Ab initio calculation of interlayer exchange coupling in Co-based synthetic antiferromagnet with alloy spacer

Rina Takashima,Takamitsu Ishihara,Takayuki Tsukagoshi,Tadashi Kai

doi:10.1063/1.5129719

Abstract

Synthetic antiferromagnets (SAF) are widely used in magnetic tunnel junctions in order to reduce the stray field of the pinned layer. In SAF, the antiparallel spin alignment is stabilized by interlayer exchange coupling (IEC) between two ferromagnetic layers that are separated by a spacer layer. The strength of IEC depends on the spacer material. It is known that strong IEC is required for stable operation of magnetic random access memory with a low error rate, and hence a spacer material that shows strong IEC has been highly sought after. In this study, we investigate the IEC of ordered alloy spacers in Co multilayers by ab initio calculations based on density functional theory. We find that the IEC of binary alloy spacers composed of group 8 and 9 elements, namely, RuIr, RuRh, and OsIr, are greater than that of Ru and Ir spacers, which have commonly been used in SAF pinned layers. We also present dependence of IEC strength on the composition ratio of these alloy spacers. The relation between the strength of IEC and the lattice length is discussed.

Highlights

Interlayer exchange coupling (IEC),1–3 which is coupling between ferromagnetic layers separated by a spacer layer, plays a crucial role in spintronic devices such as hard disk drives and magnetic random access memory (MRAM)
We find that binary alloy scitation.org/journal/adv spacers composed of group 8 and 9 elements show stronger IEC than spacers composed of unalloyed Ru and Ir, which have typically been used in magnetic tunnel junction (MTJ)
We investigated the IEC of several ordered alloy spacers by ab initio calculation

Summary

INTRODUCTION

Interlayer exchange coupling (IEC), which is coupling between ferromagnetic layers separated by a spacer layer, plays a crucial role in spintronic devices such as hard disk drives and magnetic random access memory (MRAM). Antiferromagnetic IEC is widely used to decrease the stray field from the pinned layer.. In SAF pinned layers, when the IEC is not strong enough, the spin state of the SAF pinned layer is unstable and random spin flips can occur under high voltage.. In SAF pinned layers, when the IEC is not strong enough, the spin state of the SAF pinned layer is unstable and random spin flips can occur under high voltage.8 Such spin flips cause write errors in MRAM. The oscillation period and IEC strength depend on the spacer material.. In MTJs, Co/Ru/Co9,10 and Co/Ir/Co11 show relatively strong antiferromagnetic IEC and have been used as SAF pinned layers. The oscillation period of IEC depends on the composition ratio of the spacer alloy.. Our findings provide promising new candidates for SAF materials in MTJs

METHOD

RESULTS AND DISCUSSION

CONCLUSION