Perturbed angular correlation study of a nanostructured HfO2 film

The hyperfine field at ¹⁸¹Ta lattice sites in a nanostructured HfO₂ thin film was studied by the perturbed angular correlation (PAC) technique. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The thickness was about 25 nm. The radioactive ¹⁸¹Hf ions were produced by neutron activation of the very thin film in the Portuguese research reactor by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out at room temperature after annealing at different temperatures up to 1,473 K in air. The PAC technique allows determining the electric field gradient at the ¹⁸¹Ta probe sites. The ¹⁸¹Ta isotopes appear in the sample as disintegration product of ¹⁸¹Hf.     

Electric field gradient in nanostructured SnO2 studied by means of PAC spectroscopy using 111Cd or 181Ta as probe nuclei

Electric quadrupole interactions were studied in pure and Mn-doped powder samples and thin films of SnO₂ using perturbed γγ angular correlation spectroscopy (PAC). The powder samples were prepared by Sol gel method and the thin film were prepared on the Si (100) substrate by sputtering technique using Sn in the oxygen atmosphere. The samples were characterized by x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The thickness of the film was 100 nm. The average particle size of the SnO₂ powder samples was determined to be smaller than 60 nm. The radioactive ¹¹¹In and ¹⁸¹Hf tracers were introduced in the powder samples during the sol gel chemical process. Radioactive ¹¹¹In was implanted on the SnO₂ thin films using the University of Bonn ion implanter (BONIS). PAC measurements were carried out in a four BaF₂ detector spectrometer in the temperature range of 77–973 K for samples annealed at different temperatures. The PAC results for both nuclear probes show the presence of two electric quadrupole interactions. The major fractions in both cases correspond to the substitutional sites in the rutile phase of SnO₂. The results are compared with previous PAC measurements.     

Momentum dependence of spin polarization for beta emitting nuclei produced through charge exchange reaction at intermediate energy

Hyperfine interactions were studied in the intermetallic compound GdRh₂Si₂ by perturbed angular correlation (PAC) technique using ¹⁸¹Hf(¹⁸¹Ta) probe nuclei. The measurements were performed in the temperature range 15–285 K. The PAC spectra above the antiferromagnetic ordering temperature of the GdRh₂Si₂ compound (T _N ～ 106 K), were analyzed using a model that included only electric quadrupole interactions. The observed major fraction was assigned to the ¹⁸¹Hf(¹⁸¹Ta) probe substituting the Gd atoms. The PAC spectra below Néel temperature were analyzed using combined electric quadrupole and magnetic dipole interactions. The B_hf value at Gd, measured at 15 K was found to be 1.4(1) T which, is smaller, when compared with the values obtained in this compound using other nuclear probes, ¹⁵⁵Gd (B_hf ～ 30 T) and ¹⁴⁰Ce (B_hf ～ 26 T). The present result using ¹⁸¹Hf(¹⁸¹Ta) probe is quite interesting since it shows that the contribution to B_hf at Gd due the host is smaller than other components which contribute to the hyperfine field. The temperature dependence of B_hf shows an anomalous behavior.     

The Nonanuclear [Mo(IV){(CN)Fe(III)(3-ethoxy-saldptn)}8]Cl4 Complex Compound Exhibits Multiple Spin Transitions Observed by Mössbauer Spectroscopy

The hyperfine interactions at ¹⁸¹Ta ions on Fe³⁺ sites in α-Fe₂O₃ (hematite) were studied in the temperature range 11–1100 K by means of the perturbed angular correlation (PAC) technique. The ¹⁸¹Hf(β⁻)¹⁸¹Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B_hf and the electric field gradient (EFG) at the impurity sites. The angles between B_hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using ¹¹¹Cd probe. ^aAlso at Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe ¹⁸¹Hf(¹⁸¹Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 10¹² at/cm². PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000oC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 340 MHz for 30% of the probe nuclei. Annealing up to 600oC reduces the damping of this frequency without an increase of the probe atom fraction f_s in these sites. Above 600oC f_s grows rapidly until after the 900oC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of ν_Q= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800oC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000oC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric field gradients in Hf2Fe

