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 T-B similar or equal to 50 K in D similar or equal to 5 nm NPs but only diamagnetism in 12 nm NPs. For T < T-B=50 K, the strong temperature dependence of coercivity H-c, saturation magnetization M-s, and exchange bias H-e (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with H-c similar or equal to 250 Oe, H-e similar or equal to-40 Oe, and M-s similar or equal to 10(-2) emu/g at 5 K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below T-B. A magnetic moment mu similar or equal to 0.006 mu(B) per Au atom attached to DT is determined using a model which yields M-s 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. (c) 2007 American Institute of Physics.