Characterizing Dielectric And Ferromagnetic Nanofilms By Electrostatic/magnetic Force Probes: Finite-element Electromagnetic Field Simulations

With the development of energy technologies,the dielectric and magnetic materials apprehensively used in the electrical power equipment such as insulation cable,power-generating or driven machinery,voltage transformer have attracted high interest of interdisciplinary researches.Electrostatic probe microscopy is focused for investigating the electric potential,charge and dielectric permittivity on the surfaces of dielectric materials,while magnetic probe microscopy is primarily exploited to study the magnetic micro-structures relevant to magnetic performances of magnetic materials,such as magnetic moments and magnetization characteristics of magnetic domains,etc.In the present study,the electromagnetic field simulations on the electrostatic and magnetic probe characterizations of dielectric and ferromagnetic nanofilm are implemented with the finite-element method.The correlations between the geometry and size of detecting probe and nanofilm sample and the electrostatic and magnetic forces are elucidated to evaluate their limitations on the probe microscopy of detecting and imaging the dielectric and magnetic nanofilms,which provides a theoretical basis for the probe microscopy technology of characterizing nanomaterials.3D finite-element models of the conductive probe with a dielectric nanofilm on metal substrate are constructed and analyzed in terms of electrostatic force and electric potential,in which the critical lateral dimension(circular film diameter)is required and determined for accurately characterizing the dielectric permittivity of dielectric nanofilms.The thickness and diameter of the modeled dielectric films ranges in1nm～10μm and 100nm～10μm respectively,with the relative dielectric constant altering in 2～1000 to represent various dielectric materials,while the probe tilt-angle is raised from 0° to 20°.The critical diameter of nanoscale dielectric film depends not only on the geometry and tilt angle of electrostatic force probe,but also on the thickness of dielectric nanofilm: for the dielectric films with a thickness greater than100 nm,the critical diameter is almost independent of probe geometry,while mainly relying on the thickness and dielectric permittivity of dielectric film,which follows an exponential relationship.For the ideal dielectric nanofilms with an approximately infinite lateral dimension(lateral size is larger than critical diameter)and an ultra-thin limit(thickness h<10～100nm)that satisfying accurate measurements of dielectric permittivity by electrostatic force probe,an effective analytical model incorporating probe tilt-angle is established based on the results of electrostatic field simulations.A general scheme of analyzing probe electrostatic force is suggested to accurately measure the dielectric permittivity of dielectric nanofilms by electrostatic force microscopy.By means of magnetostatic field simulations on the testing system composed of a permanent magnet probe and a ferromagnetic nanofilm(single magnetic domain sample),the magnetic forces between the detecting probe and the tested nanofilm are analyzed in relations to the probe/film geometry and sample magnetic moment to explore their limitations on the scanning imaging resolution of magnetic probe,and thus propose a reasonable scheme of magnetic probe microscopy.The varying magnetic signal of scanning probe is systematically analyzed in aspects of the shape and magnetic-film thickness of detecting probe,the magnetic moment orientation and magnetization of the tested sample,and the probe tilt angle.The direction of magnetic force detected by the permanent magnet probe depends on the magnetic moment orientation of magnetic nanosample;compared with half-angle of probe cone probe,the curvature radius of probe tip exerts more influence on magnetic force;magnetic force correlates positively with the sample magnetization intensity and the thickness of magnetic film on probe surface.The magnetic force probe detecting the magnetic moment of magnetic domains in ferromagnetic nanofilm relies significantly on sample thickness.When sample thickness is larger by several times than lateral diameter to reach a critical value,the magnetic force signal detected by probe originates dominantly from the magnetic contribution on magnetic domain surface,which incapacitates magnetic force probe to accurately characterize magnetic microstructures of ferromagnetic nanofilms.