Skyrmions are topologically stable vortex-like objects with a swirling spin configuration which

International audienceBarium ruthenate Ba4Ru3O10, in which Ru3O12 trimers are connected together to form a checkered twodimensional framework, has been synthesized and its structural, magnetic, and transport properties studied between 300 and 2 K. The paramagnetic to antiferromagnetic transition at TN ≈ 105 K evidenced on the susceptibility curve coincides with an increase of electron localization in transportmeasurements. Thermoelectric power and Hall coefficientmeasurements both exhibit dramatic changes at TN, characteristic of a reconstruction of the bands structure near the Fermi level. No pronounced structural changes are observed at TN in this compound. The magnetic scattering signal on the neutron powder diffraction patterns below TN is weak but can be tentatively modeled with an antiferromagnetic ordering of the spins at both ends of a trimer, the spin of the more symmetric Ru site remaining idle. Crystal field and strong spin-orbit coupling at the Ru4+ site seem to be the key parameters to understand the magnetic state of Ba4Ru3O10

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We explore the field-temperature phase diagram of the XY pyrochlore antiferromagnet Er2Ti2O7 by means of magnetization and neutron diffraction experiments. Depending on the field strength and direction relative to the high symmetry cubic directions [001], [110] and [111], the refined field induced magnetic structures are derived from the zero field ψ2 and ψ3 states of the Γ5 irreducible representation which describes the ground state of XY pyrochlore antiferromagnets. At low field, domain selection effects are systematically at play. In addition, for [001], a phase transition is reported towards a ψ3 structure at a characteristic field H 001 c = 43 mT. For [110] and [111], the spins are continuously tilted by the field from the ψ2 state, and no phase transition is found while domain selection gives rise to sharp anomalies in the field dependence of the Bragg peaks intensity. For [110], these results are confirmed by high resolution inelastic neutron scattering experiments, which in addition allow us to determine the field dependence of the spin gap. This study agrees qualitatively with the scenario proposed theoretically by Maryasin et al. [Phys. Rev. B 93, 100406(R) (2016)], yet the strength of the field induced anisotropies is significantly different from theory. arXiv:1701.07220v2 [cond-mat.str-el]

Fe32.1Ge33As2 and Fe32.5Ge32P3 demonstrate complex commensurate antiferromagnetic ordering below 125 K accompanied by a spin reorientation upon further cooling.