Charge states and lattice sites of Fe ions in virgin and Mn-doped Al x Ga1−x N samples were investigated using 57Fe emission Mössbauer spectroscopy following radioactive 57Mn+ ion implantation at ISOLDE, CERN. In the undoped Al x Ga1−x N, Fe2+ on Al/Ga sites associated with nitrogen vacancies and Fe3+ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe3+ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe4+ on Al/Ga sites. Density functional theory calculations confirm the Fe4+ charge state as stabilised by the presence of substitutional Mn2+ in its vicinity. The completely filled spin up orbitals in Mn2+ (3d5) are expected to enhance magnetic exchange interactions. The population of the Fe4+ state is less pronounced at high Al concentration in Al x Ga1−x N:Mn, a behaviour attributable to hybridisation effects of 3d states to the semiconductor bands which weakens with increasing (decreasing) Al (Ga) content. Our results demonstrate that co-doping promotes the co-existence of unusual charge states of Fe4+ and Mn2+, whereas their trivalent charge states prevail with either transition metal incorporated independently in III-nitrides. Co-doping thus opens up a new avenue for tailoring novel magnetic properties in doped semiconductors.

The magnetic properties of Mn x Ga alloys critically depend on composition x, and the atomic-scale origin of those dependences is still not fully disclosed. Molecular beam epitaxy has been used to produce a set of Mn x Ga samples (x = 0.7 divided by 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting Mn-57/Fe and In-119/Sn emission Mossbauer spectroscopy at ISOLDE/CERN, the Mn and Ga site-specific chemical, structural, and magnetic properties of Mn x Ga are investigated as a function of x, and correlated with the magnetic properties as measured by superconducting quantum interference device magnetometry. Hyperfine magnetic fields of Mn/Fe (either at Mn or Ga sites) are found to be greatly influenced by the local strain induced by the implantation. However, In/Sn probes show clear angular dependence, demonstrating a huge transferred dipolar hyperfine field to the Ga sites. A clear increase of the occupancy of Ga lattice sites by Mn for x > 1 is observed, and identified as the origin for the increased antiferromagnetic coupling between Mn and Mn at Ga sites that lowers the samples' magnetization. The results shed further light on the atomic-scale mechanisms driving the compositional dependence of magnetism in Mn x Ga.

An extension of the online implantation chamber used for emission Mössbauer Spectroscopy (eMS) at ISOLDE/CERN that allows for quick removal of samples for offline low temperature studies is briefly described. We demonstrate how online eMS data obtained during implantation at temperatures between 300 K and 650 K of short-lived parent isotopes combined with rapid cooling and offline eMS measurements during the decay of the parent isotope can give detailed information on the binding properties of the Mössbauer probe in the lattice. This approach has been applied to study the properties of Sn impurities in ZnO following implantation of 119In (T½ = 2.4 min). Sn in the 4+ and 2+ charge states is observed. Above T > 600 K, Sn2+ is observed and is ascribed to Sn on regular Zn sites, while Sn2+ detected at T < 600 K is due to Sn in local amorphous regions. A new annealing stage is reported at T ≈ 550 K, characterized by changes in the Sn4+ emission profile, and is attributed to the annihilation of close Frenkel pairs.

The change in the Curie temperature of single crystalline garnet Y3Fe5O12 (YIG) sample due to lattice damage induced by ion implantation has been investigated in 57Fe emission Mössbauer Spectroscopy (eMS) following implantation of 57Mn (T½ = 1.5 min). The Mössbauer spectra analysis reveal high spin Fe3+ ions substituted on both the octahedral and the tetrahedral sites. Measurements in the temperature range 298 K-798 K show that average values of the magnetic hyperfine field are decreased by the implantation-induced damage on the local lattice structure of the YIG. The Curie temperature, however, is determined to be 651 ± 5 K, considerably higher than the value of bulk YIG (559 K). This is most likely due to lattice damage-induced changes on the spin configurations of YIG through a FeA-O-FeD distortion scheme.

