Nanomagnets for ultra-high field MRI: Magnetic properties and transverse relaxivity of silica-coated ε-Fe2O3

•ε-Fe2O3 nanoparticles in a blocked state coated with amorphous silica were studied.•Thorough characterization by XRD, TEM and Mössbauer spectroscopy was performed.•Magnetization near low-temperature magnetic transition displays anomalous behavior.•Transverse relaxivity is interpreted in a combination of MAR and SDR regimes.•Studied systems show high performance as contrast agents for ultra-high field MRI. Magnetic nanoparticles of iron oxides have received extensive attention in the biomedical research, e.g. as prospective contrast agents for T2-weighted magnetic resonance imaging (MRI). The ability of a contrast agent to enhance the relaxation rate of 1H nuclei in its vicinity is quantitatively described by its relaxivity. Among the polymorphs of the iron(III) oxide, the nanoparticulate ε-Fe2O3, distinguished by its high magnetocrystalline anisotropy resulting in a blocked state of single-domain particles up to the Curie temperature of ∼500 K, has never been studied as a contrast agent for MRI before. We analyzed r1 and r2 relaxivities of ε-Fe2O3 nanoparticles coated with amorphous silica, particularly with the aim to determine their dependences on the external magnetic field, temperature, and thickness of the silica coating. The r2 relaxivity was interpreted within the motional averaging and static dephasing regimes. MRI images at 11.75 T confirmed high applicability of ε-Fe2O3-based contrast agents in ultra-high fields. We present the first case study considering ε-Fe2O3 nanomagnets for prospective application in biomedicine, in particular for MRI in ultra-high fields. The study is complemented by a thorough investigation of magnetic properties of the nanoparticles, revealing some interesting anomalies.