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

摘要

Thin epitaxial films of gallium or scandium-doped and undoped yttrium iron garnet (Y_(3)Fe_(5)O_(12) or YIG) on nonmagnetic Gd_(3)Ga_(5)O_(12) substrates were irradiated with swift heavy ions (50 MeV ~(32)S, 50 MeV ~(63)Cu, and 235 MeV ~(84)Kr) 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/μm in this low ion-velocity range. The local order and magnetic properties of the damaged films were then studied at room temperature by ~(57)Fe 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 ~(32)S or ~(63)Cu ions, the CEMS data show that the tetrahedral Fe~(3+) 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 ~(84)Kr 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 ~(132)Xe ion irradiation. In the case of the nanophase induced by ion-beam recrystallization of the tracks with ~(32)S or ~(63)Cu 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 (YFeO_(3)) and haematite (α-Fe_(2)O_(3)) 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.