Flexible Lithium Ferrite Nanopillar Arrays for Bending Stable

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Flexible Lithium Ferrite Nanopillar Arrays for Bending Stable Microwave Magnetism Guohua Lan,† Lvkang Shen,† Lu Lu,† Cuimei Cao,‡ Changjun Jiang,‡ Huarui Fu,§ Caiyin You,§ Xiaoli Lu,∥ Chunrui Ma,⊥ Ming Liu,*,† and Chun-Lin Jia†,#

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†

School of Electronic and Information Engineering, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China ‡ School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, China § School of Material Science and Engineering, Xi’an University of Technology, Xi’an 710048, China ∥ State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China ⊥ School of Material Science and Engineering and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China # Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, D-52425 Jülich, Germany S Supporting Information *

ABSTRACT: Recent development in magnetic nanostructures has promoted flexible electronics into the application of integrated devices. However, the magnetic properties of flexible devices strongly depend on the bending states. In order to realize the design of new flexible devices driven by an external field, the first step is to make the magnetic properties insensitive to the bending. Herein, a series of LiFe5O8 nanopillar arrays were fabricated, whose microwave magnetic properties can be modulated by tuning the nanostructure. This work demonstrates that nanostructure engineering is useful to control the bending sensitivity of microwave magnetism and further design stable flexible devices.

KEYWORDS: ferromagnetic resonance, flexible spintronics, spinel ferrite oxide, nanopillar arrays, nanomagnetism ith the rapid development of flexible electronics, there is increasing interest in integrating high quality magnetic nanostructures on flexible substrates.1,2 For magnetic microwave devices, the bending status could apparently modulate the microwave magnetic properties of flexible materials, showing potential in sensors and tunable microwave devices.3 However, in order to realize the future design of external field modulated flexible devices, one must ensure that the devices can only get tuned by the external field (for example, applied electric field or optical field).4 Therefore, a technologically relevant solution is to use devices based on magnetic materials whose microwave magnetism is insensitive to bending. Ferromagnetic resonance (FMR) is one of the most effective experimental techniques for studying the static and dynamic magnetic properties of nanostructures.5 In the recent work by Liu et al., it revealed that bending could have a huge influence on the resonance field (Hr) for sensor applications.3 We argue that the anisotropy energy of the thin films should be responsible for the bending tuned Hr in magnetic materials. According to Kittle’s equation, while only considering the

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© XXXX American Chemical Society

contribution from the Zeeman energy and the anisotropy energy of the film/nanowire, the perpendicular FMR equation can be expressed as6,7 (ω/γ )2 = [Hr cos(θM − θH) + 4πNeff Ms cos 2θM ] × [Hr cos(θM − θH) − 4πNeff Ms sin 2 θM ]

(1)

Here, ω is the microwave angular frequency. γ is the gyromagnetic factor. Hr is the resonance field. Ms is the saturation magnetization. Neff is the effective demagnetization factor from the dipolar interaction. θM and θH represent the angle made by the substrate plane with the magnetization M and with the external applied field H, respectively. It seems a good strategy to tune the magnetic anisotropy in the FMR of the materials by modulating the value of Neff.8 For a continuous film, Neff ≈ 1. While for nanopillar and nanodot arrays, the ratio of length to diameter as well as the space Received: July 30, 2018 Accepted: November 5, 2018 Published: November 5, 2018 A

DOI: 10.1021/acsami.8b12954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Effect of demagnetization factor Neff on the magnetic anisotropy of LFO nanostructure in FMR tests. (b) Schematics of the chemical etching method for fabricating LFO nanopillar arrays.

