It is shown that the critical exponent for the effective shear modulus of a composite medium where a compliant polymer matrix is filled with ferromagnetic particles may significantly depend on the ...external magnetic field. The physical consequence of this dependence is the critical behavior of the relative magnetorheological effect.
•structure and microwave properties of epoxy resin composites with graphite nanoplatelets decorated by FeNi nanoparticles.•The graphite phase i the investigated nanopowder is predominant, and ...Fe20Ni80 component contains fcc FeNi3.•decoration of the GNPs surface by Fe20Ni80 nanoparticles leads to an essential improvement of EMR absorption properties of composites in high frequency range.
This paper is concerned with investigation of the structure and microwave properties of epoxy resin composites with graphite nanoplatelets (GNPs) decorated by FeNi nanoparticles prepared by the salt impregnation. It was confirmed by SEM and XRD that the method gives nanopowder where the metal componentFe20Ni80 is in the form of nanoparticles (20–40 nm in diameter) which are distributed over the surface and edges of the GNPs. The graphite phase in the investigated nanopowder is predominant, and Fe20Ni80 component mainly contains fcc FeNi3. Measurements of magnetic properties confirmed that Fe20Ni80 are small, randomly oriented assembly of spherical (or close to spherical) particles on the GNPs surface. The saturation magnetization of GNP-Fe20Ni80 particles is 25 emu/g and it weakly depends on temperature. Electrical resistivity measurements have shown that decoration of GNPs leads to essential increase of conductivities well as improvements EMR absorption properties in high frequency range (26–60 GHz) in (GNP-Fe20Ni80)/with in compression GNP/epoxy under the same volume content of 1.45 vol.%(GNP-Fe20Ni80)/L285 composites demonstrated superior broadband absorption properties with microwave absorption efficiency higher than 97% in the whole investigated frequency region.The effective absorption bandwidths are as high as 12.2 GHz were observed at the frequency range 41.9 and 53.2 GHz and 13.3 GHz at the frequency range 51.1 GHz–64.4 for this composite. It is assumed that decoration of GNPs surface by nanoscale Fe20Ni80 particles leads to a formation of the multiple dielectric and magnetic loss mechanisms, such as interfacial polarization, dipole polarization, space-charge polarization, eddy current loss, Debye dipolar relaxation, natural resonance and exchange resonance, which improve the microwave absorption properties of the investigated composites.
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•The effect of blocking of particles displacement in MAE was observed from ZFC-FC.•M-H loops change the shape at solidification temperature of MAE.
The magnetization of a ...magnetoactive elastomer (MAE) with microparticles of soft magnetic carbonyl iron embedded in a highly elastic matrix has been studied. It is shown that at high temperatures its magnetization curve has the form of a specific hysteresis loop. This hysteresis is attributed to the influence of displacement of magnetized particles in the elastically soft elastomer matrix under the effect of magnetic forces, leading to the change of magnetic interaction between the particles. In this case, there is a maximum in the field dependence of the magnetic susceptibility, the occurrence of which has been associated with the competition between re-arrangement of particles, when they are displaced in a magnetic field, and saturation of particles’ magnetization. When the MAE is cooled below approximately 225 K, both the magnetic hysteresis and the maximum in the field dependence of the magnetic susceptibility disappear. When the MAE material is cooled below the solidification temperature of the elastomer matrix, the displacements of the magnetic particles during magnetization are blocked by the rigid matrix. The magnetization reversal of the MAE is reversible. This means that the shape of subsequent magnetization loops remains constant and the sample returns into the initial non-magnetized state after the magnetic field is turned off.
Principal factors leading to the efficient alternating current (AC) field‐induced heating of magnetic fluids are determined based on the results of magnetic and calorimetric measurements on NaFeO2 ...nanoparticles and their fluids. Features of the magnetic behavior are singled out, and characteristic magnetic parameters, such as the Curie and blocking temperatures, are determined for the NaFeO2 nanoparticles. The dependence of the fluid caloric properties on the AC magnetic field amplitude and nanoparticle concentration is analyzed. It is shown that the heating efficiency of the weakly coercive NaFeO2 nanoparticles displays an amplitude threshold character, and the heating rate is critically dependent on the AC magnetic field amplitude.
