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

Abstract : 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 32S, 50 MeV 63Cu, and 235 MeV 84Kr) 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 57Fe conversion electron Mössbauer 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 32S or 63Cu 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 84Kr 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 132Xe ion irradiation. In the case of the nanophase induced by ion-beam recrystallization of the tracks with 32S or 63Cu 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 (α-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.
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Journal of Applied Physics, American Institute of Physics, 2000, 87 (4), pp.1899-1907. 〈10.1063/1.372110〉
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Jean-Marc Constantini, Jean-Michel Desvignes, Alain Pérez, F. Studer. Local order and magnetic behavior of amorphous and nanocrystalline yttrium iron garnet produced by swift heavy ion irradiations. Journal of Applied Physics, American Institute of Physics, 2000, 87 (4), pp.1899-1907. 〈10.1063/1.372110〉. 〈hal-00906590〉

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