Study of the magnetic properties of GdZn compound using PAC spectroscopy with 140Ce and 111Cd as probe nuclei

Nanocrystalline Al₆₄Cu₂₃Fe₁₃ icosahedral quasicrystal has been obtained by milling of solid quasicrystal precursors prepared by arc-melt. The local structure around Fe atoms was studied by Mössbauer spectroscopy using a quadrupole splitting distribution method. Mössbauer results of annealed and milled samples show the existence of a broadened distribution of Fe sites which is associated to intrinsic disorder. The structural characterization was determined using x-ray diffraction. The average grain-size of the nanostructured quasicrystal, obtained from the line broadening of the X-ray diffraction peaks, was estimated to be of the order of 10 nm for a sample milled by 5 h.     

Study of hyperfine interactions in the intermetallic compound CePd2Si2 using PAC technique with 111Cd as probe nuclei

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°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.     

Hyperfine interaction study of electric field gradient in hafnium phosphide using181Ta probe

The hyperfine interaction of¹⁸¹Ta in HfP has been investigated by time differential perturbed angular correlation method which yielded interaction frequencyv _Q=630.20(15) MHz. The observed electric field gradient is calculated to be 1.66(25) × 10²⁰ V/m².     

Electric quadrupole interactions in nano-structured SnO 2 as measured with PAC spectroscopy

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.     

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.     

Non-ionic contributions to the electric-field gradient at 111Cd/181Ta impurities in bixbyites

The electric-field-gradient (EFG) tensor at both cation sites of the bixbyite structure in ¹⁸¹Hf-implanted Lu- and Sm-sesquioxides was determined by the PAC technique. The cumulated EFG data at Ta-impurity sites in binary oxides enable us to discuss the “universal” character of the empirical correlation between local and ionic contributions to the EFG in these systems. An EFG factorization in terms of the electronic characteristics of the probe and the geometry of the cation coordination is proposed, which explains the experimental EFG results at Ta/Cd impurity sites in bixbyites and agrees with a simplified decomposition of the EFG valence contribution coming from ab-initio calculations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of hyperfine interactions in pure and Co-doped CeO2 nanoparticles by PAC spectroscopy using 111Cd

In this work we have used the Perturbed Angular Correlation (PAC) technique to measure the hyperfine interactions in pure nanostructured (crystallite size around 70 nm) as well as doped with 3d transition metal Co (3 at. %) CeO₂ samples. The PAC measurements were performed at different temperatures between 23 and 527°C. The results did not reveal any evidence of hyperfine magnetic interactions. Spectra obtained presented three major frequencies related to substitutional sites in CeO₂, grain boundary sites and intrinsic defect related sites.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Orientation of the oxygen-induced non-axially symmetric electric field gradient at181Ta in Ta

The interaction of dilute¹⁸¹Ta in Ta with interstitial oxygen has been investigated via the γ-γ TDPAC-technique applied to the 482 keV state in¹⁸¹Ta. The trapping of Oxygen leading to a quadrupole interaction frequency ν_Q=580(5) MHz with an asymmetry parameter of η-0.37 (1) has been observed after melting the radioactive parent isotope¹⁸¹Hf with Ta. The temperature dependence of the quadrupole interaction frequency between 17 K and 293 K was found to be very weak whereas η varied by about 10%. In a single crystal experiment the orientation of the three principal axes of the electric field gradient leading to ν_Q was determined. The V_zz-axis points in <110>, the V_yy-axis in <100> and the V_xx-axis in <110> direction. These results can be understood in the charge cloud model, suggested by Wrede et al. /1/ for a similar situation found in the HfNb system.     

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.     

Hyperfine Interactions of 181Ta in Zr2Ni Observed Using PAC Spectroscopy

We have measured nuclear electric–quadrupole interactions (EQI) at ¹⁸¹Ta impurities substituted as Hf atoms into the Zr site in Zr₂Ni. Using perturbed-angular-correlation (PAC) spectroscopy, we measured the EQI over temperatures ranging from 10 to 1200 K. Over the entire range of temperature, the Zr₂Ni crystal has a bct Al₂Cu structure that includes a single Zr site. The crystal field symmetry surrounding this site is rather low, giving rise to a highly asymmetric electric-field gradient tensor. At 10 K, the EQI is characterized by an angular frequency ω₀=601(3) Mrad s⁻¹, and an asymmetry parameter η=0.835(2). At 1200 K, ω₀ decreases to 516(3) Mrad s⁻¹, and η also decreases to 0.790(4). Although weak, the temperature dependence of ω₀ is consistent with a (1−BT ^3/2) power law, in which B=6×10⁻⁶ K^−3/2. The EQI also manifests a very narrow linewidth. We observed no evidence either for magnetic ordering or for structural phase transitions in the temperature range covered by this experiment. Moreover, the sharpness of the EQI indicates that the samples as prepared are remarkably free of strain and defects. These results indicate that the Zr₂Ni structure does not promote the formation of defects and that the power-law dependence of ω₀ on T is insensitive to the asymmetric nature of the crystal. This revised version was published online in September 2006 with corrections to the Cover Date.     

