Electric field gradients created by near neighbour impurity atoms in a cubic silver host

The electric quadrupole interactions produced by near neighbour impurity atoms of Cu, Au, Zn, In, Sn and Bi on¹¹¹Cd probe nuclei in a cubic Ag lattice were studied by the TDPAC method. The effects of the type of impurity and its concentration have been investigated. The results show the presence of a high-frequency interactionv _Q ^h superimposed to a smeared out low frequencyv _Q ^l . The high frequency interaction, in the range 20 to 600 MHz, is attributed to impurity atoms located in nearest neighbour sites, while the low frequency interaction, in the range 2 to 12 MHz, is generated by impurities distributed at various different atomic distances from the probe nuclei. Bothv _Q ^h andv _Q ^l increase with impurity concentration leaving the ratiov _Q ^h /v _Q ^l almost constant. The results show that the high frequencyv _Q ^h is linearly dependent on the solute valence, and a logarithmic function of the impurity concentration, in the range 0.5 to 4.5 at. %. Large size effects have been observed in CuAg and BiAg alloys. Instead for ZnAg and SnAg, thev _Q ^h andv _Q ^l variation is attributed basically to charge effects.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP).     

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 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.     

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.     

Nuclear magnetic resonance on oriented76,77,82Br in Fe

Nuclear magnetic resonance on oriented^76,77,82BrFe has been measured using recoil-implanted samples. The magnetic hyperfine splitting frequency of⁸²BrFe in a zero external magnetic field has been determined to be 201.90(3) MHz. The resonances of⁷⁶BrFe and⁷⁷BrFe were also observed in an external magnetic field of 0.2 T asv(⁷⁶BrFe)=340.9(3) MHz andv(⁷⁷BrFe)=403.5(2) MHz. With the known values of theg-factors, the hyperfine fields have been deduced:B _HF(⁸²BrFe)=81.397(27) T,B _HF(⁷⁶BrFe)=81.38(7) T. Theg-factor of⁷⁷Br was determined to be |0.6487(4)|.     

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.     

Satellite line structure in NMR-ON on206BiFe and182ReFe

Nuclear magnetic resonance on oriented²⁰⁶BiFe and¹⁸²ReFe has been measured using recoil implanted samples. Clearly resolved satellite structures were found for both systems. By annealing at an appropriate temperature, the resonance width was reduced. The satellite line structure for¹⁸²ReFe was studied with various annealing temperatures. The ratio of the resonance strength changed with the annealing temperature. The resonances for¹⁸²ReFe were also measured in external magnetic fields of 0.1, 0.2 and 0.4 T. The resonance frequencies for the satellite structure at an external magnetic field of 0.2 T were determined:v ₁=691.7(3),v ₂=684.4(2),v ₃=687.2(2) MHz for²⁰⁶BiFe;v ₁=231.15(5),v ₂=230.20(5) for¹⁸²ReFe. The origin of the satellite structure is discussed. The effective relaxation time of¹⁸²ReFe at 8 mK and external magnetic field of 0.2 T was determined to be 18.6(6) s using a single-exponential fit.     

Nuclear orientation and NMR/ON on implanted114mIn in Fe and Co

Nuclear orientation and NMR/ON of^114mIn implanted into Fe at an energy of 80 keV and dose of 3–5×10¹⁴ cm^–2 is reported. The zero applied field resonance frequencyv ₀=203.65(6) MHz is combined with the recently determinedB _hf(InFe) of 286.8(3) kG to yield (^114mIn)=+4.658(14) nm. The nuclear spin-lattice relaxation time for^114mIn in iron is measured to be 88(18) s at 18 mK and the applied field dependence of the NMR/ON resonance frequency gives the Knight shift for the system as –2.4(6)%. The absence of measurable nuclear orientation in similarly prepared^114mInCo sources is discussed.     

The electric field gradient at188Os nuclei in Re metal

The electric quadrupole interaction of the first excited 2⁺ state of¹⁸⁸Os in hexagonal rhenium metal was investigated by means of the time-differential perturbed angular correlation technique. From the observed quadrupole frequencyV _Q=170(7) MHz, we deduce an electric field gradient value of |V_zz|=4.77(23)·10²¹V/m² for the system OsRe. The half-life of the 2⁺ state was measured to be 641(4)ps.     

The sign of the electric field gradient at the nuclear site of Zn in Zn metal

By use of low temperature nuclear quadrupole orientation the quadrupole coupling constant of^69mZn in a Zn single crystal has been determined asv _Q=eQV _zz/h=–28(3) MHz.A positive sign forV _zzat the nuclear site of Zn in Zn is deduced. The half-life of^69mZn has been remeasured asT _1/2=13.756 (18)h.     

