Diamagnetic Behavior in Nanoparticle Hematite

A paper by Mi et al. [1] suggested that certain nano-sized hematite (α-Fe2O3) particles had diamagnetic properties at room temperature. Since diamagnetic behavior is not a property normally attributed to hematite particles (hematite is generally regarded as a canted antiferromagnetic material at room temperature) we decided to test the validity of the suggestions in [1] by performing magnetic susceptibility and magnetic hysteresis measurements on a series of hematite nanoparticles with average sizes of 8 nm, 30 nm and 40 nm in diameter. We initially considered two possible explanations for the apparent diamagnetic behavior of the nanoparticles in [1]: either 1) the hematite nanoparticles themselves exhibited this unusual diamagnetic behavior, or 2) the diamagnetic response was simply the signal created by a diamagnetic dispersant that was overriding a weak positive magnetic susceptibility signal of the hematite nanoparticles. Our experiments strongly suggested the latter explanation that the apparent “diamagnetic” behavior seen in [1] was caused by a diamagnetic dispersant dominating the magnetic properties of the dispersed hematite nanoparticles.