F. Studer

Swift heavy ions in insulating and conducting oxides: tracks and physical properties

Damage induced in several oxide materials by swift heavy ions is presented. The discussion is based on results obtained on the following materials [Y3Fe5O12, AFe12O19 (A = Ba, Sr), BFe2O4 (B = Ni, Mg, Zn), ZrSi2O4, SiO2 quartz, Al2O3, high Tc superconductors (YBa2Cu3O7 − δ and Bi2Sr2CaCu2O8)] which have been irradiated by ions with atomic number ranging between 6 (12C) and 92 (238U) and energies between 0.05 GeV and 6 GeV. The damage cross section A has been deduced using several physical characterisations like Mossbauer spectrometry, saturation magnetisation measurements, channeling Rutherford backscattering, infrared absorption and electrical resistance measurements. Depending on the material and on the value of the electronic stopping power (dE/dx) the damage cross section varies between 10−17 and 10−12 cm2. Using medium and high resolution transmission electron microscopy and chemical etching of the latent track, an electronic stopping power evolution of the damage morphology has been observed leading to the definition of an effective radius Re of the latent track which can be linked to the damage (amorphous) cross section A by the relation Re = √A/π. Moreover there is a direct correlation between these values and the damage morphology: for Re > 3 nm the latent tracks are long and cylindrical, conversely for Re < 3 nm the damage is inhomogeneous along the latent track. The effect of the irradiation temperature, of the crystallographic orientation, of the initial electrical resistivity and of the oxygen stoichiometry will be presented. In opposition to what has been usually believed it will be shown that alumina (Al2O3) is indeed sensitive to the electronic stopping power. Moreover the velocity of the incident ion has a direct influence on the damage production: the lower the velocity, the higher the damage.

Track creation in SiO2 and BaFe12O19 by swift heavy ions: a thermal spike description

Abstract The thermal spike model is used in order to calculate the track radii variation versus electronic stopping power Se in two radiolysis resistant oxides: SiO2 quartz and BaFe12O19. The mean diffusion length λ of the energy deposited on the electrons is determined by fitting latent track radii versus Se: 4.0 ± 0.3 and 8.2 ± 1.3 nm respectively for both materials. A decrease in the band gap Eg (12 and 1 eV respectively) means an increase in λ.

Irradiation damage in magnetic insulators

Abstract Using high energy heavy ion irradiation, the damage induced in magnetic insulators (Y3Fe5O12, BaFe12O19, SrFe12O19, NiFeO4, MgFe2O4, ZnFe2O4, Fe3O4) in the electronic stopping power (dE/dx) regime is studied. The amorphization cross section Ap is extracted from the paramagnetic fraction observed on Mossbauer spectra. Electronic stopping power threshold for damage creation appears. The damage efficiency ϵ = A/(dE/dx) is calculated and can be fitted by a general amorphization law ϵ = ϵmax(1− exp(− k(dE/dx)4)). The damage morphology has been correlated to the damage efficiency. Spherical extended defects appear for low values of ϵ at low values of dE/dx. When increasing dE/dx and consequently ϵ, the spherical defects overlap to give discontinuous cylindrical defects. Then for the higher values of dE/dx, the defects are continuous cylinders of amorphous phase. The change of the physical properties induced by the irradiation has been studied. Depending on the shape of the defects, the evolution of the electrical conductivity and the change in the orientation of the hyperfine magnetic field are different. Specific crystallographic sites in BaFe12O19 are more sensitive to the irradiation than others. Local order in the new amorphous phase is determined using X-ray absorption at Fe K-edge and Mossbauer spectroscopies. The creation of magnetization is observed in irradiated ZnFe2O4 which initially shows only a paramagnetic behavior at room temperature.

Saturation in the damage efficiency in magnetic insulators irradiated by high energy heavy ions

Abstract Magnetic garnet Y3Fe5O12 and magnetic ferrite BaFe12O19 have been irradiated at room temperature by Kr and Xe ions using the GANIL accelerator at Caen and by U ions using the Unilac accelerator at Darmstadt. These experiments allow to cover a wide range of electronic stopping power values (between 7 MeV/μm and 45 MeV/μm) using the ion beam at energy between 40 MeV/a.m.u. and 8 MeV/a.m.u. Transmission Mossbauer spectrometry has been used to determine the damage cross section A for the corresponding value of electronic stopping power. High resolution electron microscopy observations confirm the A determination for the highest value of dE/dx. The main feature is the appearance of a saturation in the damage efficiency e = A/dE/dx above an electronic stoping power value T M . Using all the previous results, damage evolution and damage morphology description will be proposed and comparison with the chemical etching sensitivity of the magnetic garnet will be done.

