Magnetochromism is the term applied when a chemical compound changes colour under the influence of a magnetic field. In particular the magneto-optical effects exhibited by complex mixed metal compounds are called magnetochromic when they occur in the visible region of the spectrum. Examples include K<sub>2</sub>V<sub>3</sub>O<sub>8</sub>, lithium molybdenum purple bronze Li<sub>0.9</sub>Mo<sub>6</sub>O<sub>17</sub>, and related mixed oxides. Reported magnetochromic compounds are multiferroic manganese tungsten oxide and multiferroic bismuth ferrite.
MagneticallyâÂÂinduced color change can also occur in aqueous solutions of colloidal Fe<sub>3</sub>O<sub>4</sub> nanoparticles that are ~10 nm in diameter. Paramagnetic Fe<sub>3</sub>O<sub>4</sub> particles are extracted from a petroleumâÂÂbased ferrofluid or synthesized in a laboratory and then suspended in water. When exposed to a strengthening magnetic field these particles organize into chains that diffract light and cause the solution to change color from a brown to red, yellow, green and then blue. Manufacturers encapsulate microscopic droplets of this solution in a thin plastic film to create a magnetochromic magnetic field viewing screen.