Figure 2. (a) and (b) Typical XRD θ−2θ scans for (a) the LF1M2 nanocomposite film and (b) the etched LFO nanopillar arrays on F-mica substrate. (c) XRD φ scans for LFO {004} in nanopillar arrays (top), for overlapped LFO {004} and MgO {002} in LF1M2 nanocomposite film (middle), and for F-mica {202} (bottom) substrate. (d) Plan-view SEM image of LFO arrays with nanopillar length of ∼68 nm. (e) Cross-sectional STEM image of the LFO arrays with nanopillar length of ∼68 nm. (f) Average diameters of the nanopillar arrays with error bar collected from the SEM results.

distribution can significantly affect the value of Neff,9,10 and it was reported that anisotropy energy of the nanostructure plays

a very critical role on the microwave magnetism of the nanometer-sized materials.11−13 For future flexible electronic B

DOI: 10.1021/acsami.8b12954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces devices on anomalous substrates, the change of Hr is influenced by the misorientation between the applied field and the film plane under bending. If the nanostructure material has almost unchanged FMR spectra along different directions, it might reduce the bending sensitivity of Hr to the maximum extent,3 as shown in Figure 1a. Lithium ferrite (LiFe5O8, LFO) has attracted research interest for several decades, because of its potential applications as cathode material and as a component of microwave magnetic devices, such as microwave monolithic integrated circuits, gyrators, and isolators.1,14−16 In relevant work, the LFO films were successfully prepared with a good epitaxial quality at 200 °C by sputtering, which exhibited sizable magnetization and excellent FMR spectra at X-band with low loss.17,18 Hence, it can be taken as a model sample for testing the nanomagnetism. In this work, LFO:(MgO)x (simplified as LF1Mx) nanocomposite thin films with different volume ratios x of the MgO phase and different film thicknesses were fabricated on a fluorophlogopite mica (F-mica) substrate by pulsed laser deposition. Then, a series of LFO nanopillar arrays were realized by chemically etching the MgO phase in the nanocomposite thin films,19 as shown in Figure 1b (see Materials and Methods in the Supporting Information). The LFO nanopillar arrays exhibited excellent magnetic properties with higher Ms than that of the pure LFO film. Almost unchanged FMR spectra under bending and narrow differences in Hr under different applied field directions were found in the LF1M2 nanopillar arrays. It demonstrates that tuning the nanopillar structure is a good strategy to realize the magnetic isotropy of FMR for future flexible stable devices related applications.3 Typical X-ray diffraction (XRD) θ−2θ scans were performed to confirm the phase configuration of the as-grown LF1M2 nanocomposite films and the chemically etched nanopillar arrays. As shown in Figure 2a, for the as-grown LF1M2 nanocomposite film, only the (111) peaks of the LFO and the MgO phases and the (001) peaks of the F-mica substrate can be found, whereas for the etched LFO arrays shown in Figure 2b, the (111) peaks of LFO slightly shifted to a lower degree (as shown in Figure S1), which should be attributed to the disappearance of the secondary MgO phase. The crystal relationships are investigated by the φ scanning taken around the {004} reflection of the LFO phase and the {202} reflection of the F-mica substrate, as shown in Figure 2c. All of the peaks exhibit an excellent 6-fold symmetry. This result indicates the in-plane (IP) multidomain nature in the epitaxial LFO nanopillar and the LF1M2 nanocomposite film on the F-mica substrate. Panels d and e of Figure 2 display the surface morphology images of the etched 68 nm length LF1M2 arrays by the scanning electron microscope (SEM) and scanning transmission electron microscope (STEM), respectively, both of which show a separated cylindrical nanopillar shape of the LFO phase.20 SEM images of the etched 68 nm length LF1M3 and LF1M4 nanopillar arrays are displayed in Figure S2, according to which we collected the average diameter D of the nanopillar unit in Figure 2f. With the increase of x, the average diameter D of the nanopillar unit decreases. In addition, it can be seen that the nanopillar units in the etched 68 nm length LF1M2 arrays randomly distributed on the F-mica substrate, as shown in Figure 2d. For the LF1M3 and LF1M4 nanopillar arrays, the nanopillar units began to distribute as a clump. As

