C. Hassel

Element-Specific Magnetic Hysteresis of Individual 18 nm Fe Nanocubes

Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies.

Observation of ultrafast lattice heating using time resolved electron diffraction

We use ultrafast electron diffraction to study lattice heating of 20nm noble metal films after femtosecond optical excitation with moderate excitation fluences. Using the Debye–Waller effect, the rise times of the lattice temperature were measured to be 1.1ps in copper (5.9mJ∕cm2 incident fluence) and 4.7ps in gold (0.9mJ∕cm2).

Magnetic characterization of iron nanocubes

Nearly perfect single crystalline Fe core-shell nanocubes with (100) facets and 13.6 nm edge length were prepared by wet-chemical methods. While the core is metallic, the shell is composed of either Fe3O4 or γ-Fe2O3. The cubes were deposited onto GaAs substrates with monolayer coverage as proved by scanning electron microscopy. Oxygen and hydrogen plasmas were used to remove the ligand system and the oxide shell. Both types of samples were investigated by ferromagnetic resonance. While the g-factor (g=2.09) and crystalline anisotropy (K4=4.8×104 J/m3) of the pure iron cubes show up with bulk values, the saturation magnetization is reduced to (M(5K)=(1.2±0.12)×106 A/m) 70% of bulk value and the effective damping parameter (α=0.03) is increased by one order of magnitude with respect to bulk Fe.

Visualization of spin dynamics in single nanosized magnetic elements

The design of future spintronic devices requires a quantitative understanding of the microscopic linear and nonlinear spin relaxation processes governing the magnetization reversal in nanometer-scale ferromagnetic systems. Ferromagnetic resonance is the method of choice for a quantitative analysis of relaxation rates, magnetic anisotropy and susceptibility in a single experiment. The approach offers the possibility of coherent control and manipulation of nanoscaled structures by microwave irradiation. Here, we analyze the different excitation modes in a single nanometer-sized ferromagnetic stripe. Measurements are performed using a microresonator set-up which offers a sensitivity to quantitatively analyze the dynamic and static magnetic properties of single nanomagnets with volumes of (100 nm)(3). Uniform as well as non-uniform volume modes of the spin wave excitation spectrum are identified and found to be in excellent agreement with the results of micromagnetic simulations which allow the visualization of the spatial distribution of these modes in the nanostructures.

Influence of growth parameters on the perpendicular magnetic anisotropy of [Co/Ni] multilayers and its temperature dependence

[Co/Ni] multilayer films are grown in various conditions to study the influence of growth parameters on the anisotropy of the films in order to optimize the perpendicular uniaxial anisotropy. These multilayers are expected to be useful for current induced magnetization switching experiments due to its softmagnetic behavior by which the critical current density can be reduced dramatically. The polycrystalline films are prepared by electron beam evaporation with varying buffer layer, cap layer, as well as Co layer thickness and number of repetitions of Co and Ni layers. Measurements of the magneto-optic Kerr effect, ferromagnetic resonance, and superconducting quantum interference device magnetometry are used to analyze the magnetic anisotropy and magnetization. Depending on the magnitude of the uniaxial anisotropy, the overall easy direction of magnetization is orientated either in plane or out of plane depending on the amplitude of the demagnetization field. Moreover, we present the results for the temper...

Magnetic imaging with femtosecond temporal resolution

A scanning Kerr microscope with a temporal resolution of <230 fs and a spatial resolution of 210 nm is presented. Equipped with a large temporal and spatial scanning range of 8 ns and 320 microm, respectively, the microscope allows studying nonuniform magnetization dynamics on many different time scales over a large area. For demonstration, we study the magnetization dynamics in Fe/Gd multilayer dot arrays exhibiting a spin reorientation transition (SRT) on three different time scales, namely, femtosecond, picosecond, and nanosecond scales. The dynamics on all time scales varies from one dot to another. This is attributed to the high sensitivity of the SRT to the variations of the layer thicknesses and the Fe/Gd interface structure.

Shape, orientation, and crystalline composition of silver islands on Si(111)

Photoemission electron microscopy and spot profile analyzing low-energy electron diffraction have been used to study the temperature-dependent growth of Ag islands on a Si(111) surface. Depending on growth temperature, various island shapes can be formed. At low temperatures, polygonic islands are formed, consisting of both Ag(001) and Ag(111) crystal orientations. At higher temperatures, islands consist mostly of Ag(111) orientation and are predominantly of triangular shape. As the islands grow, it is possible that the crystalline composition of an island changes. We observed that Ag(001)-oriented areas convert into areas of Ag(111) orientation. The rotational orientation of the Ag islands with respect to the substrate is explained by a modified coincidence-site lattice approach.

Monopolelike probes for quantitative magnetic force microscopy: Calibration and application

A local magnetization measurement was performed by magnetic force microscopy (MFM) to determine magnetization in domains of an exchange coupled [Co/Pt]/Co/Ru multilayer with predominant perpendicular anisotropy. The quantitative MFM measurements were conducted with an iron-filled carbon nanotube tip, which is shown to behave like a monopole. As a result we determined an additional in-plane magnetization component of the multilayer, which is explained by estimating the effective permeability of the sample within the μ∗-method.

Tailoring Spin Relaxation in Thin Films by Tuning Extrinsic Relaxation Channels

The importance of extrinsic spin relaxation processes such as magnon-magnon scattering for the overall damping in ferromagnetic thin films has been shown for low relaxation rate systems. Due to its anisotropic behavior, it offers an opportunity for controlling and tailoring the spin relaxation. In this paper, a ferromagnetic resonance study of a system with pure Gilbert damping Fe94.5Si5.5/MgO(001) is shown. Fe3Si/MgO(001) systems with native magnon-magnon scattering are discussed. Possibilities for inducing magnon-magnon scattering by volume and surface defects in these systems are presented, offering a method for controlled tailoring of the overall damping in thin films.

Transient (000)-order attenuation effects in ultrafast transmission electron diffraction

We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films.