Monte Carlo Study of the Finite Size Effects on the Magnetization of Maghemite Small Particles

Monte Carlo simulations of a model for $\gamma$-Fe$_2$O$_3$ (maghemite) single particle of spherical shape are presented aiming at the elucidation the specific role played by the finite size and the surface on the anomalous magnetic behaviour observed in small particle systems at low temperature. The influence of the finite-size effects on the equilibrium properties of extensive magnitudes, field coolings and hysteresis loops is studied an compared to the results for periodic boundaries. It is shown that for the smallest sizes the thermal demagnetization of the surface completely dominates the magnetization while the behaviour of the core is similar to that of the periodic boundary case, independently of $D$. The change in shape of the hysteresis loops with $D$ demonstrates that the reversal mode is strongly influenced by the presence of broken links and disorder at the surface.Comment: Presented at the 8th Joint MMM-Intermag Conference, San Antonio, Texas, 7-11 January 2001. Session HF-09. To be published in J. Appl. Phys. 3 pages, 3 figure

Magnetic Domains and Surface Effects in Hollow Maghemite Nanoparticles

In the present work, we investigate the magnetic properties of ferrimagnetic and noninteracting maghemite (g-Fe2O3) hollow nanoparticles obtained by the Kirkendall effect. From the experimental characterization of their magnetic behavior, we find that polycrystalline hollow maghemite nanoparticles are characterized by low superparamagnetic-to-ferromagnetic transition temperatures, small magnetic moments, significant coercivities and irreversibility fields, and no magnetic saturation on external magnetic fields up to 5 T. These results are interpreted in terms of the microstructural parameters characterizing the maghemite shells by means of an atomistic Monte Carlo simulation of an individual spherical shell model. The model comprises strongly interacting crystallographic domains arranged in a spherical shell with random orientations and anisotropy axis. The Monte Carlo simulation allows discernment between the influence of the structure polycrystalline and its hollow geometry, while revealing the magnetic domain arrangement in the different temperature regimes.Comment: 26 pages, 8 figures. In press in Phys. Rev.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice [Dataset]

6 pages. -- S1. Transversal electric field distribution for the SLR at 1.57 eV. -- S2. Simulated electric field and charge distributions for a threesome of slits. -- S3. Simulated electric field and charge distributions for the simplest local dark mode of the inverted honeycomb lattice. -- S4. Profiles of the EELS signal and the simulated electric field along the slits for the antiferroelectric dark modes. -- S5. Array of the magnetic dipoles over the structure used to simulate antiferroelectric dark modes.Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5–2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.Peer reviewe

Tuning the magnetic properties of Co-ferrite nanoparticles through the 1,2-hexadecanediol concentration in the reaction mixture

This work reports on the effect of the 1,2-hexadecanediol content on the structural and magnetic properties of CoFe2O4 nanoparticles synthesized by thermal decomposition of metal–organic precursors in 1-octadecene. Although pseudo-spherical particles having an average size of about 8 nm and similar stoichiometry have been observed in all studied samples, a high level of variability in the crystal quality and, in turn, in the magnetic properties has been found as a function of the amount of 1,2-hexadecanediol added to the reaction mixture. The magnetic study reveals that samples progress from glassy magnetic behavior to bulk-like, ferrimagnetic order as the crystal quality improves. The analysis of the reaction mixtures by Fourier transform infrared spectroscopy at various stages of the reaction shows the key role of the 1,2-hexadecanediol in favoring the decomposition of the metal–organic precursor, formation of an intermediate Co2+Fe3+–oleate complex and, finally, the nucleation of nanoparticles at lower temperatures.This work was supported by Spanish MINECO (MAT2011-23641, MAT2012-33037, MAT2013-48054-C2), Catalan DURSI (2014SGR220) and European Union FEDER funds (Una manera de hacer Europa). C. Moya acknowledges Spanish MINECO for a PhD contract (BES-2010-038075) and a three month stay at the ICMM-CSIC (Madrid)