Introduction

Amorphous iron thin films and coatings on different substrate surfaces can have different properties, advantageous for technology, depending on their preparation method, composition and structure. Production of amorphous iron and some related phases poses a major challenge for both science and technology, as it requires specific, non-equilibrium techniques, but these materials can be used advantageously in many fields, such as photovoltaic technology [1], amorphous iron motors [2], nanomedicine [3], communication and computer technology [4] as well as catalysis [5, 6].

The coatings which are composed of amorphous iron and nanoparticles of iron oxides may combine the advantages of both random arrangements of atoms in amorphous state as well as those of nano dimension. The difficulties in their successful synthesis can inspire researchers to study this area. In our present work, we aimed at preparing and studying a system of this type, with special attention to the effect of swift heavy ion irradiation on thin metallic iron layer vacuum deposited onto SiO2/Si substrate.

It is well-known that amorphous iron can be produced in a metal form alloyed with certain metals and/or metalloids by rapid quenching at which iron solidifies from the molten state at a cooling rate of more than ~ 105 K s−1 [7]. However, amorphous pure iron, which does not contain any component other than iron, cannot be produced by this method because the cooling rate required for this is not feasible. Some properties of amorphous pure iron have been predicted by extrapolation from measurements on amorphous iron alloys [68]. Since our target is metal, it is not reasonable to take into account another mechanism, so-called Coulomb explosion [66, 69]. We also exclude any possible contribution of inelastic processes, like nuclear reactions, in view of projectile mass and energy [63, 64].

Figure 6
figure 6

Schematic illustration of the creation of thermal spikes in the case of swift heavy ion irradiated iron metal.

Full size image

Therefore, the observed structural changes in the material are mainly due to the thermal spike mechanism.

In our present case the stop** power values, calculated using the SRIM-2013 code [70], for Fe irradiated with 160 MeV Xe ions are (dE/dx)e = 35.45 keV nm−1 for electron stop** and (dE/dx)n = 17.81 × 10–2 keV nm−1 for nuclear stop**. The penetration depth of these ions in Fe is 8.07 μm. The threshold value of electronic stop** power for formation of thermal spikes reported for bulk iron is 40 keV nm−1 [71]. Although our value is somewhat lower than this threshold, we should take into consideration that our case is a thin deposit film surface when in contrast to bulk samples an enhanced electronic scattering from grain boundaries, surface and interfaces may cause substantial decrease in electron mobility and an increase in electron–phonon coupling. This would result in a decreased value of the threshold electronic stop** power [72,73,74,75] required for the formation of thermal spikes able to induce significant atomic rearrangements. This is supported by the result of a previous work [12] when amorphous iron phase was observed in swift heavy ion irradiated electrodeposited iron thin films due to 246 MeV Kr irradiation when the value of (dE/dx)e = 24.1 keV nm−1 was sufficient to produce enough thermal spikes. Based on the data above, one can conclude that in our case, the main mechanism responsible for the formation of the amorphous iron phase is the thermal spike. This mechanism explaining the formation of the irradiation-induced amorphous iron phase is the same as that given in previous works [12, 13, 40, 45].

The formation of the considerable amount of the silicon-iron-oxides due to 160 MeV Xe ion irradiation can be explained by the accelerated diffusion of ions via the molten tracks and ion beam mixing at the interfaces similarly as reported in other works [26, 76]. Namely, significant atomic transport occurs, by interdiffusion, from one layer to the other when the ion track passes through the interface of a layered system and the electronic stop** power exceeds the energy needed to melt the components along the trajectory of the ion [76]. The oxide formation processes depend on the electronic energy loss of the incident ions. Diffusion controlled dissolution of oxygen in iron takes place when the electronic energy loss is below the track formation threshold. In iron, chemical reaction and homogeneous oxide phase formation take place in a surface layer.

If the track formation threshold in iron is exceeded, the swift heavy ion will generate a molten track inside this layer, which allows the oxygen to diffuse from the oxide-iron interface to the deep layers and thereby to extend the oxide phase area to larger depths [26]. The effective diffusion constant estimated for swift heavy ion-induced oxygen diffusion in iron is larger by a factor of about 1000 than that of thermally induced one. If the electronic stop** power exceeds the track formation thresholds of both interface forming materials, the swift heavy ion irradiation will cause strong intermixing effects, which can lead to the formation of new oxide phases. It was found that the mixing rate varies with the square of the electronic energy deposition above the threshold [76]. The values of the electronic energy deposition Se obtained for our present 160 MeV energy Xe irradiation case are 418 eV (atoms nm−2)−1 for iron, 274.5 eV (atoms nm−2)−1 for Fe2O3, 266.6 eV (atoms nm−2)−1 for Si and 208.1 eV (atoms nm−2)−1 for SiO2. These values are much higher than the corresponding interface mixing threshold energies Sec of 216 and 113 eV (atoms nm−2)−1 for Fe/SiO2, 53 and 41 eV (atoms nm−2)−1 for Fe2O3/SiO2, for the top layer and backing layer, respectively, and of 149 eV (atoms nm−2)−1 for Si as well as 15–30 eV (atoms nm−2)−1 for SiO2 [76]. The formation of radiation-induced silicon-iron-oxide phases we obtained at present can be understood by the explanation above. It is consistent with the above consideration, why we found only a small amount of oxide phase in Fe films in previous studies when the effects of high-energy heavy ion irradiation were investigated. Namely, the electron energy losses were either below the threshold energy in the case of Cu substrate [12] or were much lower than those in the present case. This latter scenario may apply when Fe films deposited onto SiO2/Si were irradiated with 209 MeV energy Kr ions [11]. However, it cannot be ruled out entirely that for thicker iron films studied earlier, CEMS was not sensitive enough to give information on what is happening at the interface and in the substrate.

