The microscopic origin of ultrafast modification of the ratio between the symmetric (J) and antisymmetric (D) exchange interaction in antiferromagnetic iron oxides is revealed, using femtosecond laser excitation as a pump and terahertz emission spectroscopy as a probe. By tuning the photon energy of the laser pump pulse we show that the effect of light on the D/J ratio in two archetypical iron oxides FeBO3 and ErFeO3 is maximized when the photon energy is in resonance with a spin and parity forbidden d-d transition between the crystal-field split states of Fe3+ ions. The experimental findings are supported by a multielectron model, which accounts for the resonant absorption of photons by Fe3+ ions. Our results reveal the importance of the parity and spin-change forbidden, and therefore often underestimated, d-d transitions in ultrafast optical control of magnetism.

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To know the properties of a particle or a wave, one should measure how its energy changes with its momentum. The relation between them is called the dispersion relation, which encodes essential information of the kinetics. In a magnet, the wave motion of atomic spins serves as an elementary excitation, called a spin wave, and behaves like a fictitious particle. Although the dispersion relation of spin waves governs many of the magnetic properties, observation of their entire dispersion is one of the challenges today. Spin waves whose dispersion is dominated by magnetostatic interaction are called pure-magnetostatic waves, which are still missing despite of their practical importance. Here, we report observation of the band dispersion relation of pure-magnetostatic waves by developing a table-top all-optical spectroscopy named spin-wave tomography. The result unmasks characteristics of pure-magnetostatic waves. We also demonstrate time-resolved measurements, which reveal coherent energy transfer between spin waves and lattice vibrations.

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Control over the organization of assemblies from molecular dimensions up to the macroscopic length scale is an outstanding challenge in science, above all for materials in high dilution. Instead of inducing order by generating very long and stiff structures, an alternative approach is studied: a two-component assembly of a semiflexible polymer with a (self-assembling) chromonic liquid crystal. By following the structure formation in time using different techniques, a mechanistic model is proposed that explains how such unusually well-defined materials can be created from flexible components. It is concluded that at this very low concentration (>99.6% water), these macroscopically organized structures can only be formed when the energies between different assembly states and their interconversion rates are properly balanced. This may, however, be in reach for a wide range of materials, which makes this a generic route toward high definition at low concentration without the need for long and rigid building blocks.@en

The macroscopic consequences of displacive modulation on the morphology of incommensurate single crystals are confirmed. Bands of faces in the neighborhood of stable normal crystal faces have been observed on spherically shaped Rb2ZnBr4 crystals. An interpretation is given in terms of classical morphological theory extended to include (four-dimensional) superspace group symmetry. This leads to the view that the formation of these bands involves, at least partly, so-called satellite faces and gives a simple explanation of why the set of bands has a lower point-group symmetry than the set of normal faces.@en