Spin morphologies and heat dissipation in spherical assemblies of magnetic nanoparticles

Abstract

Aggregates of magnetic nanoparticles (MNPs) exhibit unusual properties due to the interplay of small system size and long-range dipole-dipole interactions. Using the micromagnetic simulation software OOMMF, we study the spin morphologies and heat dissipation in micron-size spherical assemblies of MNPs. In particular, we examine the sensitivity of these properties to the dipolar strength, manipulated by the interparticle separation. As OOMMF is not designed for such a study, we have incorporated a novel scaling protocol for this purpose. We believe that it is essential for all studies where volume fractions are varied. Our main results are as follows: (i) Dense assemblies exhibit strong dipolar effects which yield local magnetic order in the core but not on the surface, where moments are randomly oriented. (ii) The probability distribution of ground-state energy exhibits a long high-energy tail for surface spins in contrast to small tails for the core spins. Consequently, there is a wide variation in the energy of surface spins but not the core spins. (iii) There is strong correlation between ground-state energy and heating properties on application of an oscillating magnetic field h(t) = h o cos 2πf t: the particles in the core heat uniformly, while those on the surface exhibit a wide range from cold to intensely hot. (iv) Specific choices of h o and f yield characteristic spatial heat distributions, e.g., hot surface and cold core, or vice versa. (iv) For all values of h o and f that we consider, heating was maximum at a specific volume fraction. These results are especially relevant in the context of contemporary applications such as hyperthermia and chemotherapy, and also for novel materials such as smart polymer beads and superspin glasses.

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

“…Understanding the precise influence of dipolar interactions on the magnetization behavior of magnetic nanoparticles is of utmost importance for potential technological or biomedical applications [1,2]. In particular the magnetic hyperthermia performance of nanoparticle ensembles can significantly depend on the core arrangement [3][4][5][6][7][8][9][10][11], mainly due to the sensitivity of magnetic relaxation on induced dipolar interaction energy [12][13][14][15][16][17][18][19][20]. Experimentally, interacting nanoparticle ensembles have been much characterized via temperaturedependent magnetometry techniques [21][22][23][24][25][26][27].…”

Magnetization Reversal in Two-dimensional Ensemble of Nanoparticles with Positional Defects

“…Let the particle diameter be D and the lattice spacing be l. Let the magnetocrystalline anisotropy constant be K eff and particle volume V = πD 3 /6. So, the anisotropy energy of single particle can be expressed as [52,53].…”

Micromagnetic simulations of clusters of nanoparticles with internal structure: Application to magnetic hyperthermia

“…Arrays of NPs packed in different arrangements such as spheres, cubes and fcc structures have been studied and reported in the literature [15,18,23,36]. Fu et al [18] investigated the dipole interaction effects on the heating performance of a cluster of 64 and 63 superparamagnets in simple cubic and fcc structures, respectively.…”

“…Different studies have used the MS approximation to investigate NP clustering in a variety of arrangements such as chains, rings, cubes, face-centered cubic (fcc), 2D hexagonal lattice, spheres or disordered structures, and reported productive or destructive effects of dipole interactions on the collective heating performance of NPs depending on the particle arrangement [12,14,15,18,21,23,[36][37][38][39][40]. For example, Anand [14] used Monte Carlo simulations to study the effect of dipole interactions on the heating efficiency of a chain of NPs when their uniaxial anisotropy axes make an angle θ with respect to the chain axis.…”

“…They found that the chain formation of NPs could improve the heating performance even if the chains are not aligned with respect to one another. Anand et al [36] explored the heating behavior of micron-sized spherical clusters of NPs by changing the amplitude and frequency of the applied field. They found different heating behavior for the core and surface NPs, which highly depends on the field parameters used.…”