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.

Various series of permalloy thin films were grown by dc-sputtering on Si (100) and glass substrates at room temperature and different argon pressure values using a Fe₂₀ Ni₈₀ target. The increase of argon pressure leads to a decrease of the Fe concentration in the films from 17 at.% to 15 at.%, an increase of the root mean square roughness of film surfaces, and a decrease of the sharpness of the crystalline texture of the samples. The increase of the film thickness leads to an increase of the coercive field. The transition to the "transcritical" state was observed at a critical thickness that decreases from 220 to 50 nm as the argon pressure in the chamber increases. This state was confirmed by the characteristic shapes of hysteresis loops, rotatable magnetic anisotropy, and the appearance of stripe domains.

Magnetotactic bacteria are aquatic microorganisms that internally biomineralize chains of magnetic nanoparticles (called magnetosomes) and use them as a compass. Here it is shown that magnetotactic bacteria of the strain Magnetospirillum gryphiswaldense present high potential as magnetic hyperthermia agents for cancer treatment. Their heating efficiency or specific absorption rate is determined using both calorimetric and AC magnetometry methods at different magnetic field amplitudes and frequencies. In addition, the effect of the alignment of the bacteria in the direction of the field during the hyperthermia experiments is also investigated. The experimental results demonstrate that the biological structure of the magnetosome chain of magnetotactic bacteria is perfect to enhance the hyperthermia efficiency. Furthermore, fluorescence and electron microscopy images show that these bacteria can be internalized by human lung carcinoma cells A549, and cytotoxicity studies reveal that they do not affect the viability or growth of the cancer cells. A preliminary in vitro hyperthermia study, working on clinical conditions, reveals that cancer cell proliferation is strongly affected by the hyperthermia treatment, making these bacteria promising candidates for biomedical applications.

We hereby present experimental and theoretical insights on the use of biomineralized magnetite nanoparticles, called magnetosomes, as heat nanoinductors in the magnetic hyperthermia technique. The heating efficiency or specific absorption rate of magnetosomes extracted from Magnetospirillum gryphiswaldense bacteria and immersed in water and agarose gel, was directly determined from the hysteresis loops obtained at different frequencies and magnetic field amplitudes. We demonstrate that heat production of magnetosomes can be predicted in the framework of the Stoner–Wohlfarth theory of uniaxial magnetic anisotropy subjected to significant dipolar interactions, which can be described in terms of an interaction anisotropy superimposed to that of each particle. Based on these findings, we propose optimal magnetic field amplitude and frequency values in order to maximize the heat production while keeping the undesired eddy current effects below safe and tolerable limits. The efficiency of magnetosomes as heat generators and their impact on cell viability has been checked in macrophage cells. Our results clearly indicate that the hyperthermia treatment causes both cell death and inhibition of cell proliferation. Specifically, only 36% of the treated macrophages remained alive 2 h after alternating magnetic field exposure, and 24 h later the percentage fell to 22%.