Magnetic Behavior, Electromagnetic Multiresonances, and Microwave

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Magnetic Behavior, Electromagnetic Multi-Resonances and Microwave Absorption of the Interfacial Engineered Fe@FeSi/SiO2 Nanocomposite Muhammad Javid, Yuanliang Zhou, Dongxing Wang, Da Li, Gui mei Shi, UnChol Kim, Lei Zhou, Xinglong Dong, and Zhidong Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00055 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018

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ACS Applied Nano Materials

Magnetic Behavior, Electromagnetic MultiResonances and Microwave Absorption of the Interfacial Engineered Fe@FeSi/SiO2 Nanocomposite Muhammad Javid†, Yuanliang Zhou†, Dongxing Wang†, Da Li‡, Guimei Shi§, Unchol Kim†, Lei Zhou†, Xinglong Dong†* and Zhidong Zhang‡* †

Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of

Education), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, P.R. China ‡

Shenyang National Laboratory for Materials Science, International Centre for Materials Physics,

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China §

Shenyang University of Technology, No.111, Shenliao West Road, Economic & Technological Development Zone, Shenyang 110870, P.R. China

ACS Paragon Plus Environment

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ABSTRACT: In this study, the interfacial engineered Fe@FeSi/SiO2 nanocomposite has been synthesized by one-step route of DC arc-discharge plasma. Higher energy states of Ar/Fe/Si ions in the plasma were also diagnosed by means of online optical emission spectroscopy (OES), supplying energetic information on the configuration of Fe@FeSi nanocapsules embedded in SiO2 matrix. It is indicated that the determined electron temperatures of Ar, Fe and Si ions are 23513 K (2.02 eV), 23225 K (2.00 eV) and 23063K (1.99 eV), respectively. Electromagnetic parameters display three prominent resonance peaks at 9.7, 14.3 and 16.8 GHz, those are the result from synergetic effect of heterogeneous interfaces among Fe@FeSi/SiO2 nanocomposite. The optimized reflection losses at these resonant frequencies are -33, -20 and -38 dB in certain thicknesses, respectively. Excellent microwave absorption of Fe@FeSi/SiO2 nanocomposite is readily tunable by the consequence from multi-resonance behavior and electromagnetic synergetic effect in the interface-rich nanocomposite. The revealed multi-resonance phenomena are significant in design and fabrication of electromagnetic materials as well the correlative devices, with effective absorption losses at distinctive frequencies.

KEYWORDS: Fe@FeSi/SiO2, Nanocomposite, Arc-Discharge Plasma, Optical Emission Spectroscopy (OES), Multi-interfaces, Superparamagnetic, Reflection Loss

1. Introduction Recently, the radiated electromagnetic waves have become a severe environmental problem, thanks to widespread applications of electromagnetic waves (EMW) in aerospace scopes, communication technology, radar systems, which does not only disturb the functionalities of electronic devices but also influence community health1-5. Excellent electromagnetic wave


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absorption

materials

are

necessary

to

waste

harmful

radiation

through

efficient

complementarities between magnetic/dielectric losses in the materials. Single functional absorbent is usually restrained in appropriate impedance matching, broad absorption bandwidth and integrated performances. Hence, development of novel EMW absorption materials is devoted with respect to multiplicate components, microstructures, geometry/morphology and excellent performance in EM depletion. The core/shell nanocapsules (NCs) with magnetic/dielectric parts are expected to be an excellent microwave absorber, because their designable microstructure could sponsor new mechanisms for EM loss, i.e. interface polarization, multiple dissipation, resonance effects, etc. 68

. At core/shell interfaces, phenomena such as electronic transfer, reconstruction, ionic orbital

and spin ordering would be induced and favor to consume EMW

9-11

. Understanding on EMW

response of the interfaces could provide a theoretical basis for successfully engineering new interfaces and architecting novel functional nanostructures

12-14

. Amongst the nominees,

magnetic transition metals (Fe, Co, Ni) and their ferrites have attracted pronounced interests. In comparison with Co (150 emu/g) and Ni (57.5 emu/g), Fe is a competitive candidate with assets of high saturation magnetization (171 emu/g), large anisotropic field, steady permeability, high Curie temperature, low coercivity and high Snoek’s limit in gigahertz frequency range

15-16

.

However pure Fe nanostructures have shown demerits of higher oxidation activity, strong magnetic coupling and severe agglomeration, which may result in serious structural instability and unexpected eddy current, and thus consequently limit their applications at high frequencies 17

. Moreover, the self-passivated products of Fe oxides (Fe3O4, α-Fe2O3), generated from

chemical instability and corrosion of Fe nanoparticles are ferrimagnetic in nature, usually minor in absorption of EMW energy 18-19.


