Surface effects in nanoparticles: application to maghemite $\gamma$-Fe_{2}O_{3}

We present a review of recent advances in the study of many-body effects in magnetic nanoparticles. Considering classical spins on a lattice coupled by the exchange interaction in the presence of the bulk and surface anisotropy, we investigate the effects of finite size, free boundaries, and surface anisotropy on the average and local magnetization for zero and finite temperatures and magnetic fields. Superparamagnetism of magnetic particles necessitates introducing two diffe...

Experimental results and a model are presented to explain the observed unusual enhancement of the effective magnetic anisotropy Keff with decreasing particle size D from 15 nm to 2.5 nm in {\gamma}-Fe2O3 nanoparticles (NPs). The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3 and 7.0 nm investigated here, with the results on other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after con...

We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization taking account of temperature, applied field, core and surface anisotropy, and dipolar inter-particle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly mag...

This chapter is aimed at studying the anomalous magnetic properties (glassy behaviour) observed at low temperatures in nanoparticles of ferrimagnetic oxides. This topic is discussed both from numerical results and experimental data. Ferrimagnetic fine particles show most of the features of glassy systems due to the random distribution of anisotropy axis, interparticle interactions and surface effects. Experiments have shown that the hysteresis loops display high closure field...

The particle size (D) dependence of the effective magnetic anisotropy Keff of magnetic nanoparticles (NPs) usually shows Keff increasing with decreasing D. This dependence is often interpreted using the Eq.: Keff = Kb + (6Ks/D) where Kb and Ks are the anisotropy constants of the spins in the bulk-like core and surface layer, respectively. Here, we show that this model is inadequate to explain the observed size-dependency of Keff for smaller nanoparticles with D < 5 nm. Instea...

The magnetization curves of densely packed single domain magnetic nanoparticles (MNP) are investigated by Monte Carlo simulations in the framework of an effective one spin model. The particles whose size polydispersity is taken into account are arranged in spherical clusters and both dipole dipole interactions (DDI) and magnetic anisotropy energy (MAE) are included in the total energy. Having in mind the special case of spinel ferrites of intrinsic cubic symmetry, combined cu...

We use Monte Carlo simulations to study ${\rm Ni Fe_2O_4}$ nanoparticles. Finite size and surface effects differentiate them from their bulk counterparts. A continuous version of the Wang-Landau algorithm is used to calculate the joint density of states $g(M_z, E)$ efficiently. From $g(M_z, E)$, we obtain the Bragg-Williams free energy of the particle, and other physical quantities. The hysteresis is observed when the nanoparticles have both surface disorder and surface aniso...

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...

We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic ${\gamma}-Fe_{2}O_{3}$ nanoparticles. We have identified a critical concentration c*, that marks the transition between two different regimes in the evolution of the blocking temperature ($T_{B}$) with interparticle interactions. At low concentrations (c < c*) magnetic particles behave as an ideal non-interacting system with a constant $T_{B}$. At...

We investigate the magnetization dynamics of ferromagnetic nanoparticles in the atomistic approach taking account of surface anisotropy and the spin misalignment it causes. We demonstrate that such inhomogeneous spin configurations induce nutation in the dynamics of the particle's magnetization. More precisely, in addition to the ordinary precessional motion with frequency $f_{p}\sim10\,{\rm GHz}$, we find that the dynamics of the net magnetic moment exhibits two more resonan...