Local order and magnetic behavior of amorphous and nanocrystalline yttrium iron garnet produced by swift heavy ion irradiations

Thin epitaxial films of gallium or scandium-doped and undoped yttrium iron garnet (Y3Fe5O12 or YIG) on nonmagnetic Gd3Ga5O12 substrates were irradiated with swift heavy ions (50 MeV S-32, 50 MeV Cu-63, and 235 MeV Kr-84) in the electronic slowing down regime. The mean electronic stopping power in the films was always larger than the threshold for amorphous track formation in YIG which is around 4.5 MeV/mu m in this low ion-velocity range. The local order and magnetic properties of the damaged films were then studied at room temperature by Fe-57 conversion electron Mossbauer spectroscopy (CEMS) and x-ray absorption spectroscopy (XAS) at the iron K edge in the fluorescence mode. In the case of paramagnetic gallium or scandium-substituted films (YIG:Ga, YIG:Sc) irradiated with S-32 or Cu-63 ions, the CEMS data show that the tetrahedral Fe3+ sites are preferentially damaged, while the octahedral sites are conserved. This is confirmed by the decrease of the pre-edge peak in the XAS data of the ferrimagnetic undoped YIG films showing that the number of tetrahedral iron sites is decreased in the amorphous phase obtained with Kr-84 ion irradiation, due to the formation of fivefold-coordinated pyramidal sites, as already found in a previous study on undoped YIG sinters amorphized by 3.5 GeV Xe-132 ion irradiation. In the case of the nanophase induced by ion-beam recrystallization of the tracks with S-32 or Cu-63 irradiations, a further decrease of the pre-edge peak is found. This is interpreted by (i) an increase of the fivefold-coordinated pyramidal sites and/or (ii) a probable decomposition of the garnet into orthoferrite (YFeO3) and haematite (alpha-Fe2O3) under the high-pressure and high-temperature conditions in the thermal spike generated by the ions. The CEMS data of irradiated undoped YIG also show that both the amorphous and nanocrystalline phases have a paramagnetic behavior at room temperature. The nanophase magnetic behavior is analyzed on the basis of a superparamagnetic relaxation above the blocking temperature, whereas the amorphous phase behavior is ascribed to a speromagnetic state. (C) 2000 American Institute of Physics. [S0021-8979(00)01404-3].