Micromagnetics Simulation On Magnetic Nanostructures And Microwave Devices Based On Magnetic Nanostructures

Some typical nanostructured magnetic materials have been studied using the micromagnetics theory and a software named OOMMF, for instance, ellipsoidal nano flakes, porous nano flakes, nanotube array and nanowires array. As for the ellipsoidal nano flakes, it is found that ellipsoidal nano-flakes with different thickness have different patterns of magnetic domains, and have a single domain structure when the thickness is less than 18-20 nm. Ellipsoidal nano flakes with a single domain structure had strong shape anisotropy. Only one sharp resonance peak is found in the spectrum of imaginary parts of permeability. For ellipsoidal nano flakes, the coercivity value is dependent on the thickness, and becoming larger with increasing the thickness which is opposite to the structures with multi domains. In the case of porous nano flakes with strong in-plane shape anisotropy, the presence of nano holes can increase the quantity of magnetic loss peaks. Since some loss peaks can overlap, the width of loss peaks is broaden by this way. In other words, the porous nano flakes can be a good candidate for electromagnetic wave absorbing materials with lightweight and broad absorbing bandwidth. As for nanotubes array, it is found that the distance between neighboring nanotubes and the wall thickness of nanotubes have a great impact on the configurations of magnetic domains and the shape of hysteresis loops, which can be thought as the results of different localized interactions among nanotubes. For nanowires array, it is found that the nanowires array shows strong shape anisotropy, larger coercivity and remanence due to their large aspect ratio. It is also pointed out that the magnitude of microwave permeability is dependent on the volume fraction of magnetic nanowires in a template. For instance, the real parts of microwave permeability increase with volume fraction of nanowires increasing. The resonant frequency in the spectra of imaginary parts of the permeability is linearly related to the volume fraction.Based on the measured high frequency properties of ?-Fe nanowire array embedded in AAO templates, we have designed two passive microwave magnetic devices. For the circulator we designed, the microstrip line is used as the transmission line. The center frequency of circulator is 12 GHz. The insertion loss is-0.88 dB. The degree of isolation is-28.7 dB. The relative bandwidth is about 10%. For the microwave isolator we designed, the transmission line is based on the substrate integrated waveguide structure. The operation frequency is 9.1 GHz ~ 10 GHz. The insertion loss was-0.72 dB~-0.86 dB. The isolation is-20 dB~-31 dB. The standing wave ratio is about 1.1. The relative bandwidth is about 9%. The performances of devices have met the requirements of real devices. It should be stressed that owing to the high remanence of nanowire array, the operation of these two microwave devices can be self-biased, and don’t need permanent magnets anymore which are often found in traditional microwave magnetic devices. The self-biased microwave devices are attractive in reducing the weight and volume of devices.