shown in Figure S2, due to the weak adhesion force between nanopillar arrays and F-mica substrate as well as the decreasing lateral scale of the nanopillar unit, the uneven distributed nanopillar arrays begin to fall down and even squeeze the neighbor unit together. The elastic and interfacial energy between the MgO phase and the LFO phase should be responsible for the random distribution of the LF1Mx arrays,21 which calls for further studies to improve the morphology of the nanopillar arrays. A vibrating sample magnetometer (VSM) was applied to measure the magnetic hysteresis (M−H) loops along both the IP and out-of-plane (OOP) directions of the LFO samples. In all the results shown here, the backgrounds have been eliminated and the magnetic moments have been normalized by per volume of LFO phase. Figure 3 compares the results of

Figure 3. M−H loops of the etched LF1M2 nanopillar arrays and the pure LFO film along IP and OOP directions on F-mica substrates. Insets collect the corresponding Ms.

the pure LFO film and the etched LF1M2 arrays. The difference in Ms between IP and OOP for the LFO film should result from the strain during the growth process, which has been found in various epitaxial films.16,22 For the nanopillar arrays, the difference in the Ms becomes smaller, due to the different growth condition and relatively relaxed strain state after etching the MgO phase.19 Decreased coercivity and remanent magnetization (Mr) were found in the LFO arrays, compared with that in the pure LFO film. In addition, the Ms of the LFO arrays is higher than that for the pure LFO film. Figure S3 shows the M−H loops of the LF1M3 and LF1M4 nanopillar arrays, and the corresponding Ms values of the nanopillar arrays with error bar were collected in Figure S4. A gradually increased Ms for per volume of the LFO phase was found with increasing x. The increasing air−LFO surface in the LFO nanopillar arrays should play an important role in the increase of Ms. According to the report by Arora et al., the termination of the LFO plane might lead to a noncompensation effect between A- and B-site Fe3+ ions, which was reported to induce a giant magnetic moment in ultrathin Fe3O4 films.23 Thus, it reveals that Ms increases with the increase of the surface-to-volume ratio of the LFO phase, which is similar to the former results.24,25 The different proportion of the dead layer should also contribute to the different Ms values, due to the diverse chemical environment during the film growth process.19 It is reported that the decreased coercivity has been found in various kinds of nanocomposites grown by pulsed laser deposition, which should be attributed to the interfacial diffusion or the C

DOI: 10.1021/acsami.8b12954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a)−(d) Perpendicular angle θH dependent FMR resonance field for the pure LFO film with (a) unbending and (b) bending (R = 4.0 mm) states, and for the LFO nanopillar arrays with (c) unbending and (d) bending (R = 4.0 mm) states on F-mica substrates, respectively. (e) Schematic illustration of the experimental setup for LFO sample during the FMR spectroscopy tests. (f) Corresponding elevation view for the sample under unbending and bending states. (g) and (h) Hr along OOP direction for (g) the LFO samples with the same thicknesses and (h) the LF1M2 nanopillar arrays with different thicknesses. The double arrows between the dash lines present the maximum change of Hr under bending.