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A plentiful research has been done concerning the surface modification on metallic nanoparticles (NPs), such as coatings of polymer, carbon, ceramic or heterogeneous metals. Carbon-coated and graphene encapsulated metallic nanocapsules had shown good anti-oxidation capability and satisfactory performances

20-23

As a versatile modifier, silica (SiO2) coating has

demonstrated potential with its excellent stability, nontoxicity, high physicochemical inertness and easy conjugation with various functional groups, thus enabling its coupling with NPs for fabrication of multicomponent nanostructures3, 24-26. Silica shell can also enhance the colloidal stability and favour to control the distance between the cores through its coating thickness. In electromagnetic research field, SiO2 is assumed to be EM transparent in the gigahertz (GHz) range and has minute effect on EM absorption due to its low dielectric constant

27

, however it

can still significantly change EM absorption performance by enclosing the core/shell absorber or combining into the structures, thus an electrical conductive network can be avoided by silica due to its insulating behavior. Shao et al. had reported that Ni@SiO2 core-shell structure showed a far better EM absorption than Ni nanoparticles

28

. Zhen et al. found that EM absorption ability of

FeNi3 could be sharply improved if it is coated with a thin layer of silica 29. Neo and Ding found that carbonyl iron coated by a silica layer presented a better impedance match especially at higher frequency range and consequently better EM absorption ability

30

. In addition, the

introduced SiO2 shell was found to isolate the cross-particle diffusion and prevent the aggregation of Co particles, as well as enhance the anti-oxidation capability. Ni et al. found that SiO2 coating enabled Fe particles to maintain better dispersity by reducing their magnetic coupling effect

31

. Yang et al. synthesized the core/shell iron-silica nanocomposites with better

EM properties at radio frequencies (1 MHz – 1 GHz)

32

. Liu et al. had synthesized a series of

multicomponent hierarchical microspheres, i.e. CoNi, CoNi@SiO2, CoNi@SiO2@TiO2 and


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CoNi@Air@TiO2, and demonstrated their novelties generated from the combination of strong magnetic loss (CoNi cores), excellent dielectric loss (TiO2 shells), or the in-between air/SiO2 intermediate layer as the impedance matching mediator

33

. Zhang et al. synthesized various

core/shell metal/heterogeneous oxide nanocapsules by arc-discharge method, and found that by integrating dielectric silica shells with magnetic cores the electromagnetic loss can be greatly affected in the whole range of 2-18 GHz

34

. In the above mentioned literatures, most of them

focused on tuning EMW impedance matching or protecting metallic cores from oxidation by silica coating, but the electromagnetic response at interfaces generated by silica layers was rarely examined. Recently, materials such as graphene have attracted much attention due to their excellent physicochemical properties, however the graphene based microwave absorber are very costly yet and cannot used in real applications.35 In the present work, one-step procedure of arc-discharge plasma was applied in synthesis of the nanocomposite consisting of superparamagnetic Fe@FeSi nanocaspules (NCs) embedded in SiO2 matrix. The electromagnetic multi-resonances in 2 – 18 GHz range, i.e. natural resonance and polarization resonances, and their contributions in microwave absorption were particularly investigated and discussed. An attempt on understanding the energy state of Ar arc plasma has been completed by mean of optical emission spectrum (OES) diagnosis, which may give an indepth insight into the controllable synthesis and formation mechanism for the Fe@FeSi/SiO2 nanocomposite. 2. Experimental Section 2.1 Synthesis of Fe@FeSi/SiO2 nanocomposite The Fe@FeSi/SiO2 nanocomposite powders have been prepared by an arc-discharge plasma technique 36. In the preparation process, a mixture of micron-sized Fe and Si powders with purity


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higher than 99.9 wt. % and weight ratios of Fe:Si = 9:1, was uniformly intermingled in mortar and pistol for 30 minutes, and then pressed into a cylindrical block as raw target which served as the anode of arc-discharge, while the cathode was a tungsten rod. The raw target was placed on a water-cooled copper stage, and the distance between two electrodes was set as about 3 mm. Argon gas as the preparation atmosphere was introduced alone into the evacuated chamber (5.0 ×10−3 Pa) without any active gas such as hydrogen. During the preparation process, the arc current was maintained at 120 A, while the potential was retained at 30 V. After being passivated in traces of air for 8 hours, the product was collected from the top and wall of the working chamber.

2.2 Characterization, OES and magnetic/electromagnetic measurements Crystal structures of the as-prepared Fe@FeSi/SiO2 nanocomposite powders were characterized by means of X-ray diffraction (PANlytical Empyrean) with CuKα (λ= 0.154 nm) irradiation at a voltage of 40 kV and a current of 40 mA at a scan step of 0.016. High resolution transmission electron microscopy (HRTEM, Tecnai G220S-Twin) and scanning transmission electron microscopy (STEM) (NOVA NanoSEM 450, 300 keV) were used to analyze the morphologies and phase compositions of Fe@FeSi/SiO2 nanocomposite. Surface species on Fe@FeSi/SiO2 nanocomposite were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB™ 250Xi) utilizing monochromatic Al Kα (hv = 1486.6 eV) radiation as probe range from 190 to 800 nm. On-line atomic spectra of DC arc plasma were recorded by optical emission spectroscopy (OES) spectrometer (AvaSpec-ULS2048L). Static magnetic properties were measured with a vibrating sample magnetometer (VSM) in a physical property measurement system. To measure the electromagnetic parameters, the Fe@FeSi/SiO2 nanopowders were


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homogeneously mixed into a parafﬁn matrix with a mass ratio of 1:1 and compressed into a toroidal shape with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. This testing sample was used to obtain its complex permittivity and permeability by an Agilent PAN A5222A network analyzer.

