Ultrafine FeCo Nanoparticles Isolated by Ultrathin Dielectric Shells for

Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3570−3576

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Ultrafine FeCo Nanoparticles Isolated by Ultrathin Dielectric Shells for Microwave Application Guohua Bai, Jiaying Jin, Chen Wu,* and Mi Yan* School of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Key Laboratory of Novel Materials for Information Technology of Zhejiang Province, Zhejiang University, Hangzhou 310027, China

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S Supporting Information *

ABSTRACT: Low-dimensional soft magnetic materials are technologically important for miniaturization of magnetic devices. It is, however, challenging to maintain ferromagnetic response in nanosized magnetic material. In this work, ultrafine FeCo nanoparticles have been prepared by reactive pulsed laser deposition, which can be exchange coupled through ultrathin dielectric shells, even though their diameters are far below the superparamagnetic blocking limit. This unique core/shell structure guarantees ferromagnetic coupling and insulation of neighboring magnetic nanoparticles simultaneously. The nanoparticle film exhibits large saturation magnetization of 1.13 T, high electrical resistivity of 4200 μΩ·cm, maximum permeability of 120, a cutoff frequency of 1.7 GHz, and it is a promising material for microwave application. KEYWORDS: ultrafine nanoparticles, exchange coupling, electrical resistivity, magnetic properties

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INTRODUCTION Magnetic nanoparticles have drawn intensive attention in cutting-edge nanotechnologies due to their unique physical and chemical properties.1−3 On one hand, magnetic nanoparticles have been widely used for interdisciplinary applications ranging from diagnostics,4 drug targeting,5,6 catalyst recycling,7 and sensing.8 On the other hand, magnetic nanoparticles can be easily insulated for large electrical resistivity, making them widely used in magnetic devices such as magnetic recording,9,10 on-chip inductor,11 microwave absorption,12,13 and magnetic shielding.14,15 Such devices are usually required to work at high frequencies for increased data transmission rate.16 Consequently, magnetic nanoparticles are required of both large saturation magnetization (Ms) and high electrical resistivity (ρ) for minimized device volume, reduced eddy current, and impedance match17 at high frequency. Metallic magnetic materials possess large saturation magnetization compared with ferrites. Much effort has been made to fabricate metallic nanoparticle assemblies by awet chemical method,18 dealloying,19 chemical evaporation,20−22 and sputtering.23,24 It is necessary to minimize the diameter of metallic nanoparticles as much as possible to reduce intraparticle eddy current loss in high-frequency applications. However, due to the competition between spontaneous thermal fluctuation and magnetic anisotropy,21,25 magnetic nanoparticles become unstable when their sizes approach the superparamagnetic blocking limit (db), on the premise of no magnetic interaction between these isolated magnetic nanoparticles. Under such circumstances, ferromagnetic response © 2019 American Chemical Society

could only be achieved for magnetic nanoparticles with diameter above db, making it difficult to reduce intraparticle eddy current loss. For reduced interparticle eddy current loss, dielectric shells surrounding nanoparticles are also required to avoid electric conduction between neighboring nanoparticles. Insulating materials such as oxides (Al2O3, SiO2, ZnO),16,26,27 nitrides (SiN, FeNx),24,28 carbon13 and polymers (PPy)18 have been used as the dielectric shells surrounding metallic magnetic nanoparticles, with thickness above several nanometers. The interparticle insulation can be achieved in these cases but usually at the sacrifice of saturation magnetization, due to magnetic dilution caused by excessive nonmagnetic materials. To address these problems, a core/shell structure with ultrafine metallic magnetic nanoparticles insulated by ultrathin dielectric shells is designed in this work. Amorphous nanoparticle film with FeCo cores (average diameter of ∼2.69 nm) insulated by Al2O3 dielectric shells (average thickness of ∼0.63 nm) has been successfully fabricated by reactive pulsed laser deposition (PLD). The FeCo nanoparticles are exchange coupled through the Al2O3 dielectric shells even though their diameters are far below superparamagnetic blocking limit. Such unique core/shell nanoparticle film exhibits large magnetization, excellent ferromagnetic response and high electrical resistivity, making it a suitable candidate for microwave applications. Received: March 22, 2019 Accepted: May 20, 2019 Published: June 3, 2019 3570

DOI: 10.1021/acsanm.9b00537 ACS Appl. Nano Mater. 2019, 2, 3570−3576

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ACS Applied Nano Materials

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electrical resistivity of the film was measured by four-point probe method (RTS-8). The resistivity at low temperature was measured by a physical property measurement system (PPMS) while the resistivity at high temperatures were achieved by four-point probe method via annealing under N2 atmosphere.

