135 research outputs found
The properties of isolated chiral skyrmions in thin magnetic films
Axisymmetric solitonic states (chiral skyrmions) have been predicted
theoretically more than two decades ago. However, until recently they have been
observed in a form of skyrmionic condensates (hexagonal lattices and other
mesophases). In this paper we report experimental and theoretical
investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers
two years ago (Science 341 , 636 (2013)). The results of spin-polarized
scanning tunneling microscopy analyzed within the continuum and discrete models
provide a consistent description of isolated skyrmions in thin layers. The
existence region of chiral skyrmions is restricted by strip-out instabilities
at low fields and a collapse at high fields. We demonstrate that the same
equations describe axisymmetric localized states in all condensed matter
systems with broken mirror symmetry, and thus our findings establish basic
properties of isolated skyrmions common for chiral liquid crystals, different
classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics.Comment: 12 pages, 12 figure
Electrical detection of magnetic skyrmions by non-collinear magnetoresistance
Magnetic skyrmions are localised non-collinear spin textures with high
potential for future spintronic applications. Skyrmion phases have been
discovered in a number of materials and a focus of current research is the
preparation, detection, and manipulation of individual skyrmions for an
implementation in devices. Local experimental characterization of skyrmions has
been performed by, e.g., Lorentz microscopy or atomic-scale tunnel
magnetoresistance measurements using spin-polarised scanning tunneling
microscopy. Here, we report on a drastic change of the differential tunnel
conductance for magnetic skyrmions arising from their non-collinearity: mixing
between the spin channels locally alters the electronic structure, making a
skyrmion electronically distinct from its ferromagnetic environment. We propose
this non-collinear magnetoresistance (NCMR) as a reliable all-electrical
detection scheme for skyrmions with an easy implementation into device
architectures
Field-free deterministic ultra fast creation of skyrmions by spin orbit torques
Magnetic skyrmions are currently the most promising option to realize
current-driven magnetic shift registers. A variety of concepts to create
skyrmions were proposed and demonstrated. However, none of the reported
experiments show controlled creation of single skyrmions using integrated
designs. Here, we demonstrate that skyrmions can be generated deterministically
on subnanosecond timescales in magnetic racetracks at artificial or natural
defects using spin orbit torque (SOT) pulses. The mechanism is largely similar
to SOT-induced switching of uniformly magnetized elements, but due to the
effect of the Dzyaloshinskii-Moriya interaction (DMI), external fields are not
required. Our observations provide a simple and reliable means for skyrmion
writing that can be readily integrated into racetrack devices
Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Magnetic skyrmions are chiral spin structures with a whirling configuration.
Their topological properties, nanometer size and the fact that they can be
moved by small current densities have opened a new paradigm for the
manipulation of magnetisation at the nanoscale. To date, chiral skyrmion
structures have been experimentally demonstrated only in bulk materials and in
epitaxial ultrathin films and under external magnetic field or at low
temperature. Here, we report on the observation of stable skyrmions in
sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero
applied magnetic field. We use high lateral resolution X-ray magnetic circular
dichroism microscopy to image their chiral N\'eel internal structure which we
explain as due to the large strength of the Dzyaloshinskii-Moriya interaction
as revealed by spin wave spectroscopy measurements. Our results are
substantiated by micromagnetic simulations and numerical models, which allow
the identification of the physical mechanisms governing the size and stability
of the skyrmions.Comment: Submitted version. Extended version to appear in Nature
Nanotechnolog
Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy
Magnetic skyrmions are highly promising candidates for future spintronic
applications such as skyrmion racetrack memories and logic devices. They
exhibit exotic and complex dynamics governed by topology and are less
influenced by defects, such as edge roughness, than conventionally used domain
walls. In particular, their finite topological charge leads to a predicted
"skyrmion Hall effect", in which current-driven skyrmions acquire a transverse
