Formation of quasi - solitons in transverse confined ferromagnetic film media

The formation of quasi-2D spin-wave waveforms in longitudinally magnetized stripes of ferrimagnetic film was observed by using timeand spaceresolved Brillouin light scattering technique. In the linear regime it was found that the confinement decreases the amplitude of dynamic magnetization near the lateral stripe edges. Thus, the so-called effective dipolar pinning of dynamic magnetization takes place at the edges. In the nonlinear regime a new stable spin wave packet propagating along a waveguide structure, for which both transversal instability and interaction with the side walls of the waveguide are important was observed. The experiments and a numerical simulation of the pulse evolution show that the shape of the formed waveforms and their behavior are strongly influenced by the confinement. We report on the observation of a new type of a stable, two-dimensional nonlinear spin wave packet propagating in a magnetic waveguide structure and suggest a theoretical description of our experimental findings. Stable two-dimensional spin wave packets, so-called spin wave bullets, were previously observed, however solely in long and wide samples of a thin ferrimagnetic film of yttrium-iron-garnet (YIG) [1, 2, 3], that were practically unEmail address: [email protected] Email address: [email protected] 1 ar X iv :0 70 4. 00 24 v1 [ nl in .P S] 3 1 M ar 2 00 7 bounded in both in-plane directions compared to the lateral size of the spin wave packets and the wavelength of the carrier spin wave. In a waveguide structure, where the transverse dimension is comparable to the wavelength, up to day only quasi one-dimensional nonlinear spin wave objects were observed, which are spin wave envelope solitons. Here a typical system is a narrow (' 1-2mm) stripe of a YIG ferrite film [4, 5]. Both for solitons and bullets the spreading in dispersion is compensated by the longitudinal nonlinear compression. Concerning the transverse dimension, solitons have a cosinelike amplitude distribution due to the lateral confinement in the waveguide, whereas bullets show a transverse nonlinear instability compensating pulse widening due to diffraction and leading to transverse confinement. Here we report on the observation of a new stable spin wave packet propagating along a waveguide structure, for which both transversal instability and interaction with the side walls of the waveguide are important. The experiments were carried out using a longitudinally magnetized long YIG film stripe of 2.5mm width and 7μm thickness. The magnetizing field was 1831Oe. The spin waves were excited by a microwave magnetic field created with a microstrip antenna of 25μm width placed across the stripe and driven by electromagnetic pulses of 20ns duration at a carrier frequency of 7.125GHz. As is well known the backward volume magnetostatic spin wave (BVMSW) [6] excited in the given experimental configuration is able to form both envelope solitons and bullets [4], depending on the geometry. The spatio-temporal behavior of the traveling BVMSW packets was investigated by means of spaceand time-resolved Brillouin light scattering spectroscopy [7]. The obtained results are demonstrated in Fig. 1 where the spatial distributions of the intensity of the spin wave packets are shown for given moments of time. The spin wave packets propagate here from left to right and decay in the course of their propagation along the waveguide because of magnetic loss. The left set of diagrams corresponds to the linear case. The power of the driving electromagnetic wave is 20mW. The right set of diagrams corresponding to the nonlinear case was collected for a driving power of 376mW. Differences between these two cases are clearly observed. First of all the linear spin wave packet is characterized by a cosine-like lateral profile while the cross section of the nonlinear pulse is sharply modified relative to the linear case and has a pronounced bell-like shape. Second, the intensity of the linear packet decays monotonically with time while the intensity of the nonlinear packet initially increases because of its strong transversal compression (see the second diagram from the top in Fig. 1). Both of these nonlinear features provide clear evidence for the develop-