Jesco Topp

Spin-wave interference in three-dimensional rolled-up ferromagnetic microtubes.

We have investigated spin-wave excitations in rolled-up Permalloy microtubes using microwave absorption spectroscopy. We find a series of quantized azimuthal modes which arise from the constructive interference of Damon-Eshbach-type spin waves propagating around the circumference of the microtubes, forming a spin-wave resonator. The mode spectrum can be tailored by the tubes radius and number of rolled-up layers.

Spin wave diffraction and perfect imaging of a grating.

We study the diffraction of Damon-Eshbach-type spin waves incident on a one-dimensional grating realized by microslits in a thin Permalloy film. By means of time-resolved scanning Kerr microscopy, we observe unique diffraction patterns behind the grating which exhibit replications of the spin wave field at the slits. We show that these spin wave images, with details finer than the wavelength of the incident Damon-Eshbach spin wavelength, arise from the strongly anisotropic spin wave dispersion.

Spin-wave confinement in rolled-up ferromagnetic tubes

We investigate the spin-wave dispersion in rolled-up Permalloy microtubes based on self-rolling strained semiconductor layers. Using microwave absorption spectroscopy we find that these structures exhibit a characteristic spin-wave mode spectrum. The magnetization and spin-wave resonance at zero external magnetic field is determined by curvature induced dynamic demagnetization fields. At high magnetic fields transverse to the tube axis, the three-dimensional shape anisotropy of the tube results in spin-wave confinement in well-defined regions along the tube perimeter.

Reprogrammable magnonic crystals formed by interacting ferromagnetic nanowires

Spin-wave (SW) modes are addressed which are confined in thin individual Ni80Fe20 nanowires with widths ranging from 220 to 360 nm. In periodic arrays with an edge-to-edge separation of down to 100 nm, confined modes of neighboring nanowires are found to couple coherently and form allowed minibands and forbidden frequency gaps. This gives rise to a one-dimensional magnonic crystal. We present all-electrical SW spectroscopy data and micromagnetic simulations. We find that the nanowire arrays allow us to reprogram the relevant magnonic band structure via the magnetic history. A forbidden frequency gap of up to about 1 GHz is controlled by an in-plane magnetic field being as small as a few mT.

Spin waves and domain wall modes in curved magnetic nanowires

The confinement of spin waves in inhomogeneous fields and spin wave interaction with domain walls has attracted interest due to possible applications in magnonics. We investigate spin waves in curved ferromagnetic nanowires. The field dispersion and localization of spin waves is revealed by comparison to known modes in stripes and taking into account the specific field reversal of the curved wire. In small wires we find a strongly altered mode spectrum in a certain field regime. Micromagnetic simulations show an extended domain wall within the wire in this field region. The domain wall shows several dynamic modes and changes the remaining spin wave modes. We find mode suppression as well as newly arising modes due to the strong inhomogenous internal field of the wall.

Formation and control of internal spin-wave channels in arrays of densely packed Permalloy nanowires

Broadband spin-wave spectroscopy is performed on two arrays of long 300-nm-wide Ni20Fe80 wires which exhibit two different edge-to-edge separations a of 700 and 200nm. When the in-plane field H is applied a few degrees off from the hard axis direction, an intermediate field regime is found where the central region and the edges of a wire are magnetized in different directions. Here, resonances are pronounced and reflect spin waves confined on the sub-100nm scale within a wire. For small a, the confinement effect is found to occur over a broad regime of H. These results are relevant for high-density integration of magnonic waveguides.