Z. Nemati

Anisotropy effects in magnetic hyperthermia: A comparison between spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles

Spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles, with different FeO:Fe3O4 ratios, have been prepared by a thermal decomposition method to probe anisotropy effects on their heating efficiency. X-ray diffraction and transmission electron microscopy reveal that the nanoparticles are composed of FeO and Fe3O4 phases, with an average size of ∼20 nm. Magnetometry and transverse susceptibility measurements show that the effective anisotropy field is 1.5 times larger for the cubes than for the spheres, while the saturation magnetization is 1.5 times larger for the spheres than for the cubes. Hyperthermia experiments evidence higher values of the specific absorption rate (SAR) for the cubes as compared to the spheres (200 vs. 135 W/g at 600 Oe and 310 kHz). These observations point to an important fact that the saturation magnetization is not a sole factor in determining the SAR and the heating efficiency of the magnetic nanoparticles can be improved by tuning their effective anisotropy.

FeCo nanowires with enhanced heating powers and controllable dimensions for magnetic hyperthermia

A detailed study of the magnetic properties and heating capacities of electrodeposited FeCo nanowires with varying lengths (2–40 μm) and diameters (100 and 300 nm) is reported. We find that specific absorption rate (SAR) increases rapidly with increasing wire length up to 10 μm, followed by a gradual increase for larger lengths. Magnetic and hyperthermia measurements have revealed the important effect of dipolar interactions between the nanowires on their magnetic and inductive heating responses. Both calorimetric and AC magnetometry methods consistently show that the physical movement contribution of the nanowires to the SAR is small, and that for applied fields exceeding the coercive field, the nanowires tend to align parallel to the field, thus enhancing the SAR. Maximum SAR values of ∼1500 W/g have been achieved for the largest wires at H = 300 Oe and f = 310 kHz.

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

The exploration of exchange bias (EB) on the nanoscale provides a novel approach to improving the anisotropic properties of magnetic nanoparticles for prospective applications in nanospintronics and nanomedicine. However, the physical origin of EB is not fully understood. Recent advances in chemical synthesis provide a unique opportunity to explore EB in a variety of iron oxide-based nanostructures ranging from core/shell to hollow and hybrid composite nanoparticles. Experimental and atomistic Monte Carlo studies have shed light on the roles of interface and surface spins in these nanosystems. This review paper aims to provide a thorough understanding of the EB and related phenomena in iron oxide-based nanoparticle systems, knowledge of which is essential to tune the anisotropic magnetic properties of exchange-coupled nanoparticle systems for potential applications.

Laser Target Evaporation Fe 2 O 3 Nanoparticles for Water-Based Ferrofluids for Biomedical Applications

Maghemite spherical magnetic nanoparticles (MNPs) were prepared by laser target evaporation. X-ray diffraction, transmission electron microscopy, specific surface area, and dynamic light scattering studies were performed. For water-based suspensions prepared on the basis of obtained MNPs, the zeta potential was measured. Magnetic and microwave measurements were performed both for MNPs and ferrofluids. To estimate the inductive magnetic heating of electrostatically self-stabilized or electrostatically stabilized by adsorbed citrate ions ferrofluids, magneto-inductive heating experiments were performed that showed heating efficiency. For the study of cytotoxicity and maghemite MNPs accumulation process, two non-pathogenic Exophiala nigrum (black) and its mutant strain (red) yeasts were studied. In both cases, no significant alterations of cell morphology were observed.

Core/shell iron/iron oxide nanoparticles: are they promising for magnetic hyperthermia?

Core/shell iron/iron oxide nanoparticles have been proposed as a promising system for biomedical applications, because they combine a core (iron) with a high magnetic moment and a shell (iron oxide) with good biocompatibility. However, due to the interdiffusion of atoms between the core and the shell, with increasing time the core progressively shrinks until the particles eventually become hollow or nearly hollow, and as a result, the magnetic properties of these nanoparticles progressively deteriorate, negatively affecting their biomedical capabilities. In this article, we have studied the change of the morphology of the nanoparticles, from core/shell to hollow, depending on their size, and analyzed how this affects their magnetic and heating properties for magnetic hyperthermia. We have synthesized three core/shell samples with average sizes of 8, 12, and 14 nm. We have observed that with increasing size, the magnetic properties and the heating efficiency of the core/shell nanoparticles are improved and at the same time, they become more stable and retain their core/shell morphology for a longer period of time, making them more desirable for biomedical applications. As the nanoparticles become hollow, their saturation magnetization continuously decreases, and the heating efficiency also decays, rendering them less useful for magnetic hyperthermia application.

