagnetic simulations with a significant strength of interfacial anisotropy, as shown in Figure S7d, Supporting Information. On the other hand, the interfacial anisotropy of “type II” nanocrystals is much smaller than that of “type I;” therefore type II nanocrystals keep the same in-plane easy axis as the Fe3O4 coreFigure 5. Dependence of anisotropy on core–shell volume ratio (Vcore/Vshell) of a) “type I” CoO only nanocrystals and possess less depend(core)–CFO (shell) and b) “type II” CFO (core)–CoO (shell) nanocrystals grown on STO (001) ence on Vcore/Vshell (see Figure 5b and substrates. The upper axes show the interface-to-ferrimagnetic-volume ratio (Ainterface/VFiM) Figure S7d, Supporting Information). calculated from Vcore/Vshell. We can further tailor the interfacial anisotropy by cooling the nanocrystals observed in “type I” nanocrystals with Vcore/Vshell higher below Néel temperature of AFM (298 K for CoO) with magthan 0.25. netic field.[11,27] Exchange bias field, defined as the field shift Here we discuss the anisotropy discrepancy between of hysteresis loop, can be induced in AFM–FiM systems due the “type I” and “type II” core–shell nanocrystals. Since the to the aligned spins at interface.[12] Figure 6a,b shows the out-of-plane [001] and in-plane [100] directions are equiva- hysteresis loops measured at 50 K after cooling from room lent in CFO, Fe3O4, and CoO crystals, the magnetocrystalline temperature under 3 T magnetic field. The cooling magnetic anisotropy cannot lead to the observed difference between field is applied along the easy axis of nanocrystals, that is, [100] and [001] directions. Magnetostriction is also excluded [001] and [100] for “type I” and “type II,” respectively. Both because the results of X-ray diffraction show the same types reveal clear loop shifts, reconfirming the presence of strain state (0.4%) in “type I” and “type II” nanocrystals interfacial exchange coupling. For the “type I” nanocrystals (see Figure 2a and Figure S6b, Supporting Information, for (Figure 6a), exchange bias field (Hex) and deduced interfacial “type I” and “type II” nanocrystals, respectively). Other pos- exchange energy (Jex = tFiM HexMs, product of FiM thickness, sible factors are shape anisotropy and interfacial anisotropy. exchange bias field, and saturation magnetization)[27] reach Micromagnetic simulations are carried out and the results are 2.4 kOe and 0.4 mJ m−2 in out-of-plane direction, respectively. described in Figure S7, Supporting Information. The simula- On the other hand, the Hex and Jex in “type II” nanocrystals tion results reveal that the easy axis would lie in the (001) (Figure 6b) are 1.6 kOe and 0.27 mJ m−2 in in-plane direction, plane (i.e., in-plane) for all core–shell nanocrystals if only which are 30% smaller than those in “type I.” The weaker shape anisotropy is considered. Interfacial anisotropy must interfacial coupling in the “type II” nanocrystals is also conbe added into the model to derive out-of-plane (i.e., [001]) sistent with our observation on the less anisotropy variations easy axis, as shown in Figure S7c, Supporting Information. In with Ainter/VFiM (shown in Figure 5b), that is, weaker inter“type I” core–shell nanocrystals, the interface-to-ferrimag- facial anisotropy. Our results demonstrate that core–shell netic-volume ratio (Ainter/VFiM) is increased by increasing sequence can be an important tuning knob to manipulate the interfacial exchange energy and thus magnetic anisotropy in core–shell nanocrystals. In summary, we take the CoO–CFO as a model system to demonstrate a generic approach of epitaxial core–shell nanocrystals, which possess aligned orientation and reversible core–shell sequence that are hard to achieve in conventional core– shell nanocrystals. The epitaxial core–shell nanocrystals could be synthesized generally with a melted and evaporable additive as well as a substrate with suitable lattice mismatch. Our results demonstrate that magnetic anisotropy of CoO–CFO core– shell nanocrystals is conserved due to Figure 6. Interfacial exchange coupling controlled by the core–shell sequence. Hysteresis loops at 50 K after field-cooling. a) “Type I” nanocrystals with field along out-of-plane [001]. aligned orientations among nanocrystals. b) “Type II” nanocrystals with field along in-plane [100]. Hex represents the exchange bias By controlling the out-of-plane crystallographic orientations while maintaining field of loop.
