G. Marest

Optical, channeling and MÖssbauer studies of high dose iron implanted ionic crystals

Abstract The association of optical absorption, channeling and conversion electron Mossbauer techniques has been applied to the study of high dose iron implantation (6 × 1016 57Fe+.cm−2) phenomena in MgO single crystals. After implantation defects in the oxygen sublattice (F-type centers) and in the magnesium sublattice (V−-centers) have been observed as well as superparamagnetic iron precipitates (size ∽20 A) and Fe2+ ions. After thermal annealing for one hour at 700°C in argon atmosphere, all the iron species are converted mainly into Fe3+ ions contributing to the stabilization of F-type centers. A part of them is magnetically ordered even at room temperature, the other remaining paramagnetic down to 4.2 K. This could correspond to a bipartition in the Fe2O3 particle sizes: 50 to 80 A and <20 A respectively. Preliminary results in LiF implanted with 2 × 1016 57Fe+ ions. cm−2 are also reported

Electrical conductivity modification in iron implanted MgO

Abstract Electrical conductivity measurements of MgO single crystals implanted at room temperature with 100 keV iron ions and doses ranging from 1016 up to 1017 ions·cm−2 exhibit an insulating-conducting transition located around 3 × 1016 ions·cm−2. Above this critical dose, the conductivity increases rapidly to reach a value of ∼ 100 Ω−1·cm−1 at 1017 ions·cm−2. The annealing behaviour at temperatures up to 600°C shows a decrease of conductivity and insulating property recovery. The comparison of these results with previous ones on defect characterization by optical absorption and implanted impurity characterization by conversion electron Mossbauer spectroscopy permits a qualitative interpretation of the electrical conductivity modifications observed.

Spinel ferrite formation in iron implanted MgO crystals

Abstract Optical absorption, conversion electron Mossbauer spectroscopy and transmission electron microscopy have been combined to study high dose iron implantation effects in MgO single crystals. After implantation at room temperature with 6 × 10 16 ions cm −2 , large concentrations of defects are observed in the anionic sublattice (F, F + and F 2 centers) and in the cationic sublattice (V − -centers) as well as superparamagnetic iron precipitates (∼20 A in diameter), and Fe 2+ ions. The thermal annealing effects at 700°C are to anneal all the point defects previously mentioned and to convert the iron mainly into Fe 3+ ions. The major fraction of these species is aggregated into Fe 2 O 3 particles, while a smaller fraction part is substitutionally located in the MgO matrix. A significant change is observed after annealing at 800°C. At this stage the aggregates are sufficiently large (∼200 A) to be observed well by electron microscopy. The electron diffraction pattern reveals the formation of a spinel ferrite: MgFe 2 O 4 or Fe 3 O 4 . The annealing effect at a temperature higher than 800°C is to increase the ferrite particle size (∼300 A at 900°C).

Conversion electron Mössbauer study of LiF implanted with 57Fe ions

Abstract Lithium fluoride crystals implanted with 57Fe ions at doses from 5×1015 at/cm2 to 6×1016 at/cm2 have been studied with conversion electron Mossbauer spectroscopy. It was found that iron enters the implanted zone in three well defined charge states: Fe3+, Fe2+ and Fe0 (metal). At low doses the relative fractions of Fe3+ and Fe2+ are close to 80% and 20%, which corresponds to the respective ionic fractions in an iron doped LiF sample obtained by vacuum co-deposition. With the increase in the implantation dose the Fe2+ and Fe0 fractions increase to about 40% and 30%, respectively. Annealing in vacuum results in a growth of metallic iron precipitates, whereas annealing in the presence of oxygen leads to precipitation of superparamagnetic particles of some, still unidentified, ferric compound. The Mossbauer data were related to the role of F- and F2-centres observed by optical absorption measurements. Good agreement has been achieved between the quadrupole splitting data for Fe3+ in fluorides LiF, NaF and KF and the calculated values in frames of the model of V−-Fe3+-V− defect clusters presented in our earlier work.

Metallic alloy precipitates in high dose indium implanted MgO

Abstract MgO implanted at room temperature with 150keV In+ ions and doses ranging from 1015 to 1017 ions cm−2 was studied by optical absorption and transmission electron microscopy (TEM). Creation of defects in the anionic sublattice (F-, F+-, F2-centers) and in the cationic sublattice (V−-centers) are observed. Subsequent annealings of the implanted crystals have shown different behaviours depending on the implanted dose. For medium dose (2 × 1016 ions cm−2), the formation of In3+ species seems to be the preponderant phenomenon. At higher dose (8 × 1016 ions cm−2), metallic precipitates are formed between 400 and 700°C. The identification of these precipitates has been achieved using TEM: they are formed of a metallic alloy Mg3In with a hexagonal structure and their orientation relationship with respect to the MgO matrix is: (0001)Mg3In//(111)Mgo and [1120]Mg3In// [l10]MgO.

Ion implanation effects in bubble garnet materials

Abstract (YSmLuCa)3(FeGe)5O12 garnet films grown by liquid phase epitaxy on a GGG substrate have been implanted at room temperature with 6×101657Fe+ ions at energy 100 keV. Ion channeling and conversion electron Mossbauer spectroscopy techniques have been used to characterize the damage and the implanted ions as well as their annealing behaviour at temperatures up to 800°C. Directly after implantation, the implanted layer is amorphized and the iron atoms are observed in three forms: metallic precipitates, Fe2+ and Fe3+ ions. For annealings at temperatures up to 500°C, the oxidation of iron into Fe3+ species takes place but the system remains paramagnetic down to 77 K. At 650°C, the crystal recovery is partially achieved and a fraction of the iron ions is observed in octahedral (a) and tetrahedral (d) sites of the garnet. After annealing at 800°C, a complete recrystallization of the implanted zone is obtained and the majority of the implanted ions is distributed in a and d sites. However, at this stage a fraction of the implanted iron is precipitated in the form of α-Fe2O3 particles.

Perturbed angular correlation associated with optical absorption for the study of high dose In implanted LiF

Abstract LiF crystals implanted at room temperature with 2 × 10 16 stable indium ions/cm 2 and subsequently in the same conditions with 4 × 10 10 radioactive 111 In ions/cm 2 have been used to study the metallic aggregate formation. After implantation and after thermal treatments up to 650°C, the optical absorption measurements combined with time differential perturbed angular correlation measurements have been performed. We observe that the metallic aggregate formation takes place between room temperature and 350°C. Above 350°C we observe their dissolution in the matrix. The maximum fraction of implanted ions aggregated in metallic clusters reaches 37% at 350°C. The other fraction of implanted indium is distributed into four parts exposed to different interactions and showing different evolutions during the annealing procedure. The first part is composed of ions in sites perturbed by defects at long distances. The three other sites experience well defined interactions with quadrupole frequencies at about 90, 150 and 200 MHz. We do not observe indium in unperturbed cubic sites at any step of the thermal treatment.

Oriented indium metallic aggregates in implanted LiF single crystals

Abstract After implantation at room temperature with 2×1016 indium ions/cm2 and an annealing at 350°C we observe metallic aggregate formation in LiF single crystals. Using the perturbed angular correlation technique it can be proved that these aggregates are an indium tetragonal metal with its c-axis directed along the 〈110〉 axis of the single crystal and with an axial ratio c a smaller than the normal indium tetragonal ratio.

Aggregate Formation in High Dose Iron Implanted MgO Single Crystals Studied by Optical Absorption and Conversion Electron Mössbauer Spectroscopies

The association of optical absorption, Rutherford backscattering and conversion electron Mossbauer spectroscopies have been applied to the study of high dose (6.10 16 ions, cm −2 ) iron implantation phenomena in MgO single crystals. After implantation defects in the oxygen sublattice (F-type centers) and in the magnesium sublattice (V-type centers) have been observed as well as superparamagnetic iron precipitates (size ∼20A) and Fe 2+ ions. After thermal annealing at 700°C in argon atmosphere all the iron species are converted mainly into Fe 3+ ions. A part of them is magnetically ordered even at room temperature, the other part remaining paramagnetic down to 4.2 K. This could correspond to a bipartition in the Fe 2 O 3 particle sizes: 50 to 80 A and

Metallic Aggregate Formation of Indium Implanted in Lif Studied By Optical and Nuclear Hyperfine Methods

Optical absorption and nuclear hyperfine method such as time dependent perturbed angular correlation have been associated to study the formation of metallic clusters in LiF crystals implanted at room temperature with 2.10 16 stable indium ions/cm 2 and 4.10 10 radioactive 111 In ions/cm 2 . The maximum fraction of implanted ions aggregated into metallic clusters reaches 37% at 350°C and a dissolution phenomena is observed above. Influence of the host matrix on the structure of metallic aggregates and the existence of indium-defect interactions are discussed.