Fe3O4 nanoparticles (NPs) with different shapes have been prepared by a ‘solventless’ synthesis approach to probe shape anisotropy effects on the magnetic and inductive heating properties. Various shapes including spheres, octahedrons, cubes, rods, wires, and multipods are obtained through alterations in reaction conditions such as the ratio of precursor to surfactant content and heating rate. Magnetic and Mössbauer measurements reveal better stoichiometry in anisotropic-shaped Fe3O4 NPs than that in the spherical and multipod NPs. As a result, the magnetization value of the anisotropic-shaped NPs approaches the value for bulk material (∼86 emu g−1). More surprisingly, the Verwey transition, which is a characteristic phase transition of bulk magnetite structure, is observed near 120 K in the anisotropic-shaped NPs, which further corroborates the fact that these NPs possess better stoichiometry compared to the spherical and multipod-shaped NPs. Other than the improved magnetic properties, these anisotropic-shaped NPs are more effective for hyperthermia applications. For example, compared to the conventional spherical NPs, the nanowires show much higher SAR value up to 846 W g−1, making them a potential candidate for practical hyperthermia treatment. In particular, the octahedral NPs shows an SAR value higher than the same size spherical NPs, which demonstrates the importance of occurrence of the Verwey transition in Fe3O4 NPs for better stoichiometric and higher heating.

Brown's theorem on coercivity of ferromagnetic materials has predicted that the coercivity level is substantially higher than in practice for all the materials studied in experiments in the past seven decades, which is known as the Brown's paradox. In this paper, a system with a coercivity close to the one predicted by Brown's theorem is investigated. Cobalt nanowires are obtained by chemical synthesis that give rise to coercive forces significantly higher than the magnetocrystalline anisotropy field, verifying the Brown's theorem. It is found that the coercivity is strongly dependent on the nanowire diameter, the alignment of the wires in an assembly, and the packing density of the assembly. An analysis based on the current experimental results and related literature reveals a coercivity ceiling in consideration of geometrical dimensions and the effective magnetic anisotropy. Quantitative information is obtained about the proximity effect on the coercivity and the magnetization which shows the correlation between the energy product and the packing density. Furthermore, it is found that by coating the nanowires with Fe, the energy density can be enhanced. These findings provide a guideline for materials design of future high‐performance permanent magnets that take advantage of shape anisotropy at the nanoscale.

Magnetic cobalt ferrite CoFe2O4 is rich with physical phenomena, owing to its crystalline and magnetic structures. When such a ferrite is produced in a modulated nanoscale size, the increased specific surface area gives rise to even more complex behavior in its magnetism, particularly in relation to magnetic hardening. By correlating nanoparticle size (from 3.5 nm to 80 nm) with crystallite size and magnetic properties, we can observe interesting relations between particle size and magnetic coercivity. On exceeding the superparamagnetic limit of about 10 nm, room-temperature coercivity and remanence values are found to increase with increasing nanoparticle size, up to a maximum value of 4.1 kOe and 52 emu g−1, respectively, at a size of approximately 45 nm. Above this critical size, the nanoparticles are comprised of multiple crystallites, and demonstrate the existence of a cooperative phenomenon, the so-called interaction domains, which leads to a decrease in coercivity and remanence values. More interestingly, the ultrasmall-sized CoFe2O4 nanoparticles (3.5–16 nm) show an anomalous coercivity enhancement and irreversible behavior at low temperatures, as compared to the large-sized nanoparticles, which may be ascribed to enhanced effective magnetic anisotropy due to the surface spin-canting effect. Furthermore, training behavior in the exchange bias field, together with field-dependent blocking behavior, indicate that ultrasmall CoFe2O4 nanoparticles possess highly frustrated surface spins, which rearrange much more slowly than their interior spins, resulting in magnetic hardening at low temperatures.

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In this report, hematene (2D α-Fe2O3 nanosheets) with an exceptionally high coercivity of up to 7.5 kOe has been synthesized via a soft-chemical exfoliation process. The high coercivity correlates with the surface magnetic anisotropy that originates from enhanced uncompensated spin canting as a result of the 2D morphology. This observation is different from the behavior of the bulk counterpart that exhibits collinear antiferromagnetic ordering with no net magnetization at low temperatures. In addition, our study shows a suppression of the Morin transition in 2D nanosheets, which further confirms that the surface spins deviate strongly from the collinear antiferromagnetic ordering. We also observed a spin-glass-like transition with a rapid increase in saturation magnetization and a decrease in anisotropy in the ultra-thin α-Fe2O3 nanosheets at temperatures below 48 K. The spin-glass behavior is correlated with the observed exchange bias and the magnetic field dependence of spin-glass freezing temperature.