Perturbed Angular Correlations (PAC) technique was applied to measure the electric field gradients (EFG) at ¹⁸¹Ta sites in Hf₂Fe. The compound has the cubic structure of the Ti₂Ni prototype with two non‐equivalent crystallographic sites for Hf atoms. EFGs for the two sites were measured as a function of the temperature. In addition, one more EFG was observed, which was assigned to the presence of defects in the lattice. The ratio of the measured EFG at the regular lattice sites has been used to assign charges to the ions through a simple calculation using the point charge model and it is shown that the lattice symmetry can explain the very different EFG at both sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of electric field gradient in TbCoO3

The temperature dependence of the electric field gradient (efg) in TbCoO₃ perovskite was measured by perturbed angular correlation (PAC) technique using ¹¹¹Cd and¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced into the compound through a chemical process during sample preparation. The electric quadrupole interactions at ¹¹¹Cd show two different sites, assigned to probe substituting Tb and Co atoms. The temperature dependence of quadrupole frequencies show sharp discontinuities which have been interpreted as thermally activated spin state transitions from low-spin ground state configuration to the intermediate-spin state and from intermediate-spin to high-spin state of Co^3?+? ion. For ¹⁸¹Ta only one interaction was observed, which was assigned to probe at Co site. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out. No magnetic order was observed till 77 K.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

Anomalous Temperature Dependence of the EFG in AlN Measured with the PAC-Probes 181Hf and 111In

¹⁸¹Hf and ¹¹¹In ions were implanted into AlN-layers in order to investigate their immediate lattice site environment and its temperature dependence by means of the Perturbed Angular Correlation (PAC) technique. After rapid thermal annealing at 1273 K up to 50% of the probe atoms were incorporated on undisturbed lattice sites defined by an electric field gradient (EFG) of 33 MHz for In and 572 MHz for Hf for measurement at room temperature. PAC-spectra taken at temperatures between 25 and 1200 K show that the EFG measured at the site of the undisturbed probes changes with temperature. While for Hf it decreases by 3%, for In it increases by 25% within the measured temperature range. Thus, the change cannot be due only to the thermal lattice expansion. In the case of In the fraction of probe atoms on substitutional sites increases with temperature until it reaches nearly 100% at 973 K. These effects are fully reversible. For the Hf probe, an additional EFG was detected at temperatures above 300 K.     

Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

PAC Measurements on New Ferromagnetic Compound Pd2TiSn

Magnetic hyperfine field (mhf) acting on ¹⁸¹Ta at the Ti site has been investigated in the alloy Pd₂TiSn by Perturbed Angular Correlation (PAC) measurements using the (133–482) keV – cascade in ¹⁸¹Ta following the ^– decay of ¹⁸¹Hf. The magnetic hyperfine field was measured as a function of temperature in the range of 45–1000K. The magnetization measurements were carried out with a vibrating sample magnetometer. The interpretation of the experimental results is based on the assumption that the probe nuclei occupy both the regular Ti sites as well as the Ti atom sites randomly distributed on the Pd sublattice.     

Perturbed angular correlation study of order– disorder transition in HfW2O8

The perturbed angular correlation (PAC) technique has been used to characterize the degree of atomic order in the neighbourhood of radioactive ¹⁸¹Hf isotopes in HfW₂O₈. PAC measurements were carried out at temperatures between 14 and 723 K. The compound was synthesized starting with the oxides HfO₂ and WO₃, using a method involving ball milling, high temperature annealing and quenching in liquid nitrogen. Fast cooling allows to have the compound at temperatures below 1050 K. The compound has a high degree of stability below such temperature and around 430 K atomic ordering occurs. This transition order–disorder is reversible. ^aAlso at Comisión de Investigasiones Científicas de la Provincia de Buenos Aires, Argentina.     

Electric field gradients at 181Ta probe in ZrNi: Results from perturbed angular correlation and first-principles calculations

Results of temperature dependent perturbed angular correlation (PAC) measurements in the equiatomic ZrNi alloy have been reported for the first time using ¹⁸¹Hf probe. At room temperature, values of quadrupole frequency and asymmetry parameter for the major component (~80%) are found to be ω_Q=26.8(4) Mrad/s, and η=0.413(7). The resulting electric field gradient comes out to be V_zz=2.99 ×10¹⁷ V/cm² and this corresponds to the probe nuclei occupying the regular substitutional Zr sites. In ZrNi system, no magnetic interaction is observed down to 77 K indicating absence of any magnetism in this material. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies on an inactive but similarly prepared sample confirm the dominant presence of the orthorhombic ZrNi phase in the sample. A complementary density functional theory (DFT) calculation results in V_zz=−2.35×10¹⁷ V/cm², η=0.46 at the ¹⁸¹Ta probe impurity site and zero magnetic moment on each atomic site, in close agreement with the experimental results. Furthermore, it is found that electric field gradient for the regular component follows a T^3/2 temperature dependence between 77 and 353 K, beyond which it varies linearly with temperature.     