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the {111} facets, we can modify the shape and magnetic anisotropy of nanocrystals. Due to the core–shell structure, nanocrystals possess more degrees of freedom to manipulate their properties. By changing the core–shell volume ratio or and core–shell sequence, we can alter the interface-to-volume ratio and interfacial exchange coupling; therefore, the magnetic anisotropy can be further tuned. For example, we can obtain out-of-plane magnetic easy axis in CoO (core)–CFO (shell) “type I” nanocrystals in contrast to the Fe3O4 coreonly and CFO (core)–CoO (shell) “type II” nanocrystals with in-plane anisotropy. We also clearly show the dependence of core–shell interfacial coupling on core–shell sequence by results of exchange bias field and interfacial anisotropy. We believe that our proposed approach can be applied to other oxide systems. For most of oxide materials, Bi2O3 would be a good melted add-in to get nanostructures. The oxide systems with small lattice mismatch, such as MgO/NiO, β-Mn3O4/ Fe3O4, and CrO2/TiO2, should be potentially grown by using this method. The aligned orientation and variable core–shell structure in epitaxial nanocrystals shine a path toward further functional design of nanostructures.
Experimental Section Sample Fabrication: The CoO–CoFe2O4 core–shell nanocrystals were grown by pulsed laser deposition with a 248 nm KrF excimer laser under 10 Hz. The core/shell volume ratio (Vcore/Vshell) was controlled by varying the number of laser pulses of Bi2O3–CoO and BiFeO3 targets. The Vcore/Vshell values of samples are around one except the samples used in Figure 5 and Figure S7d, Supporting Information. Structural Analysis: The X-ray RSM and φ-scan were performed on an eight-ring diffractometer with synchrotron X-ray source in BL17A at National Synchrotron Radiation Research Center (NSRRC), Taiwan. RSM was performed by 2D scanning along [001]STO and [110]STO directions. The lattice constant and strain of CoO and CFO were calculated by taking SrTiO3 (113) diffraction peak as standard. Scanning electron microscopy was taken with JEOL JSM-6700F with a field-emission gun. The crystallographic direction of nanocrystal edge was determined by the angle between nanocrystal edges and substrate edges, known as SrTiO3 〈100〉 directions. Atomic force microscopy was taken with Veeco Multimode 8 under the ScanAsyst mode. Plane-view surface element mapping by Auger electron spectroscopy was taken in PHI 700 Scanning Auger Nanoprobe with energy resolution of 0.1% and lateral resolution of 8 nm. Cross-sectional transmission electron analyses and element mapping were taken in JEOL JEM-2010F and JEOL JEM-2200F with field-emission guns, respectively. Magnetic Analysis: Room-temperature and low-temperature hysteresis loops were taken by MicroMag 3900 vibrating sample magnetometer and Quantum Design MPMS-XL superconducting quantum interference device, respectively. Due to epitaxy, we can obtain the hysteresis loops measured along various crystallographic directions of nanocrystals, the same as crystallographic orientation of substrate. X-ray magnetic dichroism was done with bending-magnet X-ray source in BL11A at the NSRRC, Taiwan.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements S.-C. Liao and Y.-L. Chen contributed equally to this work. This work at National Chiao Tung University and National Tsing Hua University was supported by the Ministry of Science and Technology of Republic of China, Taiwan, under Contract Nos. MOST-103-2119-M009-003-MY3 and MOST-102-2221-E-007-043-MY2, respectively.
