Surface vacancy mediated pinning of the magnetization inγ−Fe2O3nanoparticles: A micromagnetic simulation study

Dipolar ferromagnetism in three-dimensional superlattices of nanoparticles

“…In this section we demonstrate that the picture that emerges from these simulations and those presented in Ref. 12 are at least qualitatively consistent with the experimental data of Ref. 8, and, more importantly perhaps, how simulations studies similar to those presented in the current work and Ref.…”

“…In this paper we present results from a series of atomistic finite temperature simulations on a model of an FCC lattice of maghemite nanoparticles using the stochastic Landau-Lifshitz-Gilbert (sLLG) equation. These simulations, when taken together with those presented in our earlier studies on ensembles of non-interacting maghemite nanoparticles 12 , provide an interesting complement to recent experiments on magnetoferritin (Fe 3 O 4 /γ-Fe 2 O 3 ) particles 8 in which the particles can be self-assembled to form a FCC superlattice with typical length of the order of 1.5-2.0 µm but which can be disassembled following the application of an optical stimulus. Comparing the results from before and after the disassembly clearly show the effects of the FCC ordering on the magnetic properties of the particles.…”

“…The origin of this additional disordering implied by the reduced order parameter σ nl observed in Fig. 3 may be understood from our previous simulation studies on ensembles of non-interacting maghemite nanoparticles 12 , in which it was shown that the magnetic moment of the nanoparticles were pinned by the surface vacancies in the octahedral B-sites by the surface magnetization. This pinning effect is a result of the frustration that arises as consequence of the competition between the exchange and the surface anisotropy.…”

“…Because of this competition it was shown in Ref. [12] that for each nanoparticle there exists a Néel-like domain wall in the surface magnetization at the equatorial plane separating the north and south magnetic poles in which the spins, located at the sites within this domain wall, are highly frustrated. As a consequence the presence of a vacancy located at a site within the domain wall will generally result in a lower energy than if the same vacancy were located at a site close to one of the poles.…”

Competing ferro- and antiferromagnetic exchange drives shape-selective $$\hbox{Co}_3\hbox{O}_4$$ nanomagnetism

“…As the surface spins freeze we would expect to see a large increase in the coercivity and exchange bias field 30 , 31 . The plates show a constant coercivity at temperatures below , and all shapes show similar behaviour in indicating that no freezing is occurring at the surface 32 – 34 .…”