Reconfigurable magnetism in a bi-stable nanomagnetic array: Artificial Spin-Vortex Ice.

Abstract

Artificial spin ices (ASI) are magnetic metamaterials comprising geometrically-tiled interacting nanomagnets. Typically each nanoisland is magnetized along its long axis giving an Ising ‘macrospin’. There has been significant recent interest in these systems for functional magnonics due to their large reconfigurable dipolar field landscape. Here we present recent advances using a width modification scheme where artificial spin ice is modified by widening a subset of bars relative to the rest of the array. There are two principal benefits to the width modification. Firstly, Increasing width decreases the coercive field of the macrospins, so certain geometric tilings of the wide bars allow preparation of any ordered macrospin vertex state via simple global-field protocols1. Secondly at a certain width the macrospin spin state will be energetically equivalent with the vortex state so the wide bars can be tuned to be metastable in the macrospin and vortex states, yielding Artificial Spin-Vortex Ice (ASVI)2. The precise control of both nanoisland magnetism and vertex configuration by the width modification enables us to demonstrate the efficacy of zero-field ferromagnetic resonance measurements of the spin-wave spectrum as a fingerprint of the local dipolar field environment and vertex populations in magnetic nanostructures3. Artificial Spin-Vortex Ice allows a cross over experimentally from the classic 2-state Ising ASI to a 4-state Potts model system where the macrospins can convert into closed vortices. Unlike in a ‘clock model’ Potts system, the dipolar interactions are almost completely eliminated in the vortex state.4 The enhanced bi-texture microstate space gives rise to emergent physical memory phenomena, with ratchet-like vortex training and history-dependent nonlinear training dynamics. We observe vortex-domain formation alongside MFM tip vortex-writing. Tip-written vortices dramatically alter local reversal and memory dynamics. The precise control and the dramatic differences in local dipolar environment are very promising for reconfigurable magnonics. In the width modified macrospin ASI, we obtain shifts in ferromagnetic resonance frequency of the order of hundreds of MHz and access states with mode hybridization. ASVI yield much stronger contrast in dipolar environment such that the gap between the macrospin and the vortex peak is around 3.8 GHz.