Multiferroicity in an organic charge-transfer salt that is suggestive of electric-dipole-driven magnetism
Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, as for conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic chargetransfer salt, thus representing a new class of multiferroics.We propose a charge-order-driven mechanism leading to electronic ferroelectricity in this material. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.
Sudden slowing down of the charge carrier dynamics at the Mott metal-insulator transition
We investigate the dynamics of correlated charge carriers in the vicinity of the Mott metal–insulator (MI) transition in the quasi-twodimensional organic charge-transfer salt k-(D8-BEDT-TTF)2Cu[N(CN)2]Br by means of fluctuation (noise) spectroscopy. The observed 1/ f -type fluctuations are quantitatively very well described by a phenomenological model based on the concept of non-exponential kinetics. The main result is a correlation-induced enhancement of the fluctuations accompanied by a substantial shift of spectral weight to low frequencies in the vicinity of the Mott critical endpoint. This sudden slowing down of the electron dynamics, observed here in a pure Mott system, may be a universal feature of MI transitions. Our findings are compatible with an electronic phase separation in the critical region of the phase diagram and offer an explanation for the not yet understood absence of effective mass enhancement when crossing the Mott transition.
Magnetization dynamics of a single CrO2 grain
By using micro-Hall magnetometry we have studied the magnetization dynamics of a single, micronsize CrO2 grain. With this techniques we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain's easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps.
The figure shows hysteresis loops measured for magnetic fields applied almost parallel and perpendicular to the easy magnetization direction. Lower right inset: SEM image of the CrO2 grain placed between two Hall crosses (labeled 1 and 2). Upper left inset: Barkhausen jumps corresponding to the displacement of a single domain wall.
For more information, see Appl. Phys. Lett. 97, 042507 (2010).
Magnetic Nanoparticles grown by STM-assited CVD
Magnetically driven electronic phase separation in the semimetallic ferromagnetc EuB6
Combined measurements of fluctuation spectroscopy and weak nonlinear transport of the semimetallic ferromagnet EuB6 reveal unambiguous evidence for magnetically driven electronic phase separation consistent with the picture of percolation of magnetic polarons (MP), which form highly conducting magnetically ordered clusters in a paramagnetic and “poorly conducting” background. These different parts of the conducting network are probed separately by the noise spectroscopy/nonlinear transport and the conventional linear resistivity. We suggest a comprehensive and “universal” scenario for the MP percolation, which occurs at a critical magnetization either induced by ferromagnetic order at zero field or externally appliedmagnetic fields in the paramagnetic region.
For more information, see Phys. Rev. B 86, 184425 (2012).
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