3,106 research outputs found
Heusler 4.0: Tunable Materials
Heusler compounds are a large family of binary, ternary and quaternary
compounds that exhibit a wide range of properties of both fundamental and
potential technological interest. The extensive tunability of the Heusler
compounds through chemical substitutions and structural motifs makes the family
especially interesting. In this article we highlight recent major developments
in the field of Heusler compounds and put these in the historical context. The
evolution of the Heusler compounds can be described by four major periods of
research. In the latest period, Heusler 4.0 has led to the observation of a
variety of properties derived from topology that includes: topological metals
with Weyl and Dirac points; a variety of non-collinear spin textures including
the very recent observation of skyrmions at room temperature; and giant
anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic
structures. Here we give a comprehensive overview of these major achievements
and set research into Heusler materials within the context of recent emerging
trends in condensed matter physics
Micromagnetic simulations of spinel ferrite particles
This paper presents the results of simulations of the magnetization field
{\it ac} response (at to GHz) of various submicron ferrite particles
(cylindrical dots). The ferrites in the present simulations have the spinel
structure, expressed here by MZnFeO (where M stands for a
divalent metal), and the parameters chosen were the following: (a) for : M
= \{ Fe, Mn, Co, Ni, Mg, Cu \}; (b) for : M = \{ Fe, Mg \} (mixed
ferrites). These runs represent full 3D micromagnetic (one-particle) ferrite
simulations. We find evidences of confined spin waves in all simulations, as
well as a complex behavior nearby the main resonance peak in the case of the M
= \{ Mg, Cu \} ferrites. A comparison of the and cases for fixed
M reveals a significant change in the spectra in M = Mg ferrites, but only a
minor change in the M = Fe case. An additional larger scale simulation of a
by particle array was performed using similar conditions of the FeO
(magnetite; , M = Fe) one-particle simulation. We find that the main
resonance peak of the FeO one-particle simulation is disfigured in the
corresponding 3 by 3 particle simulation, indicating the extent to which
dipolar interactions are able to affect the main resonance peak in that
magnetic compound.Comment: 35 pages, 11 figures, Journal of Magnetism and Magnetic Materials, in
press
Proximity effect of vanadium on spin-density-wave magnetism in Cr films
The spin-density wave (SDW) state in thin chromium films is well known to be
strongly affected by proximity effects from neighboring layers. To date the
main attention has been given to effects arising from exchange interactions at
interfaces. In the present work we report on combined neutron and synchrotron
scattering studies of proximity effects in Cr/V films where the boundary
condition is due to the hybridization of Cr with paramagnetic V at the
interface. We find that the V/Cr interface has a strong and long-range effect
on the polarization, period, and the N\'{e}el temperature of the SDW in rather
thick Cr films. This unusually strong effect is unexpected and not predicted by
theory.Comment: 7 figure
Magnetic properties of submicron Co islands and their use as artificial pinning centers
We report on the magnetic properties of elongated submicron magnetic islands
and their influence on a superconducting film. The magnetic properties were
studied by magnetization hysteresis loop measurements and scanning-force
microscopy. In the as-grown state, the islands have a magnetic structure
consisting of two antiparallel domains. This stable domain configuration has
been directly visualized as a 2x2-checkerboard pattern by magnetic-force
microscopy. In the remanent state, after magnetic saturation along the easy
axis, all islands have a single-domain structure with the magnetic moment
oriented along the magnetizing field direction. Periodic lattices of these Co
islands act as efficient artificial pinning arrays for the flux lines in a
superconducting Pb film deposited on top of the Co islands. The influence of
the magnetic state of the dots on their pinning efficiency is investigated in
these films, before and after the Co dots are magnetized.Comment: 6 pages including figure
Microscopic tunneling experiments on atomic impurities in graphene and on magnetic thin films
This thesis presents investigations on hydrogenated graphene by scanning tunneling microscopy and spectroscopy (STM/STS) as well as the implementation of spin-polarized STM. Preparation processes for a magnetic standard sample and spin-sensitive chromium tips are developed.
The measurements on graphene reveal specific hydrogen adsorption sites in low coverage and the formation of a pattern at higher coverage. Both is found to be in agreement with previous predictions and calculations. Upon hydrogenation, an impurity midgap state emerges in the density of states which is measured directly for the first time. Complementing angle resolved photoemission experiments confirm that this state is dispersionless over the whole Brillouin zone.
A routine is developed to prepare the standard sample system of ultra-thin iron films on tungsten (Fe/W(110)). Investigations on this system confirm the magnetic properties known from literature, including the presence of a spin spiral, and prove that it is well suited for the characterization of spin-polarized tips. Different approaches for the preparation of tips from the antiferromagnetic material chromium are tested. Among these, a promising new method is presented: The coating of crystalline chromium tips with fresh chromium material suggests reproducibility of the tip characteristics. The performance of the produced tips in STM measurements is excellent in regard to a fixed spin-polarization, high resolution and stability.
