Chiral Magnonic Edge States in Ferromagnetic Skyrmion Crystals Controlled by Magnetic Fields

TL;DR

This paper demonstrates that an external magnetic field can induce a topological phase transition in ferromagnetic skyrmion crystals, enabling control over chiral magnonic edge states and magnon spin currents for energy-efficient spintronics.

Contribution

It reveals for the first time that magnetic fields can switch topological magnonic edge states in skyrmion crystals, offering a new method to control magnon spin currents.

Findings

Magnetic field induces a topological phase transition in skyrmion crystals.

Two chiral magnonic edge states exist below the critical field.

Edge states unravel and become trivial above the critical field.

Abstract

Achieving control over magnon spin currents in insulating magnets - where dissipation due to Joule heating is highly suppressed - is an active area of research that could lead to energy-efficient spintronics applications. However, magnon spin currents supported by conventional systems with uniform magnetic order have proven hard to control. An alternative approach that relies on topologically protected magnonic edge states of spatially periodic magnetic textures has recently emerged. A prime example of such textures is the ferromagnetic skyrmion crystal which hosts chiral edge states providing a platform for magnon spin currents. Here, we show, for the first time, an external magnetic field can drive a topological phase transition in the spin wave spectrum of a ferromagnetic skyrmion crystal. The topological phase transition is signaled by the closing of a low-energy bulk magnon gap at a critical field. In the topological phase, below the critical field, two topologically protected chiral magnonic edge states lie within this gap, but they unravel in the trivial phase, above the critical field. Remarkably, the topological phase transition involves an inversion of two magnon bands that at the $\Gamma$ point correspond to the breathing and anticlockwise modes of the skyrmions in the crystal. Our findings suggest that an external magnetic field could be used as a knob to switch on and off magnon spin currents carried by topologically protected chiral magnonic edge states.