velocity component analogous to charged particles in the conventional Hall
effect. Here, we present nanoscale pump-probe imaging that for the first time
reveals the real-time dynamics of skyrmions driven by current-induced spin
orbit torque (SOT). We find that skyrmions move at a well-defined angle
{\Theta}_{SH} that can exceed 30{\deg} with respect to the current flow, but in
contrast to theoretical expectations, {\Theta}_{SH} increases linearly with
velocity up to at least 100 m/s. We explain our observation based on internal
mode excitations in combination with a field-like SOT, showing that one must go
beyond the usual rigid skyrmion description to unravel the dynamics.Comment: pdf document arxiv_v1.1. 24 pages (incl. 9 figures and supplementary
information
Femtosecond control of electric currents at the interfaces of metallic ferromagnetic heterostructures
The idea to utilize not only the charge but also the spin of electrons in the
operation of electronic devices has led to the development of spintronics,
causing a revolution in how information is stored and processed. A novel
advancement would be to develop ultrafast spintronics using femtosecond laser
pulses. Employing terahertz (10 Hz) emission spectroscopy, we
demonstrate optical generation of spin-polarized electric currents at the
interfaces of metallic ferromagnetic heterostructures at the femtosecond
timescale. The direction of the photocurrent is controlled by the helicity of
the circularly polarized light. These results open up new opportunities for
realizing spintronics in the unprecedented terahertz regime and provide new
insights in all-optical control of magnetism.Comment: 3 figures and 2 tables in the main tex
Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces
Spin-orbit coupling (SOC) describes the relativistic interaction between the
spin and momentum degrees of freedom of electrons, and is central to the rich
phenomena observed in condensed matter systems. In recent years, new phases of
matter have emerged from the interplay between SOC and low dimensionality, such
as chiral spin textures and spin-polarized surface and interface states. These
low-dimensional SOC-based realizations are typically robust and can be
exploited at room temperature. Here we discuss SOC as a means of producing such
fundamentally new physical phenomena in thin films and heterostructures. We put
into context the technological promise of these material classes for developing
spin-based device applications at room temperature
Creation and annihilation of topological meron pairs in in-plane magnetized films
Merons which are topologically equivalent to one-half of skyrmions can exist only in pairs or groups in two-dimensional (2D) ferromagnetic (FM) systems. The recent discovery of meron lattice in chiral magnet Co8Zn9Mn3 raises the immediate challenging question that whether a single meron pair, which is the most fundamental topological structure in any 2D meron systems, can be created and stabilized in a continuous FM film? Utilizing winding number conservation, we develop a new method to create and stabilize a single pair of merons in a continuous Py film by local vortex imprinting from a Co disk. By observing the created meron pair directly within a magnetic field, we determine its topological structure unambiguously and explore the topological effect in its creation and annihilation processes. Our work opens a pathway towards developing and controlling topological structures in general magnetic systems without the restriction of perpendicular anisotropy and Dzyaloshinskii-Moriya interaction
Dynamics and inertia of skyrmionic spin structures
Skyrmions are topologically protected winding vector fields characterized by a spherical topology. Magnetic skyrmions can arise as the result of the interplay of various interactions, including exchange, dipolar and anisotropy energy in the case of magnetic bubbles and an additional Dzyaloshinskii-Moriya interaction in the case of chiral skyrmions. Whereas the static and low-frequency dynamics of skyrmions are already well under control, their gigahertz dynamical behaviour has not been directly observed in real space. Here, we image the gigahertz gyrotropic eigenmode dynamics of a single magnetic bubble and use its trajectory to experimentally confirm its skyrmion topology. The particular trajectory points to the presence of strong inertia, with a mass much larger than predicted by existing theories. This mass is endowed by the topological confinement of the skyrmion and the energy associated with its size change. It is thereby expected to be found in all skyrmionic structures in magnetic systems and beyond. Our experiments demonstrate that the mass term plays a key role in describing skyrmion dynamics.
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