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 |v _Q| =e ² qQ/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.     

Electric-field gradients at Ta donor impurities in Cr2O3(Ta) semiconductor

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf(→¹⁸¹Ta)-implanted corundum Cr₂O₃ powder samples in order to determine the magnitude and symmetry of the electric-field gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbitals (FP−APW+lo) calculations. The results are also compared with EFG results coming from PAC experiments in isomorphous α-Al₂O₃ and α-Fe₂O₃ doped with ¹¹¹In→¹¹¹Cd and ¹⁸¹Hf→¹⁸¹Ta tracers. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf (→¹⁸¹Ta)-implanted corundum α-Fe₂O₃ single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Hafnium binding to rat serum transferrin

Using the TDPAC-technique binding parameters for Hf were determined afterin vivo uptake of¹⁸¹Hf in rat plasma. As much as 98.5% of the metal ions proved to be bound to protein, essentially to transferrin. The main fraction of the¹⁸¹Hf ions experiences a well defined electric quadrupole perturbation frequency (vQ1=(1516 ± 15)MHz, δ₁=(5.3 ± 0.8)%) connected with a marked relaxation damping (λ = (46 ± 8)MHz). The remaining Hf nuclei are subject to a fairly broad distribution of electric field gradients (v_Q2=(1014 ± 37)MHz, δ₂=(16±3)%). The results are compared with data obtained within vitro ¹⁸¹Hf-labeled human transferrin.     

Temperature dependence of the electric quadrupole interaction at scandium impurities in zirconium and hafnium

The PAC probe⁴⁴Sc was employed to study the magnitude and temperature dependence of the electric quadrupole interaction at the site of the transition metal Sc in the hexagonal host lattices Zr and Hf. In Zr ∼ 100% of the probe atoms were situated on regular lattice sites and a quadrupole interaction frequency of υ_Q=7.8(1)MHz was measured at room temperature. For Hf the fraction of the probe atoms on regular lattice sites was smaller and a damping of the interaction pattern (υ_Q=9.3(5)MHz at 293 K) could not be excluded. The temperature dependence of the electric field gradient in both host lattices was found to be weak.     

Hf3Al2 and Zr3Al2 Isostructural Aluminides Studied by PAC with 181Ta and 111Cd Probes

The electric quadrupole hyperfine interactions for ¹⁸¹Hf/¹⁸¹Ta and for ¹¹¹In/¹¹¹Cd probes in polycrystalline Zr₃Al₂ and Hf₃Al₂ compounds were measured in the temperature range 24–1100 K. The hyperfine quadrupole interaction parameters were determined after different doping techniques and heat treatments of the samples.¹⁸¹Hf/¹⁸¹Ta was found to substitute the Hf/Zr sites and the ¹¹¹In/¹¹¹Cd impurities appear to substitute both the 8(j) Al sites and the three nonequivalent Hf/Zr sites.     

Decoupling experiments for the study of electric and magnetic hyperfine interactions in ferromagnetic compounds

The decoupling experiments involving the time-differential perturbed angular correlation (TDPAC) of the 133–482 keV γ-γ cascade in the presence of an external magnetic field applied along the quantization axis have been performed to measure the electric quadrupole and the magnetic hyperfine interactions experienced by the¹⁸¹Ta nuclei at Hf sites in the pseudobinary compounds HfFe_2-xSi_x, withx=0.1 andx=0.3. The hyperfine magnetic fields measured at 298 K areH _hf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe_1.9Si_0.1 andH _hf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe_1.7Si_0.3. The decoupling technique has also been used to obtain a —ve sign for the hyperfine field experienced by¹⁸¹Ta nuclei at the Ti or Hf sites in the Heusler compound Co₂Ti_0.8Hf_0.2Sn and a+ve sign for the hyperfine field at Zr sites in the cubic (C15) Laves phase compound ZrFe₂.