Microscopic understanding the metal-to-insulator transition (MIT) in strongly correlated materials is critical to the design and control of modern "beyond silicon" Mott nanodevices. In this work, the local MIT behaviors in single crystalline V2O3 thin films were probed on an atomic scale by online Fe-57 emission Mossbauer spectroscopy (eMS) following dilute (<10(-3) at.%) implantation of Mn-57(+) (T-1/2 = 90 s). Both the epitaxial and the textured V2O3 thin films grown by direct current magnetron sputtering were studied. Three structural components were resolved and identified in the eMS spectra with parameters characteristic of Fe in the 2+ valence state, which are attributable to Fe in either lattice damage or structural defects and Fe in the intrinsic crystal structure of V2O3, respectively. The results prove that the oxygen vacancies are common in the V2O3 thin films. With co-existence of both the non-stoichiometry and epitaxial strain in the thin films, the epitaxial strain plays a dominant role in controlling the global MIT properties of the film. The atomic scale structural transition captured by the eMS affirms the early-stage dynamics of the MIT of V2O3 thin film reported previously. These results approve the feasibility to tune the electronic transport of the V2O3 thin films for the next-generation Mott nanodevices by the epitaxial strain via the sample growth parameters.

The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission Fe-57 Mossbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (1 0 0). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2(+) valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy V-N and interstitial N-I) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial N-I mediated site-exchange between N-I and V-N inside the crystal grain are deduced via a N-2 dimmer like diffusion of N-I through the crystal grains in the temperature range of 540-620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy (V-Ti) is only detected at the grain boundaries. Annealing up to 813 K, both the V-N and N-I are annihilated in the crystalline grains and the V-Ti is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of Mn-57/Fe-57 and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.

The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission Fe Mössbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (100). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2 valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy VN and interstitial NI) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial NI mediated siteexchange between NI and VN inside the crystal grain are deduced via a N2 dimmer like diffusion of NI through the crystal grains in the temperature range of 540 620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy (VTi) is only detected at the grain boundaries. Annealing up to 813 K, both the VN and NI are annihilated in the crystalline grains and the VTi is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of Mn/Fe and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.

We review the research carried out using the apparatus for surface physics and interfaces (ASPIC), at ISOLDE, CERN. We give an overview of the research highlights since 2000, focusing on magnetic and non-magnetic metallic surfaces, and introduce the scientific program that will follow the upgrade which is currently underway, focusing on two-dimensional materials. ASPIC was formerly used for the growth of ultrathin metallic films and their characterization by means of perturbed angular correlation spectroscopy. Past research has mainly focused on the determination of the magnetic hyperfine field at the probe atom located on different sites at the surface such as terraces, kinks, steps as well as on the investigation of the static magnetic polarization at the interface between ferromagnetic and paramagnetic layers. Future research on two-dimensional materials using ASPIC is foreseen to focus on the investigation of structural and electronic properties of adatoms (adsorption sites, hybridization effects, intra-atomic charge transfer, magnetic moments, etc). We emphasize, in this context, the exceptional capabilities of ASPIC in terms of broad applicability, high precision and low detection limits.

The common charge states of Sn are 2+  and 4+. While charge neutrality considerations favour 2+  to be the natural charge state of Sn in ZnO, there are several reports suggesting the 4+  state instead. In order to investigate the charge states, lattice sites, and the effect of the ion implantation process of dilute Sn atoms in ZnO, we have performed 119Sn emission Mössbauer spectroscopy on ZnO single crystal samples following ion implantation of radioactive 119In (T½  =  2.4 min) at temperatures between 96 K and 762 K. Complementary perturbed angular correlation measurements on 111mCd implanted ZnO were also conducted. Our results show that the 2+  state is the natural charge state for Sn in defect free ZnO and that the 4+  charge state is stabilized by acceptor defects created in the implantation process.

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.

57Fe emission Mössbauer spectroscopy has been applied to study the lattice location and properties of Fe in gadolinium gallium garnet Gd3Ga5 O 12 (GGG) single crystals in the temperature interval 300 – 563 K within the extremely dilute (<10−4 at.%) regime following the implantation of57Mn (T 1 / 2= 1.5 min.) at ISOLDE/CERN. These results are compared with earlier Mössbauer spectroscopy study of Fe-doped gadolinium gallium garnet Gd3Ga5 O 12(GGG), with implantation fluences between 8×1015 and 6×1016 atoms cm−2. Three Fe components are observed in the emission Mössbauer spectra: (i) high spin Fe2+ located at damage sites due to the implantation process, (ii) high spin Fe3+ at substitutional tetrahedral Ga sites, and (iii) interstitial Fe, probably due to the recoil imparted on the daughter57∗Fe nucleus in the β − decay of57Mn. In contrast to high fluence57Fe implantation studies the Fe3+ ions are found to prefer the tetrahedral Ga site over the octahedral Ga site. No annealing stages are evident in the temperature range investigated. Despite the very low concentration, high-spin Fe3+ shows fast spin relaxation, presumably due to an indirect interaction between nearby gadolinium atoms.

