The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies.

A study of the crystallographic and magnetic structure of ${\text{SrCoO}}_{2.5}$ with a brownmillerite-type structure has been carried out from neutron powder-diffraction (NPD) measurements at temperatures ranging from 10 to 623 K, across the Néel temperature $\left(T_N=537\text{K}\right)$ of this antiferromagnetic oxide. The study has been complemented with differential scanning calorimeter (DSC), $dc$ susceptibility and magnetization measurements. Although the refinement of the crystal structure from NPD data is possible in the orthorhombic $Pnma$ and $Ima2$ space groups, the support of ab-initio band-structure calculations has allowed us to select, without ambiguity, the $Ima2$ space group as the ground state for ${\text{SrCoO}}_{2.5}$ brownmillerite. In $Ima2$ the crystallographic structure of ${\text{SrCoO}}_{2.5}$ is described as layers of corner-sharing $\text{Co}1{\text{O}}_6$ octahedra alternating along the $a$ axis with layers of vertex-sharing $\text{Co}2{\text{O}}_4$ tetrahedra, conforming chains running along the [0 0 1] direction. The magnetic structure below $T_N=537\text{K}$ is $G$-type with the magnetic moments directed along the $c$ axis. This magnetic arrangement is stable from $T_N$ down to 10 K. At $T=10\text{K}$, the magnetic moment values for Co1 and Co2 atoms are $3.12\left(13\right){\mu }_B$ and $2.88\left(14\right){\mu }_B$, respectively, compatible with a ${\text{Co}}^{2+}\underset{̱}{L}$ state, where $\underset{̱}{L}$ stands for a ligand hole. The magnetic susceptibility curves show, below 200 K, a divergence of zero-field cooling and field cooling curves, displaying broad maxima which are interpreted as due to the presence of ferromagnetic clusters embedded into an antiferromagnetic matrix. These inhomogeneities are inherent to the synthesis process, by quenching microcrystalline samples of ${\text{SrCoO}}_{3-x}$ composition from high temperature, where cubic, ferromagnetic perovskites have been identified by diffraction methods.

We show both experimental evidences and Monte Carlo modeling of the effects of interparticle dipolar interactions on the hysteresis losses. Results indicate that an increase in the intensity of dipolar interactions produce a decrease in the magnetic susceptibility and hysteresis losses, thus diminishing the hyperthermia output. These findings may have important clinical implications for cancer treatment.