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

Abstract

Thermal activation tends to destroy the magnetic stability of small magnetic\nnanoparticles, with crucial implications in ultra-high density recording among\nother applications. Here we demonstrate that low blocking temperature\nferromagnetic (FM) Co nanoparticles (TB<70 K) become magnetically stable above\n400 K when embedded in a high N\\'eel temperature antiferromagnetic (AFM) NiO\nmatrix. The origin of this remarkable TB enhancement is due to a magnetic\nproximity effect between a thin CoO shell (with low N\\'eel temperature, TN; and\nhigh anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix\n(with high TN but low KAFM). This proximity effect yields an effective AFM with\nan apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to\nNiO. In turn, the Co core FM moment is stabilized against thermal fluctuations\nvia core-shell exchange-bias coupling, leading to the observed TB increase.\nMean-field calculations provide a semi-quantitative understanding of this\nmagnetic- proximity stabilization mechanism.\n