Temperature dependence of the local electromagnetic field at the Fe site in multiferroic bismuth ferrite

In this paper, we present a study of the temperature-dependent characteristics of electromagnetic fields at the atomic scale in multiferroic bismuth ferrite (BiFeO$_3$ or BFO). The study was performed using time differential perturbed angular correlation (TDPAC) spectroscopy on implanted 111In (111Cd) probes over a wide temperature range. The TDPAC spectra show that substitutional $^{111}$In on the Fe$^{3+}$ site experiences local electric polarization, which is otherwise expected to essentially stem from the Bi$^{3+}$ lone pair electrons. Moreover, the TDPAC spectra show combined electric and magnetic interactions below the Néel temperature $T_N$. This is consistent with simulated spectra. X-ray diffraction (XRD) was employed to investigate how high-temperature TDPAC measurements influence the macroscopic structure and secondary phases. With the support of ab initio DFT simulations, we can discuss the probe nucleus site assignment and can conclude that the $^{111}$In ($^{111}$Cd) probe substitutes the Fe atom at the B site of the perovskite structure

57Fe Mössbauer study of epitaxial TiN thin film grown on MgO(100) by magnetron sputtering

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 $^{57}$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 (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 $^{57}$Mn/$^{57}$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

Charge and spin state of dilute Fe in NaCl and LiF

There is an apparent mismatch between electron paramagnetic resonance and Mössbauer spectroscopy results on the charge and spin states of dilute Fe impurities in NaCl; Mössbauer spectroscopy data have been interpreted in terms of high-spin Fe$^{2+}$, while electron paramagnetic resonance studies suggest low-spin Fe$^{1+}$. In the present study, the charge and spin states of dilute substitutional Fe impurities in NaCl and LiF have been investigated with $^{57}$Mn → $^{57}$Fe emission Mössbauer spectroscopy. A scheme is proposed which takes into account the effects of nearest-neighbor distances and electronegativity difference of the host atoms on the Mössbauer isomer shift and allows for the unequivocal differentiation between high-spin Fe$^{2+}$ and high/low-spin Fe$^{1+}$ in Mössbauer spectroscopy. From these considerations, the Mössbauer results are found to be consistent with dilute Fe impurities in NaCl and LiF in a low-spin Fe$^{1+}$  state. These conclusions are supported by theoretical calculations of isomer shifts and formation energies based on the density-functional theory. The experimental results furthermore suggest that charge compensation of dilute Mn$^{2+}$ dopants in NaCl and LiF is achieved by Na vacancies and F$^-$ interstitials, respectively

Charge states and lattice sites of dilute implanted Sn in ZnO

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 (T ½  =  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

57^{57}Fe emission Mössbauer spectroscopy following dilute implantation of 57Mn^{57} Mn into In 2O3_2O_3

Emission Mössbauer spectroscopy has been utilised to characterize dilute $^57$Fe impurities in In $_2O_3$ following implantation of $^57$Mn ($T_{1/2}$ = 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 Fe$^{2+}$. 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 Fe$^{2+}$ state in In $_2O_3$. 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 $_2O_3$. A gradual transition from an amorphous to a crystalline state is observed with increasing implantation/annealing temperature

The ^68mCu/⁶⁸Cu isotope as a new probe for hyperfine studies: The nuclear moments

Time Differential Perturbed Angular Correlation of γ-rays (TDPAC) experiments were performed for the first time in the decay of 68mCu (6-, 721 keV, 3.75 min) produced at the ISOLDE facility at CERN. Due to the short half-life of the source isotope, the measurements were carried out online. The intermediate state (2+, 84.1 keV, 7.84 ns) offers the unique opportunity to study the electromagnetic fields acting at a copper probe in condensed matter via hyperfine interactions. The present work allowed determination of the nuclear moments for this state. The electric quadrupole moment |Q(2+, 84.1 keV)| = 0.110(3) b was obtained from an experiment performed in Cu2O and the magnetic dipole moment |µ| = 2.857(6) µN from measurements in cobalt and nickel foils. The results are discussed in the framework of shell model calculations and the additivity rule for nuclear moments with respect to the robustness of the N = 40 sub-shell

Atomic-scale study of the amorphous-to-crystalline phase transition mechanism in GeTe thin films

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

The Cu-68m/Cu-68 isotope as a new probe for hyperfine studies: The nuclear moments

Time Differential Perturbed Angular Correlation of gamma-rays (TDPAC) experiments were performed for the first time in the decay of Cu-68m (6(-), 721 keV, 3.75 min) produced at the ISOLDE facility at CERN. Due to the short half-life of the source isotope, the measurements were carried out online. The intermediate state (2(+), 84.1 keV, 7.84 ns) offers the unique opportunity to study the electromagnetic fields acting at a copper probe in condensed matter via hyperfine interactions. The present work allowed determination of the nuclear moments for this state. The electric quadrupole moment vertical bar Q(2(+), 84.1 keV)vertical bar = 0.110(3) b was obtained from an experiment performed in Cu2O and the magnetic dipole moment vertical bar mu vertical bar = 2.857(6) mu(N) from measurements in cobalt and nickel foils. The results are discussed in the framework of shell model calculations and the additivity rule for nuclear moments with respect to the robustness of the N = 40 sub-shell. Copyright (C) EPLA, 201