One may think that the incorporation of Xe can also occur in the irradiation-induced iron oxide phases since xenon iron oxides were predicted [77] forming under very high pressure in Earth’s lower mantle. Such a phase formation which ought to happen more intensely near the penetration depth of ions, which is ~ 8 µm in our case, would not be observable using conversion electron Mössbauer spectroscopy.

Comparing our results with those of previous relevant works, it could be concluded that they are basically in line with each other. The partial transformation of α-iron into other phases has already been observed in different iron films vacuum or electrodeposited onto SiO2/Si and other substrates, irradiated by swift heavy ions [11, 12, 26, 40, 76] as well as with low energy heavy ion implantation [13, 43]. The amorphous iron phase shown at present is consistent with those reported earlier [11,12,13, 39, 40, 45]. The formation of a small amount of FeII oxide phase has already been also indicated upon 180 MeV Kr ion irradiation [26]. However, due to the fact that, in the present case, an Fe layer was evaporated with a thickness comparable to the thickness of the SiO2 layer and we used swift heavy ion irradiation with electron energy deposition much more above the threshold energy than applied in the previous works it was possible to observe that the majority of the metallic α-iron film was converted dominantly to silicon-iron-oxides and, to a less extent, to amorphous iron. Furthermore, the temperature-dependent conversion electron Mössbauer spectroscopy gave a possibility for more accurate phase analysis as earlier.

Conclusions

Irradiation-induced transformation upon swift heavy ion irradiation applying 160 MeV 132Xe26+ ions with a fluence of 5 × 1013 ion cm−2 was found in the as-deposited, dominantly metallic, crystalline α-Fe (~ 85%) and minor FeIII oxide/oxyhydroxide (~ 15%) thin layer produced by vacuum deposition of 57Fe metal onto SiO2/Si substrate. The formation of amorphous pure iron (~ 21%), i.e., metallic iron without alloying elements in amorphous state, and silicon-iron-oxides (~ 47%), which latter can be associated with Si-substituted magnetite γ-Fe1.5SiO4, develops upon swift heavy ion irradiation, while ~ 32% of the total iron retains the crystalline α-Fe state. The irradiation-induced silicon-iron-oxide phases, of which ~ 19% is in FeII state, show superparamagnetic behavior with a transition in a wide temperature range starting from 150 K. The swift heavy ion irradiation-induced formation of amorphous iron and Fe-Si oxide phases in the metallic 57Fe layer vacuum deposited onto SiO2/Si can be interpreted in terms of the thermal spike model of the energy deposition of heavy ions involving irradiation accelerated diffusion and atomic mixing. Additional swift heavy ion irradiation experiments to further promote the irradiation-induced transformation of the as-deposited crystalline α-Fe phase are in progress.

Methods

Iron metal enriched in 57Fe (95 wt.%) was vapor-deposited, at a pressure of 2 × 10–5 Pa, onto thermally oxidized Si wafers, with approximately 10 nm thick SiO2 layer present on the Si surface. The thickness of the evaporated iron layer was 10 nm. The deposition process was monitored by a calibrated quartz crystal. The surface dimension of the deposited samples was 14 × 14 mm2.

The swift heavy ion irradiation of this metallic 57Fe layer was carried out with 160 MeV energy 132Xe26+ ions with a fluence of 5 × 1013 ion cm−2 at room temperature, at a current density of 0.01 μA cm−2 and a pressure of about 10–3 Pa, at the IC-100 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia. The Si substrate sides of the samples were mounted onto a Cu target backing using a double-sided carbon tape. The plane of samples was oriented perpendicular to the ion beam direction. The temperature of target backing was controlled during the irradiation.

Conversion electron Mössbauer measurements on the metallic 57Fe thin layers before and after the swift heavy ion irradiation were performed by conventional Mössbauer spectrometers (WISSEL) with flowing gas (96% He, 4% CH4) proportional counters and 57Co(Rh) sources of 1.85 GBq activity, at room temperature (RT). CEM measurements of the irradiated sample were also performed in a wide temperature range between 17 and 295 K (at 17 K, 50 K, 70 K, 100 K, 120 K, 150 K and 295 K) using a CEM apparatus with a cryostat (Iwatani Cryo Mini D310). The low temperature CEM spectra were recorded using a proportional counter filled with a counter gas of 1 bar pressure at RT. Hydrogen was used as counter gas for the measurements at 17 K and 50 K, while 98%He-2%CH4 gas mixture was applied for the measurements at temperatures higher than 70 K. The CEM apparatus with the proportional counter was described in detail elsewhere [78, 79]. The major part of the information with this method can be obtained mainly from sample depths less than 100 nm [80,81,82]. Isomer shifts were given relative to α-iron. The evaluation of Mössbauer spectra was performed by least-square fitting of the lines using the MOSSWINN code [83].

The magnetization measurements were performed using a Quantum Design Physical Properties Measurement System (PPMS Dynacool system) with the vibrating sample magnetometer (VSM) option. The temperature dependence of the magnetization was recorded in a sweep mode of 1 K min−1 in the zero-field-cooled (ZFC) and field-cooled (FC) measuring regimes. The hysteresis loops were recorded at temperatures of 280 K and 5 K, in external magnetic fields ranging from − 5 to + 5 T.

The surface morphology of the material was characterized by a JSM6490LV scanning electron microscope (SEM, JEOL). The samples were coated with carbon for this purpose.