Fe3O4 thin films: controlling and manipulating an elusive quantum material

Fe 3 O 4 (magnetite) is one of the most elusive quantum materials and at the same time one of the most studied transition metal oxide materials for thin-film applications. The theoretically expected half-metallic behaviour generates high expectations that it can be used in spintronic devices. Yet, despite the tremendous amount of work devoted to preparing thin films, the enigmatic first-order metal–insulator transition, and the hallmark of magnetite known as the Verwey transition, is in thin films extremely broad and occurs at substantially lower temperatures as compared with that in high-quality bulk single crystals. Here we have succeeded in finding and making a particular class of substrates that allows the growth of magnetite thin films with the Verwey transition as sharp as in the bulk. Moreover, we are now able to tune the transition temperature and, using tensile strain, increase it to substantially higher values than in the bulk. Spintronics: controlled Verwey transition in Fe3O4 thin films Magnetite Fe3O4 is of considerable interest for spintronic applications. Fe3O4 is known to undergo a complex structural distortion below 125 K, giving rise to a ground state in which the localized electrons are distributed over three Fe sites. However, the Verwey transition tends to occur at lower temperatures with increased widths in magnetite thin films. Now a team led by Liu Hao Tjeng at Max Planck Institute for Chemical Physics of Solids in Germany reports a spinel structured Co2TiO4 substrate that allows the growth of Fe3O4 films which not only exhibit a transition as sharp as that of the bulk, but also increase the transition temperature well beyond the bulk value. The key principle is to realize sufficiently large domains and narrow domain size distribution in the film utilizing the strain imposed by a properly matched substrate.