Fe₃O₄ nanoparticles are one of the most promising candidates for biomedical applications such as magnetic hyperthermia and theranostics due to their bio-compatibility, structural stability and good magnetic properties. However, much is unknown about the nanoscale origins of the observed magnetic properties of particles due to the dominance of surface and finite size effects. Here we have developed an atomistic spin model of elongated magnetite nanocrystals to specifically address the role of faceting and elongation on the magnetic shape anisotropy. We find that for faceted particles simple analytical formulae overestimate the magnetic shape anisotropy and that the underlying cubic anisotropy makes a significant contribution to the energy barrier for moderately elongated particles. Our results enable a better estimation of the effective magnetic anisotropy of highly crystalline magnetite nanoparticles and is a step towards quantitative prediction of the heating effects of magnetic nanoparticles.

Most of the magnetic devices in advanced electronics rely on the exchange bias effect, a magnetic interaction that couples a ferromagnetic and an antiferromagnetic material, resulting in a unidirectional displacement of the ferromagnetic hysteresis loop by an amount called the ‘exchange bias field’. Setting and optimizing exchange bias involves cooling through the Néel temperature of the antiferromagnetic material in the presence of a magnetic field. Here we demonstrate an alternative process for the generation of exchange bias. In IrMn/FeCo bilayers, a structural phase transition in the IrMn layer develops at room temperature, exchange biasing the FeCo layer as it propagates. Once the process is completed, the IrMn layer contains very large single-crystal grains, with a large density of structural defects within each grain, which are promoted by the FeCo layer. The magnetic characterization indicates that these structural defects in the antiferromagnetic layer are behind the resulting large value of the exchange bias field and its good thermal stability. This mechanism for establishing the exchange bias in such a system can contribute towards the clarification of fundamental aspects of this exchange interaction.

The characteristic relaxation frequency of a granular antiferromagnet $f_0$ in the Néel–Arrhenius expression has been calculated from a high resolution measurement of the time dependence of the median blocking temperature of a CoFe/IrMn exchange biased system. Calculations based on a granular model give $f_0=\left(2.1\pm 0.4\right)\times {10}^{12}{\text{s}}^{-1}$ , three orders of magnitude greater than values commonly found in the literature.

Due to the advent of antiferromagnetic (AF) spintronics, there is a burgeoning interest in AF materials for a wide range of potential and actual applications. Generally, AFs are characterized via the ordering at the Néel temperature (T_N), but to have a stable AF configuration, it is necessary that the material has a sufficient level of anisotropy so as to maintain the orientation of the given magnetic state fixed in one direction. Unlike the case for ferromagnets, there are little established data on the anisotropy of AFs and, in particular, its origins, other than it being magneto-crystalline, and those factors which control it. In this perspective article, these factors are reviewed in light of recent and established experimental data. The anisotropy can be found indirectly via the exchange bias phenomenon. This technique is reviewed and, in particular, the implications for the nature of the anisotropy that is measured and its distribution. Finally, a strategy is proposed that would allow for the development of AF materials with controlled anisotropy for future applications.