Enhanced Microwave Absorption Properties of Intrinsically Core/shell Structured La0.6Sr0.4MnO3Nanoparticles
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The intrinsically core/shell structured La0.6Sr0.4MnO3nanoparticles with amorphous shells and ferromagnetic cores have been prepared. The magnetic, dielectric and microwave absorption properties are investigated in the frequency range from 1 to 12 GHz. An optimal reflection loss of −41.1 dB is reached at 8.2 GHz with a matching thickness of 2.2 mm, the bandwidth with a reflection loss less than −10 dB is obtained in the 5.5–11.3 GHz range for absorber thicknesses of 1.5–2.5 mm. The excellent microwave absorption properties are a consequence of the better electromagnetic matching due to the existence of the protective amorphous shells, the ferromagnetic cores, as well as the particular core/shell microstructure. As a result, the La0.6Sr0.4MnO3nanoparticles with amorphous shells and ferromagnetic cores may become attractive candidates for the new types of electromagnetic wave absorption materials.
KeywordsLa0.6Sr0.4MnO3nanoparticles Core/shell structure Microwave absorption Electromagnetic matching
In recent years, serious electromagnetic interference pollution arising from the rapidly expanding business of communication devices, such as mobile telephones and radar systems, has attracted great interest in exploiting effective electromagnetic (EM) wave absorption materials with properties of wide frequency range, strong absorption, low density, and high resistivity. Magnetic nanoparticles, besides it is important technical applications in magnetic refrigerators, magnetic recording, magnetic fluids , and biomedicine , can be a potential candidate for microwave absorption at high frequency over gigahertz, ascribed to the high Snoek’s limit [3, 4]. Nevertheless, the relative complex permeability of metallic magnetic materials may decrease due to eddy current phenomenon induced by electromagnetic wave .
Recently, core/shell nanostructures have received intense attention due to their improved physical and chemical properties over their single-component counterparts , which are of great importance to a potentially broader range of applications in electronics, magnetism, and optics. A number of core/shell structured materials, like CdSe/ZnS [7, 8], CdS/ZnS , and ZnO/ZnS [10, 11] have been studied. Concerning the disadvantage of magnetic absorber, the fabrication of materials with core/shell microstructure is a promising way to solve this problem. Consequently, many core/shell structured materials with a magnetic metallic core and a dielectric shell have been investigated, in which the magnetic metallic materials act as a magnet that increases the permeability of the composites. While dielectric materials act not only as centers of polarization, which increases the dielectric loss, but also as an insulating matrix among magnetic metallic particles that reduces the eddy current loss. Several groups have reported good microwave absorption properties of core/shell structured materials, such as α-Fe/Y2O3, Fe/Fe3B/Y2O3, Ni/C , CoFe2O4/carbon nanotube . Nevertheless, it is difficult to prepare monodispersed magnetic nanoparticles due to the small sizes and high active surface areas of nanoparticles that lead to aggregation easily. The complex fabrication process and uneasily controllable experimental parameters of preparing core/shell heterogeneous system are of great challenge for putting such nanocomposites absorber into practical applications. The particular electronic structure and unusual electromagnetic characteristics of the nanocrystalline perovskite manganite indicate that it has high application as microwave absorption materials. Though several works have reported the microwave absorption properties of bulk manganites [16–19], the excellent microwave absorption properties originating from the intrinsically core/shell structured nanoparticles are not reported as far as we know.
In our present work, we investigate the microwave absorption properties of half-metallic soft magnetic La0.6Sr0.4MnO3(LSMO) nanoparticles. Our experimental results demonstrate that LSMO nanoparticles with intrinsically core/shell structure are promising for the application to produce broadband and effective microwave absorbers.
The La0.6Sr0.4MnO3(LSMO) nanoparticles were prepared by the traditional sol–gel method. The stoichiometric amounts of La2O3, Sr(NO3)2, and 50% Mn(NO3)2solutions were used as starting materials, and La2O3was converted into metal nitrates by adding nitric acid. These metal nitrates were dissolved in distilled water to obtain a clear solution. After stirred for 2 h, citric acid (the molar ratio of LSMO to citric acid is 2:1) was added with constant stirring, and then an appropriate amount of urea was added to the solution. Subsequently, the solution was evaporated to get a gel. The gel was firstly decomposed at about 250 °C for 24 h. The resulting powder was separated into several parts with equal mole and annealed at different temperatures of 700, 900 and 1100 °C for 6 h to obtain samples with different average particle sizes.
Phase analysis of the products was performed by powder X-ray diffraction (XRD) technique. Morphology observation of particles was conducted with transmission electron microscope (TEM), the detailed morphology of the nanoparticles was studied by means of a high-resolution transmission electron microscope (HRTEM) JEOL-2010 with an emission voltage of 200 kV. Infrared (IR) transmission spectra were collected at room temperature, in which KBr was used as a carrier. Magnetic properties were measured using a superconducting quantum interference device magnetometer (SQUID). The relative complex permeability (μ r ) and the relative complex permittivity (ɛ r ) of the particle/wax composites were measured on a vector network analyzer (Agilent Technologies, HP8720ES) using transmission/reflection mode . The prepared powders were mixed with wax by the ratio of 2:1 in weight and pressed into a mode to prepare the specimen, the coaxial cylindrical specimen was 3.04 mm in inner diameter, 7.00 mm in outer diameter, and 2.00 mm in thickness.
