Magneto-ionic suppression of magnetic vortices

Yu Chen,Aliona Nicolenco,Pau Molet,Agustin Mihi,Eva Pellicer,Jordi Sort

doi:10.1080/14686996.2021.1988830

Abstract

Magneto-ionics refers to the non-volatile control of the magnetic properties of materials by voltage-driven ion migration. This phenomenon constitutes one of the most important magnetoelectric mechanisms and, so far, it has been employed to modify the magnetic easy axis of thin films, their coercivity or their net magnetization. Herein, a novel magneto-ionic effect is demonstrated: the transition from vortex to coherent rotation states, caused by voltage-induced ion motion, in arrays of patterned nanopillars. Electrolyte-gated Co/GdO x bilayered nanopillars are chosen as a model system. Electron microscopy observations reveal that, upon voltage application, oxygen ions diffuse from GdO x to Co, resulting in the development of paramagnetic oxide phases (CoO x ) along sporadic diffusion channels. This breaks up the initial magnetization configuration of the ferromagnetic pillars (i.e., vortex states) and leads to the formation of small ferromagnetic nanoclusters, embedded in the CoO x matrix, which behave as single-domain nanoparticles. As a result, a decrease of the net magnetic moment is observed, together with a drastic change in the shape of the hysteresis loop. Micromagnetic simulations are used to interpret these findings. These results pave the way towards a new potential application of magnetoelectricity: the magneto-ionic control of magnetic vortex states.

Highlights

Magnetic vortex is one of fundamental magnetization states that occurs in micro-/nanosized ferromagnetic structures, e.g., disks, ellipses or nanowires, etc., due to geometrical spin confinement.[1,2,3,4] This state is highly appealing for non-volatile magnetic memories and spintronic devices since the magnetic information can be stored by encoding both (i) the vortex chirality, i.e. the direction of in-plane magnetization rotation, and (ii) the polarity, i.e. the out-of-plane magnetization of the nano-scale vortex core.[5]
We investigate a new room-temperature magneto-ionic effect that is induced at the nanoscale: the transition from ‘magnetic vortex state’ to ‘coherent rotation’ in electrolytegated Co/GdOx bilayered nanopillars
Our work demonstrates a new magneto-ionic effect: a proof of concept of voltage-driven magneto-ionic switching from vortex to single domain magnetization reversal states in 13 heterostructured Co/GdOx nanopillars

Summary

Introduction

Magnetic vortex is one of fundamental magnetization states that occurs in micro-/nanosized ferromagnetic structures, e.g., disks, ellipses or nanowires, etc., due to geometrical spin confinement.[1,2,3,4] This state is highly appealing for non-volatile magnetic memories and spintronic devices since the magnetic information can be stored by encoding both (i) the vortex chirality, i.e. the direction of in-plane magnetization rotation (clockwise or counterclockwise), and (ii) the polarity, i.e. the out-of-plane magnetization of the nano-scale vortex core (up or down).[5]. Micromagnetic simulations first predicted the possibility to switch between the magnetic vortex and polar states in FeGa nanodots grown onto a ferroelectric PZT substrate.[15] Later on, experimental works showed annihilation of magnetic vortices in Ni disks via uniaxial compressive strain transferred from PMN-PT[43] or BaTiO3.[44] Electric-field-assisted switching of magnetic vortex chirality was shown in Co/PMN-PT.[45] While these results are very appealing for voltage-control of vortex states, the complicated behavior of the ferroelectric domains of these substrates (including ferroelectric relaxor phenomena[46] and presence of surface skin layers[47]) poses some challenges to obtain uniform magnetization reversal modes in all disks comprised in the patterned arrays This adds to other important issues of strain-mediated ME systems, such as clamping effects, mechanical fatigue, high fabrication costs (in the case of PMN-PT) or need of relatively large applied voltages (e.g., 400 V).. The challenges and future research directions for further optimization and control of this newly observed magneto-ionic effect are described

Experimental section

Results and Discussion

Conclusions