Magnetic Force Microscopy on Nanofibers—Limits and Possible Approaches for Randomly Oriented Nanofiber Mats

Magnetic force microscopy (MFM) belongs to the methods that enable spatially resolved magnetization measurements on common thin-film samples or magnetic nanostructures. The lateral resolution can be much higher than in Kerr microscopy, another spatially resolved magnetization imaging technique, but since MFM commonly necessitates positioning a cantilever tip typically within a few nanometers from the surface, it is often more complicated than other techniques. Here, we investigate the progresses in MFM on magnetic nanofibers that can be found in the literature during the last years. While MFM measurements on magnetic nanodots or thin-film samples can often be found in the scientific literature, reports on magnetic force microscopy on single nanofibers or chaotic nanofiber mats are scarce. The aim of this review is to show which MFM investigations can be conducted on magnetic nanofibers, where the recent borders are, and which ideas can be transferred from MFM on other rough surfaces towards nanofiber mats.

Magnetization Reversal in Concave Iron Nano-Superellipses

Square magnetic nanodots can show intentional or undesired shape modifications, resulting in superellipses with concave or convex edges. Some research groups also concentrated on experimentally investigating or simulating concave nano-superellipses, sometimes called magnetic astroids due to their similarity to the mathematical shape of an astroid. Due to the strong impact of shape anisotropy in nanostructures, the magnetization-reversal process including coercive and reversibility fields can be expected to be different in concave or convex superellipses than that in common squares. Here, we present angle-dependent micromagnetic simulations on magnetic nanodots with the shape of concave superellipses. While magnetization reversal occurs via meander states, horseshoe states or the 180° rotation of magnetization for the perfect square, depending on the angle of the external magnetic field, more complicated states occur for superellipses with strong concaveness. Even apparently asymmetric hysteresis loops can be found along the hard magnetization directions, which can be attributed to measuring minor loops since the reversibility fields become much larger than the coercive fields.

Half-hedgehog spin textures in sub-100 nm soft magnetic nanodots

Permalloy hemispherical nanodots are able to host three-dimensional chiral structures (half-hedgehog spin textures) with non-zero topological charge at room temperature and in absence of DMI interaction.

Vortex lines in a cubic magnetic nanodot: structure and dynamics

Langevin simulations of cubic magnetic nanodots were performed using the Landau-Lifshitz equation with exchange and dipolar interactions. Vortices tend to organize as lines: we establish the structure and dynamics thereof for a large range of the dipolar versus exchange ratio d. These lines tend to be bent and twisted. For large values of the dipolar interaction, a complex network of vortex lines arises. Dynamics evidences low frequency collective gyrotropic motions of vortex lines which maintain their distance during motion.