The low temperature quadrupole orientation of103Ru in ruthenium

The quadrupole frequencyv _Q =e ² qQ/h of¹⁰³Ru (Z=44,N=59) in a ruthenium single crystal has been measured using the technique of low temperature quadrupole orientation to bev _Q (¹⁰³RuRu)=−14.7(5) MHz. Temperatures below 2 mK were reached in this experiment using a PrNi₅ demagnetization stage attached to a³He−⁴He dilution refrigerator. Using the measured magnitude of the RuRu electric field gradient (EFG) at low temperatures |eq(RuRu)|=1.02(3)×10¹⁷ V.cm⁻² [1] and adopting the sign ofeq(RuRu) to be negative from systematics, this result yields a value for the ground-state electric quadrupole moment of¹⁰³Ru ofQ(¹⁰³Ru)=+0.59(2) b. This moment may be interpreted using the weak coupling model of de-Shalit [2]. A Korringa constant for¹⁰³RuRu ofC _K=39(6) Ks was measured in this experiment. Taking advantage of a small iridium contamination of the ruthenium single crystal, the quadrupole moment of the¹⁹²Ir ground state was determined to beQ(¹⁹²Ir)=+2.12(25) b. The sign of the IrRu electric field gradient was found to be negative as a result of this work.     

Quadrupole interaction studies of Hg in Sb

Time differential perturbed angular correlation and nuclear orientation studies of the electric quadrupole interaction for Hg in Sb have been performed. The effective field gradients |V _zz ^eff (HgSb)|=1.43(18)×10¹⁷V cm^–2 at room temperature andV _zz ^eff (HgSb)=+1.8(2) × 10¹⁷V cm^–2 below 0.05 K have been derived. These two values are no indication for an anomalous temperature dependence of the effective field gradient for Hg in Sb. The value of the electric field gradient fits well into the systematics for Hg in other hosts. It is shown that the electronic enhancements of the field gradients are correlated to the valence of the impurities and are rather insensitive to the host properties.On leave of absence from: University of Lisboa, Portugal     

The effect of temperature and concentration on the electric field gradient in binary indium alloys

The concentration and temperature dependence of the quadrupole hyperfine interaction of¹¹¹Cd in InTl (hcp),InPb (fct),InTl (fct) andInCd (fct and fcc) alloys were studied using the perturbed angular correlation technique. The change in the observed quadrupole interaction frequency with concentration can be described by a linear dependence on the axial ratioc/a in all cases. In the alloys with identical crystal structures the strength of thec/a dependence is independent of the solute, in contrast to the strength of the concentration dependence. In all cases where no phase transition occurs, the change in the electric field gradient with temperature follows the empirical relationV _zz (T)=V _zz(0) · (1–B·T ^3/2), where the coefficientB depends on the lattice structure, on the solute-solvent combination and on the concentration. The phase transitions ofInCd alloys at 293 K could clearly be seen as discontinuities in the temperature curves. A similar series of discontinuities observed around 116 K suggests the existence of a cubic low temperature phase.     

The quadrupole interaction of100Rh in various intermetallic compounds of palladium

The nuclear quadrupole interaction of the 75 keV excited state of¹⁰⁰Rh in the ordered intermetallic compounds PdHg, PdPb₂ PdSb and PdTe was measured. Using an estimate for the nuclear quadrupole moment of the 75 keV state in¹⁰⁰Rh and point ion lattice sums for the lattice electric field gradient (EFG) at the Rh site, the electronic contributionV _zz ^el to the total EFG was derived.V _zz ^el was found to be correlated with the average valence electron concentration in the compounds studied. In the case of PdPb₂, the temperature dependence of the quadrupole interaction was also studied and found not to follow theT ^3/2 rule which is observed in virtually all pure noncubic metals.Work partially supported by U.S. Department of Energy.     

