Magnetic skyrmions at room temperature and far-from-equilibrium
Laboratory for Neutron Scattering and Imaging
Paul Scherrer Institut, Switzerland
In condensed matter physics, the term magnetic skyrmion is applied to describe a novel twist in a smooth magnetization field characterized by a topological winding number. First discovered in 2009 as a lattice of whirlpool-like twists in a chiral cubic magnet, magnetic skyrmions have since been explored extensively both in other bulk crystals, and in synthetic systems such as magnetic multilayers. Regardless of the host system, the topological winding of skyrmions is the common ingredient that leads to unique physical properties that attract significant interest. For example, due to the topological winding it is not possible to smoothly deform a skyrmion into a topologically distinct state such as a ferromagnet; instead in a physical system the rupture of the topology requires overcoming an energy barrier. Thus, skyrmions when created display a robustness against perturbation and decay that is generally referred to as topological protection. This is a central concept underlying the long-term stability of skyrmions and their suitability for use in applications.
Here we will describe recent experiments done at PSI on Co-Zn-Mn compounds, a family of high-temperature cubic magnet discovered recently to host skyrmions. We find that dependent on the composition, magnetic skyrmions can be stabilized either well above, at, or well below room temperature as thermodynamic equilibrium phases. Amongst other remarkable aspects in this system, we will focus on how the skyrmions phases fall out of thermal equilibrium remarkably easily on supercooling to form robust metastable skyrmion states. These metastable skyrmion states extend unprecedentedly across the majority of the phase diagram, and can even display a thermally-reversible structural transition that surprisingly does not trigger a decay to the thermodynamic ground state. Overall, the results provide insight into the generic aspects of topological skyrmions far-from-equilibrium, itself a nascent research area with its own rich potential.
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