Transient forces between nanoscale objects on surfaces govern friction, viscous flow, and plastic deformation, occur during manipulation of matter, or mediate the local wetting behavior of thin films. To resolve transient forces on the (sub) microsecond time and nanometer length scale, dynamic atomic force microscopy (AFM) offers largely unexploited potential. Full spectral analysis of the AFM signal completes dynamic AFM. Inverting the signal formation process, we measure the time course of the force effective at the sensing tip. This approach yields rich insight into processes at the tip and dispenses with a priori assumptions about the interaction, as it relies solely on measured data. Force measurements on silicon under ambient conditions demonstrate the distinct signature of the interaction and reveal that peak forces exceeding 200 nN are applied to the sample in a typical imaging situation. These forces are 2 orders of magnitude higher than those in covalent bonds.T ime-dependent forces mediate adsorption, ordering phenomena, and visco-elasticity that are important in rheology and tribology, as well as in biology and catalysis. The importance of dynamic aspects becomes obvious when looking at the viscoelastic properties of polymers (1, 2). Even on the level of a single biomolecule under external stress, velocity dependence can be observed: the stability of the molecule increases with the applied force rate (3). However, dynamic forces occurring during processes at surfaces, in thin and confined lubrication films, or on the level of nanoscopic objects are experimentally not easily accessible.In this context, atomic force microscopy (AFM) offers a large potential to investigate and manipulate material at the (sub) microsecond time scales and nanometer length scale. AFM (4) and related techniques (5) have gained increasing importance in many fields of research and industrial applications. Raster scanning the sample with a sharp tip attached to the end of the microfabricated cantilever allows not only the visualization of objects in shape and size of single molecules, but also the ability to touch and squeeze, pull and push them (6). In dynamic force microscopy, the motion of the cantilever is externally modulated. In tapping-mode AFM, the most common dynamic mode, the cantilever is excited to oscillate at its fundamental resonant frequency. Once each oscillation cycle, the tip interacts with the surface, and information about the tip-sample interaction is transferred into the time course of the signal. The signal formation process is depicted in Fig. 1, and the inset shows schematically the setup of tapping-mode AFM.To obtain the acting forces from the dynamics of the oscillating cantilever, there are basically two routes. First, under ultra high vacuum conditions, the change of the resonant frequency of the force-coupled cantilever is used to estimate the interaction potential (7-9). These methods highly depend on the high quality factor of the oscillation under ultra high vacuum. Second, under ambient condi...

In tapping-mode atomic force microscopy, the phase shift between excitation and response of the cantilever is used as a material-dependent signal complementary to topography. The localization of information in the phase signal is demonstrated with 1.4-nm lateral resolution on purple membrane of Halobacterium salinarum in buffer solution. In a first-order approximation, the phase signal is found to correlate with modulations of the tip oscillation amplitude, induced by topography. Extending the analysis to contributions of the tip-sample interaction area as a second-order approximation, a method is proposed to extract information about the interaction from the phase signal for surfaces with a roughness in the order of the tip radius.

We present an experimental analysis of the nonlinear tip–sample interaction in tapping-mode atomic-force microscopy by exploiting anharmonic contributions of the cantilever motion. Two aspects of a concept aiming at a full reconstruction of the tip–sample interaction are demonstrated: higher flexural eigenmode vibrations excited by the impact of the oscillating tip on the sample are used to measure the tip–sample interaction time; by imaging at higher harmonics of the driving frequency material contrast is obtained.

We numerically investigate the interaction between propagating spin waves and a transverse domain wall in a nanowire by using micromagnetic simulations. In order to understand the mechanisms that lead to domain wall motions, we calculate domain wall velocity in a defect-free nanowire and the depinning fields for a pinned domain wall that is depinned in and against the direction of the spin-wave propagation. We find that the physical origin of the spin-wave-induced domain wall motion strongly depends on the propagating spin-wave frequency. At certain spin-wave frequencies, transverse domain wall vibrations lead to transverse wall displacements by the spin waves, while at other frequencies, large spin-wave reflection drives domain wall motion. By analyzing the depinning field calculations, the different underlying physical mechanisms are distinguished.

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape.