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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Ships sailing in ice require a propeller that is able to endure both extreme loads and fatigue loads and operate efficiently in ice and open water. Knowledge and descriptions of the physical processes of propeller-ice interaction are essential to model the interaction with its dominant parameters and finally predict the loads. The research described in this paper uses an experimental setup to determine if the crushing strength of ice, or in general a solid, is a dominant parameter in propeller-ice interaction as stated in empirical and theoretical models. Warm model ice, a paraffin based material to be used at room temperature, with ex-situ tested crushing strength, density and elasticity, is supplied to an in-situ model propeller at different rpms. One blade of the propeller is equipped with a six-component load sensor. Impacts are recorded in the time domain and synchronised with high speed footage. The data is analysed to understand and explain the impact behaviour by comparing it with rotational speed, load and footage. Scaling of the warm model ice properties is discussed as well due to density differences between warm model ice and sea ice.@en

To predict loads on propellers in ice, model tests can be used. Using regular (refrigerated) cold model ice in ice basins is a valid option. However, these tests are expensive, difficult to reproduce and bound to time and location, due to the required cooling in ice model basins. An alternative would be to use warm model ice, a material with the properties of model ice at room temperature. This paper proposes one variety, using only materials available from DIY stores. Based on theoretical propeller-ice interaction models, it is assumed that the loads come from a crushing process. Hence, the compressive strength follows as dominant material property of ice. To match compressive strength of weak cold model ice, a large particle composite is proposed. Expanded Polystyrene (EPS) beads are used as particles, with paraffin as matrix to produce warm ice specimens. The compressive strength of these specimens were measured with a uniaxial compression test and matched with weak model ice. The specimens were designed for in-situ use in model scale propeller impact tests.@en