669 research outputs found
Magnetism of Covalently Functionalized Carbon Nanotubes
We investigate the electronic structure of carbon nanotubes functionalized by
adsorbates anchored with single C-C covalent bonds. We find that, despite the
particular adsorbate, a spin moment with a universal value of 1.0 per
molecule is induced at low coverage. Therefore, we propose a mechanism of
bonding-induced magnetism at the carbon surface. The adsorption of a single
molecule creates a dispersionless defect state at the Fermi energy, which is
mainly localized in the carbon wall and presents a small contribution from the
adsorbate. This universal spin moment is fairly independent of the coverage as
long as all the molecules occupy the same graphenic sublattice. The magnetic
coupling between adsorbates is also studied and reveals a key dependence on the
graphenic sublattice adsorption site.Comment: final version, improved discussion about calculations and defect
concentratio
Relativistic domain-wall dynamics in van der Waals antiferromagnet MnPS3
The discovery of two-dimensional (2D) magnetic van der Waals (vdW) materials has flourished an endeavor for fundamental problems as well as potential applications in computing, sensing and storage technologies. Of particular interest are antiferromagnets, which due to their intrinsic exchange coupling show several advantages in relation to ferromagnets such as robustness against external magnetic perturbations. Here we show that, despite of this cornerstone, the magnetic domains of recently discovered 2D vdW MnPS3 antiferromagnet can be controlled via magnetic fields and electric currents. We achieve ultrafast domain-wall dynamics with velocities up to ~3000 m s−1 within a relativistic kinematic. Lorentz contraction and emission of spin-waves in the terahertz gap are observed with dependence on the edge termination of the layers. Our results indicate that the implementation of 2D antiferromagnets in real applications can be further controlled through edge engineering which sets functional characteristics for ultrathin device platforms with relativistic features
Properties and dynamics of meron topological spin textures in the two-dimensional magnet CrCl3
Merons are nontrivial topological spin textures highly relevant for many
phenomena in solid state physics. Despite their importance, direct observation
of such vortex quasiparticles is scarce and has been limited to a few complex
materials. Here we show the emergence of merons and antimerons in recently
discovered two-dimensional (2D) CrCl3 at zero magnetic field. We show their
entire evolution from pair creation, their diffusion over metastable domain
walls, and collision leading to large magnetic monodomains. Both quasiparticles
are stabilized spontaneously during cooling at regions where in-plane magnetic
frustration takes place. Their dynamics is determined by the interplay between
the strong in-plane dipolar interactions and the weak out-of-plane magnetic
anisotropy stabilising a vortex core within a radius of 8-10 nm. Our results
push the boundary to what is currently known about non-trivial spin structures
in 2D magnets and open exciting opportunities to control magnetic domains via
topological quasiparticles.Comment: Nature Communications 12, 185 (2021). Editors' Highlights sectio
All-optical control of spin in a 2D van der Waals magnet
Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI(3). While the magnetism of bare CrI(3) layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI(3) with a monolayer of WSe(2). The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe(2) and CrI(3) is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology
Universal Magnetic Properties of sp-type Defects in Covalently Functionalized Graphene
Using density-functional calculations, we study the effect of sp-type
defects created by different covalent functionalizations on the electronic and
magnetic properties of graphene. We find that the induced magnetic properties
are {\it universal}, in the sense that they are largely independent on the
particular adsorbates considered. When a weakly-polar single covalent bond is
established with the layer, a local spin-moment of 1.0 always appears
in graphene. This effect is similar to that of H adsorption, which saturates
one orbital in the carbon layer. The magnetic couplings between the
adsorbates show a strong dependence on the graphene sublattice of
chemisorption. Molecules adsorbed at the same sublattice couple
ferromagnetically, with an exchange interaction that decays very slowly with
distance, while no magnetism is found for adsorbates at opposite sublattices.
Similar magnetic properties are obtained if several orbitals are
saturated simultaneously by the adsorption of a large molecule. These results
might open new routes to engineer the magnetic properties of graphene
derivatives by chemical means
Scalable photonic sources using two-dimensional lead halide perovskite superlattices
Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling
Transient magneto-optical spectrum of photoexcited electrons in the van der Waals ferromagnet Cr2Ge2Te6
Femtosecond optical control of magnetic materials shows promise for future ultrafast data storage devices. To date, most studies in this area have relied on quasimonochromatic light in magneto-optical pump-probe experiments, which limited their ability to probe semiconducting and molecule-based materials with structured optical spectra. Here, we demonstrate the possibility of extracting the magneto-optical spectrum of the electrons in the conduction band in the two-dimensional van der Waals ferromagnet Cr2Ge2Te6 (CGT), which is made possible due to broadband probing in the visible spectrum. The magneto-optical signal is a sum of contributions from electrons in the conduction and valence bands, which are of opposite sign for CGT. Depending on the probe wavelength used, this difference could lead to an erroneous interpretation that the magnetization direction is reversed after excitation, which has important consequences for understanding spin toggle switching phenomena
Breaking through the Mermin-Wagner limit in 2D van der Waals magnets
The Mermin-Wagner theorem states that long-range magnetic order does not
exist in one- or two-dimensional (2D) isotropic magnets with short-ranged
interactions. The theorem has been a milestone in magnetism and has been
driving the research of recently discovered 2D van der Waals (vdW) magnetic
materials from fundamentals up to potential applications. In such systems, the
existence of magnetic ordering is typically attributed to the presence of a
significant magnetic anisotropy, which is known to introduce a spin-wave gap
and circumvent the core assumption of the theorem. Here we show that in
finite-size 2D vdW magnets typically found in lab setups (e.g., within
millimetres), short-range interactions can be large enough to allow the
stabilisation of magnetic order at finite temperatures without any magnetic
anisotropy for practical implementations. We demonstrate that magnetic ordering
can be created in flakes of 2D materials independent of the lattice symmetry
due to the intrinsic nature of the spin exchange interactions and finite-size
effects in two-dimensions. Surprisingly we find that the crossover temperature,
where the intrinsic magnetisation changes from superparamagnetic to a
completely disordered paramagnetic regime, is weakly dependent on the system
length, requiring giant sizes (e.g., of the order of the observable universe
~10 m) in order to observe the vanishing of the magnetic order at
cryogenic temperatures as expected from the Mermin-Wagner theorem. Our findings
indicate exchange interactions as the main driving force behind the
stabilisation of short-range order in 2D magnetism and broaden the horizons of
possibilities for exploration of compounds with low anisotropy at an atomically
thin level
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