Skyrmions at Vanishingly Small Dzyaloshinskii-Moriya Interaction or Zero Magnetic Field

TL;DR

This paper demonstrates that nanoscale skyrmions can exist at zero magnetic field with negligible Dzyaloshinskii-Moriya interaction, supported by a model including biquadratic exchange and anisotropy, aligning with experimental observations.

Contribution

It introduces a novel theoretical model showing skyrmions at vanishing DMI and zero magnetic field, extending understanding of magnetic structures in such conditions.

Findings

Nanoscale skyrmions observed at zero DMI and magnetic field.

Short pitch spin-spiral structures supported by the model.

Phase diagrams of various magnetic states predicted.

Abstract

By introducing biquadratic together with usual bilinear ferromagnetic nearest neighbor exchange interaction in a square lattice, we find that the energy of the spin-wave mode is minimized at a finite wavevector for a vanishingly small Dzyaloshinskii-Moriya interaction (DMI), supporting a ground state with spin-spiral structure whose pitch length is unusually short as found in some of the experiments. Apart from reproducing the magnetic structures that can be obtained in a canonical model with nearest neighbor exchange interaction only, a numerical simulation of this model with further introduction of magnetic anisotropy and magnetic field predicts many other magnetic structures some of which are already observed in the experiments. Amongst many observed structures, nanoscale skyrmion even at vanishingly small DMI is found for the first time in a model. The model provides the nanoscale skyrmions of unit topological charge at zero magnetic field as well. We obtain phase diagrams for all the magnetic structures predicted in the model.