88 research outputs found
Experimental observation of switching in ferromagnetic nanoscale double disks
We investigated a system of two overlapping ferromagnetic permalloy disks, called a hysteron. For sufficiently small disk diameters the hysteron contains only one vortex which is displaced off the center of the disk hosting it. By swapping the vortex from one side of the disk to the other via an in-plane magnetic field, the magnetization in the appended disk is reversed. The magnetization reversal process based on these off-center magnetic vortex states was theoretically found to have low switching fields, as magnetization reversal does not require wall nucleation. Here we investigate hysterons experimentally by studying magnetization reversal and configuration by means of Lorentz transmission electron microscopy and electron holography. For the smallest hysterons with individual disk diameters below 200 nm we found the peculiar switching scheme suggested recently
Spin structure relation to phase contrast imaging of isolated magnetic Bloch and Neel skyrmions
Magnetic skyrmions are promising candidates for future storage devices with a
large data density. A great variety of materials have been found that host
skyrmions up to the room-temperature regime. Lorentz microscopy, usually
performed in a transmission electron microscope (TEM), is one of the most
important tools for characterizing skyrmion samples in real space. Using
numerical calculations, this work relates the phase contrast in a TEM to the
actual magnetization profile of an isolated Neel or Bloch skyrmion, the two
most common skyrmion types. Within the framework of the used skyrmion model,
the results are independent of skyrmion size and wall width and scale with
sample thickness for purely magnetic specimens. Simple rules are provided to
extract the actual skyrmion configuration of pure Bloch or Neel skyrmions
without the need of simulations. Furthermore, first differential phase contrast
(DPC) measurements on Neel skyrmions that meet experimental expectations are
presented and showcase the described principles. The work is relevant for
material sciences where it enables the engineering of skyrmion profiles via
convenient characterization.Comment: 6 pages, 3 figure
Position controlled self-catalyzed growth of GaAs nanowires by molecular beam epitaxy
GaAs nanowires are grown by molecular beam epitaxy using a self-catalyzed,
Ga-assisted growth technique. Position control is achieved by nano-patterning a
SiO2 layer with arrays of holes with a hole diameter of 85 nm and a hole pitch
varying between 200 nm and 2 \mum. Gallium droplets form preferentially at the
etched holes acting as catalyst for the nanowire growth. The nanowires have
hexagonal cross-sections with {110} side facets and crystallize predominantly
in zincblende. The interdistance dependence of the nanowire growth rate
indicates a change of the III/V ratio towards As-rich conditions for large hole
distances inhibiting NW growth.Comment: 9 pages, 4 figure
Shifting and pinning of a magnetic vortex core in a permalloy dot by a magnetic field
Magnetic pinning in thin films seems to be a major research subject in the near future, as it is involved in all switching processes which include a movement of a domain wall or a magnetic vortex. We used Lorentz transmission electron microscopy and vortex pinning at artificial pinning sites to investigate the pinning behavior of magnetic vortices for the first time with high spatial resolution
Scanning transmission electron microscopy strain measurement from millisecond frames of a direct electron charge coupled device
A high-speed direct electron detection system is introduced to the field of transmission electron microscopy and applied to strain measurements in semiconductor nanostructures. In particular, a focused electron probe with a diameter of 0.5 nm was scanned over a fourfold quantum layer stack with alternating compressive and tensile strain and diffracted discs have been recorded on a scintillator-free direct electron detector with a frame time of 1 ms. We show that the applied algorithms can accurately detect Bragg beam positions despite a significant point spread each 300 kV electron causes during detection on the scintillator-free camera. For millisecond exposures, we find that strain can be measured with a precision of 1.3  × 10−3, enabling, e.g., strain mapping in a 100×100 nm2 region with 0.5 nm resolution in 40 s
Entropy-limited topological protection of skyrmions
Magnetic skyrmions are topologically protected whirls that decay through singular magnetic configurations known as Bloch points. We used Lorentz transmission electron microscopy to infer the energetics associated with the topological decay of magnetic skyrmions far from equilibrium in the chiral magnet Fe1-xCoxSi. We observed that the lifetime tau of the skyrmions depends exponentially on temperature, tau similar to tau(0) exp(Delta E/k(B)T). The prefactor tau(0) of this Arrhenius law changes by more than 30 orders of magnitude for small changes of the magnetic field, reflecting a substantial reduction of the lifetime of skyrmions by entropic effects and, thus, an extreme case of enthalpy-entropy compensation. Such compensation effects, being well known across many different scientific disciplines, affect topological transitions and, thus, topological protection on an unprecedented level
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