Geometric aspects of the dipolar interaction in lattices of small particles

Abstract

The hysteresis curves of systems composed of small interacting magnetic particles, regularly placed on stacked layers, are obtained with Monte Carlo simulations. The remanence as a function of temperature, in interacting systems, presents a peak that separates two different magnetic states. At low temperatures, small values of remanence are a consequence of antiferromagnetic order due to the dipolar interaction. At higher values of temperature the increase of the component normal to the lattice plane is responsible for the small values of remanence. The effect of the number of layers, coordination number and distance between particles are investigated.

2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals

“…The total vorticity of the arrays increases with the thickness, in good agreement with previous simulations. 27 Thus, the vorticity and the remanent magnetization in Fig. 5 are inversely correlated.…”

Magnetic dipolar interaction in ultrathin films: Structural and size effects

“…Realistic values of this parameter correspond, for instance, to the tetragonal distortion of cubic cells, which is crucial in the understanding of experimentally reported planar/perpendicular configuration transitions [8,9]. Rather large or small values of the actual crystallographic structural aspect ratio are not achieved in metal-on-metal films; nevertheless, they result of interest not only for an easier identification of these effects in such systems, but also because of the possible realization of other similar systems, bearing in mind that by magnetic Q2D crystal structures we mean only those with nodes occupied by atomic dipoles, as the set of rare-earth atoms in Perovskite structures of some magnetic compounds [1], the case when regular arrays of vacancies are present inside magnetic ultrathin films [12], and systems composed of small interacting magnetic particles, regularly placed on stacked layers [11].…”

“…Dealing with the magnetic moments related to the atomic spins (or eventually the sum of spin and orbital moments [10]) allows applications not only to atoms, but also to other magnetic nanoparticles embedded in non-magnetic matrix and considered as point dipoles arranged in a Q2D crystal array [11], once the following starting assumptions can be accepted. Distinct magnetizations along the growth direction are in principle allowed, particularly when the number of atomic layers is large enough.…”

“…When r ! t, it can be shown that the term ÀV d erfðjr À tjxÞ=jr À tj, with erfðuÞ the error function, must be added to (11). The Q2D Lorentz potential is then…”

“…Different kinds of theoretical calculations (see, e.g., [10]) and Monte Carlo simulations (see, e.g., [11,12]) have been developed. Simple phenomenological expressions are frequently employed when attention is mainly focused on magnetization switching: since the exchange interaction gives the same value for both planar and perpendicular magnetization, it can be left out of consideration [1,3]; the magnetocrystalline energy has a volume contribution associated to the crystal anisotropy, as well as a surface contribution associated to the perpendicular anisotropy, which depends on the film thickness and is responsible for the spin reorientation transition [1,4,7]; the dipolar energy is then assumed as related to the demagnetization field due to the magnetic poles at both planar and lateral film surfaces, but this is just the dipolar shape contribution to the most fundamental term of the above mentioned Hamiltonian.…”