Effect of FePd alloy composition on the dynamics of artificial spin ice

TL;DR

This study investigates how varying FePd alloy composition affects the thermal dynamics and ground state formation in artificial spin ice arrays, revealing tunable interaction strengths and pathways.

Contribution

It demonstrates that co-sputtering can finely tune the magnetic interaction strength and dynamics in artificial spin ice, with experimental validation against theoretical models.

Findings

Higher magnetization FePd samples reach >90% ground state vertices at 493 K.

Lower magnetization samples fluctuate at lower temperatures, reaching only 25% ground state vertices.

Experimental blocking temperatures differ significantly from simple theoretical predictions.

Abstract

Artificial spin ices (ASI) are arrays of single domain nano-magnetic islands, arranged in geometries that give rise to frustrated magnetostatic interactions. It is possible to reach their ground state via thermal annealing. We have made square ASI using different FePd alloys to vary the magnetization via co-sputtering. From a polarized state the samples were incrementally heated and we measured the vertex population as a function of temperature using magnetic force microscopy. For the higher magnetization FePd sample, we report an onset of dynamics at $T = 493$ K, with a rapid collapse into $>90\%$ ground state vertices. In contrast, the low magnetization sample started to fluctuate at lower temperatures, $T = 393$ K and over a wider temperature range but only reached a maximum of $25\%$ of ground state vertices. These results indicate that the interaction strength, dynamic temperature range and pathways can be finely tuned using a simple co-sputtering process. In addition we have compared our experimental values of the blocking temperature to those predicted using the simple N\'{e}el-Brown two-state model and find a large discrepancy which we attribute to activation volumes much smaller than the island volume.