[1] L. M. Liz-Marzán, P. Mulvaney, J. Phys. Chem. B 2003, 107, 7312. [2] S. Kim, B. Fisher, H.-J. Eisler, M. Bawendi, J. Am. Chem. Soc. 2003, 125, 11466. [3] E. Janik, A. Wachnicka, E. Guziewicz, M. Godlewski, S. Kret, W. Zaleszczyk, E. Dynowska, A. Presz, G. Karczewski, T. Wojtowicz, Nanotechnology 2010, 21, 015302. [4] C. Cagli, F. Nardi, B. Harteneck, Z. Tan, Y. Zhang, D. lelmini, Small 2011, 7, 2899. [5] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao, S. Sun, Adv. Mater. 2010, 22, 2729. [6] X.-J. Hao, T. Tu, G. Cao, C. Zhou, H.-O. Li, G.-C. Guo, W. Y. Fung, Z. Ji, G.-P. Guo, W. Lu, Nano Lett. 2010, 10, 2956. [7] A. N. Kulak, P. Iddon, Y. Li, S. P. Armes, H. Cölfen, O. Paris, R. M. Wilson, F. C. Meldrum, J. Am. Chem. Soc. 2007, 129, 3729. [8] G. Salazar-Alvarez, J. Sort, S. Suriñach, M. D. Baró, J. Nogués, J. Am. Chem. Soc. 2007, 129, 9102. [9] D. W. Kavich, J. H. Dickerson, S. V. Mahajan, H. A. Hasan, J.-H. Park, Phys. Rev. B 2008, 78, 174414. [10] Y. Ijiri, T. C. Schulthess, J. A. Borchers, P. J. van der Zaag, R. W. Erwin, Phys. Rev. Lett. 2007, 99, 147201. [11] E. Lima, E. L. Winkler, D. Tobia, H. E. Troiani, R. D. Zysler, E. Agostinelli, D. Fiorani, Chem. Mater. 2012, 24, 512. [12] P. J. van der Zaag, Y. Ijiri, J. A. Borchers, L. F. Feiner, R. M. Wolf, J. M. Gaines, R. W. Erwin, M. A. Verheijen, Phys. Rev. Lett. 2000, 84, 6102. [13] J.-C. Yang, Q. He, Y.-M. Zhu, J.-C. Lin, H.-J. Liu, Y.-H. Hsieh, P.-C. Wu, Y.-L. Chen, S.-F. Lee, Y.-Y. Chin, H.-J. Lin, C.-T. Chen, Q. Zhan, E. Arenholz, Y.-H. Chu, Nano Lett. 2014, 14, 6073. [14] J. D. Ferguson, G. Arikan, D. S. Dale, A. R. Woll, J. D. Brock, Phys. Rev. Lett. 2009, 103, 256103. [15] K. A. Bogle, J. Cheung, Y.-L. Chen, S.-C. Liao, C.-H. Lai, Y.-H. Chu, J. M. Gregg, S. B. Ogale, N. Valanoor, Adv. Funct. Mater. 2012, 22, 5224. [16] R. Kannan, M. S. Seehra, Phys. Rev. B 1987, 35, 6847. [17] A. Goldman, Modern Ferrite Technology Van Nostrand-Reinhold, NY, USA 1990. [18] F. W. Lytle, J. Appl. Phys. 1964, 35, 2212. [19] D. Risold, B. Hallstedt, L. J. Gauckler, J. Phase Equilib. 1995, 16, 223. [20] J. Cheung, K. Bogle, X. Cheng, J. Sullaphen, C.-Y. Kuo, Y.-J. Chen, H.-J. Lin, C.-T. Chen, J.-C. Yang, Y.-H. Chu, N. Valanoor, J. Appl. Phys. 2012, 112, 104321. [21] K. A. Bogle, V. Anbusathaiah, M. Arredondo, J.-Y. Lin, Y.-H. Chu, C. O’Neill, J. M. Gregg, M. R. Castell, V. Nagarajan, ACS Nano 2010, 4, 5139.
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communications www.MaterialsViews.com [22] B. J. Touzelin, J. Less Common Met. 1981, 77, 11. [23] P. F. Fewster, Crit. Rev. Solid State 1997, 22, 69. [24] D. Wang, X. Ma, Y. Wang, L. Wang, Z. Wang, W. Zeng, X. He, J. Li, Q. Ping, Y. Li, Nano Res. 2010, 3, 1. [25] I. C. Nlebedim, J. E. Snyder, A. J. Moses, D. C. Jiles, IEEE Trans. Magn. 2012, 48, 3084. [26] The interface-to-ferrimagnetic volume-ratio ( S = Ainter/VFiM) of type I nanocrystals is calculated by following equation: S = 4 3 r 2 ( r + 1) ,
where r and b represent the Vcore/Vshell and length of 100] edge of core–shell nanocrystals, respectively. [27] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J. S. Muñoz, M. D. Baró, Phys. Rep. 2005, 422, 65.
