The influence of shape and structure on the Curie temperature of Fe and Co nanoparticles

We have investigated the effect of lattice fluctuations on the magnetic properties of nanoparticles of Fe and Co. Atomic structures were simulated using a molecular-dynamic approach, with the system slowly cooled into the ordered phase. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian taking into account the long-range nature and atomic separation dependence of the exchange. The magnetic properties are found to be affected by both the particle shape and the lattice fluctuations. For a perfectly ordered lattice we find that a spherical particle has a larger magnetization for a given temperature than a cube containing the same number of atoms. We have also studied the effect of lattice fluctuations. This involves a comparison of M T for two cases, firstly, a nanoparticle with a fixed lattice corresponding to the low-temperature annealed state T=20 K , and secondly a nanoparticle with a lattice structure equilibrated at the temperature T, the latter case being subject to fluctuations in the lattice spacing, and the nanoparticle shape. The dynamic structure gives rise to a reduction in the magnetization and Tc, which is a finite size effect to be considered beyond others such as the reduction in coordination at the nanoparticle surface.