formation of antiphase domains with lower coercivity.26 Moreover, the much smaller transverse sizes of the nanopillar arrays might lead to the formation of the superparamagnetic phase in the LFO arrays, which decreases the coercivity and remanence.27 FMR measurement was used to investigate the dynamic magnetic properties of the etched LF1Mx nanopillar arrays and pure LFO film. Panels a−d of Figure 4 collect the relationship between the Hr and the perpendicular angle θH in the FMR spectrum for both the pure LFO film and the LF1M2 arrays. The corresponding experimental setup of the bending axis shown in Figure 4e,f, which is set as the x-axis. The y-axis and z-axis are set along the LFO [112̅] and [111] orientations of the central nonbending unit cell, respectively. R represents the bending radius of the film. According to Figure 4a,b, an apparent change of Hr was found in the bending pure LFO film, especially when θH was set around 90°, while for the etched LF1M2 nanopillar arrays in Figure 4c,d, the Hr shows a narrow change at different θH and a nearly unchanged value under bending states. Figure S5 shows the Hr of the nanopillar arrays under different bending states and the corresponding results for the LF1M3 and LF1M4 nanopillar arrays are displayed in Figures S6 and S7, respectively, all of which further proves that bending has little impact on the FMR spectra of the LFO nanopillar arrays. Figure 4g shows the Hr along the OOP (θH = 90°) direction for the 68 nm thick LFO film and the 68 nm length LF1Mx nanopillar arrays, which also reveals that the change of Hr under bending is much lower in the nanopillar arrays. The decreased demagnetization field should be responsible for the result. According to eq 1, the θH dependent Hr highly relies on the value of 4πNeffMs. This equation is only suitable for the unbending film/arrays. While under bending, the included angle θH′ between the tangent plane of the bending substrate and the magnetic field will not be the original θH, but a series of values for each small region. Hence, it is clear that the relationship between the θH′ and Hr can effectively influence the bending result. If Hr remains unchanged for all θH, the change of Hr under bending should also decreases. According to eq 1, reducing the absolute value of Neff is one of the most effective solutions to realize the unchanged Hr. Not only the demagnetization of the single nanopillar but also the dipole interaction between the neighbor

nanopillar unit should be considered. In this paper, the range for the length to diameter ratio is from 3 to 7, whose influence is smaller than that from the dipole interaction between the neighbor nanopillar unit.28 Hence, we first consider the secondary factor. For ideal infinite cylindrical shape arrays, the Neff can be calculated as7 Neff = (3f − 1)/2 = ( 3π D2 /2r 2 − 1)/2

(2)

f is a filling factor that describes the volume fraction in the nanocomposite structure. r presents the central distance between the neighbor nanopillar arrays. For a continuous film, f = 1 and Neff = 1, whereas the limit of isolated infinite nanopillar is obtained by f = 0. The f for ideal infinite nanopillar arrays that are uniformly distributed can be obtained by f = 1/(1 + x). For LF1M2, Neff = 0. Therefore, the LF1M2 nanopillar arrays should theoretically exhibit the lowest magnetic anisotropy and bending tunability of Hr. Figure S8 shows the FMR spectra of the pure LFO film, the etched LF1M2 and LF1M4 nanopillar arrays on the unbending F-mica substrates. Compared with the pure LFO film, both of the etched LF1M2 and the LF1M4 nanopillar arrays show fewer differences in Hr along the IP and OOP directions, which accords well with the result we discussed above. However, an unexpected trough was found in the FMR spectra of LF1M4 arrays along the OOP direction, as shown in Figure S8, which could be result from the possible existence of LFO nanopillars with different magnetic states. According to the SEM result discussed above, in the LF1M3 and LF1M4 samples, the space distributions of the nanopillars are not very uniform and even show the trend to form clumps. The neighbor nanopillars in the clumps might show different macro magnetic performances with those neighbor nanopillars that have enough distance with each other, which needs further research. As shown in Figure 4g, the change of Hr under bending for the LF1M3 and LF1M4 is also not monotonous, due to the complex space distribution of the nanopillar arrays. In this paper, we mainly discuss the FMR result for the LF1M2 samples under different degrees of bending. In fact, the length of the nanopillar arrays l cannot be ignored in our case. To figure out the relationship between the bending tunability of Hr and the length-to-diameter ratios, the LF1M2 nanopillar arrays with different l are displayed in D

