Finite-size effect on Néel temperature in antiferromagnetic nanoparticles

Abstract

Muon spin relaxation/rotation $(\ensuremath{\mu}\mathrm{SR})$ and magnetic susceptibility measurements were carried out on antiferromagnetic nanoparticles of CuO. Nanoparticles with center size of around 5 nm were prepared by ball-milling from single crystals of CuO and investigated using $\ensuremath{\mu}\mathrm{SR}$ measurements. In the $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ assembly, the ${T}_{\mathrm{N}}$ was reduced drastically to $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, compared with the bulk ${T}_{\mathrm{N}}=229\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A similar effect was observed in a system of 2 to 3 nm diameter nanorods, which was synthesized by a direct solution reaction method, where ${T}_{\mathrm{N}}$ was suppressed further to 13 K. The present work reports direct evidence of a dramatic finite-size effect on the magnetic transition temperature in antiferromagnetic systems.