Microwave Magnetic Properties Of Magnetic Metal And Alloy Nanoparticles With Different Shapes

The high frequency magnetic materials obey the Snoek limit, thus it is difficult toimprove. This study bases on principle that the ferromagnetic metal and alloynanoparticles of different shapes can exceed the Snoek's constant by the large surfaceanisotropy and may have potential for microwave application. As the permeability andresonance frequency can be increased at the same time. The ferromagnetic metal andalloy nanoparticles of different shapes may have good microwave property in the GHzfrequency range and to be good microwave absorption filler.In this work, nano spheroid (carbonyl iron), wire (nickel nanowire), flake-shaped(FeCuNbSiB) particles have been synthesized and investigated. The composite sampleswere prepared by mixing the metal and alloy particles with paraffin wax with differentvolume concentration of the particles. The complex permeability of the compositesamples was obtained using an Agilent E8363B vector network analyzer in the 0.1-18GHz range. The main results are shown as follows:(1). For the spheroid carbonyl iron, the imaginary part of permittivity of nanoparticleis much higher than the microparticle, but the imaginary part of permeability is lower. Asthe magnetic loss is the main loss of carbonyl iron composition, the microwave propertyof nanoparticles (Reflection Loss and Absorption Width) is not better thanmicroparticles;(2). For the composites with different volume concentration of the particles, we canget the intrinsic permeability of 20～30 nm carbonyl iron nanoparticle at low frequency is7, the intrinsic resonance frequency is 11.5 GHz. At the same time, the intrinsicpermeability of carbonyl iron microparticle is 23, the intrinsic resonance frequency is 3.4GHz;(3) According to the intrinsic permeability and resonance frequency, both of them arenot beyond the Snoek's limitation, this can be ascribed the easily domination of surfaceanisotropy;(4). The value of the Ni nanowires' resonance peak can be explained by the shapeanisotropy. The natural resonance mode and the exchange resonance mode lead a wideimaginary part of permeability in Ni nanowires with diameter 100 nm; (5). As the exchange mode is sensitive to the condition of the boundary, it can be usedto analyzes the surface anisotropy. The large surface anisotropy means large pinning, andthe small surface anisotropy means small pinning. The fit to the experimental data is poorfor large pinning, thus implying that the surface anisotropies in the Ni nanowires aresmall;(6). The natural resonance mode and exchange resonance mode of Ni nanowires havedifferent responses to the dipolar field, the natural resonance frequency is linearity to thedipolar field by Kittel's equation; the exchange resonance mode is nonlinearity to thedipolar field. At the same time, the dipolar field will suppress the intensity of exchangeresonance mode;(7). According to the intrinsic permeability and resonance frequency of Ni nanowire,the product of them is not beyond the Snoek's limitation;(8). As the use of the shape anisotropy, the flake soft magnetic particles FeCuNbSiBovercome the difficulty of relatively small intrinsic anisotropies and increase the naturalresonant frequencies to GHz range so as to lead the higher real part and the imaginarypart of the permeability;(9). The resonance peak of flake particles was simulated by using the combination ofthe Landau-Lifshitz-Gilbert equation and Bruggeman's effective medium theory,considering a random spatial distribution of magnetic easy axes. The magnetic lossmechanism in flake particles is mainly caused by the natural resonance. The dampingfactor mainly comes from the coupling of magnon and conduct electron by skin effect;(10). The product of initial permeability and resonance frequency of FeCuNbSiB isbeyond the Snoek's limitation, this may results from the planar magnetic anisotropy.