Especially, a recovery of the tip magnetization direction proposed in this thesis makes this new preparation method superior to all processes yielding antiferromagnetic tips reported so far.:1 Introduction
2 Basics
2.1 Scanning tunneling microscopy
2.2 Spin-polarized STM â access to magnetic information
2.3 Measurement setup
3 Probing local hydrogen impurities in quasi-free-standing graphene
3.1 Functionalization of graphene
3.2 In-situ fabrication of quasi-free-standing graphene and its functionalization
3.3 Interpretation of the results
3.4 Short summary
4 Chromium tips for spin-polarized tunneling experiments
4.1 Magnetism at the nanoscale
4.2 Growth and properties of Fe/W(110)
4.3 Preparation of tips with outstanding properties
4.4 Short summary
5 Summary and outlookInhalt der vorliegenden Arbeit sind Untersuchungen von hydogeniertem Graphen mittels Rastertunnelmikroskopie und -spektroskopie (RTM/RTS) sowie die EinfĂŒhrung spin-polarisierter RTM. Im Rahmen dessen wurden PrĂ€parationsprozesse fĂŒr magnetische Standardproben und spin-sensitive Chrom-Spitzen entwickelt.
Die Messungen an Graphen zeigen spezifische Wasserstoff-Adsorptionsstellen bei geringer Bedeckung und die Ausbildung eines Musters bei höherer Bedeckung, jeweils in Ăbereinstimmung mit Vorhersagen und Berechnungen. Der durch Hydrogenierung entstehende Störstellenzustand in der BandlĂŒcke der Zustandsdichte wurde zum ersten Mal direkt gemessen. ErgĂ€nzende winkelaufgelöste Photoelektronenspektroskopieexperimente bestĂ€tigen, dass dieser Zustand in der gesamten Brillouinzone dispersionsfrei ist.
Ein Verfahren zur Herstellung magnetischer Standardproben aus ultradĂŒnnen Eisenfilmen auf Wolfram (Fe/W(110)) wurde entwickelt. RTM-Untersuchungen an diesem System bestĂ€tigen die bereits aus der Literatur bekannten magnetischen Eigenschaften, insbesondere das Vorhandensein einer Spinspirale. Damit ist Fe/W(110) hervorragend geeignet fĂŒr die Charakterisierung spin-polarisierter Spitzen. Verschiedene AnsĂ€tze, die zur Herstellung von Spitzen aus dem antiferromagnetischen Material Chrom verfolgt wurden, werden prĂ€sentiert, darunter auch eine vielversprechende neue Methode: Das Aufwachsen eines frischen Chromfilms auf kristalline Spitzen desselben Materials verspricht eine Reproduzierbarkeit von Spitzeneigenschaften. Der Einsatz von so hergestellten Spitzen in RTMMessungen ist geprĂ€gt von einer festgelegten Spin-Polarisation, hohem Auflösungsvermögen und StabilitĂ€t.
Insbesondere die mögliche Reproduzierbarkeit der Magnetisierungsrichtung, die in dieser Arbeit diskutiert wird, macht diese Methode allen bisher berichteten Herstellungprozessen antiferromagnetischer Spitzen ĂŒberlegen.:1 Introduction
2 Basics
2.1 Scanning tunneling microscopy
2.2 Spin-polarized STM â access to magnetic information
2.3 Measurement setup
3 Probing local hydrogen impurities in quasi-free-standing graphene
3.1 Functionalization of graphene
3.2 In-situ fabrication of quasi-free-standing graphene and its functionalization
3.3 Interpretation of the results
3.4 Short summary
4 Chromium tips for spin-polarized tunneling experiments
4.1 Magnetism at the nanoscale
4.2 Growth and properties of Fe/W(110)
4.3 Preparation of tips with outstanding properties
4.4 Short summary
5 Summary and outloo
Edge-Mediated Skyrmion Chain and Its Collective Dynamics in a Confined Geometry
The emergence of a topologically nontrivial vortex-like magnetic structure,
the magnetic skyrmion, has launched new concepts for memory devices. There,
extensive studies have theoretically demonstrated the ability to encode
information bits by using a chain of skyrmions in one-dimensional nanostripes.
Here, we report the first experimental observation of the skyrmion chain in
FeGe nanostripes by using high resolution Lorentz transmission electron
microscopy. Under an applied field normal to the nanostripes plane, we observe
that the helical ground states with distorted edge spins would evolves into
individual skyrmions, which assemble in the form of chain at low field and move
collectively into the center of nanostripes at elevated field. Such skyrmion
chain survives even as the width of nanostripe is much larger than the single
skyrmion size. These discovery demonstrates new way of skyrmion formation
through the edge effect, and might, in the long term, shed light on the
applications.Comment: 7 pages, 3 figure
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