title: Ferromagnetic behaviour of ZnO: the role of grain boundaries.
abstract: The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with "magnetic atoms" such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation.
authors.affiliation: Karlsruher Institut für Technologie, Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; Institute of Solid State Physics, Russian Academy of Sciences, Ac. Ossipyan str. 2, 142432 Chernogolovka, Russia; Max-Planck-Institut für Intelligente Systeme, Heisenbergstrasse 3, 70569 Stuttgart, Germany; National University for Research and Technology "MISiS", Leninsky prospect 4, 119991 Moscow, Russia.
authors.forename: Boris Borisovich
authors.initials: BB
authors.lastname: Straumal
authors.affiliation: Institute of Solid State Physics, Russian Academy of Sciences, Ac. Ossipyan str. 2, 142432 Chernogolovka, Russia; Max-Planck-Institut für Intelligente Systeme, Heisenbergstrasse 3, 70569 Stuttgart, Germany.
authors.forename: Svetlana G
authors.initials: SG
authors.lastname: Protasova
authors.affiliation: Karlsruher Institut für Technologie, Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; Institute of Solid State Physics, Russian Academy of Sciences, Ac. Ossipyan str. 2, 142432 Chernogolovka, Russia.
authors.forename: Andrei A
authors.initials: AA
authors.lastname: Mazilkin
authors.affiliation: Max-Planck-Institut für Intelligente Systeme, Heisenbergstrasse 3, 70569 Stuttgart, Germany.
authors.forename: Eberhard
authors.initials: E
authors.lastname: Goering
authors.affiliation: Max-Planck-Institut für Intelligente Systeme, Heisenbergstrasse 3, 70569 Stuttgart, Germany.
authors.forename: Gisela
authors.initials: G
authors.lastname: Schütz
authors.affiliation: National University for Research and Technology "MISiS", Leninsky prospect 4, 119991 Moscow, Russia; Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninsky prospect 49, 117991 Moscow, Russia.
authors.forename: Petr B
authors.initials: PB
authors.lastname: Straumal
authors.affiliation: Karlsruher Institut für Technologie, Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Baretzky
keywords: ferromagnetism; grain boundaries; zinc(II) oxide (ZnO)