3. Results and Discussions 3.1 Crystal structure and microstructure of Fe@FeSi/SiO2 nanocomposite XRD patterns of the Fe@FeSi/SiO2 nanocomposite with three major phases are exhibited in Figure 1. The diffraction peaks observed related to the (111), (200) and (220) crystallographic planes, at 44.78°, 65° and 83° are in agreement with the cubic Fe pattern (JCPDS 98-7194). The diffraction peaks related to (110), (200), (210) and (211) lattice planes at 28.09°, 34.59°, 45.12°and 49.72° are corresponding to phase of FeSi intermetallic compound (JCPDS 79-0619) and the diffraction peaks related to (111), (101) and (200) lattice planes, at 21.98°, 32.49° and 36.19° are in good agreement with SiO2 (JCPDS 01-0649, polymorphs β-cristobalite). In the same time, no peaks of iron oxide are present, implying that the SiO2 matrix along with FeSi shell can efficiently protect the Fe cores against oxidation in the passivation process.


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Figure 1. X-ray diffraction of Fe@FeSi/SiO2 nanocomposite powders.

The morphology and microstructure of Fe@FeSi/SiO2 nanocomposite powders are characterized by TEM and HRTEM analyses, as shown in Figure 2. The Fe@FeSi/SiO2 nanocomposite look like interconnected network embedded with well-defined core/shell nanocapsules (NCs) (Figure 2a). HRTEM image of Figure 2b reveals that the nanocapsule consists of Fe cores with diameter of approximately 20 nm and FeSi shell about 5 nm in thickness, surrounding the Fe@FeSi NCs are bigger SiO2 matrix which has formed in passivation process. The structural details of core (I), shell (II) and matrix (III) in Figure 2c are further confirmed by inverse Fast Fourier Transformation (FFT) on the lattices as shown in the inset, indicating the lattice spacings of region Ι (Fe phase), ΙΙ (FeSi phase) and ΙΙΙ (SiO2 phase)


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are 2.033 nm [(110) plane of BCC], 2.019 nm [(210) plane of simple cube] and 4.067 nm [(110) plane of polymorphs β-cristobalite], respectively. The heterogeneous phases involved in Fe@FeSi/SiO2 nanocomposite essentially create large numbers of intricate interfaces, which becomes a prominent feature of this nanostructured material. In high-temperature arc-discharge plasma procedure, the formation of Fe@FeSi/SiO2 nanostructures is followed several steps that the crystal seeds of Fe would be nucleate at first, then the Si atoms have probability to grow on them, finally; the mixture layer of Fe and Si atoms would be further combined into Fe-Si alloy through diffusion. It is expected that Fe and Si may form certain alloying compounds such as Fe2Si, Fe3Si, Fe5Si3 and FeSi, among them the FeSi compound possesses the lowest formation energy justified by that relative order of stability: FeSi (-39.3 kJ/mol) > Fe2Si (-30.6 kJ/mol) > Fe3Si (-25.8 kJ/mol) 37, consequently the FeSi shell has been finally formed at the surface of Fe core. Moreover, formation process of SiO2 is detail explained in sporting information Figure S-1, EDX in Figure S-2 and also by HRTEM diagram in Figure S-3. To achieve further comprehensive evidence for the structure of Fe@FeSi/SiO2 nanocomposite, the cross section analysis on a nanocapsule is carried out using a focused ion beam (FIB) procedure and acquired as shown in the STEM image of Figure 3a. The line mapping result (Figure 3c) indicates that in the inner area Fe atoms are overlapped and more concentrated than Si atoms surrounded, providing evidences for Fe core and FeSi shell. This structural information also suggests that Fe core is not directly exposed to the external environment; that is, Fe@FeSi NCs are protected by the SiO2 matrix. This structural feature is also further supported by the energy dispersive X-ray spectroscopy (EDS) mapping as displayed in Figure 3b.


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Figure 2. (a) TEM image shows interconnected network morphology of the Fe@FeSi/SiO2 nanocomposite powders; (b) HRTEM image reveals well-defined Fe@FeSi NCs and SiO2 matrix; and (c) The regions of Fe core (I), FeSi shell (II) and SiO2 matrix (III) in left side HRTEM image, those are further confirmed by the inverse Fast Fourier Transformation (FFT) in right side picture.


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Figure 3. Structural and elemental analysis of Fe@FeSi/SiO2 nanocomposite powders. (a) Highangle annular dark field (HAADF)/STEM image; (b) EDS elemental mapping of Fe, Si, and O in the same area of HAADF; and (c) EDS intensities (Fe, Si, O) profiles along the green line on a Fe@FeSi/SiO2 composite particle in (a).

