Direct determination of large spin-torque nonadiabaticity in vortex core dynamics.

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be ß=0.15±0.07, which is more than an order of magnitude larger than the damping constant α, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.

Damping by slow relaxing rare earth impurities in Ni80Fe20.

Doping Ni80Fe20 by heavy rare earth atoms alters the magnetic relaxation properties of this material drastically. We show that this effect can be well explained by the slow relaxing impurity mechanism. This process is a consequence of the anisotropy of the on site exchange interaction between the 4f magnetic moments and the conduction band. As expected from this model the magnitude of the damping effect scales with the anisotropy of the exchange interaction and increases by an order of magnitude at low temperatures. In addition, our measurements allow us to determine the relaxation time of the 4f electrons as a function of temperature.

Laser-induced magnetization dynamics of lanthanide-doped permalloy thin films.

We investigate the effect of Ho, Dy, Tb, and Gd impurities on the femtosecond laser-induced magnetization dynamics of thin Permalloy films using the time-resolved magneto-optical Kerr effect. Varying the amount of Ho, Dy, Tb content from 0% to 8%, we observe a gradual change of the characteristic demagnetization time constant from approximately 60 to approximately 150 fs. In contrast, Gd concentrations up to 15% do not influence the time scale of the initial photoinduced magnetization loss. We propose a demagnetization mechanism that relies on strong magnetic inertia of the rare-earth dopant which stabilizes the ferrimagnetic ordering and thereby delays the demagnetization.

Longitudinal detection of ferromagnetic resonance using x-ray transmission measurements.

We describe a setup for the x-ray detection of ferromagnetic resonance in the longitudinal geometry using element-specific transmission measurements. Thin magnetic film samples are placed in a static magnetic field collinear with the propagation direction of a polarized soft x-ray beam and driven to ferromagnetic resonance by a continuous wave microwave magnetic field perpendicular to it. The transmitted photon flux is measured both as a function of the x-ray photon energy and as a function of the applied static magnetic field. We report experiments performed on a 15 nm film of doped Permalloy (Ni(73)Fe(18)Gd(7)Co(2)) at the L(3)/L(2)-edges of Fe, Co, and Ni. The achieved ferromagnetic resonance sensitivity is about 0.1 monolayers/square root(Hz). The obtained results are interpreted in the framework of a conductivity tensor based formalism. The factors limiting the sensitivity as well as different approaches for the x-ray detection of ferromagnetic resonance are discussed.