Local order and magnetic behavior of amorphous and nanocrystalline yttriumiron 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 irradiate d with swift heavy ions (50 MeV S-32, 50 MeV Cu-63, and 235 MeV Kr-84) in t he electronic slowing down regime. The mean electronic stopping power in th e 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 loc al 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 fluoresc ence mode. In the case of paramagnetic gallium or scandium-substituted film s (YIG:Ga, YIG:Sc) irradiated with S-32 or Cu-63 ions, the CEMS data show t hat the tetrahedral Fe3+ sites are preferentially damaged, while the octahe dral 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 t he number of tetrahedral iron sites is decreased in the amorphous phase obt ained with Kr-84 ion irradiation, due to the formation of fivefold-coordina ted pyramidal sites, as already found in a previous study on undoped YIG si nters amorphized by 3.5 GeV Xe-132 ion irradiation. In the case of the nano phase induced by ion-beam recrystallization of the tracks with S-32 or Cu-6 3 irradiations, a further decrease of the pre-edge peak is found. This is i nterpreted by (i) an increase of the fivefold-coordinated pyramidal sites a nd/or (ii) a probable decomposition of the garnet into orthoferrite (YFeO3) and haematite (alpha-Fe2O3) under the high-pressure and high-temperature c onditions in the thermal spike generated by the ions. The CEMS data of irra diated undoped YIG also show that both the amorphous and nanocrystalline ph ases have a paramagnetic behavior at room temperature. The nanophase magnet ic behavior is analyzed on the basis of a superparamagnetic relaxation abov e the blocking temperature, whereas the amorphous phase behavior is ascribe d to a speromagnetic state. (C) 2000 American Institute of Physics. [S0021- 8979(00)01404-3].