3.2 Optical emission spectroscopy (OES) diagnosis on the DC arc-discharge plasma Optical emission spectroscopy is a precise and non-intrusive technique to diagnose the arc plasma by recording photons with characteristic wavelengths, which originate from the decays of electron excited states of plasma species during the evaporation of raw target. In most Ar plasma the red/near-infrared spectral region from 690 to 900 nm in OES is dominated with clear atomic Ar lines of 4p–4s transitions. As shown in Figure 4a, the on-line OES are presented for typical arc-discharge plasma in synthesis of the Fe@FeSi/SiO2 nanocomposite, the spectral lines have been identified by NIST Atomic Spectra Database (ver. 5.3). The intensity of lines in the present work is much stronger than the other studies owing to high pressure of Ar atmosphere and ultrahigh temperature of Ar ions. Compared with other non-thermal plasmas, this arc-discharge


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plasma possesses much higher temperature (up to 104 K) and much stronger electron energy. Accordingly, much more Ar atoms are excited with stronger emission intensity. Such intensity in principle is proportional to the concentration of species 38, thus OES diagnosis can give various valuable information of the excited plasma for understanding the synthesis of nanostructures

39

.

In all spectra, the strong lines for Feo atoms are found in range of 240 ‒ 280 nm and Fe+1 ions in range of 800 - 900 nm, the weak lines for Si+1,+2 ions appear at 288.1, 390.5 and 634.7 nm, while the spectral lines of Aro,+1 atom/ions emerge weakly in range of 400 - 500 nm and became more flourishing in region from 690 - 900 nm which further confirms the solitary atmosphere of Ar gas in the preparation. Characterization of the arc plasma is carried out by quantitatively analyzing the energetic states of Ar atoms utilizing the atomic Ar lines as shown in Figure 4b. The calculated electron temperature (Te) of plasma is a critical parameter to exhibit the energetic conditions for synthesis of nanostructures. It had been indicated that the inert gas, such as argon, neon or helium in the plasma, plays a vital role in controlling electron temperature through the penning excitation and ionization processes. It is also significant that the electron temperature of plasma under a concentration of Ar gas would be helpful to understand the particle’s collision and plasma reactions processes. For local thermal equilibrium (LTE) plasma, the electron temperature is identical with the excitation temperature, so that the most common spectroscopic diagnosis on the electron temperature are carried out by means of Boltzmann’s plot method with respect to the excitation temperature

40

. The population of emitting levels follows Boltzmann’s distribution in LTE

plasma, as following relationship of Eq. 1:

ln

= − + ………………..(1),


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where, Ikl is the intensity of emitting light, λkl the wavelength, gk the statistical weight of the upper level, Akl the transition probability from level k to level l, Ek is the energy of the upper level, and KB is Boltzmann constant. The data in plot of ln(Iklλkl/gkAkl) vs. Ek/KB are fitted by a straight line with a slope of -1/T, thus the electron temperature (Te) can be determined with a better precision. Boltzmann’s plots have been applied on Ar, Fe and Si ions to estimate the electron temperatures, as shown in Figure 4b ‒ 4c, respectively. From three straight lines fitted, Ar ions display Te = 23513 K (2.02 eV) with correlation coefficient R2 of 0.97, Fe ions show Te = 23225 K (2.00 eV) with R2 of 0.99, and Si ions demonstrate Te = 23063K (1.99 eV) with R2 of 0.99. The determined electron temperatures can be recognized reasonably, for each species elements with almost the same values.


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Figure 4. (a) Typical optical emission spectrum (200-1000 nm) of Ar plasma recorded from DC arc-discharge in synthesis of the Fe@FeSi/SiO2 nanocomposite; (b) - (d) Boltzmann’s plots to estimate the electron temperatures Te, for the cases of Ar, Fe and Si ions, respectively.

In this arc-discharge plasma, the Ar gas was used as the only working gas which had exhibited the ability to greatly influence the features of nanostructures 41. OES results above illustrate that the Ar atoms have been activated into ions, implying high energy state of the plasma. Higher Ar concentration sourced from its higher pressure (0.03 MPa used in this work) favors both higher energy of arc plasma and higher ionization of metallic atoms (Fe+ and Si+), meanwhile the inert Ar+ ions also have the enhanced condensation effect through energy exchange with the growth species atoms, as well an etching effect to affect the morphologies of nanostructures. The mechanism for formation of Fe@FeSi/SiO2 nanocomposite can be proposed as follows (see Figure 5.): The first step is co-evaporation of the raw target by arc plasma, thus the gaseous state of Fe and Si atoms can be generated (Figure 5., the left picture); Second step is the preferential nucleation of Fe seeds owing to its higher melting points (1808 K) than Si (1683 K); Subsequent step is the growth of crystal seeds into nanoparticles, during which the growth species of Si atoms/ions could deposit on Fe seeds and further form the stable FeSi shells through diffusion, while the Fe seeds also grow into the cores, thus Fe@FeSi NCs are fabricated with rich Si atoms on outer layers (Figure. 5, the middle picture); Final step is passivation process, in which the traces of O2 gas was introduced into the chamber soon after the reaction stop, highly chemically active outer silicon species will be oxidized into SiO2 matrix protecting the Fe@FeSi NCs from further oxidization (see Figure 5, the right picture).


