Coupled structural and magnetic properties of ferric fluoride nanostructures: Part II, a Monte Carlo–Heisenberg study

Abstract

a b s t r a c tWe present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte Carlo Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with superexchange angle dependent interactions. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic) at the grain boundary, in agreement with experimental features. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. We conclude on possible extensions of the model.

Surface anisotropy of iron oxide nanoparticles and slabs from first principles: Influence of coatings and ligands as a test of the Heisenberg model

“…The atomic positions, represented on figure 3, changed by less than 2 % in comparison to the input values after optimization, and the cell parameters changed by less than 1 %. A potential application of such a system might be found in the pinning of magnetic moments, and hence a change in hysteresis curves, from the coupling to antiferromagnetic materials [26,27] From discussions with experimental teams [19] developing ferrites made of hybrid oxide nanoparticles we then tried a combined approach, generating a simple molecular dynamics simulation of a maghemite cluster coated with cobalt oxide, at 1000K, based on simple, Buckhingam-like ionic potentials [26] and extracting several representative clusters of increasing size around a random atom. The atomic positions in these clusters were then optimized using the BFGS procedure within the Quantum Espresso suite with the same conditions discussed above, using periodic boundary conditions with a cubic cell of 32 atomic units.…”

“…In the case of the isolated cluster, since the set of J <i,j> is not too large, we allowed those to take any value, but in the case of the surfaces, the large number of J <i,j> gave an excellent fit anytime, whatever the conditions, and the distribution of values obtained was hard to read. Therefore, in this latter case, we used the same model already used in [26], namely two parameters for each kind of super-exchange(iron-iron, iron-cobalt or cobalt-cobalt) related to the super-exchange angle θ in between atoms separated by an oxygen atom, using the relation…”