Confirming the unusual temperature dependence of the electric-field gradient in Zn

The electric-field gradient (EFG) at nuclei in solids is a sensitive probe of the charge distribution. Experimental data, which previously only existed in insulators, have been available for metals with the development of nuclear measuring techniques since about 1970. An early, systematic investigation of the temperature dependence of the EFG in metals, originally based on results for Cd, but then also extended to various other systems, has suggested a proportionality to T 3/2 . However, later measurements in the structurally and electronically similar material Zn, which demonstrated much more complex behavior, were largely ignored at the time. The present experimental effort has confirmed the reliability of this unexpected behavior, which was previously unexplained

Compositional dependence of epitaxial L10-Mnx Ga magnetic properties as probed by 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy

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 ÷ 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting 57Mn/Fe and 119In/Sn emission Mössbauer 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

Unusual charge states and lattice sites of Fe in Al x Ga1-x N:Mn

Charge states and lattice sites of Fe ions in virgin and Mn-doped Al x Ga1-x N samples were investigated using Fe-57 emission Mossbauer spectroscopy following radioactive Mn-57(+) 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+ (3d(5)) 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.This work was supported by the European Union Seventh Framework through ENSAR (Contract No. 262010) and the German BMBF under Contract Nos. 05K13TSA and 05K16PGA. The work was funded by the Austrian Science Fund (FWF) through Projects No. P26830 and No. P31423. H Masenda, K Bharuth-Ram, and D Naidoo acknowledge support from the South African National Research Foundation and the Department of Science and Innovation within the SA-CERN programme. H Masenda also acknowledges support from the Alexander von Humboldt (AvH) Foundation. B Qi, H P Gislason and S lafsson acknowledge support from the Icelandic Research Fund. I Unzueta thanks the support of (MINECO/FEDER) and the Basque Government for the Grants RTI2018-094683-B-C5 (4, 5) and IT-1005-16, respectively

Investigation of Metal Ions Drift in Memristive Devices using Perturbed Angular Correlation (PAC) method

The distinctive electrical characteristics of memristive devices render them an optimal choice for enhancing the computational efficiency of computer systems and facilitating their integration into neuromorphic applications. Particularly noteworthy is their potential application in the field of Artificial Intelligence. This study aims to elucidate the atomic-level structure of memristive devices employing Perturbed Angular Correlation (PAC) spectroscopy. Specifically, our investigation seeks to scrutinize the impact of external voltage on the local electrical field gradients within memristive devices and gain insights into ion channel formation mechanisms. Our findings underscore the potential of PAC as a promising technique for probing the local structure of memristive devices. However, the examined samples were found unsuitable for PAC measurements, primarily due to the diminutive size of the active zone, which hindered the attainment of statistically significant results. Consequently, we propose an alternative device configuration for future investigations. Furthermore, our study underscores the critical importance of controlled aging and storage protocols for memristive units, attributable to the formation of silver sulfide on the contact pads of the samples. These insights are pivotal for the development and longevity of memristive device technologies

Group Theory Analysis to Study Phase Transitions of Quasi-2D Sr3Hf2O7

We present an ab-initio study performed in the framework of density functional theory, group-subgroup symmetry analysis and lattice dynamics, to probe the octahedral distortions, which occur during the structural phase transitions of the quasi-2D layered perovskite Sr3Hf2O7 compound. Such a system is characterized by a high-temperature I4/mmm centrosymmetric structure and a ground-state Cmc21 ferroelectric phase. We have probed potential candidate polymorphs that may form the I4/mmm → Cmc21 transition pathways, namely Fmm2, Ccce, Cmca and Cmcm. We found that the band gap widths increase as the symmetry decreases, with the ground-state structure presenting the largest gap width (∼5.95 eV). By probing the Partial Density of States, we observe a direct relation regarding the tilts and rotations of the oxygen perovskite cages as the transition occurs; these show large variations mostly of the O p-states which contribute mostly to the valence band maximum. Moreover, by analyzing the hyperfine parameters, namely the Electric Field Gradients and asymmetric parameters, we observe variations as the transition occurs, from which it is possible to identify the most plausible intermediate phases. We have also computed the macroscopic polarization and confirm that the Cmc21 phase is ferroelectric with a value of spontaneous polarization of 0.0478 C/m2. The ferroelectricity of the ground-state Cmc21 system arises due to a second order parameter related to the coupling of the rotation and tilts of the O perovskite cages together with the Sr displacements

Group Theory Analysis to Study Phase Transitions of Quasi-2D Sr3_{3}Hf2_{2}O7_{7}

We present an $ab-initio$ study performed in the framework of density functional theory, group-subgroup symmetry analysis and lattice dynamics, to probe the octahedral distortions, which occur during the structural phase transitions of the quasi-2D layered perovskite Sr$_{3}$Hf$_{2}$O$_{7}$ compound. Such a system is characterized by a high-temperature I4/mmm centrosymmetric structure and a ground-state Cmc21 ferroelectric phase. We have probed potential candidate polymorphs that may form the $I4/mmm$ → $Cmc2$$_{1}$ transition pathways, namely $Fmm2$, $Ccce$, $Cmca$ and $Cmcm$. We found that the band gap widths increase as the symmetry decreases, with the ground-state structure presenting the largest gap width (∼5.95 eV). By probing the Partial Density of States, we observe a direct relation regarding the tilts and rotations of the oxygen perovskite cages as the transition occurs; these show large variations mostly of the O $p$-states which contribute mostly to the valence band maximum. Moreover, by analyzing the hyperfine parameters, namely the Electric Field Gradients and asymmetric parameters, we observe variations as the transition occurs, from which it is possible to identify the most plausible intermediate phases. We have also computed the macroscopic polarization and confirm that the $Cmc2$$_{1}$ phase is ferroelectric with a value of spontaneous polarization of 0.0478 C/m$^{2}$. The ferroelectricity of the ground-state $Cmc2$$_{1}$1 system arises due to a second order parameter related to the coupling of the rotation and tilts of the O perovskite cages together with the Sr displacements