A multi-scale modeling approach is employed for the study of the effect of oleic-acid (OA) coverage on the magnetic behaviour of Co ferrite nanoparticles (CFNs), using high performance computing (HPC). Our study is performed in three different length scales: first we perform electronic structure calculations using parallel implementation of the density functional theory (DFT) method to study the magnetic properties of ultra-small OA coated CFNs. Next taking input from the DFT data, we calculate the magnetic characteristics of larger in size OA coated CFNs at an atomic scale performing Monte Carlo simulations. Finally, a mesoscopic modeling approach for interacting assemblies of nanoparticles is employed to reduce further computation time and sources for the study of the magnetic behaviour of CFNs covered with different percentage of oleic-acid, at finite temperature. The results demonstrate that the DFT magnetic moment and magnetic anisotropy of the nanoparticle decrease with the increase of the percentage of the surfactant. However, in the assembly of CFNs the interplay between the exchange and dipolar interparticle interactions results in the increase of the magnetic anisotropy and the decrease of the saturation magnetization as the percentage of OA coverage increases, in agreement with experimental findings. The proposed multi-scale computational approach, implemented in HPC environment, illustrates its ability to handle numerical calculations on complex magnetic interactions of multiple structural components. It can overcome computational limitations to predict optimum parameters for hybrid organic/inorganic nanomaterials for various applications.

This study focuses on the synthesis of metallic magnetic nanosystems embedded in mesoporous silica (SiO2), and the impact of matrix porosity, controlled by temperature treatment, on the efficiency of H2 reduction process. The reduction of FeCo oxides to the corresponding alloy nanosystems was first optimized, identifying FeCo with 50 at% Fe as the optimal composition due to its high saturation magnetization (∼242 A m2 kg-1) and oxidation onset temperature (∼440 °C). Then, the FeCo-oxide nanocomposites were synthesized into SiO2via sol-gel self-combustion under thermal treatments, to properly tune the surface area of the silica matrix. By controlling the annealing temperature, the specific surface area (SA) of the matrix decreases from ∼512(1) m2 g-1 to ∼345(1) m2 g-1 when annealed to 900 °C in air. Following topochemical reduction in H2, the structural properties of the obtained FeCo-SiO2 nanocomposites have been analyzed using X-ray powder diffraction and magnetic properties were evaluated to establish a correlation between matrix SA and reduction capability. The decrease of SA leads to incomplete reduction at higher temperatures, with the formation of FeYOX/CoXOY intermediates. This work underscores the critical role of matrix porosity in achieving a delicate balance to ensure both the efficient conversion of nanostructured oxide to their metallic state and the preservation of their magnetic and structural integrity.

Ferrofluids (FFs) based on high-moment nanoparticles have emerged as an important class of smart nanomaterials, because of their fast response to moderate strength magnetic fields. Understanding the mechanism of cluster formation stimulated by external magnetic fields, followed by cluster dissociation, is pivotal for FFs' magnetic manipulation. A new strategy is proposed here, using the diffusion limited cluster aggregation (DLCA) model, to investigate the characteristics and the optimum conditions for the formation of field-driven high-moment structures in a fluid at room temperature. The conditions for fast cluster dissociation after removing the field, studied for the first time, suggest a completely reversible process, resulting in the initial FF structures; therefore, the FFs can be reused. Two representative cases of high-moment material-based FFs are investigated: (1) the CoFe2O4 multicore particle-based FFs and (2) the FeCo alloy nanoparticle-based FFs. In both cases, each particle inside the fluid is covered with an organic surfactant shell. The numerical simulations demonstrate that (a) high magnetic moment plays a significant role in the cluster aggregation rate in the presence of the field and (b) steric interactions from the surfactant coating result in complete reversibility of the cluster process. The results open new perspectives for novel FF-based applications.

