Direct Observation of Magnetocrystalline Anisotropy Tuning

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Direct Observation of Magnetocrystalline Anisotropy Tuning Magnetization Configurations in Uniaxial Magnetic Nanomaterials Shimeng Zhu, Jiecai Fu, Hongli Li, Liu Zhu, Yang Hu, Weixing Xia, Xixiang Zhang, Yong Peng, and Junli Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00058 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Direct Observation of Magnetocrystalline Anisotropy Tuning Magnetization Configurations in Uniaxial Magnetic Nanomaterials

Shimeng Zhu1, Jiecai Fu1, Hongli Li1, Liu Zhu1, Yang Hu1, Weixing Xia2, Xixiang Zhang3, Yong Peng1,* and Junli Zhang1,3,*

1

Key Laboratory of Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou

University, Lanzhou 730000, P. R. China.

2

Key Laboratory of Magnetic Materials and Devices,

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China. 3Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 239955, Kingdom of Saudi Arabia.

* Corresponding authors Key Laboratory of Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, P. R. China E-mail: [email protected] (Yong Peng) and [email protected] (Junli Zhang)

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ABSTRACT. Discovering the effect of magnetic anisotropy on the magnetization configurations of magnetic nanomaterials is essential and significant for not only enriching the fundamental knowledge of magnetics, but also facilitating the designs of desired magnetic nanostructures for diverse technological applications, such as data storage devices, spintronic devices, and magnetic nanosensors. Herein, we present a direct observation of magnetocrystalline anisotropy tuning magnetization configurations in uniaxial magnetic nanomaterials with hexagonal structure by means of three modeled samples. The magnetic configuration in polycrystalline BaFe12O19 nanoslice is a curling structure, revealing that the effect of magnetocrystalline anisotropy in uniaxial magnetic nanomaterials can be broken by forming an amorphous structure or polycrystalline structure with tiny

grains.

Both

single

crystalline

BaFe12O19

nanoslice

and

individual

particles

of

single-particle-chain BaFe12O19 nanowire appear single domain state, revealing a dominant role of magnetocrystalline anisotropy in the magnetization configuration of uniaxial magnetic nanomaterials. These observations are further verified by micromagnetic computational simulations.

KEYWORDS. magnetic configuration, magneto-crystalline anisotropy, electron holography, micromagnetic simulation, nanostructured BaFe12O19

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Magnetic anisotropy (MA) determines the magnetization configuration of magnetic materials and plays a critical role in many impacted applications including the magnetic recording, communication devices and spintronic devices.1-4 High magnetic anisotropy is crucially demanded for the high stability of magnetic recording units. Numerous researches on ferromagnetic bulk materials have demonstrated the dominant role of crystal structure and chemical composition in the magnetic anisotropy.5-7 In comparison with bulk materials, low dimensional magnetic nanostructures can offer additional degrees of freedom to tune the magnetic anisotropy through the modulation of size and morphology.8-11 Therefore, the features of the nanostructured materials become more attractive in fundamental research and practical applications. To date, interfacial effects were found to be the main contribution to the magnetic anisotropy in nanosized cobalt clusters embedded in niobium film,1 while the perpendicular magnetic anisotropy can achieve the same order of magnitude of shape anisotropy in Co-rich Co-Pt film.11 Besides, the existence of slow magnetic relaxation in one-dimensional cyanometalate complex4 and the controllable magnetic anisotropy by electric field in polymer ferroelectric film were also reported.8 However, how the crystal orientation and the additional degrees affect the magnetic anisotropy and the magnetization configuration remain unclear. The underlying relationship between the magnetic anisotropy and magnetization configuration in magnetic nanostructures deserve to be explored. Magnetoplumbite type barium ferrite (BaFe12O19, BFO) is a typical example having high uniaxial magnetocrystalline anisotropy energy with hcp structure, which has been widely applied into magnetic recording, magneto-optic recording, motors, toys, etc.12-14 Its crystal structure typically consists of 64 ions per primitive unit cell occupied on 11 different symmetry sites and crystallizes in P63/mmc space group (Figure 1a). The net magnetic moment per cell is provided by the sum of 24 Fe3+, of which 16 are spins up located at octahedral (pink and yellow) and five-fold coordinated sites (light green), while 8 are spins down located at octahedral (blue) and tetrahedral sites (dark green) as ACS Paragon Plus Environment