HIGH RESOLUTION ELECTRON MICROSCOPY OF TRACKS IN SOLIDS

Abstract The damage induced by heavy ion irradiation in a wide palette of materials, extending from metals to insulators, is presented. The damage creation and the track morphology have been investigated by electron microscopy and especially HRTEM when available. It is shown that heavy ion irradiation in the electronic stopping power regime is a very efficient tool to amorphize the solids. But the sensitivity to irradiation of each material is strongly variable since the electronic stopping power threshold for damage creation can vary between 1 and 40 keV/nm. The wide difference in sensitivity to heavy ion irradiation between the insensitive materials such as some semiconductors and insulators and the very sensitive ones like the quartz and some magnetic ferrites is a real challenge for any general model of damage creation in solids.

Influence of the Electronic Stopping Power on the Damage Rate of Yttrium-Iron Garnets Irradiated by High-Energy Heavy Ions

Heavy-ion irradiation in the GeV range is an excellent way to investigate electronic stopping power effects in solids up to a few keV/A. Effects of 1.8 GeV Ar, 2.9 GeV Kr and 3.0 GeV Xe irradiations of yttrium-iron garnets have been investigated by magnetic measurements and transmission electronic microscopy (TEM). The dominant effect of electronic losses is demonstrated. For the first time, TEM images of latent tracks are presented.

Threshold electronic energy loss for the creation of latent tracks in Y3Fe5O12 and BaFe12O19 oxides irradiated by high energy heavy ions

The irradiation effects induced by highly energetic heavy ions (> 1 GeV) in two ferrimagnetic oxides Y 5 Fe 5 O 12 and BaFe 12 O 19 have been investigated by means of vibrating sample magnetometry and Mossbauer spectroscopy. In the case of 3.3 GeV-xenon ions, the observation of latent tracks by high resolution electron microscopy suggests that a damage mechanism based on electronic stopping power is largely predominant contrary to 1.8 GeV-argon ions which show localized defects associated with cascades of atomic collisions. As a consequence, a threshold electronic energy loss for the formation of continuous latent tracks has been considered and confirmed by 2.8 and 3.7 GeV-krypton ion irradiations. Threshold values equal to 8.5 and 12 MeV/μm have thus been determined for Y 3 Fe 5 O 12 and BaFe 12 O 19 respectively showing the structural effect of the target on the latent track formation.

HREM investigation of GeV heavy ion latent tracks in ferrites

The irradiation of ferrites Bi2Fe4O9, Y3Fe5O12, BaFe12O19 and AFe2O4 (A = Fe, Zn, Ni, Mg) by 3.0 GeV xenon ions creates paramagnetic cylinders of nearly amorphous matter around the path of the ion. The latent tracks are continuous for Bi2Fe4O9, Y3Fe5O12, BaFe12O19, NiFe2O4 and MgFe2O4 and discontinuous for Fe3O4 and ZnFe2O4 emphasizing the existence of a threshold energy deposition. Track production mechanism based on Coulomb explosion spike and statistical ion spike concepts is proposed above the threshold. Under the threshold the gap model based on successive ionization spikes could be considered.

High energy heavy ion irradiation effects in α-Al2O3☆

Abstract Single crystals of Al 2 O 3 have been irradiated at GANIL with 3.5 MeV/amu Pb ions, at a temperature of ≌ 8 K. The fluence range extended from 4×10 11 to 1.2×10 12 ions cm −2 . The effects of high electronic excitation induced in the samples have been characterized by Rutherford backscattering on channeling (RBS) in conjunction with optical absorption measurements. Moreover, some samples preliminary implanted with 10 16 57 Fe + ions cm −2 at 110 keV and annealed at 1673 K during one hour were studied using conversion electron Mossbauer specroscopy (CEMS) in order to obtain complementary informations. Preliminary RBS results (77 K irradiations) indicate a damage cross section of ∼ 10 −13 cm 2 , consistent with a track radius of about 1.8 nm. The defect efficiency has been also investigated as a function of the electronic stropping power (d E /D x ) c .

Does continuous trail of damage appear at the change in the electronic stopping power damage rate

Y3Fe5O12 samples have been irradiated by Kr ions with incident energies between 9 and 37 MeV/A and Xe ions between 9 and 25 MeV/A. In this way a range of electronic stopping power between 7 and 28 MeV/μm has been explored. The irradiated samples have been etched chemically. A homogeneous track etching radius is observed after xenon irradiation but not after krypton irradiation. This result indicates that at an electronic stopping power value (dE/dx) of 17±3 MeV/μm a change in the damage morphology occurs: above this value the latent track becomes continuous cylinder. This value of stopping power is markedly higher than the dE/dx (8±1 MeV/μm) at which we have previously observed a change in the latent track damage rate and in the damage morphology from spherical to cylindrical shape of the defects.