Magnetic and Mössbauer spectroscopy studies of nanocrystalline iron oxide aerogels

A sol-gel synthesis was used to produce iron oxide aerogels. These nanocrystalline aerogels have a pore-solid structure similar to silica aerogels but are composed entirely of iron oxides. Mössbauer experiments and x-ray diffraction showed that the as-prepared aerogel is an amorphous or poorly crystalline iron oxide, which crystallized as a partially oxidized magnetite during heating in argon. After further heat treatment in air, the nanocrystallites are fully converted to maghemite. The particles are superparamagnetic at high temperatures, but the magnetic properties are strongly influenced by magnetic interactions between the particles at lower temperatures

Evidence of random magnetic anisotropy in ferrihydrite nanoparticles based on analysis of statistical distributions

We show that the magnetic anisotropy energy of antiferromagnetic ferrihydrite depends on the square root of the nanoparticles volume, using a method based on the analysis of statistical distributions. The size distribution was obtained by transmission electron microscopy, and the anisotropy energy distributions were obtained from ac magnetic susceptibility and magnetic relaxation. The square root dependence corresponds to random local anisotropy, whose average is given by its variance, and can be understood in terms of the recently proposed single phase homogeneous structure of ferrihydrite.Comment: 6 pages, 2 figure

Micromagnetic simulations of small arrays of submicron ferromagnetic particles

We report the results of a set of simulations of small arrays of submicron ferromagnetic particles. The actions of dipolar and exchange interactions were qualitatively investigated by analysing the ferromagnetic resonance spectra at 9.37 GHz resulting from the magnetization response of con- nected and unconnected particles in the array as a function of the applied dc magnetic field. We find that the magnetization precession movement (at resonance) observed in individual particles in the array presents a distinctive behaviour (an amplitude mismatch) in comparison to isolated, one-particle reference simulations, a result that we attribute to the action of interparticle dipolar couplings. Exchange interactions appear to have an important role in modifying the spectra of connected particles, even through a small contact surface.Comment: 24 pages, 8 figures, accepted for publication in Physical Review

Magnetic relaxation in finite two-dimensional nanoparticle ensembles

We study the slow phase of thermally activated magnetic relaxation in finite two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles whose easy axes of magnetization are perpendicular to the distribution plane. We develop a method to numerically simulate the magnetic relaxation for the case that the smallest heights of the potential barriers between the equilibrium directions of the nanoparticle magnetic moments are much larger than the thermal energy. Within this framework, we analyze in detail the role that the correlations of the nanoparticle magnetic moments and the finite size of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure