Time Differential Perturbed Angular Correlation Technique Research Articles

Using radioactive beams to unravel local phenomena in ferroic and multiferroic materials

The increasing interest in using ferroic and multiferroic materials in high-tech applications requires that the underlying physical phenomena are studied at the atomic scale. Time-differential perturbed angular correlation (TDPAC) measurements have a local character and can provide important information concerning combined magnetic dipole and electric quadrupole interactions in multiferroic systems. With the application of characterization techniques and radioactive beams, this method has become very powerful, especially for the determination of the temperature dependence of hyperfine parameters, even at high temperatures. Such measurements lead to a better understanding of phase transitions, including observations of local environments in low fractions of different phases. Several facilities are in use at ISOLDE-CERN that benefit from the multitude of available ion beams that exist for the use and development of the TDPAC technique. An overview of the prior literature, combined with a discussion of measurement conditions and isotopes, is presented. Particular emphasis is given to the important case of measurements carried out at ISOLDE-CERN employing 111mCd as a probe.

Local structures at In impurity sites in ZnO probed by the TDPAC technique

Local structures in zinc oxide (ZnO) doped with In ions were investigated by means of the time-differential perturbed angular correlation (TDPAC) method with the 111Cd(← 111In) probe. Indium-concentration dependence of the TDPAC spectra suggests that the local structures formed by the impurities are sensitive to their population in the system: In ions occupy defect-free Zn sites when doped with a trace amount; they locally form microscopic unique structures widely dispersed in ZnO at 0.05 at. % or higher; and other compounds are formed at the In concentration as high as 10 at. %. Their characteristic structures are discussed based on the hyperfine-interaction parameters.

Magnetic moment of the 2083 keV level of140Ce

For the magnetic moment of the 2083 keV level of ${}^{140}$Ce, there are four published data, all obtained by applying an external magnetic field of less than 5 T to a liquid sample containing ${}^{140}$La using the time-differential perturbed angular correlation (TDPAC) technique. Although these four values are consistent within two times their uncertainties (2\ensuremath{\sigma}), the range of values in 2\ensuremath{\sigma} extends from $\ensuremath{\mu}=+3.0$ to $+$5.2 (in units of nuclear magneton, ${\ensuremath{\mu}}_{\mathrm{N}}$). This time, the TDPAC technique was successfully applied to the 2083 keV level of ${}^{140}$Ce implanted in an Fe foil. The magnetic moment of this level was determined to be $\ensuremath{\mu}=+4.00(20){\ensuremath{\mu}}_{\mathrm{N}}$, employing the known hyperfine field at ${}^{141}$Ce in Fe, \ensuremath{-}41(2) T, which agrees very well with one of the values, $\ensuremath{\mu}=+4.06(15){\ensuremath{\mu}}_{\mathrm{N}}$. The present value is compared with two shell-model calculations.

Production of 62Zn radioactive nuclear beam and on-line PAC investigation of quadrupole interaction in nano-magnetic material Fe73.5Cu1Nb3Si13.5B9

An Isotope Separator On-Line (ISOL), dedicated to on-line perturbed angular correlation (PAC) and positron annihilation spectroscopy (PAS), has been constructed at CIAE based on the HI-13 tandem accelerator, and a 30 keV radioactive beam of 62Zn with intensity of 2 ×107/s produced by it. Using the 62Zn radioactive beam the investigation of the nano-crystalline soft magnetic Fe73.5Cu1Nb3Si13.5B9 materials has been fulfiled for the first time by the on-line time differential PAC technique. Two quadrupole interaction frequencies ω01 ( = 440 Mrad/s) and ω02 ( = 90 Mrad/s) were gained, indicating there are two implantation sites of 62Zn. The frequency ω02 shows a fluctuation of the crystal lattice constants and proves the characteristic distribution model of nano-crystalline grain boundary. The frequency ω01 demonstrates that the implanted 62Zn occupies the site of the ordered grain core. The fractions of ω01 and ω02 are f01 = 38 % and f02 = 62 %, respectively.

