Investigation of Hyperfine Interactions in CeIn3 byTDPAC

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(^–)¹⁴⁰Ce probe. The values of mhf measured experimentally as a function of temperature are discussed in terms of critical behavior.     

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.     

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.     

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.     

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).     

Investigation of spin transition in GdCoO3 by measuring the electric field gradient at Co sites

In the present work, the time differential perturbed angular correlation (PAC) technique was used to study the temperature dependence of electric field gradient (efg) in GdCoO₃ perovskite using ¹¹¹Cd and ¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the oxide lattice through chemical process during sample preparation and were found to occupy only the Co sites in GdCoO₃. The efg's at ¹¹¹Cd and ¹⁸¹Ta show temperature dependence with two different fractions each that change with temperature. In the case of ¹¹¹Cd the quadrupole frequency slowly decreases, with corresponding increase of the temperature and shows a peak like structure at around 200 K and a discontinuity at 700 K. These changes 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 Co³⁺${\mathrm{Co}}^{3+}$ ion similar to LaCoO₃ compound. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out.     

Electric field gradient in bixbyite rare-earth oxides R2O3 (R=Tl,Eu, Lu,Tm) measured by perturbed angular correlation

The electric field gradient (EFG) at ¹¹¹Cd nuclei dilutely substituting the cation site in bixbyite rare-earth oxides Tl₂O₃, Eu₂O₃, Lu₂O₃ and Tm₂O₃ has been measured using perturbed angular correlation technique. The ¹¹¹In(EC) ¹¹¹Cd probe nuclei were introduced into the samples by thermal diffusion. The experimental EFG values are compared with those calculated using the point charge model (PCM). The results are discussed in terms of a correlation between the electric field gradient and cation–oxygen bond length in metal oxides. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of the magnetic hyperfine field at cerium impurity in Co

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B _hf ) at Ce impurity in Co using ¹⁴⁰La→ ¹⁴⁰Ce probe. The radioactive ¹⁴⁰La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2– 1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B _hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.