111Cd Probe Research Articles

Hyperfine interactions of111In→111Cd probe atoms on GaAs (111)B reconstructed surfaces

We have used111In→111Cd perturbed-angular-correlation (PAC) spectroscopy to measure hyperfine interactions at surface sites on two molecular-beam-epitaxy-grown GaAs (111)B (As-terminated) reconstructed surfaces. The PAC measurements performed on the 2×2 reconstructed surfaces show two well-defined nuclear electric-quadrupole interactions that occur at two inequivalent Ga-sites on the surface. The measurements performed on the √19×√19 R 23.4° reconstructed surfaces show primarily one well-defined interaction to which corresponds a large and asymmetric EFG, in which the EFGz-axis is oriented essentially perpendicular to the surface. Annealing experiments were performed to convert one reconstruction to the other and then to reconvert it back to the original reconstruction, e.g., 2×2→√19×√19 R 23.4°→ 2×2. The PAC measurements that followed each annealing step show the characteristic frequencies for each reconstruction. This result indicates that these probe sites are thermodynamically stable. In this report, we present a brief description of the experiment and, as an example, one series of measurements that illustrates the results of an extensive investigation, which reports the first measurements of group-III bonding symmetries on a compound III–V semiconductor surface.

PAC Studies of Ag-Hf Intermetallic Compounds

Abstract The hyperfine quadrupole interaction of 181Ta and 111Cd probes in HfAg and HfiAg com-pounds was studied at 24 to 1100 K with the time -differential perturbed angular correlation (TDPAC) technique. The EFGs measured in HfAg on 181Ta and 111Cd are 1.388(5)x1018 Vcm-2 and 0.340(5)x1018 Vcm -2 , respectively. For both probe atoms a linear decrease of the EFG with temperature was observed with slopes 1.11(2)x10-4 K-1 and 1.73(5)x10-4 K-1 for 181Ta and 111Cd, respectively. In Hf2 Ag, 0.469(2)x1018 Vcm-2 was determined on 181Ta nuclei and 0.428(5)x1018 Vcm-2 on 111Cd. Also a linear EFG temperature dependence with the large slope of 2.39(6)x10-4 K-1 was observed for 111Cd probes in Hf2Ag, while the EFG measured on 181Ta nuclei in this compound exhibits a very weak temperature variation which can be fitted neither with a linear T nor a T 3/2 dependence. In all cases the electronic contributions to the EFG are very small indicating that these transition element systems do not obey the universal correlation between the electronic and the ionic EFG observed in numerous other systems not belonging to the transition metal series.

PAC-studies of Sn-doped In2O3: electronic defect relaxation following the 111In(EC)111Cd-decay

Perturbed γ-angular correlation measurements as function of the measuring temperature and the Sn-content have been performed in Sn-doped cubic In2O3 using 111In(EC)111Cd and 111m Cd probes. By comparing the spectra of both isotopes, electronic relaxation phenomena, socalled decay after-effects, following the electron capture radioactive decay of 111In, were established. A time independent loss of the static quadrupole interaction amplitudes was found to be characteristic for the electronic relaxation, and its temperature and doping dependence were measured. The high statistical accuracy of the PAC-data allowed a separation between structural and dynamic effects and the observation of each lattice site’s behaviour. Relaxation rates were extracted from numerical simulations based on Blume’s theory and related to the predominant electron transport mechanisms in In2O3, especially, with rising Sn-content, to the transition to metallic conduction.

Precision PAC measurements in Er2O3 and Ho2O3 single crystals and structure refinement

The structure of C-form Ho2O3 and Er2O3 single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of111In(EC)111Cd probe ions using the perturbed γ-γ angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to X-ray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.

Electric field gradients of 111Cd in monoclinic (B-phase) rare earth sesquioxides

Perturbed gamma - gamma angular correlation measurements with 111In(EC) 111Cd probes have been performed in the monoclinic (B-phase) rare earth sesquioxides Sm2O3, Eu2O3 and Gd2O3. The spectra in all three oxides show a dominating electric field gradient of high asymmetry, eta approximately=1, and strong coupling constant, nu Q approximately=208 MHz, at room temperature. A slight modification of the structure in this phase with temperature was observed, but no phase transition occurred. Unlike the other structural isomorphs of the rare earth oxides, the cationic lattice sites in the B phase are not equally occupied by the implanted 111In ions in the temperature range investigated (10-900 K). The point-charge model and symmetry considerations were used to associate the dominating electric field gradient with substitutional 111In on the crystallographic site M(3). Two additional electric field gradients populated in small fractions were found in limited temperature ranges and tentatively attributed to defects.