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Figure 5. Schematic diagram for the formation of Fe@FeSi/SiO2 nanocomposite

3.3 Surface species of Fe@FeSi/SiO2 nanocomposite The surface of Fe@FeSi/SiO2 nanocomposite was analyzed by XPS, as shown in Figure 6. The survey spectrum of Figure 6a clearly sightsee the emissions from nanocomposite, while Figures. 6b – 6d show the detailed photoelectron core level spectra of Fe, Si and O elements, respectively. From Fe 2p electrons core level spectrum (Figure 6b), one can observe that close peaks at 706.8 eV and 720.4 eV correspond with the binding energies of Fe 2p3/2 and Fe 2p1/2 peaks for pure Fe metal 42. The binding energies of Si 2p electrons (Figure 6c) are assigned to main SiO2 matrix at 103.2 eV and minor FeSi shell at 99.5 eV and 706.8eV

43-44

. The silicon oxides are further

confirmed by the binding energies of O 1s electrons as shown in Figure 6d), in which a strong OSi bond of SiO2 at 532.5 eV 36 and a trivial O-Fe bond of Fe2O3 at 530.9 eV are presented, detail explanation of this oxidation effect is given in SI under Section S-1. These results of XPS analysis further verify the structure of Fe@FeSi/SiO2 nanocomposite, which are initially covered by Si-rich outer layer which is almost converted into SiO2 through the passivation in air.


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Figure 6. X-ray photoelectron spectra of Fe@FeSi/SiO2 nanocomposite. (a) Survey spectrum; (b) Binding energies of Fe 2p electrons; (c) Binding energies of Si 2p electrons; and (d) Binding energies of O 1s electrons. Dotted lines are experimental and solid lines are the fitted data.

3.4. Static magnetic properties of Fe@FeSi/SiO2 nanocomposite The magnetic hysteresis (M-H) loops of Fe@FeSi/SiO2 nanocomposite measured at 5 K and 300 K are presented in Figure 7a. Langevin-type curve of sample with magnetic remanence nearly zero 0.89 emug-1. This indicated that there was almost no remaining magnetization when the external magnetic field was removed, suggesting that Fe@Fe/SiO2 nanocomposite exhibit a superparamagnetic behavior. The specific saturation magnetization, Ms of the sample, is 30.5 emu g-1. This value is smaller than the reported value of Fe of 171emug-145.The reduction in the


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value of Ms could be attributed to the rather smaller size ,surface spin effects, surface spin canting of the Fe nanoparticles and the relatively low amount of Fe (26 wt.%), and existence of weak magnetic FeSi alloy shell and nonmagnetic massive SiO2 matrix are ascribed to the lower Ms. As expected, in low temperature regime, blocked (ferromagnetic) particles become preponderant and the M-H loop becomes slightly hysteretic with an increased MS of 37.5 emug-1 and Hc of 63 Oe. The measurements of zero-field-cooling (ZFC) and field cooling (FC) under field of 100 Oe were accomplished as shown in Figure 7b, giving information such as blocking temperature, and intrinsic properties of the nanocomposite. In ZFC curve, a sharp peak at 33K corresponds to a transition, the blocking temperature TB1, from ferromagnetism to superparamagnetism of Fe@FeSi/SiO2 nanocomposite. The FC curve also reveals a cups like influx appeared at 37 K, and most probably superparamagnetic behavior According to superparamagnetic theory46, the magnetocrystalline anisotropy constant and particle size can be estimated by realtion, 25⋅KBTB = KV, where KB is Boltzmann constant, TB blocking temperature, K magnetocrystalline anisotropy constant, and V the volume of the nanoparticle47. For the Fe@FeSi/SiO2 nanocomposite, using average diameter of 22 nm and the blocking temperature of 33K, thus the anisotropy constant K can be calculated as 0.03×105 erg/cm3 this increase in anisotropy is due to surface and interface effects of the sample. For the superparamagnetic nanoparticles, the different particle size will exhibit different blocking temperature. As the temperature is raised to the first transition at 33 K, Fe@FeSi/SiO2 nanocomposite exhibit a superparamagnetc behavior, and subsequently undergo a monotonic increasing to a maximum, a blunt peak at 140 K is observed which is assumed to the second blocking temperature stemmed from some bigger Fe cores. For the isolated Fe cores, a broad distribution in their size results in a gradual transition from ferromagnetic to


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superparamagnetic without well-defined temperature, but the bifurcation between ZFC and FC curves clearly points towards the superparamagnetism of Fe@FeSi/SiO2 nanocomposite. Both the temperature and field dependent magnetization of Fe@FeSi/SiO2 nanocomposite supply essential results on the magnetic and structural characteristics, for further understanding of microwave absorption capability and potential applications in high frequency range.

Figure 7. (a) Hysteresis loops of Fe@FeSi/SiO2 nanocomposite measured at 5 and 300 K. (b) The ZFC and FC curves (measured in a magnetic field of 100 Oe).