3b
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Received: March 4, 2015 Revised: May 6, 2015 Published online: May 28, 2015
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Nanocrystals
Self-Assembled Epitaxial Core–Shell Nanocrystals with Tunable Magnetic Anisotropy Sheng-Chieh Liao, Yong-Lun Chen, Wei-Cheng Kuo, Jeffrey Cheung, Wei-Cheng Wang, Xuan Cheng, Yi-Ying Chin, Yu-Ze Chen, Heng-Jui Liu, Hong-Ji Lin, Chien-Te Chen, Jeng-Yih Juang, Yu-Lun Chueh, Valanoor Nagarajan, Ying-Hao Chu,* and Chih-Huang Lai* The interfacial interaction becomes significant as the dimension of structure shrinks to the nanoscale.[1] This interaction has triggered huge attention for decades on core–shell nanostructures, such as nanocrystals and nanowires applied for photovoltaics,[2,3] resistance random-access memory,[4] magnetic resonance imaging,[5] and spintronic devices.[6] However, several limitations of conventional core–shell nanocrystals restrict degrees of freedom to manipulate their salient properties. One limitation is the lack of control on crystal orientations among nanostructures. The random orientations of nanocrystals drop the anisotropy information that originally existed in each nanocrystal.[7] Another limitation is the confined capability of core–shell inversion by conventional chemical methods, which hinders further engineering on physical properties by altering the core–shell sequence.[8,9] These limitations impede the exploration of
S.-C. Liao, Y.-Z. Chen, Prof. Y.-L. Chueh, Prof. C.-H. Lai Department of Materials Science & Engineering National Tsing Hua University Hsinchu 30013, Taiwan E-mail: [email protected] Y.-L. Chen, Dr. H.-J. Liu, Prof. Y.-H. Chu Department of Materials Science & Engineering National Chiao Tung University Hsinchu 30010, Taiwan E-mail: [email protected] W.-C. Kuo, Prof. J.-Y. Juang Department of Electrophysics National Chiao Tung University Hsinchu 30010, Taiwan J. Cheung, X. Cheng, Prof. V. Nagarajan School of Materials Science and Engineering University of New South Wales Sydney 2052, Australia W.-C. Wang Graduate Program for Science and Technology of Accelerator Light Source National Chiao Tung University Hsinchu 30010, Taiwan Dr. Y.-Y. Chin, Dr. H.-J. Lin, Dr. C.-T. Chen National Synchrotron Radiation Research Center Hsinchu 30076, Taiwan DOI: 10.1002/smll.201500627 small 2015, 11, No. 33, 4117–4122
new functions in core–shell nanostructures. In this study, an alternative and generic approach to obtain self-assembled core–shell nanocrystals based on epitaxy is proposed to overcome these limitations. The orientation alignment between nanocrystals is enabled through epitaxial relation between nanocrystals and substrate, and the core–shell inversion is simply achieved by reversing the growth sequence. Antiferromagnetic (AFM) cobalt oxide (CoO)/ferrimagnetic (FiM) cobalt ferrite (CoFe2O4, CFO) are chosen to demonstrate the unique features of epitaxial core–shell nanocrystals, where the AFM–FiM exchange coupling existing at the CoO–CFO interface can be used for modifying magnetic properties.[10–12] We first show that the morphology and magnetic anisotropy of nanocrystals can be manipulated by varying the out-ofplane orientations. We also demonstrate that changes of the core–shell sequence and core–shell volume ratio can tailor properties of nanocrystals due to the presence of interfacial exchange coupling between core and shell. Our approach opens a new avenue to engineer the functionalities in nanocrystal systems. To demonstrate aligned CoO–CFO core–shell nanocrystals, we grow nanocrystals epitaxially on a single-crystal substrate, which provides an easy way to align the crystallographic orientation of nanocrystals.[13] In the previous work, we proposed that the addition of melted materials increased the diffusion length so that discrete nanocrystals could be grown successfully.[14,15] This method, however, is not sufficient to achieve discrete core–shell nanocrystals because part of shell materials may nucleate on the substrate and form undesirable cores. To prevent this situation, substrate with larger lattice mismatch to the shell is selected to provide larger interfacial energy, which increases the energy barrier to nucleation. As a result, the shell materials would just nucleate on the existing cores with smaller lattice mismatch and form the epitaxial shell layers of the core–shell structure, as shown in Figure 1a. In the case of CoO–CFO core–shell nanocrystals, we choose SrTiO3 (STO) as the substrate because the lattice mismatch between STO and CoO/CFO (9.2%/7.4%) is much larger than that between CoO and CFO (1.5%).[16–18] Bismuth oxide (Bi2O3, with melting point of 824 °C[19] is selected as a liquid additive to improve the diffusivity; Bi2O3 is immiscible to CoO and CFO, and it could be removed by evaporation under the vacuum environment.[15,20] The fabrication
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Figure 1. a) Selective growth of shells on cores with the aid of melted materials and higher lattice mismatch between shell and substrate. b) Detailed growth process of CoO (core)–CoFe2O4 (shell) nanocrystals.