ELECTRIC FIELD GRADIENTS IN SEMICONDUCTORS AND INSULATORS: EMPIRICAL CORRELATION FOUND IN BINARY OXIDES

For the first time a well defined correlation between local and lattice contributions of the electric field gradient in semiconductors and insulators is presented. The local component of the electric field gradient in binary oxides, originated in extraionic contributions, is extracted from all quadrupole coupling constants measured in Perturbed Angular Correlation experiments with 111Cd probes. The resulting systematics reveals a linear dependence of the local component over a wide range of ionic efg values, both contributing with opposite signs. The clustering of the data points is explained in terms of the oxygen near neighbor symmetry.

New technique to dope GaAs crystals with the 111In→111Cd probe for perturbed-angular-correlation spectroscopy

Perturbed-angular-correlation (PAC) spectroscopy is an important technique for measuring defect and dopant interactions in group IV and III-V semiconductors. The probe of choice for most of the successful PAC experiments on semiconductors has been 111In→111Cd introduced by ion implantation. To expand the gamut of PAC experiments that can be performed on III-V semiconductors, we have developed a simple closed-tube, vapor-phase-epitaxy (VPE) technique to produce 111In-doped GaAs single-crystal epitaxial materials. PAC measurements on these crystals yielded nearly nonperturbed correlations that indicate that the 111In probe was incorporated substitutionally into the GaAs crystals. These correlations differ significantly from the previously reported weakly perturbed correlations that were measured on GaAs crystals implanted with 111In ions. An exploratory experiment using this VPE technique also showed that Sn can be incorporated along with 111In.

Electric field gradients at 111Cd in M2Cu2O5 cuprates (MSc, Y, In, Lu, Yb, Ho, Tb)

Abstract Perturbed angular correlation spectroscopy has been used to study the hyperfine interaction at 111In(EC)111Cd in several cuprates M2Cu2O5 (MSc, Y, In, Lu, Yb, Ho, Tb) with Ho2Cu2O5 structure. The radioactive 111In probes were either ion implanted or chemically introduced into polycrystalline powder samples. Two electric field gradients, populated with equal fractions by the 111In(EC)111Cd probes, have been attributed to the two octahedral M sites. A smooth variation of the hyperfine parameters with the ion radius of the M elements was observed and traced back to the variation of the position of the oxygen ion connecting the two M sites.

Impurity trapping in the Laves phase HfV2 detected by perturbed angular correlations

A perturbed gamma gamma angular correlation (PAC) study of the electric quadrupole interaction (QI) of 111Cd probes on substitutional V sites in the C15 Laves phase HfV2 has revealed a pronounced thermal hysteresis of the QI and the 111Cd fraction on regular V sites: when samples are cycled through 700 K in vacuum, the quadrupole frequency nu Q on the way down is found to be systematically larger than that on the way up at the same temperature. Simultaneously a sharp irreversible drop of the regular site fraction occurs upon cooling from 700 K. This behaviour indicates that HfV2 reacts with water vapour at T<or=700 K, dissociating residual H2O into its atomic components. HfV2 acts as a strong getter for the hydrogen, which is absorbed upon cooling below 700 K and again desorbed at T>800 K. The oxygen, however, is absorbed only in trace amounts. These conclusions are supported by a mass spectroscopic desorption measurement and a comparative PAC study of QI in the hydride HfV2H0.1, for which the activation energy for the hydrogen diffusion has been determined to be Ea=0.070(25) eV.

PAC studies on impurities in ZnO

Acceptor-donor pairs in ZnO are studied by the perturbed angular correlation spectroscopy (PAC) using radioactive 111In/ 111Cd probe atoms. In undoped ZnO the trapping of O-vacancies as well as the trapping of Zn-interstitials at the probe atoms which are located at substitutional Zn sites is observed after different sample treatments. In Cu-, Li-, and Na-doped ZnO the acceptor impurities form complexes with the In donors. The structure of these complexes is given and compared with theoretical calculations.

Scaling of the electric field gradient of 111Cd impurities in the bixbyite oxides of Y, Sc, Dy and Yb

The quadrupole hyperfine interaction of 111Cd in the cubic C-form sesquioxides of Sc, Y, Dy and Yb has been investigated over the temperature range 295–850 K with perturbed angular correlation (PAC) spectroscopy. The 111Cd probe nuclei were either produced in the EC-decay of 111In or in the γ-decay of the 111Cd isometric state and introduced via ion implantation into the oxide matrices. A simple scaling of the components of the electric field gradient with the lattice constant has been observed on both substitutional octahedral sites.

