Magnetic order of Au nanoparticle with clean surface

•Au nanoparticles with clean surface were prepared by the gas evaporation method.•The spontaneous magnetization was observed in Au nanoparticles.•Temperature dependent spontaneous magnetization of smaller Au particles was not monotonic.•The magnetic behavior was interpreted by the ferrimagnetic-like core–shell model.•The shell thickness and the magnetic moment per Au atom were estimated.

Au nanoparticles, which are kept in vacuum after the preparation by gas evaporation method, show ferromagnetism even in 1.7 nm in diameter. The intrinsic magnetism is examined by detecting the disappearance of spontaneous magnetization in Au bulk prepared by heating the nanoparticles without exposure to the air. The temperature dependence of spontaneous magnetization is not monotonic and the increase in magnetization is observed after Au nanoparticles are exposed to the air. The magnetic behavior can be interpreted by the ferrimagnetic-like core–shell structure with shell thickness of 0.16±0.01 nm and magnetic moment of (1.5±0.1)×10−2 μB/Au atom, respectively.