process of CoO (core)–CFO (shell) nanocrystals is shown in Figure 1b. CoO–Bi2O3 and BiFeO3 targets are used to generate CoO–Bi2O3 and Fe3O4–Bi2O3 mixtures by pulsed laser ablation. The deposition is carried out at 850 °C under high oxygen pressure (35 mTorr for CoO and 15 mTorr for Fe3O4). Each deposition stage is followed by 10 min evacuation of chamber (down to 10−6 Torr) to remove Bi2O3 by evaporation, and thus discrete nanocrystals are formed.[21] Note that Fe3O4 would further react with CoO to form CoxFe3-xO4 during deposition.[22] Consequently, CoO cores and CFO shells are formed. The growth process is monitored by in situ reflective high-energy electron diffraction (RHEED) and results are shown in Figure S1, Supporting Information. Examinations of structure and morphology of CoO (core)–CFO (shell) nanocrystals on STO (001) substrates are shown in Figure 2. The X-ray reciprocal space mapping (RSM)[23] in Figure 2a exhibits only the phases of CoO (rocksalt) and CoxFe3-xO4 (spinel). The value of x is determined as 1 by X-ray magnetic circular dichroism shown in Figure S2, Supporting Information. That is, CoFe2O4 is formed due to the reaction between CoO and Fe3O4 at the growth temperature (850 °C). The epitaxy of these two phases is confirmed by X-ray φ-scans (Figure 2b) with an epitaxial relationship of CFO (001)[100]//CoO (001)[100]//STO (001)[100]. This single epitaxial relationship ensures the orientation alignment of nanocrystals, as shown in Figure 2c: Each nanocrystal possesses a pyramid shape with all edges aligned along 〈110〉STO directions. Facets of these pyramid-shaped nanocrystals are
identified as {111}CFO by measuring the angle between facets of nanocrystals and surface of substrate with an atomic force microscope. The surface element mapping by Auger electron spectroscopy (AES) in Figure 2d confirms that each nanocrystal contains both Fe and Co, and the spacing between nanocrystals, representing the bare substrate, contains only Ti. No bismuth signal is observed in the AES spectrum, suggesting that bismuth was completely evaporated. The cross-sectional element mapping shown in Figure S3, Supporting Information, further reveals that Fe mainly lies near the surface while Co lies in the core. Figure 2e shows the image of cross-sectional high-resolution transmission electron microscopy. Core–shell interfaces (indicated by yellow arrows) are observed parallel to the facets, suggesting that the core–shell structure possesses {111} interfaces. {111} planes are also the facets of CoO coreonly nanocrystals (see Figure S4a, Supporting Information). Although {111} planes are not ones with the lowest surface energy in CoO crystals, the existence of melted Bi2O3 on CoO cores during growth may alter the surface energy.[24] Consequently, orientation of core–shell interface is controlled by interfacial energy between CoO cores and melted Bi2O3 rather than surface energy of bare CoO nanocrystals during the growth of cores. Due to the epitaxial growth of core–shell nanocrystals on STO substrates, we can change the STO orientation to explore varied properties in this core–shell system. Figure 3a shows the surface morphology of CoO (core)–CFO (shell)
Figure 2. Structure and morphology of epitaxial CoO (core)–CFO (shell) nanocrystals on STO (001) substrate. a) X-ray reciprocal space mapping. b) X-ray φ-scan. c) An image of plane-view scanning electron microscopy. d) Auger element mapping. e) An image of cross-sectional high-resolution transmission electron microscopy. The arrows indicate the core–shell interface.