EXPERIMENTAL DETAILS

Sample Preparation. FeCo nanoparticle films with core/shell structure were fabricated on Si (100) single crystal substrates by reactive PLD. Schematic drawing of the deposition system is shown in Figure 1. A (Fe0.65Co0.35)0.8Al0.2 alloy disk with a diameter of 1 in. was

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RESULT AND DISCUSSION TEM characterization in Figure 2a demonstrates the core/shell structure of as-deposited nanoparticle film. No diffractions ring is found in SAED pattern (inset of Figure 2a) indicating that both nanoparticles and interparticle shells are amorphous. The HAADF image (Figure S1 in the Supporting Information (SI)) illustrates the elemental distribution of the film, where the cores mainly consist of FeCo and the shells contain Al. Distributions of nanoparticle diameter and shell thickness are shown in Figure 2b and c, where average diameter of 2.69 ± 0.45 nm and thickness of 0.63 ± 0.12 nm are obtained. Figure 3 presents XPS spectra of Fe, Co, and Al for the asdeposited nanoparticle film referenced to C 1s emission at 284.6 eV. Two characteristic peaks at 706.8 and 720.2 eV are observed in Fe 2p spectrum (Figure 3a), which correspond to metallic Fe.30,31 The Co 2p spectrum (Figure 3b) also indicates metallic state of Co (peaks at 778.3 and 793.3 eV).32 Figure 3c shows that Al 2p peak is located at 74.6 eV, corresponding to Al2O3.33 This indicates that Al with strong reactivity has been oxidized by residual oxygen and transformed into Al2O3 dielectric shell. Generally, fully dispersed magnetic nanoparticles are superparamagnetic and exhibit a split on ZFC/FC curves at blocking temperature. The ZFC/FC curves of the FeCo nanoparticle film, however, overlap well within the whole temperature range (Figure 4a). No observation of split indicates that ferromagnetic ordering is predominant in the as-deposited film.34 For noninteracting, randomly oriented magnetic nanoparticle, when its diameter is below superparamagnetic blocking limit (db), thermal fluctuation is equivalent to the energy barrier of magnetization flips, that is, magnetic anisotropy.22 Consequently, the magnetic nanoparticle assembly will be superparamagnetic without any hysteresis. For isolated FeCo nanoparticle, the superparamagnetic blocking diameter db = 2(6kBT/K1)1/3 (where kB is Boltzmann constant, T is temperature, K1 = 18 kJ/cm3 represents the magnetocrystalline anisotropy)35 is estimated to be ∼20 nm at 300 K, which is much larger than that of the FeCo nanoparticles prepared in this work (2.69 ± 0.45 nm in Figure 2b). The overlapped ZFC/FC curves indicates that the FeCo nanoparticles are exchange coupled with each other despite of their ultrafine diameters.

