Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

In situ studies of solid state reactions by perturbed angular correlation

We report on three examples of in situ studies of solid state reactions by time differential perturbed angular correlation of¹⁸¹Ta: (i) the oxidation of hafnium metal and the doping of ZrS₂ with Hf during iodine vapour transport crystal growth; (ii) the observation of sublattice melting during polymorphic phase transitions in TaS₂; (iii) the electrointercalation of 2H-TaS₂ with silver.     

Hyperfine-field measurements inCr Rh andCr Re alloys using perturbed angular correlation technique

The hyperfine fields on¹¹¹Cd probe nuclei in Cr (1 at% Rh), Cr (0.3 at% Rh) and Cr (0.5 at% Re) matrices are measured using time-differential perturbed (–) angular correlation of 173–247 keV cascade in the decay of¹¹¹In to the levels in¹¹¹Cd. The sources prepared by ion implantation of¹¹¹In (2.81 d) activity followed by appropriate annealing procedure were found to be of good quality. The addition of impurities, located to the right of chromium in the periodic table such as Rh and Re, to the chromium matrix increases the electron to atom ratio, resulting in an increase in the magnetic moment compared to pure chromium. This in turn is expected to result in an increase in the hyperfine fields at probe nuclei in these alloys. The measured hyperfine fields are in qualitative agreement with the expected changes in these alloys.On leave from Kanpur Institute of Technology.     

Ionic and electronic transport in In2S3 studied via perturbed angular correlation spectroscopy

We report on Perturbed Angular Correlation measurements in polycrystalline In₂S₃ samples in the temperature range from 8 K to 1000 K where two different crystallographic phases β and α occur. As probes, implanted ¹¹¹In nuclei have been used. The three observed EFGs are attributed to probes residing substitutionally in the different sulfur-octahedra and -tetrahedra of β-In₂S₃. A strong damping between 150 K and 300 K has been attributed to EFG fluctuations following the ¹¹¹In(EC)¹¹¹Cd decay. The α-phase (above 680 K) is characterized by a different dynamical damping of the perturbation functions, caused by mobile In atoms. Therefore, the semiconductor In₂S₃ shows, in two different temperature ranges, dynamical PAC-spectra which correspond to different types of mobile charge carriers. Since ¹¹¹In is a self atom in In₂S₃, this compound is an ideal substance to study the charge transport phenomena by the PAC technique. This revised version was published online in August 2006 with corrections to the Cover Date.     

Defects in CdS: In detected by perturbed angular correlation spectroscopy (PAC)

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom ¹¹¹In by the perturbed γγ angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of In_Cd-V_Cd pairs which seem to be responsible for the self-compensation of In donors in CdS.     

Time differential perturbed angular correlation studies of hafnium hydrides

The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal ε phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with¹⁸¹Ta as the nuclear probe. Two EFGs were found in the ε phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the ε phase, are due to the changes in the lattice parametersa _o,c _o as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies.     

Doping of sapphire single crystals with 111In and 111Cd detected by perturbed angular correlation

The electric hyperfine interaction of ion beam implanted ¹¹¹In and ¹¹¹Cd probe atoms in sapphire (Al₂O₃) single crystals has been investigated using perturbed angular correlation spectroscopy. For both probe atoms the same distinctive electric field gradient was found, indicating that nearly all the implanted probe atoms form a stable substitutional configuration in the temperature range between 77 K and 873 K on the aluminum sublattice. A comparative study between ¹¹¹In and ¹¹¹Cd-measurements points to a dynamic interaction initiated by the electron-capture of ¹¹¹In(EC)¹¹¹Cd similar to In₂O₃ and La₂O₃. Size and orientation of the EFG are discussed in comparison to experimental results in Cr₂O₃ single crystals. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the perturbed angular correlation attenuation factor for relaxing systems

The attenuation factor of gamma-gamma perturbed angular correlation in the isotropic random-phase approximation is deduced for an axially symmetric quadrupole interaction and spins 1, 3/2, 2 and 5/2. It is shown that modifications to the usual expression are significant. Useful expressions for fitting procedures are obtained.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de estudos e Projetos (FINEP).     

Time-dependent perturbed angular correlation study of181Ta in Mg

The quadrupole interaction of¹⁸¹Ta in Mg metal has been investigated at room temperature by time differential perturbed angular correlation method yieldingv _Q=127±7 MHz. The observed EFG is calculated to be 20.8×10¹⁶ V/cm².     

