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Skyrmions which are localized magnetic vortices with particle-like properties may occur
as stable or metastable states in chiral magnets. The unique static and dynamic properties
of magnetic skyrmions driven by their nontrivial topology offer attractive perspectives for
future spintronic applications and interesting objects for fundamental research. Of particular
interests is atomic-scale skyrmions either appear isolated or in the form of a lattice,
whose stability is expected to be robust over a large range of magnetic fields and temperatures.
This brings into play ultrathin layers of chiral magnets with nonmicromagnetically
describable magnetic behavior due to the competing ferro- and antiferromagnetically coupled
exchange interactions between different atomic sites that are finally the origin of stable
exchange spin spirals of atomic length scale. In this case, the spin-orbit coupling in the presence
of broken inversion symmetry ensures strong Dzyaloshinskii-Moriya interaction which
select a particular chirality of the spirals. Based on density functional theory and atomistic
spin dynamics simulations, we have extended the micromagnetic concept of stabilizing
skyrmions by applied magnetic fields to skyrmions stabilized by interlayer exchange coupling.
This enables engineering zero field skyrmion phases in chiral magnets and provides a perspective
direction to extend the number of possible systems where magnetic skyrmions can
be observed also at elevated temperatures. It is again necessary to understand the dynamic
properties of skyrmions within the general framework of fundamental physics before it can be
exploited in devices. We further present how the topological charge of an isolated skyrmion
can be used as an information bit in a data storage devices. The individual skyrmion can
be nucleated in a finite domain by a single magnetic pulse and then another pulse is used
for controlled switching between two isolated states with opposite polarity and topological
charge. This occurs through a complex energy landscape with various local minima of transient
topological states: a chiral-achiral meron pair, an achiral skyrmion and a half-switched
skyrmion.
Additionally, we present ab initio band structure theory in a class of topological materials,
topological (semi)metals, that support non-trivial topological surface states such as Dirac
cone and surface nodal line-like features at various binding energies. The systematic study
on a ternary compound suggests a new family of materials for exploring the coexistence and
competition of multiple fundamental fermionic quantum states. |