GRAIN BOUNDARIES IN ZnO: A FERROMAGNETIC NANO-FOAM

Andrei Mazilkin ¹ Petr Straumal ⁴ Ata Myatiev ⁴ Boris Straumal ¹ Brigitte Baretzky ³ Svetlana Protasova ¹ Eberhard Goering ² Gisela Schütz ²

¹Institute of Solid State Physics, Institute of Solid State Physics RAS, Chernogolovka
²Advanced Magnetic Materials, Max-Planck-Institut für Intelligente Systeme (former Inst. Metallforschung), Stuttgart
³Institut für Nanotechnologie, Karlsruher Institut für Technologie, Eggenstein-Leopoldshafen
⁴Physical Chemistry, National University of Science and Technology MISiS, Moscow

In order to elucidate room temperature (RT) ferromagnetism (FM) in pure ZnO and ZnO doped by Co, Mn and Fe, we have analyzed a multitude of experimental publications with respect to the ratio of grain boundary (GB) area to grain volume. FM only appears, if this ratio exceeds a certain threshold value s_th. Based on these important results nano-grained pure, Co-, Mn-, and Fe-doped ZnO films have been prepared, which reveal reproducible RT-FM and magnetization proportional to the film thickness, even for pure ZnO films. ZnO films consist of crystalline grains with wurzite structure separated by amorphous layers. Our findings strongly suggest that grain boundaries and related vacancies are the intrinsic origin for RT ferromagnetism.
The financial support from Deutsche akademische Austauschdienst (DAAD) and Russian Foundation for Basic Research (grants 10-02-00086, 11-03-00029 and 11-02-92694) is acknowledged.