The 2020 skyrmionics roadmap Back, C; Cros, V; Ebert, H ...
Journal of physics. D, Applied physics,
09/2020, Letnik:
53, Številka:
36
Journal Article
Recenzirano
Odprti dostop
The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform ...that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics Roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.
Stability of single skyrmionic bits Hagemeister, J; Romming, N; von Bergmann, K ...
Nature communications,
10/2015, Letnik:
6, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The switching between topologically distinct skyrmionic and ferromagnetic states has been proposed as a bit operation for information storage. While long lifetimes of the bits are required for data ...storage devices, the lifetimes of skyrmions have not been addressed so far. Here we show by means of atomistic Monte Carlo simulations that the field-dependent mean lifetimes of the skyrmionic and ferromagnetic states have a high asymmetry with respect to the critical magnetic field, at which these lifetimes are identical. According to our calculations, the main reason for the enhanced stability of skyrmions is a different field dependence of skyrmionic and ferromagnetic activation energies and a lower attempt frequency of skyrmions rather than the height of energy barriers. We use this knowledge to propose a procedure for the determination of effective material parameters and the quantification of the Monte Carlo timescale from the comparison of theoretical and experimental data.
Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the ...charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
BACKGROUND: The prevalence of adiposity in childhood is increasing. Is breast-feeding protective as suggested by cross-sectional studies? OBJECTIVE: In a longitudinal birth cohort study, we tested ...whether breast-feeding for more than 2 months has preventive effects against overweight and adiposity at 6 y. DESIGN: Of 1314 children representing the catchment areas of six delivery units, 918 could be followed up to the age of 6 y. Height, weight, and skin-fold thickness were measured at regular visits. As the criteria of overweight, obesity, and adiposity in the children, the 90th and the 97th percentiles of BMI and skin-fold values were used. Parents with a BMI at or above the 90th percentile, which was 27 kg/m2 or more, were considered overweight. Infants bottle-fed from birth or breast-fed for less than 3 months were classified as 'bottle-fed' (BO), and those breast-fed for 3 months and more as 'breast-fed' (BR). Univariate comparisons and logistic regression analysis were performed applying SAS 6.12. The final logistic model consisted of the 480 cases for whom complete data for all variables were available. The potential effect of loss to follow-up was analysed by the Cochran-Mantel-Haenzel test: the outcomes were not significantly influenced by loss to follow-up. RESULTS: At birth BMIs were nearly identical in both groups. By 3 months, BO had significantly higher BMIs and thicker skin folds than BR. From 6 months on, compared to BR, a consistently higher proportion of BO children exceeded the 90th and the 97th percentile of BMI and skin-fold thickness reference values. From the age of 4 y to 5 and 6 y, in BO the prevalence of obesity nearly doubled and tripled, respectively. With only minor changes of obesity prevalence in BR, the difference of BMI and skin-fold thickness between groups became statistically significant. Logistic regression analysis revealed that overweight of the mother, maternal smoking during pregnancy, bottle feeding, and low social status remained important risk factors for overweight and adiposity at 6 y of age. CONCLUSION: A maternal BMI of greater than or equal to 27, bottle-feeding, maternal smoking during pregnancy, and low social status are risk factors for overweight and adiposity at 6 y of age. Early bottle-feeding brings forward the obesity rebound, predictive of obesity in later life.
Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the magnetic properties of Pd/Fe atomic bilayers on Re(0001). Two kinds of magnetic ground states are ...discovered due to different types of stacking of the Pd adlayer on Fe/Re(0001). For fcc stacking of Pd on Fe/Re(0001), it is a spin spiral propagating along the close-packed (ΓKover ¯) direction with a period of about 0.9 nm, driven by frustrated exchange and Dzyaloshinskii-Moriya interactions. For the hcp stacking, the four-site four-spin interaction stabilizes an up-up-down-down state propagating perpendicular to the close-packed direction (along ΓMover ¯) with a period of about 1.0 nm. Our work shows how higher-order exchange interactions can be tuned at interfaces.
We demonstrate a canted magnetization of biatomic zigzag Co chains grown on the (5 × 1) reconstructed Ir(001) surface using density functional theory (DFT) calculations and spin-polarized scanning ...tunneling microscopy (SP-STM) experiments. It is observed by STM that biatomic Co chains grow in three different structural configurations. Our DFT calculations show that they are all in a ferromagnetic (FM) state. Two chain types possess high symmetry due to two equivalent atomic strands and an easy magnetization direction that is along one of the principal crystallographic axes. The easy magnetization axis of the zigzag Co chains is canted away from the surface normal by an angle of 33°. This giant effect is caused by the broken chain symmetry on the substrate in combination with the strong spin-orbit coupling of Ir. SP-STM measurements confirm the stable FM order of the zigzag chains with a canted magnetization.
The search for uncompensated magnetic moments on antiferromagnetic surfaces is of great technological importance as they are responsible for the exchange-bias effect that is widely used in ...state-of-the-art magnetic storage devices. We have studied the atomic spin structure of phase domain walls in the antiferromagnetic Fe monolayer on W(001) by means of spin-polarized scanning tunnelling microscopy and Monte Carlo simulations. The domain wall width only amounts to 6-8 atomic rows. Although walls oriented along directions are found to be fully compensated, detailed analysis of -oriented walls reveals an uncompensated perpendicular magnetic moment. Our result represents a major advance in the field of antiferromagnetism, and may lead to a better understanding of the magnetic interaction between ferromagnetic and antiferromagnetic materials.
We report a transverse conical spin spiral as the magnetic ground state of a double-layer Mn on a W(110) surface. Using spin-polarized scanning tunneling microscopy, we find a long-range modulation ...along the 001 direction with a periodicity of 2.4 nm coexisting with a local row-wise antiferromagnetic contrast. First-principles calculations reveal a transverse conical spin-spiral ground state of this system which explains the observed magnetic contrast. The canting of the spins is induced by higher-order exchange interactions, while the spiraling along the 001 direction is due to frustrated Heisenberg exchange and Dzyaloshinskii-Moriya interaction.
Using spin-polarized scanning tunneling microscopy we show that the magnetic order of 1 monolayer Mn on W(001) is a spin spiral propagating along 110 crystallographic directions. The spiral arises on ...the atomic scale with a period of about 2.2 nm, equivalent to only 10 atomic rows. Ab initio calculations identify the spin spiral as a left-handed cycloid stabilized by the Dzyaloshinskii-Moriya interaction, imposed by spin-orbit coupling, in the presence of softened ferromagnetic exchange coupling. Monte Carlo simulations explain the formation of a nanoscale labyrinth pattern, originating from the coexistence of the two possible rotational domains, that is intrinsic to the system.
The influence of in-plane and canted magnetic fields on spin spirals and skyrmions in atomic bilayer islands of palladium and iron on an Ir(111) substrate is investigated by scanning tunneling ...microscopy at low temperatures. It is shown that the spin spiral propagation direction is determined by the island's border which can be explained by equilibrium state calculations on a triangular lattice. We find a different response of spin spirals to in-plane magnetic fields for a propagation direction parallel to the applied field as compared to perpendicular, which originates from their cycloidal nature. As a result, the spin spiral propagation direction may be reorientated by in-plane fields. Furthermore, it is demonstrated that also skyrmions are distorted in canted fields which allows to determine the sense of magnetization rotation as enforced by the interfacial Dzyaloshinskii-Moriya interaction.
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