We have investigated the bottom-up sol-gel synthesis of nanocomposite powders comprising two magnetic phases (hexagonal Sr ferrite and spinel Co ferrite) in order to outline a strategy to obtain permanent magnets with large coercivities via low-cost and scalable syntheses. The correlation between morphological, structural and macroscopic magnetic properties of Al-substituted SrFe12O19 and SrFe12O19/CoFe2O4 nanocomposites was analyzed in detail. The hysteretic behavior can be tuned by cation substitution and/or modulation of the super-exchange coupling at the interface of the constituting phases. The magnetic data, supported by Monte Carlo simulations, indicates enhanced magnetic coupling within the composite: this observation underscores the significance of soft crystallite size and epitaxial growth quality at the interface as key factors influencing super-exchange coupling strength, ranging from fully coupled to essentially decoupled composites. Bulk magnets with high density were manufactured by compacting these nanostructured phases using spark plasma sintering, without an applied magnetic field. Consolidation of powders significantly impacted magnetic properties, by increasing remanent magnetization and decreasing coercivity due to enhanced super-exchange coupling. The presence of two phases hindered reciprocal growth, influencing coercivity differently in various compositions. Overall, the compaction enhanced magnet performance through improved particle alignment and super-exchange coupling, offering the potential for optimized magnet design.

In this work, the influence of shell thickness on the magnetic behavior of hard cobalt ferrite (CoFe2O4) nanoparticles coated with one or more layers of magnetically soft nickel ferrite (NiFe2O4) was investigated. The materials were chosen as model soft and hard magnetics due to their significant difference in the magnetic anisotropy constant, which spans 2 orders of magnitude. The obtained magnetic nanoarchitectures are compositionally graded single crystals, i.e., they comprise a crystallographic coherence of core and shell with no visible core-shell interface at the atomic level. The CoFe2O4 core exhibited an average size of similar to 9 nm, while the NiFe2O4 shells were engineered with varying thicknesses of similar to 1, 3, and 6 nm. The core/shell nanoarchitectures behave as a single magnetic unit, i.e., in the rigid exchange coupling regime. Nonmonotonic variation of the coercivity with the shell thickness is observed that is attributed to the competition of the different magnetic anisotropies, such as magneto-crystalline and surface, interplaying with the interparticle dipolar interactions. The experimental findings are in good qualitative agreement with Monte Carlo simulation results for a mesoscopic model that includes both the nanoparticle morphology and interparticle interactions.

Exchange bias has been extensively studied in both exchange-coupled thin films and nanoparticle composite systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias is far from being understood. Here, dense soft-hard binary nanoparticle composites are used not only as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift but also as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft maghemite (γ-Fe2O3) nanoparticles (no bias—symmetric reversal) and hard cobalt doped γ-Fe2O3 nanoparticles (large exchange bias—asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe2O3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Exchange bias has been extensively studied both in exchange-coupled thin films and nanoparticle composite systems. However, the role of non -exchange mechanisms in the overall hysteresis loop bias are far from being understood. Here, dense soft-hard binary nanoparticle composites are used as a novel tool not only to unravel the effect of dipolar interactions on the hysteresis loop shift, but as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft γ-Fe 2 O 3 nanoparticles (no bias – symmetric reversal) and hard cobalt doped γ-Fe 2 O 3 nanoparticles (large exchange bias – asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe 2 O 3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Organic coatings on magnetic nanoparticles are extensively used in various applications. As demonstrated recently, these coatings affect the magnetic behavior of the nanoparticles. The modification of the magnetic properties of the nanoparticles and nanoparticle assemblies as the percentage of the oleic acid (OA) coverage of a cobalt ferrite nanoparticle increases, is investigated numerically using a multiscale modeling approach that combines density functional theory (DFT) and Monte Carlo simulations. The DFT calculations show that the increase in OA coverage results in a monotonic decrease of the mean magneto-crystalline anisotropy and in a reduction of the atomic magnetic moments and the exchange coupling constants in the nanoparticle. These effects are attributed to the gradual recovery of the bulk spinel structure at the coated surface, as the OA coverage increases. Input from the DFT calculations is used in the mesoscopic modeling for the study of the magnetic behavior of an assembly of these nanoparticles by employing the Monte Carlo simulations technique. The results demonstrate that despite the decrease of magnetic anisotropy, the coercive field increases with the increase in percentage of OA coverage in the assembly, in agreement with experimental findings. This study suggests the possibility of tailoring the magnetic properties of cobalt ferrite nanoparticles for high-performance applications by varying the organic coating concentration.

