Interfaceless Exchange Bias in CoFe2O4 Nanocrystals

The role of superlattice phases and interparticle distance in the magnetic behaviour of SPION thin films

Superparamagnetic iron oxide nanoparticles (SPIONs) with tailored surface modifications were employed to fabricate ordered thin films through a drop-casting technique. By systematically varying the ligand chain length using stearic acid, decanoic acid, and hexanoic acid, we precisely controlled the interparticle distances within the films. Comprehensive investigations utilizing superconducting quantum interference device (SQUID) magnetometry elucidated the films' superparamagnetic behaviour at room temperature, as well as notable exchange interactions at lower temperatures. Notably, these exchange characteristics exhibit a correlation with the blocking temperatures of the thin films. We postulate that these characteristics can be explained by different superlattice phases formed in the thin films, as indicated in previous studies, highlighting the profound influence of self-assembly and particle packing on the magnetic properties. To validate our hypothesis regarding the internal structure, we conducted grazing-incidence small-angle X-ray scattering (GISAXS) and scanning transmission electron microscopy (STEM) measurements, enabling us to assess the quality of internal ordering without compromising the integrity of the films. With this study we demonstrated how the use of simple building blocks, guided by the intrinsic driving force of self-assembly, can lead to remarkable magnetic properties in the resulting films.

Exchange-coupled bi-magnetic nanoparticles enhance magnetothermal/chemodynamic antibacterial therapy of poly-l-lactide scaffold

Bone implant-associated bacterial infection is a common cause of transplant failure. Chemodynamic therapy (CDT) has significant antibacterial efficacy by producing reactive oxygen species (ROS). However, the ROS permeability is restricted by the natural barrier of bacterial biofilms. Herein, a hard-soft magnetic biphasic nanoparticle (CF@MF) was synthesized by in-situ growth of the soft MnFe2O4 shell on the hard CoFe2O4 core. Then, the CF@MF was incorporated into poly-l-lactide powders to prepare porous scaffolds by selective laser sintering. The scaffold exhibited superior magnetothermal conversion efficiency, which was attributed to the interfacial exchange coupling of CF@MF. Importantly, the scaffold effectively dispersed biofilms by magnetothermal therapy (MTT), favoring numerous ROS pre-generated by CDT to enter the interior to kill bacteria. This study provided a synergistic MTT/CDT strategy to eradicate deep bacterial infections in biomedical application.

Studying the Defects in Spinel Compounds: Discovery, Formation Mechanisms, Classification, and Influence on Catalytic Properties

Spinel ferrites demonstrate extensive applications in different areas, like electrodes for electrochemical devices, gas sensors, catalysts, and magnetic adsorbents for environmentally important processes. However, defects in the real spinel structure can change the many physical and chemical properties of spinel ferrites. Although the number of defects in a crystal spinel lattice is small, their influence on the vast majority of physical properties could be really decisive. This review provides an overview of the structural characteristics of spinel compounds (e.g., CoFe2O4, NiFe2O4, ZnFe2O4, Fe3O4, γ-Fe2O3, Co3O4, Mn3O4, NiCo2O4, ZnCo2O4, Co2MnO4, etc.) and examines the influence of defects on their properties. Attention was paid to the classification (0D, 1D, 2D, and 3D defects), nomenclature, and the formation of point and surface defects in ferrites. An in-depth description of the defects responsible for the physicochemical properties and the methodologies employed for their determination are presented. DFT as the most common simulation approach is described in relation to modeling the point defects in spinel compounds. The significant influence of defect distribution on the magnetic interactions between cations, enhancing magnetic properties, is highlighted. The main defect-engineering strategies (direct synthesis and post-treatment) are described. An antistructural notation of active centers in spinel cobalt ferrite is presented. It is shown that the introduction of cations with different charges (e.g., Cu(I), Mn(II), Ce(III), or Ce(IV)) into the cobalt ferrite spinel matrix results in the formation of various point defects. The ability to predict the type of defects and their impact on material properties is the basis of defect engineering, which is currently an extremely promising direction in modern materials science.

Atomic-Scale Structural Properties in NiCo2O4/CuFe2O4 Bilayer Heterostructures on (001)-MgAl2O4 Substrate Regulated by Film Thickness

Changing film thickness to manipulate microstructural properties has been considered as a potential method in practical application. Here, we report that atomic-scale structural properties are regulated by film thickness in an NiCO2O4(NCO)/CuFe2O4(CFO) bilayer heterostructure prepared on (001)-MgAl2O4 (MAO) substrate by means of aberration-corrected scanning transmission electron microscopy (STEM). The misfit dislocations at the NCO/CFO interface and antiphase boundaries (APBs) bound to dislocations within the films are both found in NCO (40 nm)/CFO (40 nm)/MAO heterostructures, contributing to the relaxation of mismatch lattice strain. In addition, the non-overlapping a/4[101]-APB is found and the structural transformation of this kind of APB is resolved at the atomic scale. In contrast, only the interfacial dislocations form at the interface without the formation of APBs within the films in NCO (10 nm)/CFO (40 nm)/MAO heterostructures. Our results provide evidence that the formation of microstructural defects can be regulated by changing film thickness to tune the magnetic properties of epitaxial bilayer spinel oxide films.

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.