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plane (containing [100] and [010] axis) becomes the hard plane. The anisotropy of the (001)-oriented core–shell nanocrystals will be further discussed later. In addition to out-of-plane orientation, core–shell sequence can be another tuning knob to further manipulate magnetic properties. In conventional chemical synthesis, reversed core–shell sequence may not be easily achieved. However, in our fabrication process, core–shell sequence can be simply switched by reversing the deposition order. We prepare two types of core–shell nanocrystals: CoO (core)– CFO (shell) “type I” and CFO (core)– CoO (shell) “type II” nanocrystals on STO (001) substrate. The fabrication process of “type II” nanocrystals is shown in Figure S5, Supporting Information. The morphology and phase constituent of “type II” nanocrystals are found to be the same as those of “type I” nanocrystals (see Figure S6, Supporting Information). The schematic diagram of morphology and hysteresis loops of “type II” nanocrystals are shown in Figure 4. The easy axis lies in (001) plane, which is perpendicular to the Figure 3. Orientation-controlled morphology and magnetic anisotropy. a) Observed easy axis of “type I” nanocrystals. morphology, b) schematic diagrams, and c) magnetic hysteresis loops of CoO (core)–CFO We further take advantage of the (shell) nanocrystals with different out-of-plane orientations. The magnetic easy axes lie in core–shell structure to modify the magthe [100] axis, [010] axis, and (111) plane for (001), (101), and (111)-oriented core–shell netic properties of “type I” and “type II” nanocrystals and they are depicted asarrows in Figure 3b. nanocrystals. We vary the volume ratio of core and shell, which can be achieved by nanocrystals grown on differently oriented STO substrates. changing the deposited amount of core and shell. A clear The shapes of core–shell nanocrystals become stripes and tri- dependence of anisotropy on the core/shell volume ratio angular/polygonal plates as they grow on STO (101) and STO (Vcore/Vshell) is found, as shown in Figure 5. The anisotropy (111) substrates, respectively. The CoO core-only nanocrys- can be determined by comparing the squareness (S = Mr/Ms, tals on STO (101) and STO (111) substrates (Figure S4b,c, ratio of remnant to saturation magnetization) of hysteresis Supporting Information) exhibit the same shapes as the CoO loops in different directions. The “type I” core–shell nanocrys(core)–CFO (shell) nanocrystals. The observed morphology tals (Figure 5a) show that the out-of-plane squareness (SOOP) of core-only and core–shell nanocrystals suggests that both exceeds the in-plane one (SIP) with increasing Vcore/Vshell, facets and interfaces of these core–shell nanocrystals are indicating that the easy axis changes from in-plane to out-of{111} planes as depicted in Figure 3b. Neither the {111} plane. On the other hand, the “type II” core–shell nanocrysfacets nor {111} interfaces are changed by the orientation of tals (Figure 5b) show a weak dependence of anisotropy on Vcore/Vshell, and all samples show in-plane easy axes. In other substrates. The magnetic hysteresis loops of CoO (core)–CFO (shell) words, the out-of-plane (i.e., [001] direction) easy axis is only nanocrystals with various out-of-plane orientations, shown in Figure 3c, are measured by a vibrating sample magnetometer. Due to the aligned orientations in epitaxial nanocrystals, we can measure the hysteresis loops along various crystallographic directions of nanocrystals. Clear magnetic anisotropy is found in all kinds of nanocrystals, which does not exist in conventional nanocrystal systems. The observed easy axes are [001] axis, [010] axis, and on (111) plane for (001), (101), and (111)-oriented core–shell nanocrystals, respectively. It is clear that (001)-oriented core–shell nanocrystals break the symmetry of three equivalent 〈001〉 easy axes that are typically Figure 4. a) A schematic diagram and b) hysteresis loops of CFO (core)– observed in bulk single-crystalline CFO.[25] The out-of-plane CoO (shell) “type II” nanocrystals grown on STO (001) substrates. [001] axis is observed as an easy axis while the in-plane (001) Arrows in Figure 4a represent the (001) magnetic easy plane. small 2015, 11, No. 33, 4117–4122