3.5 Electromagnetic multi-resonances of Fe@FeSi/SiO2 nanocomposite To expose the electromagnetic behaviors of Fe@FeSi/SiO2 nanocomposite, the permittivity (ε′

+ iε″) and the permeability (µ′ + iµ″) were measured in a frequency range of 2 – 18 GHz and presented in Figures. 8a ‒ 8b. It shows that the real part (εʹ) of permittivity is nearly constant below 9 GHz and fluctuates in the high frequency range, while the imaginary permittivity (εʹʹ) shows the increasing trend in whole frequency spectra with particularly multiple dielectric resonances at 9.8, 13.9 and 16.8 GHz. The visible resonances are thought to be interfacial


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polarizations, so called Maxwell–Wagner–Sillars 48. Such dielectric polarization is believed to be induced by asymmetric charge distribution either in a molecule structure or in an infinitesimal dielectric interfaces. In general, the resonance behaviors of permittivity originate from electronic polarization, ionic polarization, electric dipolar polarization and space-charge polarization. Former two types can be easily disqualified because both usually occur at much higher frequency county at THz and PHz, while the last two might be produced from the heterogeneous interfaces in microwave range 49. As structural analysis in above sections, large numbers of the interfaces have been generated at core/shell/matrix boundaries in Fe@FeSi/SiO2 nanocomposite and take responsibility for the dielectric resonances, so-called Maxwell-Wagner-Sillars polarization. These interfaces of Fe/FeSi, FeSi/SiO2 species, as well the charged cores or shells would attain the multiple dielectric resonance, which may enhance the microwave absorption at these resonating frequencies. Similar cases had been also found, i.e. more dielectric resonance peaks for cobalt microflower Ni/PANi

51

50

and dual dielectric resonances for core/shell structure of Ni/C and

. Moreover as a comparison Fe metal nanoparticles are lack of these effects as

previous work done by our group52, further explore the dipolar polarizations originated from orientations of diploes, the plot of real part (ε'/f) via imaginary part (ε"/f) is presented in Figure 8c.It is well known that once the recognizable Cole-Cole semicircles come into being in above plot, then Debye dielectric relaxation would be agreed

53-54

. In the inset plot, three evident

semicircles are found those well match the numbers of dielectric resonance peaks. Consequently, the synergetic effect of Debye relaxation along with the interfacial polarization (Maxwell– Wagner–Sillars) would cooperatively contribute to the permittivity and bring on abundant dielectric responses.


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Figure 8. (a) Frequency dependence of the permittivity for Fe@FeSi/SiO2 nanocompoiste, (b) Frequency dependence of the permeability for Fe@FeSi/SiO2 nanocomposite, (c) the is ColeCole plot for the relation between real part (ε′/f) and imaginary part (ε″/f) and (d) is the eddy current loss dependence of the permittivity and dielectric loss for the Fe@FeSi/SiO2 nanocomposite

The Fe@FeSi/SiO2 nanocomposite has presented above dielectric resonances, meanwhile it also maintains analogous resonances in its permeability as shown in Figure 8b. One can observe that the value of µʹ is almost constant in the whole frequency range, while µʹʹ exhibits the multimagnetic resonances (MMR) centered at 8.48 (NR), 9.75 (ER1), 14.34 (ER2) and 16.88 (ER3) GHz. The first peak at 8.48 GHz is considered as natural resonance, and later three peaks are thought as the exchange resonances similar to those found in CoNi@C nanocapsules and Co


20

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microflowers

50, 55

. These resonances would make confident contributions to the magnetic loss,

besides probable losses by the magnetic hysteresis, domain-wall displacement and eddy-current effect, etc. 56. As confirmed by HRTEM analysis, the mean size of magnetic Fe core is about 20 nm which is less than the critical size of magnetically single domain, thus the Fe cores can be considered as single domain

57

and have no magnetic loss arising from domain-wall movement in

Fe@FeSi/SiO2 nanocomposite. The magnetic hysteresis loss is mainly induced by the time lags of magnetization vector behind the external electromagnetic field vector, and it could be insignificant in a weak applied field such as the case of microwave. Furthermore, the magnetic loss induced by the eddy-current effect is restricted, because it is well known that if magnetic loss mainly result from the eddy-current effect, the values of µʹʹ(µʹ)-2f-1 should be constant 2πµ0d2σ/358 and independent of the frequency. The Figure 8d shows the plot of µʹʹ(µʹ)-2f-1 vs. frequency, which is not constant and hence the eddy current loss can be excluded in the Fe@FeSi/SiO2 nanocomposite. In general, the natural resonance and multi-exchange resonances would become main contributors to the magnetic loss of Fe@FeSi/SiO2 nanocomposite. To understand the magnetic resonance behaviors, the Gilbert modification on Landau–Lifshitz equation (LLG) has been used to fit the dynamic frequency-dependent permeability by Equation. (2) and (3): 59 =

=

+∑-.

∑-.

!

"#! $% !

&'(/( *! "+#! $, +-#! '(/( *!

( /( #

!

! !

"+#! $%

"+#! $% +-#!

!

(2)

(3),


21


Page 22 of 34

where fi is the resonance frequency, αi the damping coefficient, Ii the intensity and B constant with value 1 . For an optimal fitting result, it turned out to be required to introduce the frequencies of resonance peaks, i.e. around 8, 10, 14 and 17 GHz. The optimal fitting lines (solid) are presented in Figure 8b and the fitted results are listed in Table 1, here NR represents the natural resonance, ER1, ER2 and ER3 resonance bands are allocated to the exchange resonance, respectively.

