Domain Patterns and Dynamics in the Magnetoelectric Switching of Spin-Spiral Multiferroics

Abstract

The magnetoelectric effect has attracted tremendous attention in the past decade because it is highly interesting for applications. In particular, magnetoelectric switching allows to switch a magnetic order by an electric field. This effect may be used in novel memory devices and is potentially very energy efficient.<br /> Also from the physics point of view, magnetoelectric switching is a fascinating topic. It is based on a complex interdependence of the electric and magnetic order in the material. Their coupling is intrinsically strong in magnetically-induced ferroelectrics, among which spin-spiral multiferroics are most prominent. Here, the ferroelectric polarization is formed as a consequence of a complex spiral magnetic order. Due to this inherent coupling spin-spiral multiferroics are promising materials for reliable magnetoelectric switching. <br /> Even though the switching speed is crucial for applications, its timescale has been unknown up to now. Moreover, hardly any work was devoted to the dynamic aspects of the actual switching process so far. <br /> In the present work, we investigate the magnetoelectric switching properties of two selected spin-spiral multiferroics, MnWO₄ and CuO using optical Second Harmonic Generation (SHG). Since ferroic switching processes are generally governed by the nucleation and grow of domain, a major focus is set on the determination and analysis of the domains structures in static, quasi-static and dynamic conditions. For this purpose, an electrical-pump–optical-probe method was developed, which allows to reconstruct the evolution of the three-dimensional domain pattern with high temporal and spatial resolution. <br /> For the first time, we determined the actual magnetoelectric switching time. The observed time scale on the order of milliseconds is surprisingly slow. High domain wall mobilities and the dependence on the applied electric field strength suggest, that the reason for the slow switching lies in the low ferroelectric polarization of spin-spiral multiferroics. This is backed by energy considerations. <br /> This work provides valuable insights in two important areas of mangetoelectric switching. First, it intensely discusses the domain stuctures, which govern the switching process. Second, it yields pioneering work on the dynamics of the switching process. The method developed here paves the way for a systematic analysis of the yet largely unexplored field of magnetoelectric switching dynamics.