Figure 1. A schematic diagram illustrating the reactive PLD system. used as the target. A static magnetic field of 250 Oe was applied parallel to substrate surface during deposition to induce in-plane uniaxial magnetic anisotropy in the film. The growth chamber was evacuated to a base pressure of 2 × 10−7 Torr prior to deposition. The wavelength of laser is 248 nm with an incident energy of 250 mJ, a repetition frequency of 10 Hz and a deposition time of 4.5 h. The asdeposited sample was subjected to vacuum annealing at 473, 573, and 673 K for 2 h. The films were also deposited on NaCl substrates from which the films could be separated for high resolution transmission electron microscopy (HRTEM) characterization by dissolving the substrates in deionized water. Characterization. Microstructure and elemental distribution of the film were examined by a high resolution TEM (JEM 2100F) equipped with high-angle annular dark field apparatus (HAADF) and selected area electron diffraction apparatus (SAED). Statistical analysis of nanoparticle diameter and shell thickness were carried out based on multiple TEM images. Elemental chemical state of the film was investigated by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). A superconducting quantum interference device (SQUID) was used to measure the static magnetic properties and zero field cooling/field cooling (ZFC/FC) curves of the nanoparticle films in temperature range of 10−300 K under an applied field of 20 Oe. Magnetization curves and hysteresis loops at elevated temperatures were obtained using a vibrating sample magnetometer (VSM, Lakeshore 7410). Ferromagnetic resonance experiment was conducted with an electron spin resonance apparatus (ESR, JES-FA200). The permeability spectra of the film were measured by shorted microstrip transmission-line perturbation method29 with a vector network analyzer (VNA, Agilent E8363B). Room temperature

Figure 2. (a) HRTEM image and SAED pattern, distribution of (b) particle diameter and (c) shell thickness of the as-deposited FeCo nanoparticle film. 3571


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radio frequency (RF) components of two magnetization vectors resonating in phase (the uniform precession). The secondary resonance peak at approximately 2250 Oe corresponds to the optic mode, which is caused by the exchange field between neighboring FeCo nanoparticles. It represents the RF components of the magnetization vectors resonating out of phase. When the applied field is perpendicular to film plane in out-of-plane FMR measurement (Figure 4c), only the acoustic mode at approximately 770 Oe is detected which is related to the uniform precession. Due to the two-dimensional nature of FeCo nanoparticle film, the interparticle exchange coupling mainly exists in film plane. When a perpendicular field is applied, the interparticle exchange field does not contribute to the dynamic response. Consequently, only the acoustic mode is observed in out-ofplane FMR measurement. To investigate the strength of exchange coupling in the FeCo nanoparticle film, thermomagnetic curve (up to 1023 K) has been measured under a field of 1000 Oe parallel to film plane (Figure 5). The corresponding hysteresis loops at

Figure 3. XPS spectra of (a) Fe, (b) Co, and (c) Al taken from the asdeposited nanoparticle film. Figure 5. M-T curve at elevated temperatures for the as-deposited FeCo nanoparticle film. The hysteresis loops at selected temperatures are also shown with the temperature dependence of coercivity plotted as the inset.

Ferromagnetic resonance (FMR) can effectively illustrate the exchange interaction between two magnetic nanoparticles through “nonmagnetic” spacer.36 Based on the LandauLifshitz-Gilbert (LLG) equation, the resonant field depends on the internal field of the magnetization.37 Figure 4b and Figure 4c show the FMR measurements of the as-deposited FeCo nanoparticle film. For the in-plane FMR measurement (field is applied parallel to the film plane), two resonance modes are observed (Figure 4b), indicating two kinds of internal field. The resonance peak at approximately 770 Oe is attributed to the acoustic mode, which is caused by the exchange field in FeCo nanoparticles. It corresponds to the

different temperatures are also shown, with the temperature dependence of coercivity plotted as the inset. The thermomagnetic curve can be divided into four stages. In the temperature range below 450 K (stage I), the normalized magnetization slightly decreases with increased temperature due to thermal perturbation of the magnetization vectors. At this stage, a typical hysteresis loop can be obtained with the coercivity exceeding 20 Oe. An anomalous change is observed by increasing the temperature to 623 K (stage II), where the

Figure 4. (a) ZFC/FC curves of the as-deposited FeCo nanoparticle film. (b) In-plane and (c) out-of-plane FMR spectra of the as-deposited FeCo nanoparticle film. 3572


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Figure 6. HRTEM images of films annealed at (a) 473 K, (b) 573 K, and (c) 673 K for 2 h.