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shown in Figure 1b.15, 16 Many reports focus on their high-quality preparation and macro-scale magnetic properties stimulated by the practical applications,15, 17-20 where the growth mechanism and macroscopic magnetic behaviors on single-particle-chain BaFe12O19 nanofibres,17 as well as microwave absorption behaviors on barium ferrite based nanocomposites have been widely reported.13,14,18 However, the investigation on magnetization configuration in individual nanostructures was rarely reported, because

a high spatial resolution is always needed for the

exploration within individual nanostructural units. Although several facilities including the Lorentz microscopy,21 magnetic force microscopy,22 scanning tunneling microscopy,23 and Kerr microscopy24 have been employed to study local magnetic properties, the limited resolution in tens of nanometers is still insufficient to observe the magnetization configuration at the nanoscale. Alternatively, the off-axis electron holography (EH)25-27 as a powerful transmission electron microscopy (TEM)-based strategy using a coherent electron wave is a promising method to realize a high-resolution visualization (~2 nm) of the electromagnetic field in an isolated nanostructure. However, its application in the investigations of magnetization configurations in nano-ferrites is rarely reported so far. In this work, we use off-axis EH method to image the magnetization configurations tuned by magnetocrystalline anisotropy in uniaxial BFO nanomaterials. Three models including polycrystalline BFO nanoslice (NS), single crystalline BFO NS and single-particle-chain BaFe12O19 nanowire (NW) were designed. Crystalline sizes and orientations were found to be the main factor to tune the magnetization configurations of these nanostructured BFO ferrites. Micromagnetic simulations further verify the observed phenomena, of which constructed 3D magnetic configurations clearly demonstrate the dominant role of magnetocrystalline anisotropy in the magnetic moment distributions in the nanostructural BFO ferrites. Our finding shows that modulation of magnetocrystalline anisotropy is an efficient way to tune the magnetization ACS Paragon Plus Environment

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configurations for designing innovative magnetic devices.

Figure 1. Crystal structure of magnetoplumbite type Barium Ferrite. (a) Ball & stick model; (b) Polyhedral model, showing a uniaxial network of O2- anions with Fe3+ cations occupying five interstitial sites: three of which are octahedral sites (marked by yellow, light purple and blue), one of which is tetrahedral site (marked by dark green), and one of which is hexahedral site (marked by light green).

RESULTS AND DISCUSSIONS Three kinds of nanostructured BFO including polycrystalline nanoslice, single crystalline nanoslice and single-particle-chain nanowire were systematically investigated, which were designed to figure out the interrelations between magnetic anisotropy and magnetization configuration in uniaxial magnetic nanomaterials. SEM images show that both polycrystalline and single crystalline BFO NS are flaky (see Supporting Information Figure S1 for details). Figure 2a shows a representative TEM image of polycrystalline BFO NS, revealing an 84 nm diameter. Numerous small crystalline grains and amorphous regions can be observed from the high-resolution TEM image in Figure 2b, of which ACS Paragon Plus Environment

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crystal orientations are randomly distributed. Quantitative analysis reveals that the average diameter of the BFO grains is about 1.5 nm. The selected area electron diffraction (SAED) pattern shown in Figure 1c further confirms its polycrystalline characteristics. Figure 2d-f show the morphological and structural characterization of single crystalline BFO NS. The size of this flaky BFO NS is measured about 310 nm (Figure 2d), where the single crystal with hcp structure is confirmed by the HRTEM image (Figure 2e) and SAED pattern (Figure 2f). This BFO NS is oriented in the [210] direction and the c-axis is parallel to its surface.

Figure 2. Morphological and structural characterization of BFO NSs. TEM, HRTEM images and SAED pattern of (a-c) polycrystalline NS and (d-f) single crystalline NS.

