Characterization of nanostructured HfO2 films using Perturbed Angular Correlation (PAC) technique

The hyperfine field at ¹⁸¹Ta lattice sites in nanostructured HfO₂ thin films was studied by the Perturbed Angular Correlation (PAC) technique. Thin oxide films were deposited by Electron Beam Evaporation on a silicon substrate. The thickness of the films was ~100 nm and ~250 nm. Radioactive ¹⁸¹Hf nuclei were produced by neutron activation of the film samples in the Brazilian Research Reactor (IPEN IEA-R1) by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out after annealing at 1473 K. The PAC technique allows the determination of the electric field gradient (EFG) at the probe sites.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Hydrogen and deuterium trapping at nitrogen in tantalum studied by PAC

PAC experiments using¹⁸¹Hf as probe atom were performed in Ta samples containing 1.1 at% of N and loaded with 0.2 at% of H or of 1.6 at% of D. At room temperature only the known frequency due to N is present. Below 100K or 140K, for H and D respectively, the amplitude of this interaction decreases and two new interaction frequencies characterized by V₀=240MHz, η=0.80 and V₀=250 MHz, η=0.40 appear. These frequencies are attributed to H(D) trapped by N in the vicinity of the probe atom¹⁸¹Hf. The disappearance of these interactions at 100K and 140K is interpreted as a release of H (D) from Hf but not from N. From the data the jump rates of H(D) around N in Ta are derived.     

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.     

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.     

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.     

Static electric quadrupole interaction of Ta- and Hf-ions in barium and lead titanate

The static electric quadrupole interaction of¹⁸¹Ta and¹⁷⁸Hf in polycrystalline barium and lead titanate at the site of titanium has been measured using time differential PAC and the Mössbauer effect. The electric field gradients (EFG) at room temperature at the¹⁸¹Ta nucleus are ¦V _zz¦=(3.6±0.2)·10¹⁷V/cm² in BaTiO₃ and ¦V _zz¦=(14.6±0.6)·10¹⁷ V/cm² in PbTiO₃. The temperature dependence of the quadrupole interaction has been studied giving the following EFG values: ¦V _zz¦=(2.4±0.2)·10¹⁷ V/cm² in the monoclinic and ¦V _zz¦=(1.1±0.3)·10¹⁷ V/cm² in the rhomboedral phase of BaTiO₃, and ¦V _zz¦=(15.7±0.6)·10¹⁷ V/cm² for¹⁸¹Ta/PbTiO ₃ at 77 °K. The EFG of¹⁷⁸Hf in PbTiO₃ has been derived from a Mössbauer effect experiment to beV _zz=+(10.7±0.5)·10¹⁷ V/cm². The results are compared with EFG's calculated in a point charge model and with experimental EFG's measured at⁴⁴Sc and⁵⁷Fe in the same titanates by other authors. Contributions of covalent bonds to the effective EFG's in perovskit crystals are discussed.     

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.     

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.     

The electric field gradient at Ta in HfB2

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 HfB₂. The measured interaction frequency is ν_Q=730±5 MHz which corresponds to an electric field gradient of |eq|=(1·19±0·05)×10¹⁸ V/cm² 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 HfB₂, we conclude that the d-electron density of states at the Fermi level is quite small in agreement with trends observed by others.     

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.     

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.     

Zr-induced structural changes in Hf1−x Zr x O2 high-k thin films

The local symmetries around the Hf sites in thin films of Hf_1?x Zr _x O₂/Si(100) were probed using grazing incidence extended X-ray absorption fine-structure spectroscopy (EXAFS). The effects of the Zr incorporation on the local crystal symmetries were investigated using Hf L₃ EXAFS at the Beamline X23A2 of the Brookhaven National Laboratory. The Zr ratios in the various films were set to between 0.0 and 1.0. Significant changes in the local environment were observed for x = 0.25 or greater values. For x = 0.0, the film local structure around Hf sites remain in the equilibrium monoclinic phase as referenced from our previous studies on HfO₂ thin films on Si(100). When Zr is introduced, tetragonal symmetry around the Hf atom appears and becomes dominant at x = 0.63. Using the EXAFS theoretical simulations and non-linear least-square fit results, the fractions of the monoclinic versus tetragonal phases were identified in each film.     

Hyperfine field at 181Ta nuclei in ferromagnetic ZrZn2

A sample of ZrZn₂ containing 2% Hf has been studied by differential perturbed angular correlations of the gamma rays of ¹⁸¹Hf. A value of -15.1 ± 0.6 kOe was found for the hyperfine field at the ¹⁸¹Ta nuclei.