DOI: 10.1021/acsami.8b12954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Figures S9 and S10. The corresponding Hr along the OOP direction under bending is collected in Figure 4h. It reveals that the bending tunability of Hr increases with l. These results indicate that the 68 nm length the LF1M2 nanopillar arrays shows the best bending stability of Hr among all samples. Moreover, we have measured the FMR spectrum of the LF1M2 nanopillar arrays under the applied field with an IP angle φH ranging from 0° to 180°, which also displays excellent stability and accords with the calculation for (111)-oriented films by Kohmoto,29 as shown in Figure S11. These results pave the way for the design of future flexible microwave magnetic devices with excellent stability in three-dimensional spaces. In summary, we present systematic research of the influence of the nanostructure of the etched LF1Mx nanopillar arrays on the bending tuned magnetic properties. The structure of the nanopillar arrays can be tuned by the different target components and the different thicknesses during the growing process. In comparison with the pure LFO film, the samples of nanopillar arrays exhibit a higher Ms and very small nonzero coercivity and nonzero remanence. It is found that Hr in FMR can be tuned by the nanostructure of the LF1Mx nanopillar arrays on flexible F-mica substrates. For the 68 nm length etched LF1M2 nanopillar arrays, an almost unchanged FMR spectrum and narrow differences in Hr were found under bending and unbending states and even at various applied field directions. The obtained results demonstrate that tailoring the nanostructure of LFO is an applicable method to realize the future stable flexible microwave magnetic devices.

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No. 51771145). X.L. acknowledges the support by the National Science Foundation of China (No. 51572211).

