A. C. Komarek

Large anomalous Hall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn3Ge

A large anomalous Hall effect is observed in the triangular antiferromagnet Mn3Ge arising from an intrinsic electronic Berry phase. It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is ~500 (ohm·cm)−1 at 2 K and ~50 (ohm·cm)−1 at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual in-plane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (ħ/e) (ohm·cm)−1, comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effect–based data storage devices.

k=0 Magnetic Structure and Absence of Ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675  K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

Non-collinear magnetic structure of manganese quadruple perovskite CdMn7O12

We report on the magnetic structure of CdMn7O12 determined by powder neutron diffraction. We were able to measure the magnetic structure of this Cd containing and highly neutron absorbing material by optimizing the sample geometry and by blending the CdMn7O12 with Aluminum powder. Below its Néel temperature TN1 all magnetic reflections can be indexed by a single commensurate propagation vector k = (0, 0, 1). This is different to the case of CaMn7O12 where the propagation vector is incommensurate and where an in-plane helical magnetic structure has been found. We observe a commensurate non-collinear magnetic structure in CdMn7O12 with in-plane aligned magnetic moments resembling the ones in CaMn7O12. However, the commensurate propagation vector prevents the appearance of a helical magnetic structure in CdMn7O12. Finally, we also observe a third structural phase transition below ~60 K that can be attributed to phase separation.

Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

Verwey transition in Fe3O4 thin films: Influence of oxygen stoichiometry and substrate-induced microstructure

We have carried out a systematic experimental investigation to address the question why thin films of

Hour-glass magnetic spectrum in a stripeless insulating transition metal oxide

{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}

Antiferromagnetic correlations in the metallic strongly correlated transition metal oxide LaNiO3

(magnetite) generally have a very broad Verwey transition with lower transition temperatures as compared to the bulk. We observed using x-ray photoelectron spectroscopy, x-ray diffraction, and resistivity measurements that the Verwey transition in thin films is drastically influenced not only by the oxygen stoichiometry but especially also by the substrate-induced microstructure. In particular, we found (1) that the transition temperature, the resistivity jump, and the conductivity gap of fully stoichiometric films greatly depends on the domain size, which increases gradually with increasing film thickness, (2) that the broadness of the transition scales with the width of the domain size distribution, and (3) that the hysteresis width is affected strongly by the presence of antiphase boundaries. Films grown on MgO (001) substrates showed the highest and sharpest transitions, with a 200 nm film having a

Oxyhalides: A new class of high-TC multiferroic materials

{\mathit{T}}_{V}

Fe3O4 thin films: controlling and manipulating an elusive quantum material

of 122 K, which is close to the bulk value. Films grown on substrates with large lattice constant mismatch revealed very broad transitions, and yet all films show a transition with a hysteresis behavior, indicating that the transition is still first order rather than higher order.

Electronic and spin states of SrRuO3 thin films: An x-ray magnetic circular dichroism study

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hour-glass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La(2-x)Sr(x)CoO(4). The search for charge stripe ordering reflections yields no evidence for charge stripes in La(1.6)Sr(0.4)CoO(4), which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.