Aspherical 4f-shell effects in rare-earth metals: Electric field gradient at Dy probe nuclei

Systematic DPAC measurements of the electric field gradient at Dy in very low concentration in pure Pr, Nd, Gd, Tb, Dy, Ho and Er metals in the paramagnetic phase have shown for the first time the host associated aspherical 4f-shell effects. The results are compared with the presently available first-principles based calculations of the electric field gradient and crystalline electric field parameters of rare-earth ions in rare-earth metals and alloys. The observed 4f-spin relaxation time of Dy ion in rare-earth metals is of the order of 2 ps. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric field gradients at (111)In/(111)Cd probe atoms on A-sites in 211-MAX phases

The method of perturbed angular correlation (PAC) was applied to selected MAX phases with 211 stoichiometry. Radioactive (111)In ions were implanted in order to measure the electric field gradients (EFG) in the key compounds Ti(2)InC and Zr(2)InC to determine the strength and symmetry of the EFG at the In-site. Further PAC studies in the In-free MAX phases Ti(2)AlN, Nb(2)AlC, Nb(2)AsC and Cr(2)GeC were performed to confirm that the In probes occupy the A-site as well. The strength of the EFG, with a quadrupole coupling constant ν(Q) between 250 and 300 MHz in these phases, is quite similar to the ones found in Ti(2)InC with ν(Q) = 292(1) MHz and in Zr(2)InC with ν(Q) = 344(1) MHz, respectively. Different annealing behavior was observed whereas in all cases a linear decrease of ν(Q) with increasing measuring temperatures was found. The experimental results are also in excellent agreement with those predicted by ab initio calculations using the APW+lo method implemented in the WIEN2k code. This study shows in an exceptional manner that (111)In?→?(111)Cd atoms are suitable probes to investigate the local surrounding at the A-site in 211-MAX phases.     

Lattice Location of 181Ta and 111Cd Probes in Hafnium and Zirconium Aluminides Studied by Perturbed Angular Correlation

The perturbed angular correlation technique was applied to study the lattice location of the ¹¹¹In/¹¹¹Cd and ¹⁸¹Hf/¹⁸¹Ta probe atoms in hafnium and zirconium aluminides. Compounds of different stoichiometries and crystallographic structures were the subject of investigation. According to the expectation, in all investigated compounds ¹⁸¹Hf/¹⁸¹Ta probes occupy the Hf(Zr) crystallographic sites. The ¹¹¹In/¹¹¹Cd probes are placed at the sites of all constituent metals—aluminum, hafnium and zirconium, depending on the crystallographic structure of compound, concentration of the constituent metals and temperature of the sample.     

Temperature dependence of the Isomer Shift and electric field gradient of181Ta in Re-metal

The 6.2-keV Mössbauer transition of¹⁸¹Ta was used to study both the Isomer-Shift (IS) and the quadrupole interaction in Re-single crystal host in the temperature range of 77 K to 517 K. Spectra were obtained for the¹⁸¹W diffused into Re-single crystal source using a single line Ta-metal absorber. The temperature change for the IS, δS/δT, is 55(1)×10^?4 mm/s·deg, which is of the order of ?24 times the second-order Doppler shift. The temperature change of the electric field gradient follows the T^3/2 relation. The relative smaller change of the EFG with respect to the IS change with temperature, reflects the strong role of the change of the s-electron density (s-d electron transfer) with temperature over other parameters.     

Study of magnetic behaviour of CeRh3B2 using111Cd probe

Using¹¹¹Cd probe nuclei, the transferredH _hf(T) is measured down to 35 K in CeRh₃B₂ using TDPAC technique. The angle β between the EFG and the direction of magnetization is obtained to be 90°. The observed smallH _hf(T) may be a consequence of the reduced moment on Ce in CeRh₃B₂.