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Figure 3. Structural analysis of the single-particle-chain BFO NW measured by probe-aberration corrected STEM. (a) TEM image; (b)-(e) The experimental high angle angular dark field-scanning transmission electron microscopy (HAADF-STEM) images of corresponding particles 1-4 in (a), revealing [110], [110], [210] and [001] orientation, respectively. All HRTEM images were collected by tilting the sample holder with small angles to reach the nearest zone axis.

The TEM image in Figure 3a reveals that the BFO NW consists of several single particles that stacked along the NWs’ axial direction. The diameter of individual particles ranges from 60 nm to 90 nm. Both SAED and convergent-beam electron diffraction (CBED) patterns confirm that the BFO NWs are hexagonal structure and individual particles on each NW are single crystal. Detailed structural analysis (Figure 3b-e) measured by probe aberration-corrected scanning transmission electron microscopy (STEM) reveal that the crystallographic orientations of particles 1-4 numbered in Figure 3a are [110], [110], [210] and [001], respectively, indicating a random distribution (see more evidences in Supporting Information Figure S2). As hexagonal crystal structure typically forms uniaxial anisotropy, these orientations mean the easy axes of particles 1-3 are along the length axis

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of this BFO NW and that of particle 4 is perpendicular with the length axis of this NW. Further X-ray diffraction spectra reconfirm the three kinds of nanostructured BaFe12O19 are hcp structure (see Supporting Information Figure S3 for details), exhibits strong magnetocrystalline anisotropy. The magnetocrystalline anisotropy strongly determines the eventual magnetization configuration and also plays a prominent role in tuning the magnetic properties of hexagonal magnetic nanomaterials in their diverse applications. We therefore performed a detailed magnetization configuration characterization in these samples.

Figure 4. Magnetic induction map, modulus of magnetic phase distribution profile, and the vectorial maps (or color graphic) of the in-plane component of magnetic induction of three modeled BFO specimens. (a-c) Polycrystalline NS; (d-f) Single-crystalline NS and (h-i) NW respectively. The color wheels indicate the direction of in-plane magnetization.

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Figure 4a shows a magnetization map of the polycrystalline BFO NS (Figure 2a) constructed from three Lorentz TEM images (in focus, under focus and over focus) based on the transport of intensity equation (TIE).28 The magnetic flux in plane exhibits centrosymmetric distribution, revealing a curling magnetic structure. The corresponding modulus of phase shift profile (Figure 4b) and a vectorial map of in-plane magnetic induction (Figure 4c) further demonstrate this magnetic construction. This result shows that the magnetization configuration of the polycrystalline BFO NS is determined by its shape anisotropy and dipolar anisotropy rather than its magnetocrystalline anisotropy. These results illustrate that polycrystalline structure with tiny grains and amorphous structure can eliminate the effect of magnetocrystalline anisotropy on the magnetization configuration in uniaxial magnetic nanomaterials. Whilst, the magnetization configuration of the single-crystalline BFO NS (Figure 2d) constructed by the TIE technique at zero magnetic field appears a totally different distribution, which shows the magnetic flux line array along its c-axis direction. This data clearly reveals this single-crystalline BFO NS is a single-domain structure. It is known that the single crystal particle is in single domain state when its radius is smaller than critical single domain radius. Theoretically, a single-domains state can be maintained when the particle radius is smaller than Rsd under demagnetization state. We herein make a comparison to further verify the correctness of the magnetization configuration in the single crystalline BFO NS. The critical single domain radius is calculated by equation 1:7

1/2

36 ( AK1 ) Rsd = µo M s 2

(1)

where A and K1 are exchange constant and magnetic anisotropic constant, 5×10-7 erg·cm-1 and 330×103 J·m-3 for our BFO; µ0 is the permeability of free space. A 338 nm critical single domain radius (Rsd) for the BFO is then calculated, which is much larger than our single crystalline BFO NS