Figure 7. (a) In-plane and (b) out-of-plane FMR spectra for FeCo nanoparticle film after annealing at 673 K.

metastable. Post annealing promotes phase separation of metallic FeCo and dielectric Al2O3, reducing the interfacial areas in-between via atom diffusion. The crystallized FeCo particles interconnect with each other to form larger size of ∼20 nm after annealing, while the insulating Al2O3 becomes isolated and aggregates as inclusions in metallic matrix. Structural transition revealed by HRTEM images in Figure 6 explains the abnormal thermomagnetic and hysteresis behavior between 450 and 773 K in Figure 5. At 450 < T < 623 K (stage II), the core/shell structure of nanoparticle film is well maintained and FeCo nanoparticles are exchange coupled with each other through Al2O3 shells. The interparticle exchange coupling cannot resist thermal fluctuation when temperature approaches 623 K, and FeCo nanoparticles become superparamagnetic. The magnetization vectors of FeCo nanoparticles, however, remains to distribute in the hemisphere along the external field. In this circumstance, the measured magnetization M can be expressed as a function of angle θ between the magnetization vector and the external field as follows:

normalized magnetization declines rapidly to 0.5 with the coercivity decreased to a minimum value of 5 Oe. The area of the hysteresis loop also decreases drastically to minimum. In the temperature range between 623 and 773 K (stage III), the normalized magnetization decreases slowly. The hysteresis loop, however, recovers with the coercivity increases to 60 Oe. With the temperatures above 773 K (stage IV), the normalized magnetization decreases sharply and remains at 0.1 up to 1023 K. The hysteresis loop turns into a linear paramagnetism shape as a result of Curie transition. In order to understand the abnormal thermomagnetic and hysteresis behavior between 450 and 773 K, the FeCo nanoparticle film is subjected to vacuum annealing at 473, 573, and 673 K for 2 h. Figure 6 shows the corresponding HRTEM images and SAED patterns. The nanoparticle film remains amorphous and presents a similar core/shell structure to the as-deposited film after annealing at 473 K (Figure 6a). When increasing annealing temperature to 573 K, the core/ shell structure is still maintained except that some FeCo nanoparticles are crystallized (Figure 6b). The diffraction spots in corresponding SAED pattern can be identified as (110) plane of body-centered cubic (BBC) Fe(Co). Annealing at 673 K gives rise to a distinct structure (Figure 6c). The dark regions in TEM image are the crystallized nanoparticles, which are interconnected with each other to form large grains. The corresponding SAED pattern reveals a polycrystalline feature for the film after annealing. Two diffraction rings are observed which are attributed to the (110) and (200) planes of BBC Fe(Co) with d-spacings of 2.024 and 1.431 Å. As FeCo metallic nanoparticles and Al2O3 dielectric shells are two immiscible phases, the core/shell structure of amorphous nanoparticle film possesses larger interfacial energy, which is

M = Mscos θ = Ms

1 2π

∫0

π /2

2π sin θ cos θdθ = 0.5Ms

The calculated magnetization of 0.5Ms is consistent with the experimental magnetization in Figure 5. The superparamagnetic nature of the FeCo nanoparticles at 623 K also leads to the minimum value of coercivity and hysteresis loop area. Further increasing the temperature to 773 K is accompanied by the crystallization and interconnection of FeCo nanoparticles, which lead to enlarged coercivity of 60 Oe and recovered hysteresis loop. 3573


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μΩ·cm which is at the same scale for metallic alloy film.38 The FeCo nanoparticle film annealed at 673 K exhibits a positive TCR (inset (b) of Figure 8,) which can be attributed to the structure transition observed in Figure 6c, where the crystallized FeCo particles interconnects with each other while the insulating Al2O3 becomes isolated and aggregates as inclusions in the metallic matrix. Consequently, electrons percolate through the interconnected FeCo grains, and higher temperature leads to the scattering of more electrons by lattice vibration and results in metallic-type electron mobility. Figure 9a shows the hysteresis loop of the as-deposited FeCo nanoparticle film, which demonstrates excellent soft magnetic properties with a saturation magnetization of 1.13 T, a hard-axis coercivity (Hch) of 4 Oe and an in-plane uniaxial anisotropy (Hk) of 44 Oe. It also exhibits good high frequency response (Figure 9b) with a maximum permeability (umax) of 120, a cutoff frequency (f r) of 1.7 GHz and a broadened Δf (full width at half-maximum) of up to 3.5 GHz. Together with the high electrical resistivity of 4200 μΩ·cm, the FeCo nanoparticle film provides excellent microwave performance.