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(1) Shen, L.; Wu, L.; Sheng, Q.; Ma, C.; Zhang, Y.; Lu, L.; Ma, J.; Ma, J.; Bian, J.; Yang, Y.; Chen, A.; Lu, X.; Liu, M.; Wang, H.; Jia, C. L. Epitaxial Lift-Off of Centimeter-Scaled Spinel Ferrite Oxide Thin Films for Flexible Electronics. Adv. Mater. 2017, 29 (33), 1702411. (2) Vemulkar, T.; Mansell, R.; Fernández-Pacheco, A.; Cowburn, R. Toward Flexible Spintronics: Perpendicularly Magnetized Synthetic Antiferromagnetic Thin Films and Nanowires on Polyimide Substrates. Adv. Funct. Mater. 2016, 26 (26), 4704−4711. (3) Liu, W. L.; Liu, M.; Ma, R.; Zhang, R. Y.; Zhang, W. Q.; Yu, D. P.; Wang, Q.; Wang, J. N.; Wang, H. Mechanical Strain-Tunable Microwave Magnetism in Flexible CuFe2O4 Epitaxial Thin Film for Wearable Sensors. Adv. Funct. Mater. 2018, 28 (10), 1705928. (4) Zhao, S.; Zhou, Z.; Peng, B.; Zhu, M.; Feng, M.; Yang, Q.; Yan, Y.; Ren, W.; Ye, Z. G.; Liu, Y.; Liu, M. Quantitative Determination on Ionic-Liquid-Gating Control of Interfacial Magnetism. Adv. Mater. 2017, 29 (17), 1606478. (5) Huang, Z. C.; Liu, W. Q.; Yue, J. J.; Zhou, Q. H.; Zhang, W.; Lu, Y. X.; Sui, Y. X.; Zhai, Y.; Chen, Q.; Dong, S.; Wang, J. L.; Xu, Y. B.; Wang, B. P. Enhancing the Spin-Orbit Coupling in Fe3O4 Epitaxial Thin Films by Interface Engineering. ACS Appl. Mater. Interfaces 2016, 8 (40), 27353−27359. (6) Cho, J.; Fujii, Y.; Konioshi, K.; Yoon, J.; Kim, N. H.; Jung, J.; Miwa, S.; Jung, M. H.; Suzuki, Y.; You, C. Y. Study of Spin Dynamics and Damping on the Magnetic Nanowire Arrays with Various Nanowire Widths. J. Magn. Magn. Mater. 2016, 409, 99−103. (7) Kartopu, G.; Yalçın, O.; Kazan, S.; Aktaş, B. Preparation and FMR Analysis of Co Nanowires in Alumina Templates. J. Magn. Magn. Mater. 2009, 321 (9), 1142−1147. (8) Trusca, O. C.; Cimpoesu, D.; Lim, J. H.; Zhang, X.; Wiley, J. B.; Diaconu, A.; Dumitru, I.; Stancu, A.; Spinu, L. Interaction Effects in Ni Nanowire Arrays. IEEE Trans. Magn. 2008, 44 (11), 2730−2733. (9) Kou, X. M.; Fan, X.; Zhu, H.; Cao, R.; Xiao, J. Q. Microwave Permeability and Tunable Ferromagnetic Resonance in Cobalt Nanowire Arrays. IEEE Trans. Magn. 2010, 46 (5), 1143−1146. (10) Noh, S.; Monma, D.; Miyake, K.; Doi, M.; Kaneko, T.; Imamura, H.; Sahashi, M. Spin Dynamics in Ferromagnetic Resonance for Nano-Sized Magnetic Dot Arrays: Metrology and Insight Into Magnetization Dynamics. IEEE Trans. Magn. 2011, 47 (10), 2387−2390. (11) Li, C.; Wu, Q.; Yue, M.; Xu, H.; Palaka, S.; Elkins, K.; Ping Liu, J. Manipulation of Morphology and Magnetic Properties in Cobalt Nanowires. AIP Adv. 2017, 7 (5), 056229. (12) Hehn, M.; Padovani, S.; Ounadjela, K.; Bucher, J. P. Nanoscale Magnetic Domain Structures in Epitaxial Cobalt Films. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54 (5), 3428−3433. (13) Merabtine, S.; Zighem, F.; Faurie, D.; Garcia-Sanchez, A.; Lupo, P.; Adeyeye, A. O. Multicracking and Magnetic Behavior of Ni80Fe20 Nanowires Deposited onto a Polymer Substrate. Nano Lett. 2018, 18 (5), 3199−3202. (14) Argentina, G. M.; Baba, P. D. Microwave Lithium Ferrites: An Overview. IEEE Trans. Microwave Theory Tech. 1974, 22, 652. (15) Jovic, N. G.; Masadeh, A. S.; Kremenovic, A. S.; Antic, B. V.; Blanusa, J. L.; Cvjeticanin, N. D.; Goya, G. F.; Antisari, M. V.; Bozin, E. S. Effects of Thermal Annealing on Structural and Magnetic Properties of Lithium Ferrite Nanoparticles. J. Phys. Chem. C 2009, 113 (48), 20559−20567. (16) Zhang, R. Y.; Liu, M.; Lu, L.; Mi, S. B.; Wang, H. StrainTunable Magnetic Properties of Epitaxial Lithium Ferrite Thin Film on MgAl2O4 Substrates. J. Mater. Chem. C 2015, 3 (21), 5598−5602. (17) Zhang, R.; Liu, M.; Lu, L.; Mi, S.-B.; Wang, H. Ultra-Low Temperature Epitaxial Growth of Lithium Ferrite Thin Films by High-Pressure Sputtering. CrystEngComm 2015, 17 (43), 8256−8263. (18) Boyraz, C.; Mazumdar, D.; Iliev, M.; Marinova, V.; Ma, J.; Srinivasan, G.; Gupta, A. Structural and Magnetic Properties of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12954.

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REFERENCES

Materials and methods, typical XRD θ−2θ scans of the LF1M2 nanopillar arrays, SEM images of the nanopillar arrays, the M−H loops of etched LF1M3 and LF1M4 nanopillar arrays, θH dependent Hr for LFO samples under bending, and FMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*M. Liu. E-mail: [email protected]. ORCID

Xiaoli Lu: 0000-0003-3689-5996 Chunrui Ma: 0000-0002-7824-7930 Ming Liu: 0000-0002-4392-9659 Chun-Lin Jia: 0000-0001-7536-9521 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National Science Foundation of China (No. 51390472) and National “973” projects of China (No. 2015CB654903). M.L. and C.M. acknowledge the support by Fundamental Research Funds for the Central Universities and China Postdoctoral Science Foundation (No. 2015M582649). C.C. and C.J. acknowledge the support from the National Natural Science Foundation of China (No. 51671099). H.F. and C.Y. acknowledge the support by the National Science Foundation of China (No. 51371140 and E