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 Fe3 O4 /Mn3 O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3 O4 and Mn3 O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3 O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3 O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials.

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 + U) to treat localized states in order to investigate the magnetic and electrostatic properties of ferrite nanoparticles (CoFe2O4 and Fe2O3) covered with COOH-terminated [oleic acid (OA)] and OH-terminated [diethylene glycol (DEG)] ligands by varying the ligands coverage. OA results in a decrease of the mean magnetic moment for both particles as the coating coverage increases. The magnetic anisotropy (MAE) significantly decreases for CoFe2O4, whereas for Fe2O3 a significant increase of MAE is found as the OA coverage percentage increases. For DEG, the variation of both types of nanoparticles in the magnetic moment and the magnetic anisotropy is not significant since DEG shows a weaker attachment on the surface. As COOH shows a larger percentage of covalent bonding than OH, a larger amount of charge is transferred to both particles when OA is attached on their surface. In this case, the particles possess a higher charge, and thus they can produce a larger electrostatic potential in the neighborhood independently of the screening by the coating. Thus, the repulsive Coulombic forces are enhanced mainly in the OA coating case, resulting in an enhancement of their colloidal stability.

Abstract Exchange bias has been extensively studied both in exchange-coupled thin films and nanoparticle systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias are far from being understood. Here, dense soft-hard binary nanoparticle systems are used as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift, as well as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft g-Fe2O3 nanoparticles (no bias – symmetric reversal) and hard cobalt doped g-Fe2O3 nanoparticles (large exchange bias – asymmetric reversal) reveal that the loop shift of the mixture can be significantly enhanced depending on the fraction of soft particles. Simple calculations indicate how this effect can be further enhanced by optimizing the parameters of the constituents (coercivity and loop asymmetry). In addition, the existence of a dipolar induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias can be induced in the initially unbiased g-Fe2O3 nanoparticles due to the dipolar interaction with the exchange biased cobalt doped g-Fe2O3 nanoparticles. These results pave the way for novel approaches to tune the loop shift in magnetic systems beyond interface exchange coupling.

A mesoscopic scale approach and the Monte Carlo (MC) method have been employed to study the exchange bias behaviour of MnFe2O4 (soft)/maghemite (soft) and CoFe2O4 (hard)/maghemite (soft) nanoparticles (NPs) of size ∼ 3 nm in dense and diluted assemblies at low temperatures. The analysis of our MC results clearly shows that in the powder samples the contribution to the exchange bias field (H ex) and the coercivity (H c) comes mainly from the intraparticle core/shell structure in the hard/soft sample and that the interplay between the internal characteristics and the interparticle interactions is more important in the soft/soft samples where the dipolar strength is enhanced. In the diluted frozen ferrofluid samples where interparticle exchange interactions are absent and the role of the dipolar interactions is not significant the exchange bias effects are reduced, and they come from the intra particle structure. The variation of H ex and H c with the applied cooling field well reproduces the experimental findings and sheds light on the key mechanisms of the observed magnetic behaviour. Our results demonstrate the possibility to control the magnetic behaviour of nanostructures by using properly chosen core/shell bimagnetic nanoparticles.

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. The higher divalent metal stoichiometry in the rock salt phase, with increasing Co content, results in a population of unoccupied tetrahedral metal sites in the spinel part, likely through oxidative shell creation, involving an ordered defect-clustering mechanism, directly correlated to core stabilization. To shed light on the effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo simulations suggest that the designed NCs, with desirable, enhanced magnetic properties (cf. exchange bias and coercivity), are developed with magnetocrystalline anisotropy which increases due to a relatively low content of Co ions in the lattice. The growth of optimally performing candidates combines also a strongly exchange-coupled system, secured through a high volumetric ratio rock salt phase, interfaced by a not so defective spinel shell. In view of these requirements, specific absorption rate (SAR) calculations demonstrate that the rock salt core sufficiently protected from oxidation and the heterostructure preserved over time, play a key role in magnetically mediated heating efficacies, for potential use of such NCs in magnetic hyperthermia applications.

