| Recently, negative parameters in physics have been obtained intensive attentions, such as negative permittivity ε, permeability μ, refraction n, poisson’s ratio as well as negative mass density et al. The reason is the deepening research on metamaterials which offered an entirely new territory to further enhance the scientific significance and utilization of materials in electricity, magnetism, microwave, optical, acoustic, mechanics and calorifics. However, metamaterials which are featured by the negative permittivity and negative permeability (double negative properties) are generally focusing on the periodic structure rather than the chemical composition and microstructure, since they have been proposed in 2000. Thus, they are far from the "real" material. Then the conventional double negative materials (DNMs) which based on the intrinsic characteristics of typical materials for double negative properties are explored, and the double negative has been found in metal/ceramic heterogeneous composite material.The provskite lanthanum strontium manganese oxide (La1-xSrxMnO3, LSMO) possess double exchange effect, the electron-phonon coupling, and super-exchange, and so on, and which will lead to the use for spin and orbital magnetic field or separation of photoinduced insulator to metal transition, and the giant magnetoresistance effect and so on, in magnetic electronics, catalysis, sensing, microwave absorption, and other fields has a very important potential applications. In addition, in the LSMO, the electron energy band of eg electron and doping concentration (0.16-0.5) with metal ferromagnetic properties are wider than that of LCMO, and the within the scope of the doped high Curie temperature (Tc=350-370 K), so LSMO has more advantages and actual application value. Howere, there are few researches on the permittivity and permeability spectrum characteristics of LSMO at present. Therefore, this is based on the intrinsic properties of single-phase material to achieve the double negative features is a very meaningful work.The sol-gel method followed by sintering process were used to prepare (La,Sr)MnO3 ceramics with different Sr content as well as other element doping. With the use of X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM) and Agilent RF Impedance/Material Analyzer (E4991A), the clear influence of sintering temperature on the microstructure and morphology, electromagnetic properties are presented, to explore the correlation between the plasma oscillation, magnetic resonance and doping composition, microstructure, and then to reveal intrinsic mechanism of double negative properties.In this paper, the main contents are summarized as follows:(1) The La1-xSrxMnO3 ceramics with different Sr content were prepared by the ordinary sol-gel process with auto-combustion method, followed by sintering process at the different sintering temperature. The electromagnetic properties were tuned, and the higher sintered temperature is effective to improve the bandwidth of negative permittivity and magnetic susceptibility, due to the phase composition and the density. It will be of great potential to push the negative permittivity and the negative susceptibility to the same frequency region by adjusting the sintered temperature, to realize double negative properties.(2) The tunable negative parameters of LSMO were investigated in detail. Double negative permittivity and permeability can be achieved at a certain frequency range. Lattice symmetry and magnetic anisotropy tunerd by Sr content are found to be effective for the realization of negative permeability over a wider frequency range and to realize the double negative properties in LSMO.(3) The microstructure and dielectric properties of La0.7Sr0.3Mn1-yMyO3 (M=Fe, Ni, Cu; y=0.3 or 0.5) are prepared by sol-gel/sintering method. The symmetry of crystal structure is improved and the dielectric properties obtained in the frequency range from 80 MHz to 1 GHz can be tuned by the doping of Fe, Ni, Cu ions. As the enhanced dielectric losses in the Ni and Cu doping samples will promote the strong absorption. Thus, the improved dielectric properties make ion-doped La0.7Sr0.3MnO3 as promising candidates for the lossy ceramics in microwave electronics. In addition, the experimental results of permittivity were checked by K-K relations.(4) The experimental data of the negative permittivity are fitted well by the Lorentz resonance and the plasma oscillation model. The negative permittivity of LSMO is realized by tuning the effective concentration of conduction electrons through the Sr-doping. The tunable negative parameters in perovskite LSMO ceramics have been successfully realized. Experimental results and theoretical simulation results indicate that the negative permittivity is attributed to the plasma oscillation of conduction electrons in the LSMO and negative permeability to the domain wall and gyromagnetic spin resonance in the ferromagnetic state. Lattice symmetry and magnetic anisotropy are found to be effective for the realization of negative permeability over a wider frequency range to tune the negative parameters in LSMO. The negative permittivity was realized, and the permittivity and ac conductivity were drastically tuned by the Sr-doping effect. It is further found that the doping elements tune the permittivity by adjusting the concentration of the effective electrons, which is further supported by the frequency dependences of the conductivity (σ’ac～f).(5) The equivalent electrical circuits of all samples with negative permittivity contain shunt inductance. Therefore, it is reasonable to suggest that the inductive behavior was a characteristic of negative permittivity phenomenon.(6) The complex permeability (μ’<0) was fitted by magnetic resonance equations. The negative permeability is mainly attributed to the domain wall resonance and gyromagnetic spin rotation in the LSMO at ferromagnetic state. Furthermore, the μ’ of LSMO may also be tuned by an external DC magnetic field, and possibly the magnetic field tunable double negative property can be realized in LSMO. That is to say, the tunability of negative permittivity and negative susceptibility makes La1-xSrxMnO3 as a promising candidate for the double negative materials.This work was supported by National Natural Science Foundation of China (50772061,51172131), Program for New Century Excellent Talents in University (NCET-10-518). |
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