Emission Mössbauer spectroscopy has been utilised to characterize dilute 57Fe impurities in In 2O3 following implantation of 57Mn (T 1/2 = 1.5 min.) at the ISOLDE facility at CERN. From stoichiometry considerations, one would expect Fe to adopt the valence state 3 + , substituting In 3+, however the spectra are dominated by spectral lines due to paramagnetic Fe2+. Using first principle calculations in the framework of density functional theory (DFT), the density of states of dilute Fe and the hyperfine parameters have been determined. The hybridization between the 3d-band of Fe and the 2p band of oxygen induces a spin-polarized hole on the O site close to the Fe site, which is found to be the cause of the Fe2+ state in In 2O3. Comparison of experimental data to calculated hyperfine parameters suggests that Fe predominantly enters the 8b site rather than the 24d site of the cation site in the Bixbyite structure of In 2O3. A gradual transition from an amorphous to a crystalline state is observed with increasing implantation/annealing temperature.

Emission Mössbauer Spectroscopy (eMS) measurements, following low fluence (<1012 cm−2) implantation of 57Mn (t 1/2 = 1.5 min.) into ZnO single crystals pre-implanted with Ar and C ions, has been utilized to test the sensitivity of the 57Fe eMS technique to the different types of defects generated by the different ion species. The dominant feature of the Mössbauer spectrum of the Ar implanted ZnO sample was a magnetic hyperfine field distribution component, attributed to paramagnetic Fe3+, while that of the C implanted sample was a doublet attributed to substitutional Fe2+ forming a complex with the C dopant ions in the 2− state at O vacancies. Magnetization measurements on the two samples, on the other hand, yield practically identical m(H) curves. The distinctly different eMS spectra of the two samples display the sensitivity of the probe nucleus to the defects produced by the different ion species.

Emission Mössbauer Spectroscopy following the implantation of radioactive precursor isotope 57 Mn + ( T 1/2 = 1.5 min) into ZnO single crystals at ISOLDE/CERN shows that a large fraction of 57 Fe atoms produced in the 57 Mn beta decay is created as paramagnetic Fe 3+ with relatively long spin-lattice relaxation times. Here we report on ZnO pre-implanted with 56 Fe to fluences of 2×10 13 , 5×10 13 and 8 × 10 13 ions/cm 2 in order to investigate the dependence of the paramagnetic relaxation rate of Fe 3+ on fluence. The spectra are dominated by magnetic features displaying paramagnetic relaxation effects. The extracted spin-lattice relaxation rates show a slight increase with increasing ion fluence at corresponding temperatures and the area fraction of Fe 3+ at room temperature reaches a maximum contribution of 80(3)% in the studied fluence range.

The VITO (Versatile Ion-polarized Techniques Online) project is a new experimental setup at the ISOLDE facility at CERN. VITO is a dedicated beam line for producing laser-induced spin-polarized beams of both, atoms and ions, and it has been commissioned in response to the continuously growing demand for the use of spin-polarized beams. The new VITO beam line is a modification of the formerly existing ultra-high vacuum beam line, connecting ASPIC (Apparatus for Surface Physics and Interfaces at CERN), and it has been under construction since the beginning of 2014. Once fully commissioned, VITO will open up numerous possibilities for carrying out multidisciplinary experiments in the areas of nuclear and solid state physics, fundamental interaction physics and biophysics. In its final stage the VITO beam line will provide three fully independent experimental stations: UHV chamber for material science applications, a β-asymmetry station where highly-polarized ions will be available, and a central open-end station suitable for travelling experiments. The VITO beam line will operate in two different modes providing either beams of spin-polarized atoms or ions, or non-polarized ion beams to all three end stations operating from 10−10mbar to 50mbar. Recent experimental campaigns with stable and radioactive beams have allowed for testing VITO’s constituent parts and have demonstrated 96% of ion beam transmission to the collection chamber installed on the central station. The first experimental results obtained with on-line Perturbed Angular Correlation (PAC) spectroscopy using 68mCu ion-beams will be briefly discussed.