Figure 7 shows the in-plane and out-of-plane FMR spectra of FeCo nanoparticle films after 673 K annealing. The FMR measurement confirms the vanishment of interparticle exchange coupling after high temperature annealing. For asdeposited FeCo nanoparticle film, interparticle exchange coupling can be identified clearly as optic mode in FMR spectrum (Figure 4b). For the film annealed at 673 K, however only the acoustic mode at low field (550 Oe) is detected in both in-plane and out-of-plane FMR measurements. The split peaks located near the acoustic mode are attributed to the magnetocrystalline anisotropic field of the continuous FeCo grains after annealing. The resonance peak corresponding to interparticle exchange coupling at 2250 Oe in Figure 4b disappears which is caused by the phase separation and interconnection of FeCo nanoparticles. Figure 8 shows the temperature dependence of electrical resistivity of FeCo nanoparticle film. The as-deposited FeCo

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CONCLUSIONS

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ASSOCIATED CONTENT

Ultrafine FeCo nanoparticles (2.69 ± 0.45 nm) isolated by ultrathin Al2O3 dielectric shells (0.63 ± 0.12 nm) have been successfully fabricated by reactive PLD. Through the ultrathin Al2O3 shells, the FeCo nanoparticles can be exchange coupled even though their diameters are far under the superparamagnetic blocking limit. This unique core/shell structure provides a way to stabilize the ultrafine FeCo magnetic nanoparticle assembly, and guarantees the ferromagnetic response and high electrical resistivity of the as-deposited nanoparticle film, thus exerts supreme electromagnetic properties (ρ of 4200 μΩ·cm, Ms of 1.13 T, μmax of 120, f r of 1.7 GHz, and Δf of 3.5 GHz) for microwave applications.

Figure 8. Temperature dependence of the normalized electrical resistivity for the as-deposited FeCo nanoparticle film measured in the range of 300−800 K. The inset (a) shows a negative TCR for asdeposited FeCo nanoparticle film, whereas inset (b) presents a positive TCR for the film subjected to annealing at 673 K for 2 h.

nanoparticle film exhibits a room temperature electrical resistivity of approximately 4200 μΩ·cm, which is 3 orders of magnitude larger than that of FeCo based alloy.38 The resistivity decreases slowly until 635 K. As shown in inset (a) of Figure 8, the as-deposited FeCo nanoparticle film presents a negative temperature coefficient of resistance (TCR). The negative TCR can be correlated with the core/ shell structure in Figure 2a. Electrons in the FeCo nanoparticle film are localized by the Al2O3 dielectric shell. The conductivity is originated from the tunneling cross the dielectric shell induced by thermal fluctuation.26 As a result, an insulator-type TCR is observed. Rapid decline of resistivity takes place above 635 K, with the normalized resistivity ρ/ρ0 falls to nearly 0.06. The corresponding resistivity is about 200

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00537. The STEM-HAADF images of as-deposited FeCo nanoparticle film; The XRD patterns of FeCo nanoparticle films annealed under different temperature (PDF)

Figure 9. (a) Hysteresis loops and (b) permeability spectra of as-deposited FeCo nanoparticle film. 3574


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AUTHOR INFORMATION

Corresponding Authors

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

Guohua Bai: 0000-0002-8820-8644 Chen Wu: 0000-0001-6783-9343 Funding

This work was supported by the National Natural Science Foundation of China (Nos. 51801181, 51571176, and 51590881), the National Key Research and Development Program of China (No. 2016YFB0700902), the Key Research and Development Program of Zhejiang Province (No. 2017C01031), and the China Postdoctoral Science Foundation (No. 2018M642424). Notes

The authors declare no competing financial interest.

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Ultrafine FeCo Nanoparticles Isolated by Ultrathin Dielectric Shells for

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