Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent

In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature TB≃50K in D≃5nm NPs but only diamagnetism in 12nm NPs. For T<TB=50K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc≃250Oe, He≃−40Oe, and Ms≃10−2emu∕g at 5K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ≃0.006μB per Au atom attached to DT is determined using a model which yields Ms varying as 1∕D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.