Rhombohedron and plate-like hematite (α-Fe2O3) nanoparticles: synthesis, structure, morphology, magnetic properties and potential biomedical applications for MRI

[Display omitted] •Rhombhohedron and plate-like nanostructures are synthesized by hydrothermal method.•The HC is strongly enhanced with changing of the particle morphology.•The Morin transition is slightly morphology-dependent.•The nanoplates have higher T2 relaxation than rhombohedrons. Hematite nanorhombohedrons and nanoplates have been prepared through a hydrothermal synthesis process. The nanoplates possess width of 100 nm and thickness of 30 nm whereas hematite nanorhombohedrons (cube-like particles) have an edge length of 50 nm. The magnetization show that the Morin transitions of the samples occur at nearly the same temperature showing slightly shape-dependent magnetic properties (TM(P) = 242 K (plates) and TM(R) = 221 K (rhombohedrons). The nanorhombohedrons exhibit hysteresis properties with low coercivity HC(R) = 22 Oe at room temperature, whereas nanoplates show much higher coercivity (∼24 times higher) HC(P) = 522 Oe and strongly shape-dependent magnetic properties. The coercivity below the Morin transition is also observed for nanorhombohedrons HC(R) = 178 Oe and for nanoplates HC(P) = 2280 Oe. The transverse relaxivity rate R2 (1/T2) measured on 15.2 T magnetic resonance imaging (MRI) scanner was higher for nanoplates than for nanorhombohedrons (12.63 vs 10.72 mM-1s-1). The properties of the samples are discussed in relation to the particle shape.