Jan Podbielski

Hysteresis and control of ferromagnetic resonances in rings

The spin dynamics in narrow ferromagnetic rings is studied in the frequency range from 45MHzto20GHz at room temperature. Our broadband spectrometer allows us to monitor the ferromagnetic resonance of characteristic spin configurations as a function of an external field μ0H. We observe hysteresis and irreversible jumps of the resonance frequencies which we attribute to onion-to-vortex and vortex-to-reversed-onion transitions.

Stress dependence of ferromagnetic resonance and magnetic anisotropy in a thin NiMnSb film on InP(001)

The authors have investigated the effect of stress on the room-temperature magnetism of a 10nm thick Heusler-alloy film NiMnSb grown epitaxially on InP. Tensile stress, which the authors applied externally along the easy axis using a piezostack, is shown to reduce the coercivity and the anisotropy coefficient. Using broadband ferromagnetic resonance measurements, the authors find that a relative variation in the lattice constant as small as 0.08% changes the in-plane uniaxial anisotropy by 20%. This explains the significant variation of anisotropy with film thickness, which was recently reported for NiMnSb.

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.

Multiple ferromagnetic resonance in mesoscopic permalloy rings

Using a broadband microwave spectrometer we have investigated the magnetization dynamics of mesoscopic ferromagnetic disks and rings in the frequency range from 45MHzto20GHz. We studied permalloy rings with an outer diameter of 2μm and different ring widths w. In the mesoscopic devices we find several ferromagnetic resonance (FMR) lines which shift with the in-plane magnetic field Hb. Among them, two pronounced FMR modes are identified. We attribute these modes to localized spin waves in different ring segments where the characteristic resonance frequency either decreases or increases as a function of w, depending on the orientation of the ring segment with respect to Hb.

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.