Improved performance of polycrystalline antiferromagnet/ferromagnet stack by nitrogen assisted deposition

TL;DR

Introducing nitrogen during sputtering of IrMn/CoFe bilayers significantly enhances exchange bias and reduces coercivity by increasing antiferromagnetic anisotropy, leading to improved magnetic performance and cost savings.

Contribution

This study demonstrates that nitrogen-assisted deposition enhances magnetic properties of AF/FM stacks and allows for reduced AF layer thickness, saving costs and improving performance.

Findings

Increased exchange bias energy with nitrogen addition.

Reduced coercivity of the ferromagnetic layer.

Enhanced anisotropy energy due to nitrogen diffusion.

Abstract

The deposition process of the IrMn$_3$/Co$_{70}$Fe$_{30}$ bilayer of antiferromagnet/ferromagnet (AF/FM) type was modified by introducing a nitrogen additive in argon plasma during the magnetron sputtering of the Co$_{70}$Fe$_{30}$ layer. This slight modification significantly enhanced the exchange bias energy of the AF/FM bilayer for an IrMn layer thickness ranging from 20{\AA} to 50{\AA} and reduced the coercivity of the FM layer. Calculations indicate that the boost in exchange bias energy and the reduction in coercivity can be attributed to the increased anisotropy energy of the antiferromagnet, resulting in more effective pinning of the ferromagnet by antiferromagnetic grains. The increase in anisotropy is caused by the diffusion of nitrogen from the FM into the AF layer, as established by X-ray diffraction, Neutron Reflectometry, and X-ray Magnetic Circular Dichroism. Our research allows to improve magnetic characteristics of exchange-coupled FM/AF structures through minor modifications in the sputtering process and/or save up to 20% of the costly IrMn$_3$ target by reducing the thickness of the AF layer.