[en] The hyperfine field at metal lattice sites in a nanostructured HfO₂ thin film was studied by the Perturbed Angular Correlation (PAC) technique using ¹⁸¹Ta as probe nuclei. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The resulting film thickness was about 25 nm. The ¹⁸¹Ta isotopes are formed in Hf sites of the sample as disintegration product of radioactive ¹⁸¹Hf, which were produced by neutron activation of the very thin film by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements of the electric field gradient at the ¹⁸¹Ta probe sites were performed with the thin film in the temperature range 295 - 1273 K in vacuum. Results showed that the film has a single crystalline structure and two different quadrupole frequencies could be observed corresponding to ¹⁸¹Ta at monoclinic structure and with oxygen vacancy in its neighborhood.

[en] The perturbed angular correlation (PAC) technique was used to study the hyperfine interactions in the antiferromagnetic and paramagnetic regions of the distorted perovskites LaCrO₃ and LaFeO₃. The dilute ¹¹¹In->¹¹¹Cd nuclear probes were introduced into the samples through a chemical process. The present measurements cover the temperature ranges from 15 to 848 K for LaCrO₃ and 77 to 1324 K for LaFeO₃. Two distinct electric-quadrupole interactions were observed in each compound. The lower quadrupole frequency was assigned to the transition-metal atom site while the higher frequency was attributed to the lanthanum site in both cases. Temperature dependence of the electric-quadrupole interaction parameters indicated structural phase transitions at around 512 and 1223 K, respectively, in LaCrO₃ and LaFeO₃. The phase transitions were associated with the change from an orthorhombic to rhombohedral structure and characterized by a sudden increase in the electric field gradient V_zz and a decrease in the asymmetry parameter η for both sites. PAC spectra measured below the Ne prime el temperature revealed that at 0 K the supertransferred magnetic hyperfine field on ¹¹¹Cd at the Cr site in LaCrO₃ (2.4 T) is much smaller than at the Fe site in LaFeO₃ (19.4 T). The magnetic field on ¹¹¹Cd at La sites in both compounds is of the order of 0.3 T. Additional measurements were made to determine the magnetic hyperfine field using the probe nucleus ¹⁴⁰La->¹⁴⁰Ce. The result reconfirmed that a relatively weak hyperfine field is supertransferred to the probe atoms at La sites

[en] The magnetic hyperfine field B_hf for Cd in the Heusler alloy Pd₂MnSn at the site of chemically introduced Ag has been investigated by PAC following the β^- decay of ¹¹¹Ag. Sign and temperature dependence of B_hf have been determined. Comparison of the result B_hf(T=0)=+8.0(1) T with earlier data and ab-initio band structure calculations leads to the conclusion that the Ag activity has been incorporated at the Mn site

[en] The magnetic hyperfine fields (MHF) acting on ¹⁴⁰Ce probes diluted in Gd and Co matrices, measured at low temperatures, were compared with those obtained from first-principles electronic structure calculations. It was found that the origin of the MHF for these two situations is completely different even though the total MHF presents similar values on both the cases. For Ce diluted in Gd, the Ce 4f shell is localized and the orbital MHF dominates. On the other hand, for Ce probes in Co matrix the Ce 4f shell is delocalized and, as a consequence, the MHF is dominated by the Fermi-contact contribution.

[en] Magnetic hyperfine fields (mhf) at ¹¹¹Cd and ¹⁴⁰Ce nuclei, dilutely substituting the In and Ce sites, respectively, have been measured in the intermetallic compound CeIn₃ using perturbed angular correlation technique. A pure electric quadrupole interaction with an axially symmetric electric field gradient was observed at ¹¹¹In(EC) ¹¹¹Cd probe nuclei at room temperature while a combined magnetic dipole and electric quadrupole interaction is observed below 10K. Below the ordering temperature, only a magnetic interaction is observed at ¹⁴⁰La(β^-) ¹⁴⁰Ce probe. The values of mhf measured experimentally as a function of temperature are discussed in terms of critical behavior

[en] The measurement of the electric field gradient (efg) with PAC spectroscopy was used to follow the heat treatment during the preparation of ZnO samples using sol-gel method. In particular, the investigation was carried out on samples of intrinsically n-type II-VI wurtzite semiconductor ZnO and Co-doped Zn_0.95Co_0.05O samples prepared by sol-gel methodology from pure metallic Zn(99.9999%). Carrier-free ¹¹¹In nuclei were introduced in the samples by thermal diffusion. ¹¹¹In solution was added to the precursor sol-gel solution prior to the formation of gel material. PAC measurements were carried out to follow the formation of the ZnO. Two undoped ZnO samples, which were heated in air and argon atmosphere, show different results. PAC measurements were also used to follow the ¹¹¹In diffusion in a commercially purchased ZnO (99.99%) sample as well as to compare the results with the measurements taken with sol-gel prepared samples. The results show that samples prepared by sol-gel process followed by heating in argon produce better quality ZnO samples. The results also show that the Co atoms in Zn_0.95Co_0.05O are in substitutional sites.

[en] The perturbed angular correlation (PAC) technique has been used to study the electric field gradient (EFG) in LaCoO₃ perovskite. The results are compared with those for LaCrO₃, LaFeO₃ measured earlier. The PAC probe, ¹¹¹In ^→111Cd, was introduced in the oxide lattice by means of chemical reaction during sample preparation. In the present work, the temperature dependence of the electric quadrupole interaction parameters, for LaCoO₃ was investigated. The resulting systematics of EFG at ¹¹¹Cd, in La(Cr,Fe,Co)O₃ perovskites, reveals a linear dependence with temperature

[en] A hybrid algorithm (HA) that blends features of genetic algorithms (GA) and simulated annealing (SA) was implemented for simultaneous fits of perturbed angular correlation (PAC) spectra. The main characteristic of the HA is the incorporation of a selection criterion based on SA into the basic structure of GA. The results obtained with the HA compare favorably with fits performed with conventional methods.

[en] Measurements of the electric quadrupole interaction were used to characterize pure and cobalt-doped samples of SnO₂ prepared by the sol-gel method. Perturbed gamma–gamma angular correlation (PAC) spectroscopy using ¹¹¹In–¹¹¹Cd probe nuclei was employed for these measurements. A methodology was developed for sample preparation that were prepared by sol-gel method from pure metallic Sn (99.9999%) and Co (99.9998%) as starting materials. Carrier-free ¹¹¹In was added to the precursor sol-gel solution prior to the formation of gel. PAC measurements were carried out to follow the formation of the SnO₂. PAC measurements were carried out in the temperature range from 10 k to 1123 K and the results show that the electric quadrupole frequency depends on the annealing temperature.

[en] Perturbed γ - γ angular correlation spectroscopy (PAC) has been used to investigate the hyperfine interactions in the intermetallic compound CePd₂Si₂ using ¹¹¹In^→111Cd probe nuclei. Samples of CePd₂Si₂ were prepared by melting constituent elements in an arc furnace under pure argon atmosphere. Carrier-free ¹¹¹In nuclei were introduced into the samples by thermal diffusion at 800 deg. C in vacuum during 12 h. The measurements were performed in the temperature range of 4.2-300 K. Above the magnetic transition temperature (T_N = 10 K), the results show two distinct and well defined quadrupole interactions that were assigned to probe nuclei occupying Ce and Si sites in the compound. The quadrupole frequencies were found to decrease linearly with increasing temperature. The PAC spectra taken below 10 K were analyzed with a model including combined electric quadrupole plus magnetic dipole interactions, from which the hyperfine magnetic field was determined.