Simultaneous energy and timing decisions in time differential perturbed angular correlation experiments using BaF2 scintillators

BaF₂ scintillation counters, coupled to constant fraction differential discriminatiors, permit simultaneous energy and timing decisions in recording delayed coincidence spectra. We explore the advantages and limitations of this innovation in time differential perturbed angular correlation experiments, and illustrate it for¹¹¹CdNi and¹⁰⁰RhNi.     

Investigation of laser irradiated poly- and monocrystalline aluminium by perturbed angular correlation measurements

Poly- and monocrystalline samples of aluminium implanted with ¹¹¹In to a dose of about 10¹³ at/cm² were irradiated at a temperature of 80 K with a Q-switched pulsed ruby laser. The energy density of the 25 ns pulses was in the range of 1–5 J/cm². Perturbed angular correlation measurements revealed the formation of several In-vacancy clusters, some of which give rise to electric field gradient tensors oriented along low-index crystallographic directions. This clearly shows that laser irradiation acts as a fast quench and that the monocrystalline structure is not destroyed, not even by the highest laser pulse intensity.     

Oxidation of indium implanted aluminium studied by the Rutherford backscattering and perturbed angular correlation methods

Aluminium foils oversaturated with 5.10¹⁵/cm² implanted¹¹⁵In⁺ ions were oxidized in 200 mbar oxygen or vacuum annealed at 370–870 K. The samples were analyzed by the Rutherford backscattering (RBS) and perturbed angular correlation (PAC) techniques, using some 10¹¹/cm² implanted radioactive¹¹¹In tracers. Furthermore, the oxygen surface profiles were also scanned with high resolution by using the nuclear resonance technique (NRA). The formation of passivating Al₂O₃ surface layers, preventing deeper oxygen diffusion and the indium diffusion into these oxidized surface layers and into the bulk, were studied. Several quadrupole interaction frequencies previously attributed to strained cubic indium precipitates and indium (-vacancy) clusters were observed. When the samples were oxidized above 750 K, the formation of In-O complexes and of substitutional¹¹¹In in Al₂O₃ was observed.     

Intrinsic and extrinsic defects in semiconductors studied by perturbed γγ angular correlation spectroscopy

The application of the perturbed γγ angular correlation technique as an analytical tool for the characterisation of atomic defect configurations is discussed. Using the radioactive probe atom¹¹¹In/¹¹¹Cd, recent results on the compensation of acceptor and donor atoms in different II–VI semiconductors will be discussed, in particular the role of the cation vacancy defect.     

Anisotropy of the angular correlation in high temperature single crystal Cu

A small anisotropy in the two-γ angular correlation of annihilation radiation in a single crystal copper was detected at 925°C by the rotating specimen method. The ratio of the anisotropy of the momentum distribution of pair electrons annihilated at a vacancy site has been estimated.     

Study of nanoconfigurations in Zircon-Mullite composites using perturbed angular correlations

It has been already published that, at nanoscopic level, zircon exhibits wide regions of aperiodic material not detected by the ordinary techniques used to analyse the obtained product in the production of ceramic materials. In this paper it is reported how the Perturbed Angular Correlations (PAC) technique has assisted the interpretation of a mismatch between experiment and theoretical estimation of a mechanical property in some zircon-mullite composites. In fact, it has been proved that the difference observed between the calculated and the experimental value of the elastic modulus in composites of the form (1?x) ZrSiO₄ ? x 3Al₂O₃.2SiO₂ (with x = 15, 25, 35 and 45 wt.%) behaves very similarly as the relative amount of aperiodic zircon determined by PAC. This result allows to re-interpret the mullite role during the materials preparation.     

Analysis of perturbed angular correlation spectra of metal ions bound to proteins with rotational correlation times in the intermediate region

The formulation by Dattagupta of the strong-collision model, describing the effect on the perturbation function,G ₂(t) by the isotropic tumbling of an electric field gradient, is generalized to electric field gradients with no axial symmetry. The effect on the perturbation function by strong collisions is compared to the effect of rotational diffusion in the adiabatic limit. The comparison is carried out for decays with an intermediate state of spin 5/2 and for non-axially symmetric electric field gradients. It shows that the strong-collision model can be used for interpretation of PAC spectra of molecules with correlation times between the adiabatic and the fast relaxation limits. The strongcollision model is then used to determine the rotational diffusion of the cadmium substituted copper, zinc superoxide dismutase at 3°C and 25°C from^111mCd TDPAC spectra. For these analyses, the model is incorporated into a conventional least-squares fitting routine.     

Determination of nuclear multipole relaxation times by measurements of statistically perturbed angular correlations

It is pointed out that phenomena of nuclear multipole relaxation can also be studied by the observation of perturbed angular correlations. The corresponding formulae are derived and compared with the results of other authors. A compilation of experimental results is presented.