Progress in Magnetic Force Microscopy

Abstract

Since its early days, magnetic force microscopy (MFM) has become a truly widespread and commonly used characterization technique that has been utilised in a variety of fundamental and applied research and in industrial applications.In this work, we review the current state-of-the-art, analyse the challenges and outline the future of this fascinating field [1]. In comparison to other magnetic imaging techniques, MFM presents some advantages: high spatial resolution, down to 10nm; ability to work in different environments as vacuum, air or liquids [2] (despite the technical difficulties that are solved, MFM in liquids facilitates the study of biomagnetic materials) and under applied magnetic fields [3] (to study the magnetization reversal process of thin films, nanoparticles and nanostructures). Additionally, MFM exhibits high versatility and simplicity in operation.Advanced operation modes based on Variable Field MFM have proven to be very useful to visualize the magnetization reversal process [4] in one dimensional nanostructures (Figure 1). Isolated multisegmented nanowires (120nm in diameter) formed by segments of FeCo with variable length (increasing from 250nm at the left edge to 400nm the right edge) separated by Cu segments of 30nm have been characterized. The reversal process propagates always unidirectionally (ratched effect) irrespectively of the external field direction, due to the broken symmetry induced by the increasing length of the magnetic segments. Moreover, MFM studies allow us to determine the pinning centers (associated to Cu layers) that may vary from scan to scan. The so called 3D MFM imaging is used is these experiments to obtain a non-standard image where the slow scan corresponds to a continuous variation of the external magnetic field.Some emerging aspects of MFM imaging are also tackled in this work. It is worth mentioning the challenging quantitative and accurate interpretation of the MFM images [5], the probe-engineering alternatives [6] (including the analysis of the active role of the probe what could be used to gain information about the sample stray field with ultra-high resolution) or the MFM experiments on complex magnetic configurations as the skyrmionic states [7,8].Variable Field MFM and customized MFM probes [6] (growth by Focused Electron Beam Induced Deposition onto AFM probes) have been used for studying skyrmionic states. Figure 2 shows that permalloy hemispherical nanodots with a diameter of 70 nm and height of 30 nm (prepared by hole mask colloidal lithography) are able to host half-hedgehog spin textures with non-zero topological charge. Notice that they are observed at room temperature, in absence of DMI interaction and they can be further stabilized by the magnetic field arising from the Magnetic Force Microscopy probe. The movement of the structure core depends on its chirality and it is controlled by the tip stray field. Micromagnetic simulations have been used to determine the low energy magnetic configuration compatible with the MFM results.Future perspectives of MFM imaging as the characterization of 2D materials and devices for spintronic or straintronic are also address in this work; namely, the in situ combination of magnetic imaging with magnetoelectric, thermoelectric or thermomagnetic characterization. **