From core/shell to hollow Fe/γ-Fe2O3 nanoparticles: evolution of the magnetic behavior

High quality Fe/γ-Fe2O3 core/shell, core/void/shell, and hollow nanoparticles with two different sizes of 8 and 12 nm were synthesized, and the effect of morphology, surface and finite-size effects on their magnetic properties including the exchange bias (EB) effect were systematically investigated. We find a general trend for both systems that as the morphology changes from core/shell to core/void/shell, the magnetization of the system decays and inter-particle interactions become weaker, while the effective anisotropy and the EB effect increase. The changes are more drastic when the nanoparticles become completely hollow. Noticeably, the morphological change from core/shell to hollow increases the mean blocking temperature for the 12 nm particles but decreases for the 8 nm particles. The low-temperature magnetic behavior of the 12 nm particles changes from a collective super-spin-glass system mediated by dipolar interactions for the core/shell nanoparticles to a frustrated cluster glass-like state for the shell nanograins in the hollow morphology. On the other hand for the 8 nm nanoparticles core/shell and hollow particles the magnetic behavior is more similar, and a conventional spin glass-like transition is obtained at low temperatures. In the case of the hollow nanoparticles, the coupling between the inner and outer spin layers in the shell gives rise to an enhanced EB effect, which increases with increasing shell thickness. This indicates that the morphology of the shell plays a crucial role in this kind of exchange-biased systems.

Superparamagnetic nanoparticles encapsulated in lipid vesicles for advanced magnetic hyperthermia and biodetection

A multifunctional nanocomposite formed by superparamagnetic maghemite nanoparticles of 12.8 ± 1.7 nm diameter encapsulated in lipid unilamellar vesicles (i.e., magnetoliposomes) was prepared using size exclusion chromatography (SEC). The quality of the synthesized nanoparticles was characterized by transmission electron microscopy and X-ray diffraction measurements. Using a modified Langevin model, we analyzed the magnetic measurement data. We found that the SEC prepared magnetoliposomes possess superparamagnetic characteristics. We also performed calorimetric based magnetic hyperthermia measurement to quantify field dependent heating efficiency of the obtained magnetoliposomes. A heating efficiency of ∼160 W/g at 800 Oe and 310 kHz was obtained. Finally, we used magnetoreactance-based biodetection to explore the effect of magnetoliposomes on magneto-impedance (MI) and magneto-reactance (MX) ratios. Compared to pure vesicles, magnetoliposomes were found to increase the MI and MX ratios by ∼1.0% and 4.5%, ...

Mössbauer Studies of Core-Shell FeO/Fe3O4 Nanoparticles

FeO/Fe3O4 nanoparticles were synthesized by thermal decomposition. Electron microscopy revealed that these nanoparticles were of the core-shell type and had a spherical shape with an average size of ~20 nm. It was found that the obtained FeO/Fe3O4 nanoparticles had exchange coupling. The effect of anisotropy on the efficiency of heating (hyperthermic effect) of FeO/Fe3O4 nanoparticles by an external alternating magnetic field was examined. The specific absorption rate (SAR) of the studied nanoparticles was 135 W/g in the experiment with an external alternating magnetic field with a strength of 600 Oe and a frequency of 310 kHz. These data led to an important insight: the saturation magnetization is not the only factor governing the SAR, and the efficiency of heating of magnetic FeO/Fe3O4 nanoparticles may be increased by enhancing the effective anisotropy. Mössbauer spectroscopy of the phase composition of the synthesized nanoparticles clearly revealed the simultaneous presence of three phases: magnetite Fe3O4, maghemite γ-Fe2O3, and wustite FeO.