Signatures of a liquid-crystal transition in spin-wave excitations of skyrmions

Narayan Mohanta,Andrew D Christianson,Satoshi Okamoto,Elbio Dagotto

doi:10.1038/s42005-020-00489-w

Narayan Mohanta, Andrew D Christianson + Show 2 more

Open Access

https://doi.org/10.1038/s42005-020-00489-w

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Abstract

Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and ferromagnetic-exchange interactions with a magnetic field or anisotropy. Here, we compute the dynamical spin structure factor, using Monte Carlo and spin dynamics simulations, extracting the spin-wave spectrum in the SkX, in the vicinity of the paramagnet to SkX transition. Inside the SkX, we find six spin-wave modes, which are supplemented by another mode originating from the ferromagnetic background. Above the critical temperature Ts for the skyrmion crystallization, we find a diffusive regime, reminiscent of the liquid-to-crystal transition, revealing that topological spin texture of skyrmionic character starts to develop above Ts as the precursor of the SkX. We discuss the opportunities for the detection of the spin waves of the SkX using inelastic-neutron-scattering experiments in manganite-iridate heterostructures.

Highlights

Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order
This is interesting since specific heat measurements report a first-order phase transition[40] to the helimagnetic phase at zero magnetic field, while neutron-scattering experiments reveal a Landau soft-mode mechanism of weak crystallization to the SkX phase at a finite field[41,42,43]. These experiments suggest that the emergence of the SkX phase occurs via a precursor regime in which non-trivial topological spin textures of skyrmionic character are developed within a paramagnetic background with abundance of fluctuations, similar to the precursor phenomenon proposed in the context of liquid-crystal transitions[44]
In the diffusive regime for the skyrmion crystallization, S(q, ω = 0) profile reveals a clear transition from a ringshaped profile to a six-peak profile, characteristic of the SkX phase, confirming the previous experimental findings that the transition to the SkX phase upon cooling slowly from a higher temperature occurs via a precursor regime

Summary

Introduction

Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order. This is interesting since specific heat measurements report a first-order phase transition[40] to the helimagnetic phase at zero magnetic field, while neutron-scattering experiments reveal a Landau soft-mode mechanism of weak crystallization to the SkX phase at a finite field[41,42,43] These experiments suggest that the emergence of the SkX phase occurs via a precursor regime in which non-trivial topological spin textures of skyrmionic character are developed within a paramagnetic background with abundance of fluctuations, similar to the precursor phenomenon proposed in the context of liquid-crystal transitions[44].

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