H. G. Bauer

Inverse spin Hall effect inNi81Fe19/normal-metal bilayers

Spin pumping in ferromagnets provides a source of pure spin currents. Via the inverse spin Hall effect a spin current is converted into a charge current and a corresponding detectable DC-voltage. The ratio of injected spin current to resulting charge current is given by the spin Hall angle. However, the number of experiments more or less equals the number of different values for spin Hall angles, even for the most studied normal metal platinum. This publication provides a full study of inverse spin Hall effect and anisotropic magnetoresistance for different NiFe(Py) / normal metal bilayers using a coplanar waveguide structure. Angle and frequency dependent measurements strongly suggest that spin pumping and inverse spin Hall effect can be used to quantify spin Hall angles only if certain conditions are met. Ruling out the anisotropic magnetoresistance as a parasitic voltage generating effect measurements of the inverse spin Hall effect in Py/Pt and Py/Au yield spin Hall angles of 0.09 and 0.008 respectively. Furthermore, DC-voltages at ferromagnetic resonance for Py/Pt are studied as a function of temperature and the results are compared to theoretical models.

Nonlinear spin-wave excitations at low magnetic bias fields

Nonlinear magnetization dynamics is essential for the operation of numerous spintronic devices ranging from magnetic memory to spin torque microwave generators. Examples are microwave-assisted switching of magnetic structures and the generation of spin currents at low bias fields by high-amplitude ferromagnetic resonance. Here we use X-ray magnetic circular dichroism to determine the number density of excited magnons in magnetically soft Ni80Fe20 thin films. Our data show that the common model of nonlinear ferromagnetic resonance is not adequate for the description of the nonlinear behaviour in the low magnetic field limit. Here we derive a model of parametric spin-wave excitation, which correctly predicts nonlinear threshold amplitudes and decay rates at high and at low magnetic bias fields. In fact, a series of critical spin-wave modes with fast oscillations of the amplitude and phase is found, generalizing the theory of parametric spin-wave excitation to large modulation amplitudes.

Low-amplitude magnetic vortex core reversal by non-linear interaction between azimuthal spin waves and the vortex gyromode

We show, by experiments and micromagnetic simulations in vortex structures, that an active “dual frequency” excitation of both the sub-GHz vortex gyromode and multi-GHz spin waves considerably changes the frequency response of spin wave mediated vortex core reversal. Besides additional minima in the switching threshold, a significant broadband reduction of the switching amplitudes is observed, which can be explained by non-linear interaction between the vortex gyromode and the spin waves. We conclude that the well known frequency spectra of azimuthal spin waves in vortex structures are altered substantially, when the vortex gyromode is actively excited simultaneously.

Coupling of spinwave modes in wire structures

We investigate the magnon dispersion of spinwaves propagating in a Permalloy stripe and compare it to a plain film. We use a Scanning Kerr Microscope to directly image their spatial propagation- and decay properties as well as micromagnetic simulations to extract additional information about the static magnetization distribution. For low external fields, we find anti-crossings for the dispersion branches corresponding to different transverse wave vectors that also occur as minima of the decay length for the spinwave.

k-vector distribution of magneto-static spin waves excited by micro-fabricated antenna structures

In this article we examine the k-vector distribution of magneto-static spin waves which are excited by microfabricated antenna structures. Changing the repetition of the antenna structures from a simple coplanar wave guide like structure to a meandering type antenna the k-vector distribution can be adjusted to a more or less peaked distribution representing the pitch of the meander. However, intrinsic damping of the magnetic system counteracts this effort.