Triangular array of iron oxide nanoparticles: Simulation study of intraparticle and interparticle magnetism

Abstract

A study of spherical maghemite nanoparticles on a two dimensional triangular array was carried out using a stochastic Landau-Lifshitz-Gilbert (sLLG) approach. The simulation method was first validated with a triangular array of simple dipoles, where results show the expected phase transition to a ferromagnetic state at a finite temperature. The ground state exhibited a continuous degeneracy that was lifted by an order-from-disorder mechanism at infinitesimal temperatures with the appearance of a six-fold planar anisotropy. The nanoparticle array consisted of 7.5 nm diameter maghemite spheres with bulk-like superexchange interactions between Fe-ions in the core, and weaker exchange between surface Fe-ions and a radial anisotropy. The triangular nanoparticle array ordered at the same reduced temperature as the simple dipole array, but exhibited different behaviour at low temperatures due to the surface anisotropy. We find that the vacancies on the octahedral sites in the nanoparticles combine with the surface anisotropy to produce an effective random temperature-dependent anisotropy for each particle. This leads to a reduction in the net magnetization of the nanoparticle array at zero temperature compared to the simple dipole array. arXiv:1904.05515v1 [cond-mat.mes-hall]

Hysteresis in Two Dimensional Arrays of Magnetic Nanoparticles

“…All other parameters are the same as that of Fig. (2). There is a dominance of antiferromagnetic interaction for considerable dipolar interaction strength (h d > 0.2) and A r < 6.0 in this case also.…”

“…To see the effect of aspect ratio A r and direction of the applied magnetic field on the hysteresis, we then study magnetic hysteresis for six values of A r in Fig. (2). The magnetic field is applied along the x-direction, and temperature T is taken as 300 K. Irrespective of A r , the hysteresis loop area is negligibly tiny for very weak dipolar interaction strength (h d ≤ 0.2), indicating the dominance of superparamagnetic behaviour.…”

“…In recent years, two-dimensional arrays of magnetic nanoparticles (MNPs) have received significant attention due to their unique magnetic properties and potential technological applications [1][2][3][4][5][6][7][8]. From theoretical perspectives, they pose an exciting research topic in which competing interactions, reduced dimensionality, shape anisotropy, etc., lead to novel collective behaviours [9][10][11].…”

Size- and temperature-dependent magnetization of iron nanoclusters

“…This allows the simulation of micron-sized systems that are of technological relevance [23][24][25][26]. ASD has been recently applied to Fe oxide nanoparticles (NPs), using a triangular lattice [27].…”