High temperature magnetic stabilization of cobalt nanoparticles by an antiferromagnetic proximity effect

TL;DR

Embedding cobalt nanoparticles in a high Neel temperature antiferromagnetic NiO matrix induces a magnetic proximity effect that significantly enhances their thermal stability above 400 K, overcoming thermal activation limitations.

Contribution

This work demonstrates a novel magnetic proximity effect that stabilizes cobalt nanoparticles at high temperatures, a significant advancement over previous methods.

Findings

Cobalt nanoparticles become magnetically stable above 400 K within NiO matrix.

The proximity effect induces an effective antiferromagnetic behavior with elevated TN.

Core-shell exchange-bias coupling stabilizes the magnetic moment against thermal fluctuations.

Abstract

Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications in ultra-high density recording among other applications. Here we demonstrate that low blocking temperature ferromagnetic (FM) Co nanoparticles (TB<70 K) become magnetically stable above 400 K when embedded in a high N\'eel temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable TB enhancement is due to a magnetic proximity effect between a thin CoO shell (with low N\'eel temperature, TN; and high anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix (with high TN but low KAFM). This proximity effect yields an effective AFM with an apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed TB increase. Mean-field calculations provide a semi-quantitative understanding of this magnetic- proximity stabilization mechanism.