A particle that has all its dimensions on the nanometre scale and has magnetic order. This size means that a nanoparticle does not have domains. Magnetic nanoparticles are made of magnetic metals such as iron, cobalt, and nickel or their compounds. At high temperatures thermal fluctuations can cause superparamagnetism, i.e. random flipping of the direction of magnetization. The possible occurrence of superparamagnetism limits the use of magnetic nanoparticles in applications such as data storage.