Scanning probe microscopy of ferroic surfaces
Prof. Dr. Matthias Bode (Institut für Physik, Univ. Würzburg)

Because of their application in memory devices collective ordering phenomena, such as ferromagnetism or ferroelectricity, have recently attracted considerable interest. In my talk I will present scanning probe microscopy studies that help to unravel some nano-scale properties of ferroic surfaces. <p> In the first part I will introduce spin-resolved STM, which allows the direct imaging of the atomic scale, spin structure of surfaces. These studies reveal an unexpected variety of relatively complex magnetic structures. For example, an anitferromagnetic spin cycloid with a period of about 12 nm was observed in a Mn thin film. In combination with density functional theory calculations these data indicate that spin-orbit coupling through the so-called Dzyaloshinskii–Moriya interaction is the driving force [1]. Temperature and size dependent measurements on Mn nanoribbons reveal that the onset of the antiferromagnetic order is closely related to the ribbon width, with a decreasing Néel temperature as one moves from a 2D toward a quasi-1D system [2]. <p> In the second part on ferroelectric HoMnO3 room temperature conductive atomic force microscopy will be correlated with low temperature electrostatic force microscopy. The data reveal that the nano-scale electric conduction between the tip and the ferroelectric surface is intrinsically modulated by the polarization of ferroelectric domains. Local conductance spectra indicate that the electric conduction is described by polarization-induced Schottky-like rectification at low bias, but dominated by a space-charge limited conduction mechanism at high bias. This observation demonstrates visualization of ferroelectric domain structure by electric conduction, which may be used for nondestructive readout of nano-scale ferroelectric memories or ferroelectric sensors.
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[1] M. Bode et al., Nature 447, 190 (2007).

[2] P. Sessi et al., Phys. Rev. Lett. 103, 167201 (2009).

[3] W. Wu et al., Phys. Rev. Lett. 104, 217601 (2010).