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measured by above TEM. Therefore, it can be concluded that our experimental observation for the single crystalline BFO NS agrees with the theoretical prediction. Both experimental magnetization configurations of polycrystalline and single crystalline BFO NSs reveal that the magnetization configurations in uniaxial magnetic nanomaterials can be effectively tuned by their magnetocrystalline anisotropy via crystal size and orientation. To further demonstrate the significance of crystalline orientation, the magnetic configurations in BFO NW formed by a chain of single crystalline particles were studied. Single-particle-chain NW has a strong shape anisotropy and dipolar anisotropy due to high aspect ratio and continuous neighboring single crystals, which provide an ideal model to distinguish the importance of morphology

(shape

anisotropy

and

dipolar

anisotropy)

and

crystalline

orientation

(magnetocrystalline anisotropy) for the magnetic anisotropy in hexagonal nanostructured BFO. Figure 4g shows the magnetization configuration of the aforementioned BFO NW (Figure 3a) detected by the EH technique. It is clear that the magnetic flux lines of particles 1-3 all align along their c-axes, demonstrating single domain status. Whilst, there is no obvious magnetic flux line distribution in the particle 4 (i.e. no in-plan component), which means the magnetic flux lines in this particle are parallel to the incident electron beam and well matched with the c-axis orientation of this crystal which is perpendicular with the length axis of this BFO NW as determined above. Note that the higher density magnetic flux lines in the margin of NW is attributed to the irregular shape of NW, where the thickness information can’t be fully removed during the signal processing. This observation confirms that the shape anisotropy and dipolar anisotropy are indecisive in BFO NW, while the magnetocrystalline anisotropy is the dominant role to make the magnetic moments align the [001] direction. The phase shift resulted from the internal magnetic induction in the particle 1-3 is also analyzed as shown in Figure 4h. The modulus of the phase shift is related to the magnetic flux density and can be expressed as below:29 ACS Paragon Plus Environment

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∆ϕ =

e Bs ћ

(2)

Where ħ is the reduced Plank’s constant, e, B and s are electron charge, magnetic flux density, and the cross-section area of the specimen, respectively. The ∆ϕ is the difference of the electron wave phase shift on the left and right side of the NW as shown in green and dark blue color in Figure 4i. It can be measured from the modulus of the phase shift profile in Figure 4h, which is about 2.4 rad for this BFO NW. A 282 emu/cm3 (equal to 53.41 emu/g) magnetic flux density at remnant state can be calculated, which is close to the saturation magnetization measured by VSM (see Supporting Information Figure S4). This phase shift profile (Figure 4i) also demonstrates that the magnetic flux lines of particle 1-3 are almost parallel to the c-axis direction of each crystals. From the above experimental results and theoretical analysis, it can be seen that the micromagnetic configurations of all three nanostructured BFO are dominantly determined by their magnetocrystalline anisotropy when their grains grow into single crystals, of which shape anisotropy and dipolar anisotropy are weak.

Figure 5. The simulated 3D micromagnetic structures of three modeled BFO specimens. (a) polycrystalline NS; (b) single-crystalline NS with the diameter of 84 nm at different observation

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scale; (c) single-crystalline NS with the diameter of 310 nm; (d) single-particle-chain NW. The colors of the arrows represent the orientations of the magnetic moments.

The micromagnetic simulations were further employed to verify above experimental results by using OOMMF software. 3D distributions of magnetic flux line in above three modeled samples were then constructed. The geometrical structure (including size and crystalline orientations) and magnetic parameters used in the computational simulation were based on above TEM morphological observation and magnetic measurements. The thicknesses of NS in micromagnetic simulation are 60 nm and 80 nm respectively for the polycrystalline NSs and the larger single-crystalline NSs, as shown in Supporting Information Figure S1. The saturation magnetization was set to be 282 emu/cm3 according to the modulus of the phase shift calculation. Figure 5a shows a simulated 3D magnetic induction map of the polycrystalline BFO NS, clearly revealing that the magnetic moments appear a curling state around the center of the NS and that there is no out of plane component of the magnetic induction. This result clearly shows that the magnetocrystalline anisotropy is not taken a dominant role anymore when the grain sizes are too small in the poly-crystalline nanoslice even if its crystal structure is hexagonal. Whilst, the simulated magnetic induction map in single crystalline NS with the same size appears a single domain structure as shown in Figure 5b. The magnetic configurations of the single crystalline BFO NSs with larger size (310 nm in diameter) were also performed when the c-axis in and out of the plane. We find that both NS present single domain structure and the magnetic moments keep parallel to the c-axis as shown in Figure 5c and Supporting Information Figure S6. These observations fully confirm the dominant role of the magnetocrystalline anisotropy in BFO single-crystalline NSs when the particle size is less than the critical single domain radius. This result is well consistent with above TIE measurement (Figure 4d) and the calculation of critical single domain radius from the equation 1. Figure 5d shows the simulated 3D magnetic ACS Paragon Plus Environment

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configuration of the single-particle-chain BFO NW. It is seen that the remnant state in each particle is single magnetic domain and their magnetic moments are parallel to the c-axes of each BFO single crystals in the NW, which can also be visualized from the plan view of the magnetic moments distribution (see Supporting Information Figure S5). Only the magnetic moments at the edges and interfaces of the single particles are slightly diverged. This result further reveals that the magnetocrystalline anisotropy dominantly determines the magnetization configuration of uniaxial magnetic materials when their particles are single crystals. The role of shape anisotropy in BFO NWs with increased aspect ratio (length /diameter) is further identified (see Supporting Information Figure S7). We find that the magnetic moments in the NW can keep parallel to the easy axis (i.e., perpendicular to the NW) at remnant state even the aspect ratio increased to 5. These results indicate that the shape anisotropy is indecisive for the magnetic configuration of BaM nanostructures. The EH experimental results and 3D micromagnetic simulations of these modeles all demonstrate the decisive role of magnetocrystalline anisotropy in magnetic moment distributions in single crystalline BaM nanostructures, rather than the effect of shape anisotropy and dipolar anisotropy. Therefore, designing an amorphous structure or polycrystalline structure with tiny grains is an efficient way to break the effect of magnetocrystalline anisotropy for the uniaxial magnetic nanomaterials.

CONCLUSION We have demonstrated the influence of magnetocrystalline anisotropy on magnetization configurations in uniaxial magnetic nanomaterials by using three modeled BFO samples with hcp structure. The experimental result reveals that polycrystalline BaFe12O19 nanoslice composed of tiny grains and amorphous regions forms a curling domain structure. Both single crystalline BaFe12O19 nanoslice and single-particle-chain BaFe12O19 NW appear single domain state, of which magnetic moments are parallel to the easy axis [001] of each single crystal. Our study reveals that the

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magnetocrystalline anisotropy dominantly determines the magnetization configuration of uniaxial magnetic nanomaterials when their particles are single crystals. These observations are further verified by micromagnetic computational simulations. Our work should provide a visual evidence to understand the dominant role of magnetocrystalline anisotropy in the uniaxial magnetic materials and show a way to tune the magnetization configuration via the changes of their crystal structures, which should be significant for exploring more specific nanomaterials to satisfy the diverse applications in magnetic memory and storage devices.

METHODS Specimen

preparation.

BaFe12O19

single-particle-chain

NWs were

prepared

by

using

electrospinning method, which is the same as our previous reports.17 The electrospinning electrolyte was composed of 0.1 mmol barium nitrate (Ba(NO3)2, A.R., Alfa-Aesar Inc., USA), 1.2 mmol iron nitrate nonahydrate (Fe(NO3)3·9H2O, A.R., Alfa-Aesar Inc., USA), 0.18 g polyvinylpyrrolidone (PVP) (Mw≈1 300 000, Sigma-Aldrich Inc., USA), 1.25 ml de-ionized water (DIW) and 1.25 ml N, N-dimethyl formamide (DMF, A.R., Tianjin Chemical Corp., China). The electrospinning process was performed at 16 kV DC voltage, 15 cm spacing between needle tip and collector, and a feed rate of 0.3 mL·h-1 in a dedicated electrospinning setup. The calcination was carried out at 900 ºC for 8 h in the air with a 1 ºC/min heating rate and then cooled down to the room temperature under the same rate of 1 ºC/min for a good crystallinity. Note that the size and uniformity of BaFe12O19 particles can be controlled through the electrospinning parameters and calcination conditions accordingly.17 The polycrystalline and single crystalline BaFe12O19 nanoslices were commercially purchased (Aladdin Bio-Chem Technology, China). Structure and magnetic properties measurements. The morphologies and crystal structure of BaFe12O19 nanostructures were analyzed using transmission electron microscopy (TEM, FEI Tecnai

