Final Report Summary - TOPFIT (Topological Spin Solitons for Information Technology) The ERC project TOPFIT is concerned with exploratory research on so-called topological spin solitons for potential use in information technology. Representing a branch of mathematics, topology deals with those aspects of geometric configurations that remain unchanged under elastic deformations. This compares with solitons, representing solitary waves that propagate with little loss of energy retaining their shape and speed even after collisions. In turn, the research pursued in TOPFIT deals with the identification, classification and characterisation of spin configuration in magnetic materials that are topologically non-trivial, meaning that cannot be deformed elastically into simple magnetic arrangements such as a ferromagnetic or antiferromagnetic alignment. The origin of the non-trivial topology are thereby strong non-linear spin interactions, causing additional solitonic characteristics. A simple example for such topological spin solitons are vortex-like spin arrangements, so called skyrmions. Related to their non-trivial topology they display several highly unusual properties amenable for applications, such as a very compact particle-like character, an enhanced stability (being topologically protected) and a very efficient coupling to currents permitting to manipulate them. In turn, they are of great potential interest for the implementation of advanced concepts in data storage and data processing. As part of TOPFIT two major areas were pursued, notably magnetic phases composed of topologically protected spin solitons, and aspects of topological spin solitons suitable for applications. Major results achieved as part of TOPFIT include:(-) Identification of new materials and materials classes featuring topological spin textures with complex morphologies as controlled by different tuning parameters such as magnetic field, temperature, or strain. (-) Identification of the generic character of skyrmion lattices in chiral magnets and the associated magnetic phase diagrams. (-) Demonstration of the universal nature of spin excitations of chiral magnets using complimentary spectroscopic methods (neutrons, x-rays and magnetic resonance), as well as the addressability of specific excitation modes and the formation of magnon band structures. (-) Demonstration of an empirical connection between fluctuating topological spin solitons and non-Fermi liquid behaviour by means of advanced neutron scattering, sophisticated transport measurements and ab-initio modelling. (-) Identification of the condensation of fluctuating skyrmion textures and the kinetics of the underlying phase transitions. (-) Accurate determination of the full magnetic anisotropy potential and its influence on the magnetic textures in model systems. This set the stage for the identification of a rich new class of topological phenomena including topological defects at conventional magnetic phase transitions.(-) Identification of the mechanisms controlling the creation and destruction of skyrmions in chiral magnets, where two different classes of emergent magnetic monopoles could be identified using magnetic force and transmission electron microscopy. (-) Determination of the energetics of the stability and nature of the topological protection, revealing prominent entropy compensation effects.(-) Observation of spin transfer torque effects in skyrmion lattices in the AC susceptibility as a thermodynamic probe, where kinetic neutron scattering provided additional insights on the critical current density.(-) Observation of spin transfer torque effects at the boundary of topologically trivial magnetic phases, opening up a new field of spintronics research related to general topological defects of magnetic textures. (-) Identification of the collective dynamical properties of skyrmion domains, providing insights on the particle- and wave-like characteristics.(-) Clarification of the absence of skymrions in epitaxial thin films of selected chiral magnets, providing unexpected new insights on the key mechanisms relevant for the creation and stability of skyrmions in nano-structured systems.(-) Observation of the microscopic depth dependence of the spin order in bulk samples featuring topological spin solitons, demonstrating the importance of surfaces in nano-scaled systems.
