Magnetic hyperthermia treatment requires biocompatible magnetic nanoparticles with improved heating capacities to become a viable clinical method for cancer treatment. Although small superparamagnetic iron oxide nanoparticles under low fields have been favored (linear response theory regime), these nanoparticles present a series of limitations, including relatively low heating efficiency (specific absorption rate or SAR), that need to be overcome to make magnetic hyperthermia an efficient clinical application. We here show that by modifying the shape into deformed cubes (octopods) and tuning their size, their SAR can be greatly increased up to 70% (from 140 to 240 W/g). By using nonhydrolytic thermal decomposition, we have obtained highly crystalline monodisperse nano-octopods for different sizes (17–47 nm), and their heating response has been extensively studied in a wide range of AC fields (20–800 Oe) using combined calorimetric and AC magnetometry experiments. Our results consistently reveal that at AC fields (≤300–400 Oe), the nano-octopods with the smallest size (17 nm) possessed the largest SAR, but for higher AC fields (>400 Oe) the SAR tended to increase with increasing particle size, reaching maximum values up to 415 W/g for the 47 nm octopods. The different response has been attributed to the ratio between the applied field and the anisotropy field, which activates different heating mechanisms: mainly related to viscous losses in the case of the smallest nano-octopods, while mostly attributed to hysteresis losses in the case of the biggest ones. Our study provides important insights into the size-dependent SAR in anisotropic nanoparticles, other than what has been predicted by the linear response theory for the case of spherical nanoparticles, and paves a new pathway for the design and synthesis of novel anisotropic iron oxide nanostructures with optimal heating efficiency for enhanced hyperthermia.

We report a novel synthesis approach for the growth of core/shell FeO/Fe3O4 nanoparticles with controlled shape and size. FeO particles were partially oxidized to form core/shell FeO/Fe3O4 structures, as evidenced from transmission electron microscopy, X-ray diffraction, and magnetometry analysis. We find that the molar ratios and concentrations of surfactants are the key parameters in controlling the particle size. The particles can grow in either isotropic or anisotropic shapes, depending upon a chemical reaction scheme that is controlled kinetically or thermodynamically. The competitive growth rates of {111} and {100} facets can be used to tune the final shape of nanoparticles to spherical, cubic, octahedral, octopod, and cuboctahedral geometries. FeO particles can also be oxidized chemically or thermally to form Fe3O4 nanoparticles. By following the same synthesis technique, it is possible to synthesize rods and triangles of Fe3O4 by introducing twinnings and defects into the crystal structure of the seed. The thermally activated first-order Verwey transition at ~120 K has been observed in all the synthesized FeO/Fe3O4 nanoparticles, indicating its independence from the particle shape. These core/shell nanoparticles exhibit a strong shift in field-cooled hysteresis loops accompanied by an increase in coercivity (the so-called exchange bias effect), but the low field-switching behavior appears to vary with the particle shape.

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.

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.

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We report a comparative study of the magnetic properties of polycrystalline hollow c-Fe 2 O 3 nanoparticles with two distinctly different average sizes of 9.2 6 1.1 nm and 18.7 6 1.5 nm. High-resolution transmission electron microscopy images reveal the presence of a shell with thickness of 2 nm and 4.5 nm for the 9.2 nm and 18.7 nm nanoparticles, respectively. The field-cooled hysteresis loops show interesting features of enhanced coercivity and horizontal and vertical shifts associated with the polarity of the cooling field for both types of nanoparticles. While the anomalously large horizontal shifts and open hysteresis loop in a field as high as 9 T observed for the 9.2 nm nanoparticles corresponds to a "minor loop" of the hysteresis loop, the loop shift observed for the 18.7 nm nanoparticles manifests an intrinsic "exchange bias" (EB). Relative to the 18.5 6 3.2 nm solid nanoparticles, a much stronger EB effect is achieved in the 18.7 nm hollow nanoparticles. Our studies point to the importance of inner and outer surface spin disorder giving rise to surface anisotropy and EB and reveal a perspective of tuning EB in hollow magnetic nanoparticle systems. V

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.

Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.