Magnetochromic Sensing 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 $\alpha $-Fe$_{\mathrm{2}}$O$_{\mathrm{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 super-paramagnetic limit. These findings provide a general strategy for amplifying magnetochromism in $\alpha $ -Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ and other iron-containing nano-materials that may be useful for advanced sensing applications.