Results and Discussion
Structure and Morphology
X-ray diffraction (XRD) patterns of all the samples show single phase and free from impurities, and can be indexed to a single rhombohedral crystal structure with the symmetry [shown in Figure S1, supplementary material 1]. The increase in the calcinations temperature from 700 to 1100 °C, resulting in the sharpening of the diffraction lines [inset of Figure S1], with an increase in intensity. The X-ray linewidths provide the average particle size (D) through the classical Scherrer formulation , wherek is a constant (~0.89),λ is the wavelength of the X-ray,B is the width of the half-maximum of the peak, andθ is the diffraction angle of the peak. The values ofD are 35, 100 nm, for the 700 and 900 °C annealed samples, respectively. Scanning electron microscopy (not shown here) was used to characterize the particle size of the 1100 °C annealed sample and the corresponding particle size is 150 nm, which can be further proved in the following TEM morphology observation. The corresponding particles are labeled as S35, S100 and S150, respectively.
Magnetic and Dielectric Properties
To study the magnetic behaviors of the samples, the magnetic hysteresis loops of the samples at 300 K with different particle sizes were measured (Fig. S2). It is shown that the LSMO nanoparticles are ferromagnetic behaviors at room temperature. The inset of Fig. S2 shows the amplified image of magnetic hysteresis loop, it exhibits the soft magnetic property of prepared LSMO. The saturation magnetizations (Ms) decrease gradually with the decrease of the particle size, the values of Ms are 49, 32, and 28 emu/g for S150, S100, and S35, respectively, which are somewhat lower than that of the corresponding bulk material. For nanoparticles, the broken exchange bonds and the translational symmetry breaking of the lattice at the surfaces induce disordered spins and lead to the zero magnetization at the surface. Therefore, the increase of the relative surface contribution with decreasing particle size leads to the reduction of the Ms [25, 26]. Especially, for S35, the amorphous shell causes a greater reduction of Ms.
In Fig. 3b, it is found that both the real part ( ) and imaginary part ( ) of the relative complex permeability spectra have shown good dispersion relation. For all the samples, with increasing frequency, both and values exhibit an abrupt decrease in the range of 1–6 GHz, and then a resonance phenomenon accompanied with a broad peak at 6–12 GHz occurs. Previous investigations [33, 34] have shown that La1−x(Sr, Ba)xMnO3 micro-size powders exhibited giant microwave loss at ~10 GHz arising from natural ferromagnetic resonances. In the present La0.6Sr0.4MnO3 composition, the observed spectra as shown in Fig. 3b are in good agreement with the mechanism of natural ferromagnetic resonance arising from the magnetic anisotropy consequent on the strains in the grains. Additionally, it is found that the values decrease, while the values increase with decreasing particle size, due to the smaller saturation magnetizations Ms of small-sized particles. It is known that and values are correlated, standing for the energy storage and loss, respectively. Obviously, the inverse changes of and are attributed to the magnetic properties of LSMO nanoparticles, which play an important role in determining the magnetic behavior of the composites, endowing the composites with strong magnetic loss. Magnetic loss is caused by the time lag of the magnetization vector M behind the magnetic field vector H. The change of the magnetization vector is generally brought about by rotation of the magnetization or the domain wall displacement. These motions lag behind the change of the magnetic field and contribute to . The smaller the particle size, the weaker the spins coupling at the particles’ surface, which makes the magnetic relaxation behavior more complex, and will give rise to a magnetic loss mechanism. Additionally, the domain wall displacement loss occurs in multidomain magnetic materials, in LSMO nanoparticles where size is larger than the critical size for single magnetic domain (25 nm) , the domain wall displacement loss plays an important role in magnetic loss. Therefore, it is reasonable to deduce that the magnetic loss is due to significant contributions from both the natural ferromagnetic resonance and the domain wall displacement loss.
Microwave Absorption Properties
where f is the frequency of incident electromagnetic wave, d is the absorber thickness, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of absorber.
The dielectric loss factor ( ) and the magnetic loss factor ( ) may well explain why S35 nanoparticles have such excellent microwave absorption properties in a very wide frequency range, as shown in the inset of Fig. 4a. It is found that the dielectric loss factor shows an approximately constant value around 0.05 with a slight fluctuation, whereas the values of the magnetic loss factor exhibits a gradual increase from 0.39 to 0.47 in 1–8.2 GHz and then decreases at higher frequencies. The steady dielectric loss in the whole frequency range proves the balanced EM matching in the composites, suggesting that the enhanced microwave absorption properties result from the cooperative effect of the amorphous shells and the ferromagnetic cores. That is to say, the amorphous shells play an important role in allowing broader frequency range microwave absorption because of their steady dielectric loss ability in this range. It is evident that the excellent microwave absorption properties for the intrinsically core/shell LSMO nanoparticles are a consequence of the better EM matching due to the existence of the protective amorphous shells, the ferromagnetic cores, as well as the particular core/shell microstructure.
In conclusion, intrinsically core/shell LSMO nanoparticles exhibit excellent microwave absorption properties. The analysis of experimental data shows that the optimal reflection loss reaches −41.1 dB at 8.2 GHz with a matching thickness of 2.2 mm, the bandwidth with a reflection loss less than −10 dB is obtained in the range of 5.5–11.3 GHz for absorber thicknesses of 1.5–2.5 mm, which are attributed to the electromagnetic match in microstructure, the strong natural ferromagnetic resonance, as well as the steady dielectric loss. The LSMO nanoparticles with amorphous shells and ferromagnetic cores may have potential applications in wide-band and effective microwave absorption materials.
This work was supported by the National Key Basic Research under Contract Nos. 2007CB925001, 2007CB925002, the National Nature Science Foundation of China under Contract No. 10874051, and Anhui NSF Grant Nos. 070416233, KJ2007A084. The first author would like to thank Prof. Y. M. Zhang and Dr. M. P. Jin for their valuable discussion on this work.