Effect of Organic Coating Variation on the Electric and Magnetic Behavior of Ferrite Nanoparticles.

Organic ligand coatings can modify the surface properties of nanoparticles. With the proper choice of the type of nanoparticles and of the ligand, a targeted modification can be achieved that is suitable for specific applications. In the present work, we employ density functional theory calculations with Hubbard corrections (DFT + ) to treat localized states in order to investigate the magnetic and electrostatic properties of ferrite nanoparticles (CoFeO and FeO) covered with COOH-terminated [oleic acid (OA)] and OH-terminated [diethylene glycol (DEG)] ligands by varying the ligands coverage.

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe O /Mn O Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction.

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe O /Mn O core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented.

Effect of albumin coating on the magnetic behavior of Mn ferrite nanoclusters.

The effect of clustering induced by albumin coating on the magnetic behaviour of ultra-small MnFeO nanoparticles has been systematically investigated and compared with that in pure Mn ferrite nanoparticle dense assembly, using a mesoscopic scale approach and numerical simulations reproducing the experimental findings well. Our results provide evidence that in the coated system, the interplay between intra-particle and intra-cluster exchange interactions strongly affects the exchange bias and coercive field values, with the dipolar interactions playing a minor role. Instead, the albumin coating does not affect the thermal stability of the observed superspin glass phase, the freezing temperature being similar in the coated and uncoated systems.

Interplay between inter- and intraparticle interactions in bi-magnetic core/shell nanoparticles.

The synthesis strategy and magnetic characterisation of two systems consisting of nanoparticles with core/shell morphology are presented: an assembly of hard/soft nanoparticles with cores consisting of magnetically hard cobalt ferrite covered by a magnetically soft nickel ferrite shell, and the inverse system of almost the same size and shape. We have successfully designed these nanoparticle systems by gradually varying the magnetic anisotropy resulting in this way in the modulation of the magnetic dipolar interactions between particles. Both nanoparticle systems exhibit high saturation magnetisation and display superparamagnetic behaviour at room temperature.

Crossover From Individual to Collective Magnetism in Dense Nanoparticle Systems: Local Anisotropy Versus Dipolar Interactions.

Dense systems of magnetic nanoparticles may exhibit dipolar collective behavior. However, two fundamental questions remain unsolved: i) whether the transition temperature may be affected by the particle anisotropy or it is essentially determined by the intensity of the interparticle dipolar interactions, and ii) what is the minimum ratio of dipole-dipole interaction (E ) to nanoparticle anisotropy (K V, anisotropy⋅volume) energies necessary to crossover from individual to collective behavior. A series of particle assemblies with similarly intense dipolar interactions but widely varying anisotropy is studied.

Tailoring defects and nanocrystal transformation for optimal heating power in bimagnetic CoFeO@CoFeO particles.

The effects of cobalt incorporation in spherical heterostructured iron oxide nanocrystals (NCs) of sub-critical size have been explored by colloidal chemistry methods. Synchrotron X-ray total scattering methods suggest that cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy their vacant iron sites, offering a higher degree of resistance to oxidative conversion. Self-passivation still creates a spinel-like shell, but with a higher volume fraction of the rock salt Co-containing phase in the core.

Magnetism of Nanoparticles: Effect of the Organic Coating.

The design of novel multifunctional materials based on nanoparticles requires tuning of their magnetic properties, which are strongly dependent on the surface structure. The organic coating represents a unique tool to significantly modify the surface structure trough the bonds between the ligands of the organic molecule and the surface metal atoms. This work presents a critical overview of the effects of the organic coating on the magnetic properties of nanoparticles trough a selection of papers focused on different approaches to control the surface structure and the morphology of nanoparticles' assemblies.

Memory and superposition in a superspin glass.

