A piece of iron is usually not a magnet with a north and a south pole, such as a compass needle. This is because the internal magnetization decays in domains (Weissian districts) in order to minimize the stray field. Only by applying a static external field the magnetization can be aligned. However, iron particles in the nanometer range can be magnetically saturated when the excitation occurs with an alternating field in the GHz range (microwaves).
Surprisingly, the magnetic sample is magnetized perpendicular to the direction of the magnetic alternating field. Even more exciting is the observation that by the external excitation the order parameter is increased, although the general trend is opposite. An open system with continuous energy dissipation can, however, show local order. Life on Earth is a striking proof of this
The samples we used were Ni80Fe20 cuboids measuring 16μm x 32μm x 10nm on a coplanar waveguide (Fig. 1). The time-dependent magnetization was recorded with photoelectron emission microscopy (PEEM) under stroboscopic illumination (see Fig. 2). The gray value corresponds to the horizontal component of the magnetization vector. A vertical 180 ° domain wall separates two large domains. Within the domain wall the magnetization is shown to the left and therefore appears black. The system is similar to an oscillator driven by a periodic disturbance. The central domain wall shifts to the right. The essential mechanism is the lowering of the resonant frequency in the left domain. This leads to a dynamic adaptation of the resonant frequency to the excitation and thus increases the energy dissipation. The energy change due to the wall displacement can also be described as a force acting on the domain wall.