The lattice sites, valence states, resulting magnetic behaviour and spin-lattice relaxation of Fe ions in GaN and AIN were investigated by emission Mossbauer spectroscopy following the implantation of radioactive Mn-57(+) ions at ISOLDE/CERN. Angle dependent measurements performed at room temperature on the 14.4 keV gamma-rays from the Fe-57 Mossbauer state (populated from the Mn-57/beta(-) decay) reveal that the majority of the Fe ions are in the 2+ valence state nearly substituting the Ga and Al cations, and/or associated with vacancy type defects. Emission Mossbauer spectroscopy experiments conducted over a temperature range of 100-800 K show the presence of magnetically split sextets in the "wings" of the spectra for both materials. The temperature dependence of the sextets relates these spectral features to paramagnetic Fe3+ with rather slow spin-lattice relaxation rates which follow a T-2 temperature dependence characteristic of a two-phonon Raman process. (C) 2015 Elsevier B.V. All rights reserved.

57Fe emission Mössbauer spectroscopy has been applied to study the lattice location and properties of Fe in gadolinium gallium garnet Gd3Ga5O12 (GGG) single crystals in the temperature interval 300 – 563 K within the extremely dilute (<10−4 at.%) regime following the implantation of 57Mn (T1/2 = 1.5 min.) at ISOLDE/CERN. These results are compared with earlier Mössbauer spectroscopy study of Fe-doped gadolinium gallium This article is part of the Topical Collection on Proceedings of the International Conference on the Applications of the Mössbauer Effect (ICAME 2015), Hamburg, Germany, 13–18 September 2015 H. P. Gunnlaugsson haraldur.p.gunnlaugsson@cern.ch 1 Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Boulevard, Sofia, 1784, Bulgaria 2 CERN, PH Div, CH-1211 Geneve 23, Switzerland 3 Tokyo University of Science, Tokyo, Japan 4 Physics and Engineering Department, University of Zululand, Zululand, South Africa 5 Department of Physics, K.N. Toosi University of Technology, P.O.Box 15875-4416, Tehran, Iran 6 School of Physics, University of the Witwatersrand, Witwatersrand 2050, South Africa 7 Laboratorio MDM, IMM-CNR, Via Olivetti 2, I-20864 Agrate Brianza (MB), Italy 8 Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region, 141700, Russian Federation 9 BCMaterials & Elektrizitate eta Elektronika Saila, Euskal Herriko Unibertsitatea (UPV/EHU), 48048 Bilbao, Spain 37 Page 2 of 6 Hyperfine Interact (2016) 237:37 garnet Gd3Ga5O12(GGG), with implantation fluences between 8×1015 and 6×1016 atoms cm−2. Three Fe components are observed in the emission Mössbauer spectra: (i) high spin Fe2+ located at damage sites due to the implantation process, (ii) high spin Fe3+ at substitutional tetrahedral Ga sites, and (iii) interstitial Fe, probably due to the recoil imparted on the daughter 57∗Fe nucleus in the β− decay of 57Mn. In contrast to high fluence 57Fe implantation studies the Fe3+ ions are found to prefer the tetrahedral Ga site over the octahedral Ga site. No annealing stages are evident in the temperature range investigated. Despite the very low concentration, high-spin Fe3+ shows fast spin relaxation, presumably due to an indirect interaction between nearby gadolinium atoms.

57Fe Emission Mössbauer spectra obtained after low fluence (<1012 cm −2) implantation of 57Mn (T 1/2= 1.5 min.) into ZnO single crystal held at temperatures below room temperature (RT) are presented. The spectra can be analysed in terms of four components due to Fe 2+ and Fe 3+ on Zn sites, interstitial Fe and Fe in damage regions (Fe D ). The Fe D component is found to be indistinguishable from similar component observed in emission Mössbauer spectra of higher fluence (∼1015 cm −2)57Fe/ 57Co implanted ZnO and 57Fe implanted ZnO, demonstrating that the nature of the damage regions in the two types of experiments is similar. The defect component observed in the low temperature regime was found to anneal below RT.

In search for dilute magnetic semiconductors, the magnetic properties at the atomic-scale of Fe atoms incorporated in ZnO, in a concentration range of more than five orders of magnitude from 1 × 10−5 to 2.2 at% have been probed using emission 57Fe Mössbauer spectroscopy on implanted 57Mn and 57Co produced at ISOLDE/CERN. In the ultra-dilute regime (10−5 at%), the system shows isolated paramagnetic Fe3+ ions with a spin–lattice type of relaxation. At higher concentrations (between 0.02 and 0.2 at%) a transition to spin–spin type of relaxation between neighboring Fe3+ is observed, without any signature of magnetic ordering up to 2.2 at%. Despite the many reports of dilute magnetism in 3d-doped ZnO, this atomic level study shows no evidence of any long-range magnetic ordering between isolated Fe atoms incorporated in the ZnO lattice.