Magnetic hyperfine fields at uncovered ultrathin Ni films on Cu(100) substrates and at single-crystal Ni surfaces

Magnetic and electric hyperfine fields were measured at 111 In( 111Cd) probe atoms in the topmost atomic layer of Ni(111) and Ni(100) single-crystal surfaces as well as in ultrathin Ni films epitaxially grown on Cu(100), utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the magnetic hyperfine fields as a function of temperature were studied for both Ni surfaces as well as for different Ni(100) films with thicknesses ranging from 2 up to 10 monolayers. It was found that strength and orientation of the magnetic hyperfine fields are quite different for these systems. The critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness, whereas at the surfaces no change in the Curie temperatures was detected.

Hyperfine Interactions in Chromium Chalcogenide Spinels

By help of perturbed angular correlations (TDPAC) the hyperfine interaction in the spinels CdCr2S4 and CdCr2Se4 is investigated. Both are semiconductors. In the paramagnetic state the 111In (111Cd) probes find a dominating axially symmetric quadrupole interaction. The pure magnetic Cd-interaction in the ferromagnetic state known from NMR investigations is observed only for few probes. Single defects (anion vacacnies) which are responsible for the quadrupole interaction will be movable at higher temperatures and they annihilate with a Se excess. The number of probe – vacancy pairs depends on the implantation method of the In-probes. In the case of Cd-probes unperturbed (cubic) environments are observed.

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at 111In(111Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.

Pressure dependence of terbium magnetisation

With the 111Cd probe DPAC experiments have been performed on polycrystalline Tb at pressures up to 7 kbar and temperatures ranging from 120 to 208 K. From the measured hyperfine fields we deduce the pressure dependence dT N/dp = -0.7(1)K/kbar for the Néel temperature and d ln M 0/dp = -0.05(2) Mbar -1 for the total terbium magnetisation. The latter value is considerably lower than those reported from magnetisation measurements in strong external fields.

Interface compound formation in Ni/In thin film couples

The perturbed γγ angular correlation method has been applied to study compound formation in Ni/In thin film couples. During an isochronal anneal cycle a variety of electric field gradient situations has been observed at 111In/ 111Cd probe nuclei, corresponding to the formation of different Ni/In compounds. Structure and stoichiometry of the compounds has been determined by comparison with bulk samples of the stable Ni/In alloys.

TDPAC studies on the recovery of Ag plastically deformed at room temperature

The recovery of polycrystalline silver rolled at room temperature to different degrees of deformation (33, 48, 73 and 95%) was investigated by means of the TDPAC technique with a 111Cd probe. The dependence of quadrupole interaction parameters, extracted from time spectra, on the degree of deformation and annealing temperature was studied. At temperatures in the range 350-425 K the recovery stage occurs which is indicated by the decrease of quadrupole interaction. For lower degrees of deformation also a small fraction ( approximately 2%) of sharp quadrupole frequency of 38(1) MHz was detected after annealing at temperatures 300-375 K, associated with vacancy complexes trapped on probe atoms.

Measurements of the electric field gradient at Cd in the high T c superconductor YBa 2Cu 3O 7−x

The temperature dependence of the electric field gradient at the site of 111Cd probe nuclei has been measured with the PAC method in the superconducting system YBa 2Cu 3O 7−x between 25 and 684 K. About 50% of the probe nuclei are exposed to a defined electric field gradient with a quadrupole coupling constant of 38.6(1) MHz and an asymmetry parameter of 0.37(1) at 295 K. Calculations of the electric field gradient indicate that the Cd atoms occupy regular Y lattice sites.

Evidence against reentrant magnetic behaviour in AuFe alloys

The temperature and concentration dependence of the transferred hyperfine field in Au 1− x Fe x with 0.15< x<0.28, was measured by perturbed angular correlation spectroscopy. The temperature dependence of the hyperfine field at the 111Cd probe shows a slope change at T/ T c =0.25 for all concentrations below 20 at% Fe, but an almost continuous behaviour for higher concentrations. These results manifest a percolation-like magnetic behaviour but no magnetic double transition.

Temperature induced structural transformation in RRh 3B 2 (R  Sm, Gd) compounds

Quadrupole interaction measurements as a function of temperature in 111In doped SmRh 3B 2 compounds show a structural transformation in the vicinity of 111Cd probe nuclei. The results are consistent with a local structural change from hexagonal CaCu 5 type to the base centred monoclinic which prevails for heavy rare-earth compounds.