arXiv subjects

Manfred Albrecht

Publications and source records attributed to Manfred Albrecht.

43 records · Page 3Linked to original sources

arXiv2019

Indications for Dzyaloshinskii-Moriya Interaction at the Pd/Fe Interface Studied by \textit{In Situ} Polarized Neutron Reflectometry

Using \textit{in situ} polarized neutron reflectometry, the depth resolved evolution of the magnetism and structure in a Pd/Fe/Pd trilayer thin-film is measured during growth. The initial film structure of Pd/Fe shows a small proximity induced magnetism in the underlayer and a magnetization in the Fe layer of $\approx1.6$\,$μ_{\text{B}}$ per Fe atom, less than the expected bulk value of $2.2$\,$μ_{\text{B}}$. Deposition of the Pd capping layer initially follows an island-like growth mode with subsequent coalescence. With increasing Pd deposition the Fe moment and the proximity-induced magnetism in the Pd capping layer decrease. After final deposition of the Pd capping layer, the magnetic profile is structurally and magnetically symmetric across the Fe layer, with magnetism induced in Pd up to 0.92 \,nm from the interface. Throughout the Pd deposition the Pd/Fe/Pd trilayer structure is becoming increasingly symmetric, a fact which points to a Dzyaloshinskii-Moriya interaction as a likely cause of the observed magnetic behavior.

cond-mat.mtrl-sci↗

arXiv2017

Trapping ultracold atoms at 100 nm from a chip surface in a 0.7-micrometer-period magnetic lattice

We report the trapping of ultracold 87Rb atoms in a 0.7 micron-period 2D triangular magnetic lattice on an atom chip. The magnetic lattice is created by a lithographically patterned magnetic Co/Pd multilayer film plus bias fields. Rubidium atoms in the F=1, mF=-1 low-field seeking state are trapped at estimated distances down to about 100 nm from the chip surface and with calculated mean trapping frequencies as high as 800 kHz. The measured lifetimes of the atoms trapped in the magnetic lattice are in the range 0.4 - 1.7 ms, depending on distance from the chip surface. Model calculations suggest the trap lifetimes are currently limited mainly by losses due to surface-induced thermal evaporation following loading of the atoms from the Z-wire trap into the very tight magnetic lattice traps, rather than by fundamental loss processes such as surface interactions, three-body recombination or spin flips due to Johnson magnetic noise. The trapping of atoms in a 0.7 micrometer-period magnetic lattice represents a significant step towards using magnetic lattices for quantum tunneling experiments and to simulate condensed matter and many-body phenomena in nontrivial lattice geometries.

physics.atom-ph↗

arXiv2017

Nanoscale Magnetic Imaging using Circularly Polarized High-Harmonic Radiation

This work demonstrates nanoscale magnetic imaging using bright circularly polarized high-harmonic radiation. We utilize the magneto-optical contrast of worm-like magnetic domains in a Co/Pd multilayer structure, obtaining quantitative amplitude and phase maps by lensless imaging. A diffraction-limited spatial resolution of 49 nm is achieved with iterative phase reconstruction enhanced by a holographic mask. Harnessing the unique coherence of high harmonics, this approach will facilitate quantitative, element-specific and spatially-resolved studies of ultrafast magnetization dynamics, advancing both fundamental and applied aspects of nanoscale magnetism.

physics.optics↗

arXiv2016

Stirrers and movers actuated by oscillating fields

Locomotion via cyclic moves presents a challenge to mesoscopic objects in overdamped environments, where time reversibility may prevent directed motion. Most reported cyclic movers exploit anisotropic drag to push themselves forward. Under an oscillating drive, however, anisotropic drag enables locomotion only if the objects can change their shape. Here, we present a strategy that unexpectedly enables structurally invariant objects to move under oscillating fields. The objects are self-assembled clusters of magnetic particles that exhibit an off-centered dipole moment. By theoretical modeling and in experiments with magnetic Janus particles, we demonstrate that the interaction between such anisotropic particles in the cluster breaks time reversibility. Experimentally, we show that the magnetic configuration of a cluster determines its motion path. We realize stirrers and steerable movers with helical or directed path using the same particle system. The presented strategy based on internal interactions establishes a counterpart to locomotion via anisotropic drag.

cond-mat.soft↗

arXiv2016

Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media

We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling $1.4\,\mathrm{Tb/in}^2$ bit patterned media. Both contributions are computed for different thickness of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd($5\,\mathrm{nm}$)/FePt($5\,\mathrm{nm}$) shows a bit error rate of $10^{-4}$ with a write field of $1.16\,\mathrm{T}$.

physics.comp-ph↗

arXiv2016

Micromagnetic simulation of exchange coupled ferri-/ferromagnetic composite in bit patterned media

Ferri-/ferromagnetic exchange coupled composites are promising candidates for bit patterned media because of the ability to control the magnetic properties of the ferrimagnet by its composition. A micromagnetic model for the bilayer system is presented where we also incorporate the microstructural features of both layers. Micromagnetic finite element simulations are performed to investigate the magnetization reversal behaviour of such media. By adding the exchange coupled ferrimagnet to the ferromagnet, the switching field could be reduced by up to $40\,\%$ and also the switching field distribution is narrowed. To reach these significant improvements, an interface exchange coupling strength of $2\,\mathrm{mJ/m^2}$ is required.

physics.comp-ph↗

arXiv2012

Femtosecond nonlinear ultrasonics in gold probed with ultrashort surface plasmons

Fundamental interactions induced by lattice vibrations on ultrafast time scales become increasingly important for modern nanoscience and technology. Experimental access to the physical properties of acoustic phonons in the THz frequency range and over the entire Brillouin zone is crucial for understanding electric and thermal transport in solids and their compounds. Here, we report on the generation and nonlinear propagation of giant (1 percent) acoustic strain pulses in hybrid gold/cobalt bilayer structures probed with ultrafast surface plasmon interferometry. This new technique allows for unambiguous characterization of arbitrary ultrafast acoustic transients. The giant acoustic pulses experience substantial nonlinear reshaping already after a propagation distance of 100 nm in a crystalline gold layer. Excellent agreement with the Korteveg-de Vries model points to future quantitative nonlinear femtosecond THz-ultrasonics at the nano-scale in metals at room temperature.

physics.optics↗