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ACS Applied Materials & Interfaces Lithium Ferrite (LiFe5O8) Thin Films: Influence of Substrate on the Octahedral Site Order. Appl. Phys. Lett. 2011, 98 (1), 012507. (19) Shen, L.; Liu, M.; Ma, C.; Lu, L.; Fu, H.; You, C.; Lu, X.; Jia, C.-L. Enhanced Bending-Tuned Magnetic Properties in Epitaxial Cobalt Ferrite Nanopillar Arrays on Flexible Substrates. Mater. Horiz. 2018, 5 (2), 230−239. (20) Baek, S. H.; Jang, H. W.; Folkman, C. M.; Li, Y. L.; Winchester, B.; Zhang, J. X.; He, Q.; Chu, Y. H.; Nelson, C. T.; Rzchowski, M. S.; Pan, X. Q.; Ramesh, R.; Chen, L. Q.; Eom, C. B. Ferroelastic Switching for Nanoscale Non-Volatile Magnetoelectric Devices. Nat. Mater. 2010, 9 (4), 309−314. (21) Levin, I.; Li, J.; Slutsker, J.; Roytburd, A. L. Design of SelfAssembled Multiferroic Nanostructures in Epitaxial Films. Adv. Mater. 2006, 18 (15), 2044−2047. (22) Dhakal, T.; Mukherjee, D.; Hyde, R.; Mukherjee, P.; Phan, M. H.; Srikanth, H.; Witanachchi, S. Magnetic Anisotropy and Field Switching in Cobalt Ferrite Thin Films Deposited by Pulsed Laser Ablation. J. Appl. Phys. 2010, 107 (5), 053914. (23) Arora, S. K.; Wu, H.-C.; Choudhary, R. J.; Shvets, I. V.; Mryasov, O. N.; Yao, H.; Ching, W. Y. Giant Magnetic Moment in Epitaxial Fe3O4 Thin Films on MgO(100). Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77 (13), 134443. (24) Bai, F. M.; Yu, G.; Wang, Y. C.; Jin, L. C.; Zhong, Z. Y.; Zhang, H. W.; Ye, F. Dramatic Reduction of FMR Linewidth in Epitaxial Pb(ZrTi)O3-NiFe2O4 Nanocomposite Films. IEEE Trans. Magn. 2013, 49 (7), 4299−4302. (25) Kim, D. H.; Aimon, N. M.; Ross, C. A. Pillar Shape Modulation in Epitaxial BiFeO3−CoFe2O4 Vertical Nanocomposite Films. APL Mater. 2014, 2 (8), 081101. (26) Baber, S. M.; Lin, Q.; Zou, G.; Haberkorn, N.; Baily, S. A.; Wang, H.; Bi, Z.; Yang, H.; Deng, S.; Hawley, M. E.; Civale, L.; Bauer, E.; McCleskey, T. M.; Burrell, A. K.; Jia, Q.; Luo, H. Magnetic Properties of Self-Assembled Epitaxial Nanocomposite CoFe2O4:SrTiO3and CoFe2O4:MgO Films. J. Phys. Chem. C 2011, 115 (51), 25338−25342. (27) Newell, A. J.; Merrill, R. T. Single-Domain Critical Sizes for Coercivity and Remanence. J. Geophys. Res-Sol. Ea. 1999, 104 (B1), 617−628. (28) Sato, M.; Ishii, Y. Simple and Approximate Expressions of Demagnetizing Factors of Uniformly Magnetized Rectangular Rod and Cylinder. J. Appl. Phys. 1989, 66 (2), 983−985. (29) Kohmoto, O. Effective Demagnetizing Factors in Ferromagnetic Resonance Equations. J. Magn. Magn. Mater. 2003, 262 (2), 280−288.

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DOI: 10.1021/acsami.8b12954 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX