Magnetochromic sensing and size-dependent collective excitations in iron oxide nanoparticles

We combine optical and magneto-optical spectroscopies with complementary vibrational and magnetic property measurements to reveal finite length scale effects in nanoscale $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. Analysis of the $d$-to-$d$ on-site excitations uncovers enhanced color contrast at particle sizes below approximately 75 nm due to size-induced changes in spin-charge coupling that are suppressed again below the superparamagnetic limit. These findings provide a general strategy for amplifying magnetochromism in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and other iron-containing nanomaterials that may be useful for advanced sensing applications. We also unravel the size dependence of collective excitations in this iconic antiferromagnet.