The non-equilibrium dynamics of the superspin glass state of a dense assembly of ~ 2 nm MnFeO nanoparticles was investigated by means of magnetization, ac susceptibility and Mössbauer spectroscopy measurements and compared to the results of Monte Carlo simulations for a mesoscopic model that includes particles morphology and interparticle interactions. The zero-field cooled (ZFC), thermoremanent (TRM), and isothermal remanent magnetization (IRM) were recorded after specific cooling protocols and compared to those of archetypal spin glasses and their dimensionality. The system is found to display glassy magnetic features.

Bad neighbour, good neighbour: how magnetic dipole interactions between soft and hard ferrimagnetic nanoparticles affect macroscopic magnetic properties in ferrofluids.

Fluids responding to magnetic fields (ferrofluids) offer a scene with no equivalent in nature to explore long-range magnetic dipole interactions. Here, we studied the very original class of binary ferrofluids, embedding soft and hard ferrimagnetic nanoparticles. We used a combination of X-ray magnetic spectroscopy measurements supported by multi-scale experimental techniques and Monte-Carlo simulations to unveil the origin of the emergent macroscopic magnetic properties of the binary mixture.

Effect of albumin mediated clustering on the magnetic behavior of MnFeO nanoparticles: experimental and theoretical modeling study.

Over the last two decades, iron oxide based nanoparticles ferrofluids have attracted significant attention for a wide range of applications. For the successful use of these materials in biotechnology and energy, surface coating and specific functionalization is critical to achieve high dispersibility and colloidal stability of the nanoparticles in the ferrofluids. In view of this, the magnetic behavior of clusters of ultra-small MnFeO nanoparticles covered by bovine serum albumin, which is known as a highly biocompatible and environmentally friendly surfactant, is investigated by magnetization measurements, and numerical simulations at an atomic and mesoscopic scale.

Optimising the magnetic performance of Co ferrite nanoparticles via organic ligand capping.

Ferrofluids of CoFeO nanoparticles are gaining increasing interest due to their enhanced heating performance in biomedical applications (e.g. in magnetic hyperthermia as mediators for cancer treatment) or in energy applications (e.

Fe-Doping-Induced Magnetism in Nano-Hydroxyapatites.

Doping of biocompatible nanomaterials with magnetic phases is currently one of the most promising strategies for the development of advanced magnetic biomaterials. However, especially in the case of iron-doped magnetic hydroxyapatites, it is not clear if the magnetic features come merely from the magnetic phases/ions used as dopants or from complex mechanisms involving interactions at the nanoscale. Here, we report an extensive chemical-physical and magnetic investigation of three hydroxyapatite nanocrystals doped with different iron species and containing small or no amounts of maghemite as a secondary phase.

Superspin glass state in a diluted nanoparticle system stabilized by interparticle interactions mediated by an antiferromagnetic matrix.

In nanoparticle systems consisting of two magnetic materials (bi-magnetic nanoparticles or nanoparticles embedded in a magnetic matrix), there is a constantly growing interest in the investigation of the interplay between interparticle interactions and the nanoparticle-matrix interface exchange coupling, because of its enormous impact on a number of technological applications. The understanding of the mechanisms of such interplay is a great challenge, as it would allow controlling equilibrium and non-equilibrium magnetization dynamics of exchange coupled nanoparticles systems and finely tuning their anisotropy. Here, we provide evidence that this interplay leads to a collective superspin glass (SSG) behavior in a system of diluted ferromagnetic (FM) nanoparticles embedded in an antiferromagnetic (AFM) matrix (5% volume fraction of Co particles in Mn film matrix).

Robust Ferromagnetism of Chromium Nanoparticles Formed in Superfluid Helium.

Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are strongly ferromagnetic. The transition from antiferromagentism to ferromagnetism is attributed to atomic-scale disorder in chromium nanoparticles, leading to abundant unbalanced surface spins. Theoretical modeling confirms a frustrated aggregation process in superfluid helium due to the antiferromagnetic nature of chromium.

Enhanced magnetic properties in antiferromagnetic-core/ferrimagnetic-shell nanoparticles.

