Magnetic Nanostructures Studied By Quantitative Magnetic Force Microscopy

Magnetic nanostructures are promising candidates for the next generation nanomagnetics and spintronics devices because of their special magnetic properties. The study of these special properties should start from an analysis of the fine magnetic domain structures. Magnetic force microscope (MFM) is an important tool in imaging the surface micromagnetic structures of the magnetic materials in real space. However, it is still a technical challenge to quantitatively characterize the fine domain structures within the magnetic nanostructures in high resolution. Currently, there is still an urgent need to develop and improve quantitative and high resolution MFM methods. In this thesis, a quantitative study has been performed to explore the fine micromagnetic structures of the nanoelement arrays made of typical hard and soft magnetic materials by using the improved and newly developed MFM techniques, together with the micromagnetics calculation.A precise tip-to-sample distance control method, based on the frequency shift versus distance curves in the zero-magnetic-force-gradient regions, has been newly developed. In conjunction with the ex-situ tip magnetization reversal method, the separation of topographic and magnetic signals can be successfully achieved in non-contact mode MFM images. This has been shown applicable in the study of longitudinally magnetized recording media.The L1₀-FePt nanodot array has been imaged by using high resolution MFM. Following magnetostatic calculation, it is shown that, in these triangular prism shaped nanodots the single domain state and the double domain state both exist. The domain structures of the L1₀-FePt dots have also been quantitatively calculated by the micromagnetics, which indicates that the variety of the domain structures might be due to the fluctuation of the magnetic anisotropy energy among different nanodots.The double-disk Ni₈₀Fe₂₀ element with curious peanut shape is carefully studied by using high resolution MFM, and a magnetic vortex structure appears at its remanent state. Interestingly, the vortex core is not exactly located in the center of the element but shifted a little bit to one side. Moreover, some line profiles of the MFM signals through the core center show a visible asymmetry. The quantitative micromagnetics calculation of the fine micromagnetic structures of the Ni₈₀Fe₂₀ thin film elements confirms that the vortex core is indeed eccentric, and the lineprofile of the out-of-plane magnetic moments through the magnetic vortex core does show the same asymmetric behavior,which is due to the asymmetric distribution of the demagnetizing field induced by the asymmetric shape of the element. In addition, the micromagnetics calculation and MFM response theory are combined together to quantitatively calculate the radial distribution of the MFM signal of the magnetic vortex structure. The calculated result consists very well with the experimental measurement.At last, an attempt has been made to explore a new MFM method, by which one can carry out dynamical measurements of the local magnetic properties in submicron or even smaller scales. This method has been applied to study the magnetic properties of the Pt/Co/Pt/IrMn multilayer system, in which a series of interesting phenomena, such as inhomogeneous local exchange bias effect, have been directly observed. This will provide a new way to probe microscopic physical mechanism of the exchange biased magnetic materials.