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Vcore/Vshell (see the upper scale in Figure 5a).[26] Increased Ainter/VFiM results that the interfacial anisotropy becomes dominant, turning the easy axis from inplane (001) to out-of-plane [001] direction. The experimental observation can be well fitted in micromagnetic simulations with a significant strength of interfacial anisotropy, as shown in Figure S7d, Supporting Information. On the other hand, the interfacial anisotropy of “type II” nanocrystals is much smaller than that of “type I;” therefore type II nanocrystals keep the same in-plane easy axis as the Fe3O4 coreFigure 5. Dependence of anisotropy on core–shell volume ratio (Vcore/Vshell) of a) “type I” CoO only nanocrystals and possess less depend(core)–CFO (shell) and b) “type II” CFO (core)–CoO (shell) nanocrystals grown on STO (001) ence on Vcore/Vshell (see Figure 5b and substrates. The upper axes show the interface-to-ferrimagnetic-volume ratio (Ainterface/VFiM) Figure S7d, Supporting Information). calculated from Vcore/Vshell. We can further tailor the interfacial anisotropy by cooling the nanocrystals observed in “type I” nanocrystals with Vcore/Vshell higher below Néel temperature of AFM (298 K for CoO) with magthan 0.25. netic field.[11,27] Exchange bias field, defined as the field shift Here we discuss the anisotropy discrepancy between of hysteresis loop, can be induced in AFM–FiM systems due the “type I” and “type II” core–shell nanocrystals. Since the to the aligned spins at interface.[12] Figure 6a,b shows the out-of-plane [001] and in-plane [100] directions are equiva- hysteresis loops measured at 50 K after cooling from room lent in CFO, Fe3O4, and CoO crystals, the magnetocrystalline temperature under 3 T magnetic field. The cooling magnetic anisotropy cannot lead to the observed difference between field is applied along the easy axis of nanocrystals, that is, [100] and [001] directions. Magnetostriction is also excluded [001] and [100] for “type I” and “type II,” respectively. Both because the results of X-ray diffraction show the same types reveal clear loop shifts, reconfirming the presence of strain state (0.4%) in “type I” and “type II” nanocrystals interfacial exchange coupling. For the “type I” nanocrystals (see Figure 2a and Figure S6b, Supporting Information, for (Figure 6a), exchange bias field (Hex) and deduced interfacial “type I” and “type II” nanocrystals, respectively). Other pos- exchange energy (Jex = tFiM HexMs, product of FiM thickness, sible factors are shape anisotropy and interfacial anisotropy. exchange bias field, and saturation magnetization)[27] reach Micromagnetic simulations are carried out and the results are 2.4 kOe and 0.4 mJ m−2 in out-of-plane direction, respectively. described in Figure S7, Supporting Information. The simula- On the other hand, the Hex and Jex in “type II” nanocrystals tion results reveal that the easy axis would lie in the (001) (Figure 6b) are 1.6 kOe and 0.27 mJ m−2 in in-plane direction, plane (i.e., in-plane) for all core–shell nanocrystals if only which are 30% smaller than those in “type I.” The weaker shape anisotropy is considered. Interfacial anisotropy must interfacial coupling in the “type II” nanocrystals is also conbe added into the model to derive out-of-plane (i.e., [001]) sistent with our observation on the less anisotropy variations easy axis, as shown in Figure S7c, Supporting Information. In with Ainter/VFiM (shown in Figure 5b), that is, weaker inter“type I” core–shell nanocrystals, the interface-to-ferrimag- facial anisotropy. Our results demonstrate that core–shell netic-volume ratio (Ainter/VFiM) is increased by increasing sequence can be an important tuning knob to manipulate the interfacial exchange energy and thus magnetic anisotropy in core–shell nanocrystals. In summary, we take the CoO–CFO as a model system to demonstrate a generic approach of epitaxial core–shell nanocrystals, which possess aligned orientation and reversible core–shell sequence that are hard to achieve in conventional core– shell nanocrystals. The epitaxial core–shell nanocrystals could be synthesized generally with a melted and evaporable additive as well as a substrate with suitable lattice mismatch. Our results demonstrate that magnetic anisotropy of CoO–CFO core– shell nanocrystals is conserved due to Figure 6. Interfacial exchange coupling controlled by the core–shell sequence. Hysteresis loops at 50 K after field-cooling. a) “Type I” nanocrystals with field along out-of-plane [001]. aligned orientations among nanocrystals. b) “Type II” nanocrystals with field along in-plane [100]. Hex represents the exchange bias By controlling the out-of-plane crystallographic orientations while maintaining field of loop.
