Evidence of tetragonal distortion as the origin of the ferromagnetic ground state in γ-Fe nanoparticles

γ-Fe and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of γ-Fe nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature Mössbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters a=3.76(2)Å and c=3.50(2)Å, and a magnetic moment of 2.45(5) μB per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured γ-Fe is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of γ-Fe taking tetragonal distortion into account.The authors thank the Fund for Scientific ResearchFlanders, the Concerted Research Action of the KU Leuven (GOA/14/007), the KU Leuven BOF (STRT/14/002), the Hercules Foundation, the Portuguese Foundation for Science and Technology (CERN/FIS-NUC/0004/2015), and the European Union Seventh Framework through ENSAR2 (European Nuclear Science and Applications Research, Project No. 654002), and SPIRIT (Support of Public and Industrial Research Using Ion Beam Technology, Contract No. 227012). We acknowledge the European Synchrotron Radiation Facility (ESRF) for providing beam time (experiments 26-01-1018, 26-01-1057, 20-02-728, HC-1850, HC-2208), as well as C. Baehtz, N. Boudet, and N. Blancand for support during the experiments. We acknowledge the ISOLDE-CERN facility for providing beam time (experiment IS580) and technical assistance. The authors (L.M.C.P., F.K.) acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Advanced Microscopy, Australian National University. We also acknowledge the contribution of Prof. Mark Ridgway (Australian National University), who passed away before the work was completed

Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of Mg-27 is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra-low fluence (approximate to 2 x 10(10) cm(-2)) implantation, the interstitial fraction of Mg is highest (20-24%) in GaN pre-doped with stable Mg during growth, and lowest (2-6%) in n-GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10-12%. Both for p- and n-GaN prolonged implantations cause interstitial Mg-27 to approach the levels found for undoped GaN. Implanting above 400 degrees C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5-2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 10(14) cm(-2) result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation

Evidence of tetragonal distortion as the origin of the ferromagnetic ground state in γ\gamma−Fe nanoparticles

$\gamma$−Fe and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of $\gamma$−Fe nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature Mössbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters a=3.76(2)Å and c=3.50(2)Å, and a magnetic moment of 2.45(5) μB per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured $\gamma$−Fe is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of $\gamma$−Fe taking tetragonal distortion into account