The quadrupole interaction at181Ta in Zn

The quadrupole interaction (QI) in hexagonal close packed zinc lattice was measured using the 482 keV, 10.6 ns probe state of¹⁸¹Ta employing the time-differential perturbed angular correlation technique. The electric field gradient (EFG) at¹⁸¹Ta in Zn was derived from the measured quadrupole interaction frequency at room temperature asV _zz =12.202×10¹⁷ V/cm². The quadrupole interaction measured at various temperatures displayed normal temperature dependence similar to that seen by this probe in many non-cubic hosts.     

The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting Δv_Q of the nuclear hyperfine interaction due to an electric field gradient V_zz. MAPON has successfully been applied to measure Δv_Q as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes⁵⁶Co,⁵⁷Co,⁵⁸Co and⁶⁰Co have been studied in iron single crystal hosts, yielding Δv_Q consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe<100> system. The variation of Δv_Q with direction of magnetization, measured in⁵⁸CoFe and⁶⁰CoFe single crystal samples, is also described. Further MAPON measurements are described for a⁵⁶CoFe polycrystalline sample, for which the most probable value and width of the distribution of V_zz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.     

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.     

TDPAD measurements with highly oriented pyrolytic graphite using the19F(p,p′)19F* and19F(α, n)22Na* reactions

The TDPAD technique has been used to investigate quadrupole interactions of¹⁹F*(E _x =197 keV,J ^π=5/2⁺,T _1/2=88.5 ns) and²²Na*(E _x =583 keV,J ^π=1⁺,T _1/2=243 ns) in highly oriented pyrolytic graphite (HOPG). For¹⁹F* a single static quadrupole interaction was found with field gradientV _ZZ =3.24(19) × 10¹⁸ V/cm². For the case of²²Na* no quadrupole interaction was detected, leading to the conclusion that |QV _ZZ |<5.2(5) × 10¹⁵ b V/cm². TakingQ=0.06 b for the 583 keV state on the basis ofB(E2) measurements and theoretical estimates, we find that |V _ZZ | <8.7(8) × 10¹⁶ V/cm². The data demonstrate the advantages of using HOPG as a host material for the study of quadrupole interactions. Former Central Bureau for Nuclear Measurements (CBNM).     

Strong probe atom dependence of the electric field gradient in Bismuth

The nuclear quadrupole interaction of the 8⁺ isomers in^{206, 208}Po was measured in the semimetallic host Bi. The comparison of the extracted electric field gradient eq for PoBi with the values for the neighboring elements Bi and Pb as probe atoms in Bi reveals a very drastic increase from Pb to Bi and to Po by an order of magnitude: eq_PbBi eq_BiBi eq_PoBi =1.64(10)4.8(3)16(2)(all values at room temperature in units of 10¹⁷ V/cm²). This behavior is discussed in terms of the changing number of outerp-electrons of the probe atoms.     

Mössbauer and molecular orbital (MO) study of localized and delocalized mixed-valence organo-FeIIFeI bisandwiches

A Mössbauer study of two mixed-valence Fe^IIFe^I compounds C₅H₅Fe(C₅H₄)₂ FeC₆(CH₃)₆ (1) and ¦C₆(CH₃)₆ Fe(C₅H₄)₂ Fe C₆(CH₃)₆¦⁺ (2 ⁺) was carried out from 4.2K to room temperature. Zero-field spectra show two types of iron atoms for1 and one type for2 ⁺. Hence1 is a localized mixed-valence complex and2 ⁺ a delocalized mixed-valence complexe. High magnetic field spectra for 2⁺ give a negative sign for the EFG and show the valence electron is delocalized on the two centers. IEHT-MO calculations confirm the results and allow to explain the temperature independence of the quadrupole splitting (QS) of2 ⁺.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

Electric quadrupole orientation of iridium isotopes in rhenium

The quadrupole frequenciese ² qQ/h, for^{186, 188–190}Ir in a Re single crystal have been determined by nuclear orientation at temperatures down to 2 mK. Using a modeldependent value of +1.0 b for the quadrupole moment of the ground state of¹⁸⁹Ir, the electric field gradient of IrRe at 0 K is found to be–3.6×10¹⁷ V·cm^–2. The value is consistent with systematics proposed by Raghavan et al. Using this EFG, quadrupole moments are deduced for^{186, 188, 190}Ir.     