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G2 F30 operated at 300 kV, USA) and X-ray diffraction instrument (XRD, Philips X’pert Pro MPD, the Netherlands). Atomic resolved STEM images were performed on a probe aberration-corrected STEM (FEI, Titan Cubed Themis G2 300, USA) operated at 300 kV. The macroscopic magnetic properties of nanostructured BaFe12O19 were measured by vibrating sample magnetometer (VSM, Lakeshore, USA). Lorentz microscopy and electron holography (EH) were carried out using a specialized Lorentz microscopy (LTEM JEOL-2100F, Japan) which is equipped with specially designed objective lens providing a field free (~5 Oe) environment at the specimen. The equipped electron biprism for off-axis EH can achieve 2 nm spatial resolution. TIE measurements. The magnetization vector maps (Figure 3c and 3f) of nanoslices can be directly obtained from three Lorentz TEM images (under-focus, in-focus, and over focus) with the assistance of the transport of intensity equation (TIE) using QPt plug-in software. Magnetic induction maps are then gotten by enlarging the vector maps with a cosine function. Electron Holography. Off-axis electron holography was carried out at 200kV specialized Lorentz microscopy (LTEM JEOL-2100F, Japan) using an electron biprism operated at lower than 200 V, under which the amplitude and phase shift of electrons penetrated from the specimen were recorded. The phase shifts were caused by the in-plan component of magnetic and electrostatic potential, where the magnetic potential induced phase shift can be obtained from the half of the signal difference between the paired holograms on the positive and negative side of sample. These experiments were performed at zero magnetic field at room temperature. Micromagnetic simulations. The 3D magnetization configuration in the nanostructured BaFe12O19 was simulated by the public micromagnetic software of OOMMF.30 The geometrical parameters and crystallography orientation of the NS and NW were defined according to the observation in TEM. The unit cell size was divided into 2×2×2 nm3 and the exchange constant (A) of BaFe12O19 was set up to be 5×10-7 erg·cm-1. ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Available: Additional characterization and theoretical analysis, including morphology by SEM, crystal structure by Cs-STEM and XRD, and micromagnetic simulation by OOMMF software. The Supporting Information is available free of charge via the internet at http://pubs.acs.org. Corresponding Author E-mail: [email protected] and [email protected]

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51571104, 11604130, 51601082, 51771085 and 11274145), MOST International Cooperation Funds (2014DFA91340), the Fundamental Research Funds for the Central Universities (lzujbky-2017-176 and lzujbky-2017-177) and Open Project of Key Laboratory of Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University (LZUMMM2017003).

