Understanding thermal annealing of artificial spin ice

Xiaoyu Zhang,Yuyang Lao,Joseph T Batley,Joseph Sklenar,Chris Leighton,Justin D Watts,Nicholas S Bingham,Cristiano Nisoli,Peter Schiffer

doi:10.1063/1.5126713

Abstract

We have performed a detailed study on thermal annealing of the moment configuration in artificial spin ice. Permalloy (Ni80Fe20) artificial spin ice samples were examined in the prototypical square ice geometry, studying annealing as a function of island thickness, island shape, and annealing temperature and duration. We also measured the Curie temperature as a function of film thickness, finding that thickness has a strong effect on the Curie temperature in regimes of relevance to many studies of the dynamics of artificial spin ice systems. Increasing the interaction energy between island moments and reducing the energy barrier to flipping the island moments allow the system to more closely approach the collective low energy state of the moments upon annealing, suggesting new channels for understanding the thermalization processes in these important model systems.

Highlights

Artificial spin ice systems are two-dimensional arrays of nanoscale elements, typically composed of single domain ferromagnetic islands.1 These systems have been the subject of extensive study and have provided models for the study of a range of novel collective behaviors.2 Certain artificial spin ice geometries have well-defined collective magnetic ground states, such as the square lattice,1 while others have intrinsically disordered and complex ground states, such as the Shakti lattice.3–5 These low-energy collective states have sparked considerable interest in attempting to realize the lowest energy state of different artificial spin ice lattices.6–10 One successful approach to collective energy minimization involves annealing the arrays by heating them to temperatures near or above the Curie temperature (TC) of the ferromagnetic material.11,12 Upon cooling, the island moments arrange themselves into a low energy state via magnetostatic interactions
We fabricated our artificial square spin ice samples on Si wafers coated with a 200-nm-thick layer of Si-N deposited by low scitation.org/journal/apm pressure chemical vapor deposition
We characterized the magnetic state of the arrays in two locations using a magnetic force microscope (MFM), yielding a corresponding microstate map of the arrays

Summary

Introduction

Artificial spin ice systems are two-dimensional arrays of nanoscale elements, typically composed of single domain ferromagnetic islands.1 These systems have been the subject of extensive study and have provided models for the study of a range of novel collective behaviors.2 Certain artificial spin ice geometries have well-defined collective magnetic ground states, such as the square lattice,1 while others have intrinsically disordered and complex ground states, such as the Shakti lattice.3–5 These low-energy collective states have sparked considerable interest in attempting to realize the lowest energy state of different artificial spin ice lattices.6–10 One successful approach to collective energy minimization involves annealing the arrays by heating them to temperatures near or above the Curie temperature (TC) of the ferromagnetic material.11,12 Upon cooling, the island moments arrange themselves into a low energy state via magnetostatic interactions. Each square artificial spin ice vertex consists of four islands with 16 possible magnetic moment configurations [Fig. 1(e)] that can be classified into four types.

Results

Conclusion