Experimental Hyperfine Characterization of Adsorbed Molybdates on Oxide Surfaces

The dispersion of metal oxides on supports is a problem currently being investigated in supported catalysts. On the other hand, a nuclear spectroscopy technique, namely time differential perturbed angular correlation (TDPAC), has been used in extensive investigations on Mo-based catalysts. This technique, through the measurement of the local electric field gradient (EFG) at radioactive probes sites, can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments ofthe probes, their concentration and modifications related to in situ conditions (temperature, atmosphere, pressure, etc.) TDPAC technique was used in the present work to investigate the dispersion of molybdenum oxides on Al 2 O 3 , TiO2/Al 2 O 3 , and SiO 2 . The 99 Mo obtained by neutron irradiation of natural molybdenum present in the samples was used as TDPAC probe. Measurements at room temperature and higher ones as well as with different Mo concentrations in the samples were performed. The corresponding results allowed us through the fitted hyperfine parameters to find out the dispersion, type of adsorbed Mo-O species, and their relative abundances. For Mo concentrations below the theoretical monolayer we were able to find bulklike MoO 3 and two-dimensional-like Mo oxides. Previous results obtained with TDPAC and calculations based on a point charge model suggest different MoO 3 crystallites sites for each support. The structures of the monolayer type molybdena are probably different in each catalyst.

Observation of a Preferred Orientation in Indium Metal by the TDPAC Technique

The time-differential perturbed angular correlation(TDPAC) technique has been applied to study the microcrystalline structure of indium metal near the melting point with 111 In radioactive probes. The TDPAC function observed at 155°C just below the melting point (156.4°C) indicates the formation of a partially oriented matrix in a polycrystalline lattice.

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of111Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyEb for111In at a Cd lattice site and a Cd vacancy (111InCd−VCd−pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.

Time-differential perturbed angular-correlation-technique studies of internal oxidation of impurities in silver

The first quantitative results on internal oxidation of impurities in a noble-metal host, using the time-differential perturbed angular-correlation technique, are reported here. The system studied was indium-silver, with a few ppm of indium. The existence of a well-defined oxidation front is found for this highly dilute alloy. The kinetics of oxidation were determined for oxidating temperatures of 400 and 500\ifmmode^\circ\else\textdegree\fi{}C. Our results are in good agreement with a parabolic dependence of the depth of the internal oxidation front on the elapsed oxidation time. No unique indium-oxygen configurations were found in the range 280-900\ifmmode^\circ\else\textdegree\fi{}C.

Hyperfine magnetic field at Cd impurity site in Clb Heusler alloys PtMnSb and NiMnSb by TDPAC technique

The Time Differential Perturbed Angular Correlation (TDPAC) technique has been used to measure the temperature dependence of the hyperfine magnetic field (hmf) at the site of cadmium impurity in Clb Heusler alloys PtMnSb (Tc=575 K) and NiMnSb (Tc=720 K). Serving as probe was the 84-nsec 247-keV state of Cd-111, populated in the decay of 2.8-d In-111. Alloys were prepared by substituting for 2 atomic % of Sb by In metal which had been bombarded with 30-MeV protons to produce the In-111 activity. The measured hmf’s in kG at the Cd impurity, presumed to be at the Sb site, are (with temperatures in parentheses) in PtMnSb 166±4 (77 K), 153±2 (200 K), 138±2 (293 K), 125±2 (373 K), 95±2 (473 K), and 76±2 (488 K); in NiMnSb 213±3 (77 K), 208±4 (200 K), 194±3 (293 K), 191±4 (404 K), 169±4 (519 K), and 125±4 (630 K).

Hyperfine magnetic field at Cd impurity site in L21 Heusler alloys Rh2MnGe and Rh2MnPb by TDPAC technique

The hyperfine magnetic field (hmf) at nonmagnetic cadmium impurity in L21 Heusler alloys Rh2MnGe (Tc=450 K) and Rh2MnPb (Tc=338 K) has been measured as a function of temperature by Time Differential Perturbed Angular Correlation (TDPAC) technique. Measurement utilized the 84-nsec 247-keV state (μ=−.7656 nm) in Cd-111 populated in the decay of 2.8-d In-111. Alloys were prepared by substituting for 2 atomic % of Ge or Pb by In metal which had been bombarded with 30-MeV protons. The measured hmf’s in kG at cadmium impurity, presumed to be at Ge or Pb site, are (with temperatures in parentheses) in Rh2MnGe 188±6 (77 K) and 160±3 (293 K); and in Rh2MnPb 143±2 (77 K), 114±2 (200 K), 84±2 (273 K) and 53±1 (293 K).

