Role of dipolar interactions on morphologies and tunnel magnetoresistance in assemblies of magnetic nanoparticles

We undertake comprehensive simulations of 2d arrays (Lx×Ly) of magnetic nanoparticles (MNPs) with dipole-dipole interactions by solving LLG equations. Our primary interest is to understand the correspondence between equilibrium spin (ES) morphologies and tunnel magnetoresistance (TMR) as a function of Θ - the ratio of the dipolar to the anisotropy strength, sample size Lx, aspect ratio Ar=Ly/Lx and the direction of the applied field H→=HêH. The parameter Θ is varied by choosing three distinct particles: (i) α-Fe2O3(Θ≃0), (ii) Co (Θ≃0.37) and (iii) Fe3O4(Θ≃1.28). Our main observations are as follows: (a) For weakly interacting spins (Θ≃0), the morphology has randomly oriented magnetic moments for all sample sizes and aspect ratios. The TMR exhibits a peak value of 50% at the coercive field Hc. It is robust with respect to Lx and Ar, and isotropic with respect to êH. (b) For strong interactions (Θ>1), the moments order in the plane of the sample. The ES morphology comprises of magnetically aligned regions interspersed with flux closure loops. For fields along x or y, the maximum TMR amplitude decrease to ∼30%. For êH=ẑ, it drops to ∼3%. The TMR is robust with respect to Lx and Ar and isotropic in the x and y directions only. (c) In strongly interacting samples (Θ>1) with Lx comparable to the size of a flux closure loop, increasing Ar creates ferromagnetic chains in the sample oriented along y or -y. Consequently, for êH=ŷ, the TMR magnitude for Ar=1 is ∼33% while that for Ar=32 drops to ∼16%. For êH=x̂ on the other hand, it is ∼30% and independent of Ar. The TMR of long ribbons of MNPs has a strong dependence on Ar and is anisotropic in all three directions.