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.