Swift, Heavy Ions in Insulating and Conducting Oxides: Tracks and Physical Properties
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Damage in Oxide Materials The damage induced in oxide materials will be investigated for a wide range of electronic stopping powers (dE/dx) and ion energies (Ei). Several oxides1'2 were irradiated. In order to be as clear as possible, the morphology and the rate of the damage versus dE/dx will be described using the experimental results obtained with the yttrium iron garnet YjFesOu-3 All the observations deduced from the irradiation of this garnet are corroborated by the ones made on the other materials. Damage Morphology The interest in such oxides derives from their lack of sensitivity to a single electronic excitation. Consequently, high resolution electron microscopy (HREM) can be used to observe the damage morphology directly. Systematic observations were made4 on irradiated Y3Fe5Oi2 for a large range of dE/dx and for an incident ion energy larger than 7 MeV/ nucleon. The following description is supported by Mossbauer spectroscopy observations5 and also by the sensitivity of the tracks to chemical etching.6 Five ranges appear with increasing dE/dx: I. Below a dE/dx threshold of 4.5 keV/ nm, 7 the damage results only from nuclear collisions. II. Between 4.5 and 8 keV/nm, the damage induced by the electronic stopping power overcomes that of nuclear elastic collisions. The HREM pictures show the presence of small spherical defects of
approximately 1.5-nm radius with a defect density compatible with the ion fluence. This specific range is confirmed by t h e fact t h a t there is always an isotropic distribution of the hyperfine magnetic field as observed by Mossbauer spectroscopy.5 III. Between 8 and 14 keV/nm, the defects appear elongated and discontinuous. The radii of the cylindrical defects are still always around 1.5 nm. The hyperfine magnetic field turns parallel to the heavy-ion irradiation direction.5 IV. Between 14 and 20 keV/nm, the defects are long and cylindrical. The radius of the tracks increases from 1.5 nm to 3.1 nm with increasing dE/dx. The tracks start to cause inhomogeneous chemical etching.6 V. For dE/dx larger than 20 keV/nm, the damage becomes constant and homogeneous along the ion track. A continuous amorphous core is observed by HREM. The radius of the tracks increases as the square root of dE/dx. A summary of these electron microscopy observations is shown in Figure 1.
Damage Cross Section and Effective Radius of the Tracks Such a description was not possible as long as the damage rate variation versus the electronic stopping power dE/dx was not known. The damage induced by swift, heavy ion irradiation was measured using several physical-characterization techniques like Mossbauer spectroscopy,8 channeling Rutherford backscattering,9 and resistivity measurements10 versus the fluence for specific values of dE/dx. For each physical characterization, specific analysis is necessary to determine the damaged volume fraction C. From C, one can calculate a damage cross section A deduced from Poisson's law C = 1 - exp(-Af) where Of is the ion fluence. The damage cross sections using various
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