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Decorated Reduced Graphene Oxide Nanocomposite for Effective Electromagnetic Interference Shielding. Phys. Chem. Chem. Phys. 2015, 17, 1610-1618. 14. Ohlan, A.; Singh, K.; Chandra, A.; Dhawan, S. K., Microwave Absorption Behavior of Core-Shell Structured Poly (3, 4-Ethylenedioxy Thiophene)-Barium Ferrite Nanocomposites. ACS Appl. Mater. Interfaces 2010, 2, 927-933. 15. Xu, P.; Han, X.; Wang, M., Synthesis and Magnetic Properties of BaFe12O19 Hexaferrite Nanoparticles by a Reverse Microemulsion Technique. J. Phys. Chem. C 2007, 111, 5866-5870. 16. Collomb, A.; Wolfers, P.; Obradors, X., Neutron Diffraction Studies of Some Hexagonal Ferrites: BaFe12O19, BaMg2-W and BaCo2-W. J. Magn. Magn. Mater. 1986, 62, 57-67. 17. Zhang, J.; Fu, J.; Li, F.; Xie, E.; Xue, D.; Mellors, N. J.; Peng, Y., BaFe12O19 Single-Particle-Chain Nanofibers: Preparation, Characterization, Formation Principle, and Magnetization Reversal Mechanism. ACS Nano 2012, 6, 2273-2280. 18. Song, Y.-Y.; Das, J.; Wang, Z.; Tong, W.; Patton, C. E., In-Plane c-Axis Oriented Barium Ferrite Films with Self-Bias and Low Microwave Loss. Appl. Phys. Lett. 2008, 93, 172503. 19. Pramanik, N. C.; Fujii, T.; Nakanishi, M.; Takada, J., Development of Nanograined Hexagonal Barium Ferrite Thin Films by Sol-Gel Technique. Mater. Lett. 2005, 59, 468-472. 20. Li, Y.; Zhang, H.; Liu, Y.; Wen, Q.; Li, J., Rod-Shaped Polyaniline-Barium Ferrite Nanocomposite: Preparation, Characterization and Properties. Nanotechnology 2008, 19, 105605. 21. Brintlinger, T.; Lim, S.-H.; Baloch, K. H.; Alexander, P.; Qi, Y.; Barry, J.; Melngailis, J.; Salamanca-Riba, L.; Takeuchi, I.; Cumings, J., In Situ Observation of Reversible Nanomagnetic Switching Induced by Electric Fields. Nano Lett. 2010, 10, 1219-1223. 22. Serri, M.; Mannini, M.; Poggini, L.; Vélez-Fort, E.; Cortigiani, B.; Sainctavit, P.; Rovai, D.; Caneschi, A.; Sessoli, R., Low-Temperature Magnetic Force Microscopy on Single Molecule Magnet-Based Microarrays. Nano Lett. 2017, 17, 1899-1905. ACS Paragon Plus Environment

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23. Prévot, G.; Hogan, C.; Leoni, T.; Bernard, R.; Moyen, E.; Masson, L., Si Nanoribbons on Ag (110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain Structure. Phys. Rev. Lett. 2016, 117, 276102. 24. Romanens, F.; Pizzini, S.; Yokaichiya, F.; Bonfim, M.; Pennec, Y.; Camarero, J.; Vogel, J.; Sort, J.; Garcia, F.; Rodmacq, B., Magnetic Relaxation of Exchange Biased Pt⁄ Co Multilayers Studied by Time-Resolved Kerr Microscopy. Phys. Rev. B 2005, 72, 134410. 25. Rodríguez, L. A.; Bran, C.; Reyes, D.; Berganza, E.; Vázquez, M.; Gatel, C.; Snoeck, E.; Asenjo, A., Quantitative Nanoscale Magnetic Study of Isolated Diameter-Modulated Fecocu Nanowires. ACS Nano 2016, 10, 9669-9678. 26. Midgley, P. A.; Dunin-Borkowski, R. E., Electron Tomography and Holography in Materials Science. Nat. Mater. 2009, 8, 271. 27. Gan, Z.; Gu, M.; Tang, J.; Wang, C.-Y.; He, Y.; Wang, K. L.; Wang, C.; Smith, D. J.; McCartney, M. R., Direct Mapping of Charge Distribution During Lithiation of Ge Nanowires Using Off-Axis Electron Holography. Nano Lett. 2016, 16, 3748-3753. 28. Petersen, T. C.; Keast, V. J.; Paganin, D. M., Quantitative Tem-Based Phase Retrieval of MgO Nano-Cubes Using the Transport of Intensity Equation. Ultramicroscopy 2008, 108, 805-815. 29. Che, R.; Takeguchi, M.; Shimojo, M.; Zhang, W.; Furuya, K., Fabrication and Electron Holography Characterization of FePt Alloy Nanorods. Appl. Phys. Lett. 2005, 87, 223109. 30. Van Waeyenberge, B.; Puzic, A.; Stoll, H.; Chou, K.; Tyliszczak, T.; Hertel, R.; Fähnle, M.; Brückl, H.; Rott, K.; Reiss, G., Magnetic Vortex Core Reversal by Excitation with Short Bursts of an Alternating Field. Nature 2006, 444, 461.

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