Table 1. Resonance modes of the Fe@FeSi/SiO2 nanocomposite. Resonance modes

fexp. (GHz)

fcal. (GHz)

µkn

I

α

Natural resonance (NR)

8.48

8.10

−

0.12

0.42

Exchange resonance (ER1)

9.75

9.98

2.08

0.14

0.50


14.34

14.39

3.34

0.11

0.20


16.88

16.86

5.64

0.08

0.35

The fcal. are the calculated resonance frequencies, I the intensity of the resonance peak, α the damping coefficient and µkn the eigenvalue of derivative of spherical Bessel function jn(u). The resonance peak around 8 GHz can be attributed to the Kittel natural resonance 60. It is well recognized that the natural resonance frequency is governed by the effective anisotropy field, which is associated with the magnetocrystalline anisotropy, the size and geometry of magnetic particles, and the dipole interaction between the particles 61. The natural resonance frequency of magnetic particles can be expressed as fnr = γHeff 62, where γ is the gyromagnetic ratio and Heff = 2ǀKeffǀ/Ms is the effective anisotropy field of a cubic crystal with the saturation magnetization (MS) and the effective anisotropy constant (Keff). For magnetic nanoparticles, the anisotropy constant is strangely affected by the size and surface effect, so that the surface anisotropy constant should be involved in the effective anisotropy constant Keff = Kv + 6Ks/R with Kv the


22

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volume-anisotropy constant, Ks the surface anisotropy constant, and R the radius of the nanoparticle. For the present case of Fe cores, the mean R ≈ 20 nm (estimated from the TEM images), γ = 2.8 GHz/kOe, Kv = 0.03×105 erg/cm3 and MS 37emu/g which is equivalent 292.3 emu/cm3 using density 7.9g/cm3 63. If the surface anisotropy is also included using Ks = 0.09×104

erg/cm2, fnr is calculated to be 8.10 GHz (listed in Table 1). In comparison with 6.5 GHz, the

theoretically natural resonance frequency of bulk Fe, the shift to a high frequency is attributed to the extra contributions from the small size effect or a mass of interfaces existed in the Fe@FeSi/SiO2 nanocomposite. According to fmax= fR/(1+α2)1/2, fR = 2γKeff/Ms, and Keff = Kv + 6Ks/R

64

, it shows that fmax will shift to high frequency if R decreases and/or Ks increases.

Therefore, it is possible to tune the natural resonance frequency by altering Ks through adjusting Fe core size or the interfaces involved in Fe@FeSi/SiO2 nanocomposite. The exchange resonances are potentially observable in the nanostructured composites and can be understood by Aharoni’s theory. The resonance peaks around 9, 14 and 17 GHz could be attributed to the exchange resonance modes (ER1, ER2 and ER3), assuming that the modified Aharoni’s method still works for this Fe@FeSi/SiO2 nanocomposite given as: 65 , 0 1

=

! 2 34

5 ! 67

+ 89

(4),

where A is exchange constant (A = 2×10-6 erg/cm), µkn the roots of the differential spherical Bessel functions, γ the gyromagnetic ratio and HC the magnetostatic field. From the Equation 4, the exchange resonance frequencies are related to the squares of µkn. The first three µkn roots of differential spherical Bessel functions are constant as reported previously for Fe, Ni, Co etc. µ11 = 2.08, µ12 = 3.34 and µ13 = 5.6466-67. R is the crystal size (about 20 nm) and HC is the coercivity (HC = 65 Oe). The calculated values (fcal.) of exchange resonance frequency are 9.98, 14.39 and 16.86 GHz, those are in well agreement with the experimentally measured frequencies of 9.75,


23


Page 24 of 34

14.34 and 16.88 GHz, respectively (see Table 1). Consequently, the modified exchange mode is proven valid in explaining the resonances of ER1, ER2 and ER3 in the spectrum, thus it is rational to conclude that the resonance peaks at higher frequency are due to exchange resonances 68

. Such multi-resonances behavior could be attributed to several factors, amongst them the most

dominant is the multi-interface effect in the Fe@FeSi/SiO2 nanocomposite, at which the different spins would result in strong exchange coupling and spin precessions thus exchange resonance occurred. However, there is no clear relation between the exchange resonance mode and multiresonance effect, we are trying to find and do some work on this relation in near future.

3.6 Microwave absorption loss of Fe@FeSi/SiO2 nanocomposite The complex permeability and permittivity determine EMW reflection and attenuation characteristics of the materials. In Figure 9(a), the reflection loss (RL) curves of Fe@FeSi/SiO2 nanocomposite are calculated at given frequency and the thickness, with the following Equations: 69 3

2C(

:; = < = tanh [B ε

9

=

DE εE F]

P4

HI'F* = 20LMN O

O

P4 +

(5) (6),

where in Eq. 6, Zin is the input impedance, µr the relative permeability, εr the relative permittivity, c the velocity of EMW in free space, f the frequency of microwaves, and d the thickness of the Fe@FeSi/SiO2 nanocomposite. It is indicated that the calculated RL increases with the thickness of absorber, however the maximum losses mostly appear at about 9.8, 14.3 and 16.9 GHz, exactly at frequencies of exchange resonances as confirmed in above section. In most of EMW absorbers, absorption frequency usually shifts to low frequency side with increasing of the thickness due to the synergetic contributions of magnetic, dielectric and