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the {111} facets, we can modify the shape and magnetic anisotropy of nanocrystals. Due to the core–shell structure, nanocrystals possess more degrees of freedom to manipulate their properties. By changing the core–shell volume ratio or and core–shell sequence, we can alter the interface-to-volume ratio and interfacial exchange coupling; therefore, the magnetic anisotropy can be further tuned. For example, we can obtain out-of-plane magnetic easy axis in CoO (core)–CFO (shell) “type I” nanocrystals in contrast to the Fe3O4 coreonly and CFO (core)–CoO (shell) “type II” nanocrystals with in-plane anisotropy. We also clearly show the dependence of core–shell interfacial coupling on core–shell sequence by results of exchange bias field and interfacial anisotropy. We believe that our proposed approach can be applied to other oxide systems. For most of oxide materials, Bi2O3 would be a good melted add-in to get nanostructures. The oxide systems with small lattice mismatch, such as MgO/NiO, β-Mn3O4/ Fe3O4, and CrO2/TiO2, should be potentially grown by using this method. The aligned orientation and variable core–shell structure in epitaxial nanocrystals shine a path toward further functional design of nanostructures.
Experimental Section Sample Fabrication: The CoO–CoFe2O4 core–shell nanocrystals were grown by pulsed laser deposition with a 248 nm KrF excimer laser under 10 Hz. The core/shell volume ratio (Vcore/Vshell) was controlled by varying the number of laser pulses of Bi2O3–CoO and BiFeO3 targets. The Vcore/Vshell values of samples are around one except the samples used in Figure 5 and Figure S7d, Supporting Information. Structural Analysis: The X-ray RSM and φ-scan were performed on an eight-ring diffractometer with synchrotron X-ray source in BL17A at National Synchrotron Radiation Research Center (NSRRC), Taiwan. RSM was performed by 2D scanning along [001]STO and [110]STO directions. The lattice constant and strain of CoO and CFO were calculated by taking SrTiO3 (113) diffraction peak as standard. Scanning electron microscopy was taken with JEOL JSM-6700F with a field-emission gun. The crystallographic direction of nanocrystal edge was determined by the angle between nanocrystal edges and substrate edges, known as SrTiO3 〈100〉 directions. Atomic force microscopy was taken with Veeco Multimode 8 under the ScanAsyst mode. Plane-view surface element mapping by Auger electron spectroscopy was taken in PHI 700 Scanning Auger Nanoprobe with energy resolution of 0.1% and lateral resolution of 8 nm. Cross-sectional transmission electron analyses and element mapping were taken in JEOL JEM-2010F and JEOL JEM-2200F with field-emission guns, respectively. Magnetic Analysis: Room-temperature and low-temperature hysteresis loops were taken by MicroMag 3900 vibrating sample magnetometer and Quantum Design MPMS-XL superconducting quantum interference device, respectively. Due to epitaxy, we can obtain the hysteresis loops measured along various crystallographic directions of nanocrystals, the same as crystallographic orientation of substrate. X-ray magnetic dichroism was done with bending-magnet X-ray source in BL11A at the NSRRC, Taiwan.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements S.-C. Liao and Y.-L. Chen contributed equally to this work. This work at National Chiao Tung University and National Tsing Hua University was supported by the Ministry of Science and Technology of Republic of China, Taiwan, under Contract Nos. MOST-103-2119-M009-003-MY3 and MOST-102-2221-E-007-043-MY2, respectively.
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3b
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Received: March 4, 2015 Revised: May 6, 2015 Published online: May 28, 2015
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