Electric quadrupole interaction of210Po(6+) in Bi single crystal

The electric quadrupole coupling constant of the²¹⁰Po(6⁺) isomer in a Bi single crystal was measured as a function of the temperature using the time-differential perturbed angular correlations technique. The electric field gradient (EFG) for Po in Bi is found to be temperature independent in the range 77–477 K. Using a calculated value of the nuclear quadrupole moment, the magnitude of the EFG is derived, and in turn serves to re-examine the systematics of quadrupole moments of λh ₉ ^2/2 configurations in the lead region.     

NMRON studies of ferromagnetic hcp and fcc Co

The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions (EQI’s) and, in particular, their distributions, are obtained for⁶⁰CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e²qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically cubic ferromagnetic host. The mode efg is V_zz=−27.3(32)×10¹⁹ Vm⁻². For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM of 12.0(7) kHz yielding a mode efg of V_zz=−3.5(5)×10¹⁹ Vm⁻².     

Investigation of the hyperfine interaction of the transition element impurity193Ir in gadolinium

The combined magnetic and electric hyperfine interaction of dilute¹⁹³Ir impurities in ferromagnetic Gd has been investigated by means of the Mössbauer effect. The magnetic hyperfine field of¹⁹³Ir in Gd at 4.2 K is: |H _hf(Ir:Gd)|=624(6) kG.The electric fieldgradient at the site of Ir in Gd is:V _zz (Ir:Gd)=+19.5(5.0) × 10¹⁷ V/cm².The fieldgradient is axially asymmetric with an asymmetry parameter of 0.53(2)1.     

Temperature dependence of the quadrupole interaction of67Ge and67Zn ins-p metals

The temperature dependence of the electric quadrupole interaction was measured on the 9/2⁺ isomeric state of⁶⁷Ge in metallic Zn, Cd, In, and Sn and on the 9/2⁺ isomeric state of⁶⁷Zn in Cd metal. For all investigated systems, the temperature dependence reproduces very well aT ^1.5-relation. The analysis shows that the strength of the temperature dependence in the pure metals is correlated to 1/(M _D ² ) ( _D =Debye-temperature). This favours lattice vibrations as the main component of the temperature dependence of the electric field gradient. The strength of the temperature dependence measured on an impurity atom generally differs from the value of the pure host. Possible explanations for this effect are discussed.     

Hyperfine interaction of ZrO2 — Tetragonal phase

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beV _zz =(17.5±0.4)·10¹⁷ Vcm⁻² and axially symmetric.     

Nuclear orientation of48V in Fe and Co and NMR of48VFe

Observations of the anisotropies of the 984 and 1312 keV -rays from⁴⁸VFe and⁴⁸VCo in fields up to 4 T and temperatures down to 4 mK show extremely long nuclear spin-lattice relaxation times (3–4 h in Fe, 8 h in Co host) and indicate negative hyperfine fields in both hosts. The Fe measurements show a magnetic moment for the⁴⁸V 4⁺ ground state considerably larger than previously reported. This is confirmed by NO/NMR of⁴⁸VFe, giving (4⁺)=2.012(11) _N. The advantages of pulsed NMR techniques, especially in systems with slow relaxation, are discussed.     

High-resolution measurement of NMR-ON for191PtFe

NMR-ON measurements on¹⁹¹PtFe were repeated to look for a reported quadrupole splitting effect: a sample annealed at 850°C and measured at 15.1(6) mK gives a spectrum with a single gaussian peak with a line width of 0.39(4) MHz, the narrowest ever observed. The spectrum obtained with another sample annealed at 650°C and measured at 8.3(4) mK resembles one with a quadrupole splitting, but the ratio of the second sub-resonance to the first is inconsisten with the estimation from a model which takes account of a quadrupole splitting and fast relaxation limit. The present study shows the quadrupole splitting ofv _Q (¹⁹¹PtFe)≤0.86 MHz.     

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.