Application of TDPAC techniques to diffusion models: diffusion mechanism of Ag in In

The general problem of anomalous diffusion is discussed in connection with the way in which the TDPAC (time differential perturbed angular correlation) technique may help determine the relative fraction of substitutional or interstitial positions of the diffusing atoms. The TDPAC technique referred to the Ag-In system is outlined. The present measurements give a fraction of 0.89+or-0.21 of Ag solutes in In substitutional positions. This suggests a dissociative mechanism for Ag diffusion in In. The value of 0.6+or-0.5 is obtained for the electric field gradient asymmetry parameter eta 2 of the assumed interstitial positions. This value shows a low symmetry for the interstitial sites, indicating that these positions might be split interstitials of Ag solutes in In.

Hyperfine magnetic field measurement in Heusler alloys with rhodium-99 and indium-111 by TDPAC technique

The hyperfine magnetic field (hmf) at nonmagnetic impurities Ru and Cd in Heusler alloys Rh2MnGe and Rh2MnSn has been measured as a function of temperature by the Time Differential Perturbed Angular Correlation (TDPAC) technique. Measurement of the hmf at the Ru utilized the 20.7-nsec 90 keV state in 99Ru populated in the decay of 16-d 99Rh; alloys were prepared using Rh metal powder bombarded with ?60 MeV protons. Measurement of hmf at Cd in Rh2MnSn utilized the 84-nsec 247 keV state in 111Cd populated in the decay of 2.8-d 111In; the alloy was prepared by substituting for 2 atomic per cent of Sn with In metal bombarded with 37 MeV protons. The measured hmf’s in kG at Ru impurity at Rh site are (with temperatures in parentheses) in Rh2MnGe: 195±11 (77 K), 126±4 (293 K), and 49±4 (355 K); in Rh2MnSn: 137±2 (293 K) and 37±6 (355 K). The measured hmf’s at Cd impurity (presumably at Sn site) in Rh2MnSn are 189±5 (77 K), 113±5 (293 K), and 33±3 (373 K).

The magnetic hyperfine field at140Ce in nickel

The hyperfine interaction of140Ce in nickel has been investigated by the time-differential perturbed-angular-correlation technique (TDPAC). The probe was produced by isotope separator implantation of the fission product140Xe, the β- decay chain of which finally populates excited states of140Ce.Different spin rotation spectra were observed before and after an 8 h annealing at 415°C. The analysis of the spectra led to the conclusion that the Ce ions were in the diamagnetic 4+ state. The dominant contributions to the hyperfine interaction are two different magnetic hyperfine fields: |H hf 1|=385±7 kOe and |H hf 2|=276±12 kOe.H hf 1 disappears after annealing. The fraction of nuclei which observeH hf 2 is increased by the annealing procedure from 16% to 75%. It is assumed thatH hf 1 is the hyperfine field of CeNi in an unperturbed substitutional site andH hf 2 is attributed to Ce ions which have trapped a single vacancy.

Quadrupole interaction at Ta and lattice location of Hf in Be

The time-differential perturbed angular correlation technique (TDPAC) has been applied to the 482 keV excited state of181Ta, to determine the nuclear electric quadrupole interaction (QI) at Ta in Be. Sources were prepared by ion-implantation of181Hf in Be. Particle channeling measurements have revealed that. Hf impurities implanted in Be reside primarily at the tetrahedral interstitial site. The fundamental QI frequency obtained for Ta at this interstitial site in Be is |vQ| =e2qQ/h = 227.0 ± 2.2 MHz which corresponds to an electric field gradient |eq|=(3.71±0.15)×1017 V/cm2. This result is discussed in terms of results for Cd impurities in the same system and the systematics of the impurity QI in metals.

The electric field gradient at Ta in HfB 2

Using the time-differential perturbed-angular correlation technique, we have observed the nuclear electric quadrupole interaction at 181Ta (482 keV) as an impurity in the refractory compound HfB 2. The measured interaction frequency is ν Q =730±5 MHz which corresponds to an electric field gradient of | eq|=(1·19±0·05)×10 18 V/cm 2 at room temperature. By considering a second measurement at 4·2°K, which yields the same results as above, and by comparison with available results for Hf in HfB 2, we conclude that the d-electron density of states at the Fermi level is quite small in agreement with trends observed by others.

Crystallographic texture and differential perturbed angular correlations

The effects of crystallographic texture in samples studied by the time differential perturbed angular correlation technique (TDPAC) are described and a variation on the experimental method is suggested. The results of a representative calculation are compared with experiment.