Magnetic and calorimetric studies of biocompatible superparamagnetic NaFeO2 nanoparticles synthesized by the sol‐gel method and their fluids are presented. The heating efficiency of NaFeO2 magnetic nanoparticles displays an amplitude threshold character. The heating rate is critically dependent on the AC magnetic field amplitude. Heating productivity rapidly increases when the amplitude threshold is overcome.
We propose a theoretical approach for calculating effective electric and magnetic properties of composites, with field dependent restructuring of the filler. The theory combines the effective medium ...approximation, extended to a field-dependent (variable) percolation threshold, with an approximate treatment of the nonlinearity of material properties. Theoretical results are compared with experiments on magnetorheological elastomers, which in the context of investigated phenomena are often called magnetoactive elastomers (MAEs). In MAEs with soft polymer matrices, the mutual arrangement of inclusions changes in an applied magnetic field. This reorganization of the microstructure leads to unconventionally large changes of electrical and magnetic properties. The obtained theoretical results describe observed phenomena in MAEs well. For the magnetodielectric effect, qualitative agreement between theory and experiment is demonstrated. In the case of magnetic permeability, quantitative agreement is achieved. The theoretical approach presented can be useful for the development of field-controlled smart materials and design of intelligent structures on their basis, because the field dependence of physical properties can be predicted.
•The method of movable percolation threshold is proposed.•The effective properties of composites, with the restructuring of the filler, are calculated.•The mechanism of magnetodielectric effect in magnetoactive elastomers is proposed.•The nonmonotonic field dependence of magnetic permeability in magnetoactive elastomers is explained.
•La0.8-x□xNa0.2Mn1+xO3-Δ with strong spin-electron coupling has been prepared.•Well-developed crystalline spherical-like nanoparticles contain MnB3+, MnB4+, MnA2+.•Spin-dependent magnetism is ...observed due to eg-electron localization–delocalization.•Multicomponent nonlinear magnetic system shows complex critical behaviour near TC.•Superparamagnetic contribution of nanoparticles to magnetocaloric effect was found.
Despite extensive research on manganites owing to their potential use in modern technologies, some aspects of their behaviour associated with changes in spin, valence, and charge states remain unclear. In this work, we investigated the structural, magneto-transport, and magnetocaloric properties of La0.8-x□xNa0.2Mn1+xO3-Δ manganite nanoparticles with strong spin-electron coupling. It was found that the perovskite structure contained different valence manganese ions, MnB3+, MnB4+, and MnA2+, even in the pristine compound. With an increase in x, increases in the Curie temperature TC, spontaneous magnetization, and magnetic entropy change ΔSM, as well as a decrease in the metal-semiconducting temperature Tms, were detected. The change in the spin value of Mn ions during the transition from ferromagnetic-metallic (μFMexp < μFMtheor) to the paramagnetic-semiconducting (μPMexp > μPMtheor) state owing to the localization–delocalization process of eg-electrons on the Mn sites was found to indicate spin-dependent magnetism. The complex critical behaviour is manifested near TC with a second-order phase transition. An additional influence of the super-paramagnetism of nanoparticles on the magnetocaloric effect was determined.
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Abstract Principal factors leading to the efficient alternating current (AC) field‐induced heating of magnetic fluids are determined based on the results of magnetic and calorimetric measurements on ...NaFeO 2 nanoparticles and their fluids. Features of the magnetic behavior are singled out, and characteristic magnetic parameters, such as the Curie and blocking temperatures, are determined for the NaFeO 2 nanoparticles. The dependence of the fluid caloric properties on the AC magnetic field amplitude and nanoparticle concentration is analyzed. It is shown that the heating efficiency of the weakly coercive NaFeO 2 nanoparticles displays an amplitude threshold character, and the heating rate is critically dependent on the AC magnetic field amplitude.
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