The effect of clustering induced by albumin coating on the magnetic behaviour of ultra-small MnFe2O4 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.

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 (Edd ) to nanoparticle anisotropy (Kef 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. The Kef  is tuned through different degrees of cobalt-doping in maghemite nanoparticles, resulting in a variation of nearly an order of magnitude. All the bare particle compacts display collective behavior, except the one made with the highest anisotropy particles, which presents "marginal" features. Thus, a threshold of Kef V/Edd  ≈ 130 to suppress collective behavior is derived, in good agreement with Monte Carlo simulations. This translates into a crossover value of ≈1.7 for the easily accessible parameter TMAX (interacting)/TMAX (non-interacting) (ratio of the peak temperatures of the zero-field-cooled magnetization curves of interacting and dilute particle systems), which is successfully tested against the literature to predict the individual-like/collective behavior of any given interacting particle assembly comprising relatively uniform particles.

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.

The effect of core size on the magnetic behavior of nanoparticle assemblies of gamma-Fe2O3 core/SiO2 shell morphology is investigated. Long-range magnetostatic interactions are probed in two highly monodispersed experimental test systems of spherical nanoparticles with core diameters of 10 nm and 12.5 nm, and a shell thickness varying from 0 nm (bare particles) to similar to 50 nm. Zero-Field-Cooled magnetization curves are calculated by employing the Monte Carlo simulation technique in a mesoscopic-scale model for the assembly, assuming spin collinearity and coherent spin-reversal mechanisms. Simulation results reproduce the trend in the behavior of the Zero-Field-Cooled magnetization versus T curves in good qualitative agreement with the experimental findings. They also demonstrate that the increase of the magnetic core size results in a shift of the maximum magnetization peak, T-max, to higher temperatures due to enhanced dipolar coupling. The results shed light on how interparticle distance and magnetic core size influence the value of T-max through collective behavior and its transition to a single-particle superparamagnetic blocking temperature, T-B, as the assembly becomes magnetically diluted with increasing shell thickness.

In this chapter, we discuss the interparticle interactionInterparticle interaction effects in assemblies of magnetic nanoparticlesMagnetic nanoparticles. For our study, we have developed a mesoscopic scale model that takes into account: (a) the morphology of the assemblies and (b) the interplay between the interparticle and intra-particle characteristics of the nanoparticles. The hysteresis loopsHysteresis loop, the virgin magnetization curvesVirgin magnetization curves and the temperature-dependent (Field CooledField-Cooled magnetization curve (FC) (FC)/zero-field cooled (ZFCZero-Field-Cooled magnetization curve (ZFC))) magnetization curves have been calculated with our model. Results are presented for three case studies of different nanoparticles' morphologies assemblies and they show that our mesoscopic modelMesoscopic model reproduces well the experimentally studied systems and reveals the origin of the observed magnetic behavior.

The non-equilibrium dynamics of the superspin glass state of a dense assembly of ~ 2 nm MnFe 2 O 4 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. We illustrate in detail, by a number of experiments, the dynamical properties of the low-temperature superspin glass phase. We observe that these glassy features are quite similar to those of atomic spin glasses. Some differences are observed, and interestingly, the non-atomic nature of the superspin glass is also reflected by an observed superspin dimensionality crossover. Monte Carlo simulations—that explicitly take into account core and surface contributions to the magnetic properties of these ultrasmall nanoparticles in direct contact, as well as interparticle interactions—evidence effects of the interplay between (intraparticle) core/surface exchange coupling and (interparticle) dipolar and exchange interactions.

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. We have shown strong exchange coupling at the core/shell interface of these nanoparticles systems which was also confirmed by mesoscopic modelling. Our results demonstrate the possibility of modulating magnetic anisotropy not only by chemical composition but also by adopting the proper nano-architecture.