24

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structural benefits. For this Fe@FeSi/SiO2 nanocomposite, it is apprehensible that the exchange resonances and dielectric resonances are dominant in EMW absorption at certain frequencies, here the effect of electromagnetic multi-resonances is noticeable for the Fe@FeSi/SiO2 nanocomposite a similar case also reported previously70. The maximum RLs of -38 dB (at 16.7 GHz, 10 mm in thickness), -20 dB (at 14.5 GHz, 12 mm in thickness) and -33 dB (at 9.8 GHz 12 mm in thickness) illustrate that the normal relation between RL and the thickness still works here. From Figure 9b, the plot of dielectric loss factor (tanδE = ε"/ε') and magnetic loss factor (tanδM = µ″/µ′) vs. frequency, and the combine prominence of multi-resonance in Fe@FeSi/SiO2 nanocomposite has been further verified. In our case, the associated mechanisms for the enhanced EMW absorption properties comprise the following two features and speculation of the loss schematics have been exhibited in Figure 10. (1) Multiple dielectric loss mechanisms. In our case the dielectric loss mainly consists of two parts: Maxwell-Wagner polarization relaxation; and conduction loss. First, dipole polarization relaxation plays an important role in the EMW attenuation. In a multiphase system composed of components with different permittivity and conductivity, the accumulation of space charges at the interfaces generates interfacial polarization (Maxwell-Wagner polarization), which is regarded as an important dissipation mechanism. Furthermore, the boundaries and interfaces in Fe cores possess copious defects and dangling bonds, inducing more dipoles and polarization. Secondly, there are a large number of interfaces, like SiO2/paraffin interfaces and SiO2/FeSi alloy interfaces and Fe/FeSi interfaces. Moreover, FeSi alloy is low band gap (0.11eV) semiconductor which shows very different electric and magnetic properties. When temperature is raised due to joule heating effect of electromagnetic field incident on the resistive matrix of SiO2, this heat energy give rise the temperature of thin FeSi alloy shell, consequently it starts conducting which give rise the more


25


Page 26 of 34

conduction loss. (2) Multi-magnetic loss mechanism. In Fe@FeSi/SiO2 nanocomposite at incident EMW, the multiple resonance behaviors, such as natural resonance and exchange resonances, play a significant role during the EMW attenuation. Fe cores due to surface effects give rise the natural and exchange resonance. Conduction of charges in FeSi alloy interface act as a nano-current, moving current has a magnetic field, this field will interact the magnetic field of Fe cores, and thus the natural resonance and exchange resonance are set forth. Hence, the propagated microwave would be resonating inside the interfaces and magnetic cores. The EMW traveling through the composite of multi-interfaces by maintaining, the enhanced losses by magnetic exchange resonances and by dielectric interface polarizations, can be absorbed and exhausted at the exceptional frequencies. Additionally, the composition of raw target and preparation conditions (atmosphere, power supply, etc.) would have great contributions to the nanopowders product, thus the composition ratio and performance of Fe@FeSi/SiO2 nanocomposites could be regulated and optimized in near future in EMW application field.

Figure 9. (a) The calculated results of the reflection loss vs. frequency for Fe@FeSi/SiO2 nanocompoites with different thicknesses. (b) Frequency dependence of the dielectric and


26

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magnetic loss tangent and their combine effect for the Fe@FeSi/SiO2 nanocomposite between real and imaginary permeability vs. frequency.

Figure 10. Schematic illustration of EMW absorption for Fe@FeSi/SiO2 nanocomposites

4. Conclusions In summary, the nanocompoiste of Fe@FeSi NCs embedded in SiO2 matrix has been synthesized via one-step DC arc-discharge plasma method. Online OES analysis on the arcdischarge plasma discloses that the electron temperature has prominent effect on the formation process of Fe@FeSi/SiO2 nanocomposite. The Fe@FeSi/SiO2 nanocomposite shows a superparamagnetic characteristic with two blocking temperatures of TB1 = 37 K and TB2 = 140 K, attributed to a wide distribution of the particles size. Inspected electromagnetic properties in the 2–18 GHz demonstrated that imaginary parts of permittivity and permeability present almost


27


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parallel resonant peaks. These multi dielectric and magnetic resonances of the Fe@FeSi/SiO2 nanocomposites are related to the core-shell multi-interface interaction of magnetic spins and dielectric transition of charge transformation at the interfaces. Total microwave loss is intensely dominant at the specific frequencies of resonance regardless of the thicknesses of Fe@FeSi/SiO2 nanocomposite. ASSOCIATED CONTENT Supporting Information Available: Figure S-1. Ellingham diagrams of Fe, Si Elements, Figure S-2 the EDX spectrum and Figure S-3 HRTEM of SiO2 from the Fe@Fe/SiO2 nanocomposite. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Email: [email protected], *Email: [email protected]. Phone: +86-411-84701630. Fax: +86-411-84706130. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was sponsored by the financial support from the National Natural Science Foundation of China (Nos. 51331006 and 51271044). REFERENCES


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70. Shi, X.-L.; Cao, M.-S.; Yuan, J.; Fang, X.-Y., Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow cobalt nanochains composites with negative permeability. Appl. Phys. Lett. 2009, 95 (16), 163108.

Table of Contents Graphic and Synopsis


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Magnetic Behavior, Electromagnetic Multiresonances, and Microwave

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