Bi-magnetic core/shell nanoparticles are gaining increasing interest due to their foreseen applications. Inverse antiferromagnetic(AFM)/ferrimagnetic(FiM) core/shell nanoparticles are particularly appealing since they may overcome some of the limitations of conventional FiM/AFM systems. However, virtually no simulations exist on this type of morphology.

Susceptibility losses in heating of magnetic core/shell nanoparticles for hyperthermia: a Monte Carlo study of shape and size effects.

Optimizing the heating properties of magnetic nanoparticles is of great importance for hyperthermia applications. Recent experimental results show that core/shell nanoparticles could give an increased specific absorption rate (SAR) compared to the magnetic oxide nanoparticles currently used. We have developed a modified phenomenological model based on the linear Néel-Brown relaxation model to calculate the SAR due to susceptibility losses in complex nanoparticles with ferromagnetic (FM) core/ferrimagnetic (FiM) shell morphology.

Origin of the large dispersion of magnetic properties in nanostructured oxides: Fe(x)O/Fe3O4 nanoparticles as a case study.

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated.

Assembly-mediated interplay of dipolar interactions and surface spin disorder in colloidal maghemite nanoclusters.

Controlled assembly of single-crystal, colloidal maghemite nanoparticles is facilitated via a high-temperature polyol-based pathway. Structural characterization shows that size-tunable nanoclusters of 50 and 86 nm diameters (D), with high dispersibility in aqueous media, are composed of ∼13 nm (d) crystallographically oriented nanoparticles. The interaction effects are examined against the increasing volume fraction, φ, of the inorganic magnetic phase that goes from individual colloidal nanoparticles (φ = 0.

Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles.

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive.

Three dimensional quantitative characterization of magnetite nanoparticles embedded in mesoporous silicon: local curvature, demagnetizing factors and magnetic Monte Carlo simulations.

Magnetite nanoparticles embedded within the pores of a mesoporous silicon template have been characterized using electron tomography. Linear least squares optimization was used to fit an arbitrary ellipsoid to each segmented particle from the three dimensional reconstruction. It was then possible to calculate the demagnetizing factors and the direction of the shape anisotropy easy axis for every particle.

Stepwise behaviour of magnetization temperature dependence in iron nanoparticle assembled films.

An unusual stepwise behaviour is reported in the temperature dependence of the zero field cooled magnetization in iron nanoparticle dense films produced by ultra-short pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. This behaviour, induced by the particle system's morphology, characterized by clusters of tightly coupled nanoparticles as well as by some voids between them, is ascribed to the competition between Zeeman energy density, intracluster anisotropy energy density and intercluster exchange energy density. A phenomenological model and Monte Carlo simulations are reported, which support the proposed interpretation.

Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn(x)Fe(3-x)O4/Fe(x)Mn(3-x)O4, core/shell heterostructured nanoparticles.

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn(x)Fe(3-x)O(4)/Fe(x)Mn(3-x)O(4) with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.

Mesoscopic model for the simulation of large arrays of bi-magnetic core/shell nanoparticles.

A mesoscopic approach to simulate large arrays of core/shell nanoparticles based on the reduction of the number of simulated spins is presented. The model is used to simulate arrays of Co/CoO nanoparticles with different nanoparticle densities.

Interface exchange coupling in Co nanoparticles dispersed in a Mn matrix.

The structural and magnetic properties of 1.8 nm Co particles dispersed in a Mn matrix by co-depositing pre-formed mass-selected Co clusters with an atomic vapour of Mn onto a common substrate have been studied by using EXAFS (extended x-ray absorption fine structure), XMCD (x-ray magnetic circular dichroism), magnetometry, and theoretical modelling. At low Co volume fraction (5%) Co@Mn shows a significant degree of alloying and the well-defined particles originally deposited become centres of high Co concentration CoMn alloy that evolves from pure Co at the nanoparticle centre to the pure Mn matrix within a few nm.