Electric field gradients of111Cd in the hexagonal (A-phase) La2O3 and Nd2O3 sesquioxides

Perturbed angular correlation measurements of¹¹¹In in hexagonal La₂O₃ and Nd₂O₃ yield well defined symmetric electric field gradients with coupling constantv _Q =280 MHz (extrapolated) in La₂O₃ andv _Q =269 MHz for Nd₂O₃ at room temperature. The coupling constant in La₂O₃ linearly decreases with temperature. The spectra of La₂O₃, strongly damped at RT, rise to full amplitude at 870 K indicating dynamical interactions.     

Nuclear quadrupole interaction of199mHg-ferrocenethiol complexes

We determined the^199mHg nuclear quadrupole interaction (NQI) by time differential perturbed angular correlation in the following ferrocenethiol complexes with mercury: ferrocenethiol (v _Q =1253(4) MHz, =0); 1,1-ferrocenedithiol (47%v _Q =1555(8) MHz, =0.13(2); 25%v _Q =726(19) MHz, =0.81(2); rest unspecific); 2-ferrocenyl-ethanethiol (v _Q =1306(6) MHz, =0.17(1)); and a 1, 1-bis (2-mercapto-propylthio)ferrocene oligomer (v _Q =1411(5), =0). All NQIs are rather large with small asymmetry parameters, indicating essentially linear S-Hg-S bonds. The only exception is the minority fraction in 1,2-ferrocenedithiol which suggests the formation of a 1,3-dithia-2-mercura[3]ferrocenophane.     

Quadrupole interaction of117In in Zn metal

We have used the TDPAC method to measure the temperature dependence of the quadrupole interaction frequencyv _Q of¹¹⁷In in Zn lattice. The observedv _Q reduces from 187.4 (20) MHz at 77°K to 155.3 (20) MHz at 673°K. The variation ofv _Q is seen to be proportional toT ^3/2.     

Magnetic hyperfine fields of Nb andMo in nickel by NMR-ON of90Nb and93mMo

The magnetic hyperfine splitting frequencies of⁹⁰NbNi and^93mMoNi in an external magnetic field of 0.2 T have been determined by the NMR-ON method to be 18.52(7) and23.73(10) MHz, respectively. With the assumption of Knight shift factorK=0 and with the knowng-factors, the hyperfine fields of⁹⁰NbNi and^93mMoNi were deduced asB _HF(⁹⁰NbNi)=-4.118(16) T andB _HF(^93mMoNi)=-3.491(33) T. The rather long spin-lattice relaxation time of 32(5) min was observed for⁹⁰NbNi at an external magnetic field of 0.2T and8 mK.     

The magnetic moments of (vg 9/2)-levels in odd Zn isotopes

^69m,71mZn have been implanted with an isotope separator on-line into a cold iron host matrix. Nuclear magnetic resonance of the low-temperature oriented isotopes has been observed. The resonance frequencies for zero external magnetic field are v_L(^69mZnFe@#@) =36.814(35) MHz andv _L (^71mZnFe)=33.47(19) MHz. From these the magnetic moments of the 9/2⁺ iosmeric states have been derived as μ(^69mZn)=(?)1.138(18) n.m. andμ(^71mZn)=(?)1.035(18) n.m. The experimentally known magnetic moments of (vg _9/2)-levels in odd zinc isotopes are compared to theoretical estimates.     

Nuclear orientation of111m Cd in Zn and Be and the quadrupole moment of the 245keV state

The 48.7 m^111m Cd activity was implanted in Zn and Be single crystals which were soldered to the cold finger of a dilution refrigerator and kept below 0.2 K during implantation. Subsequent nuclear orientation experiments allowed the determination of the quadrupole interaction frequencyv _Q of the 11/2^? isomeric state of¹¹¹Cd in Zn and Be as ?139 (15) MHz and +43(16) MHz respectively. With these results we derive the quadrupole moment of the 5/2⁺ 245 keV level of¹¹¹Cd including sign asQ = +0.83(13) b and the sign of the electric field gradient for Cd in Be. The half-life of^111m Cd was redetermined as 48.67 (6) m.     

Nuclear magnetic resonance on oriented194Ir (T 1/2=19.4 h) in Fe and Ni

The hyperfine interaction of¹⁹⁴Ir (j ^π =1^?;T _1/2=19.4 h) in Fe and Ni has been investigated with the technique of nuclear magnetic resonance on oriented nuclei. For both systems the electronic-orbital-momentum induced electric quadrupole splitting could be resolved. The magnetic and electric hyperfine splitting frequencies,v _M =¦gμ _N ^B _HF/h¦ andv _Q =e ² qQ/h, respectively, were measured as:¹⁹⁴IrFe:v _m =408.54 (23) MHz;v _q =?2.47(20) MHz;¹⁹⁴IrNi:v _M =135.24(5) MHz;v _q =?1.23 (3) MHz. Taking into account a 3% uncertainty arising from hyperfine anomalies theg-factor is deduced as ¦g¦=0.39 (1). The electric quadrupole moment,Q=+0.352 (18)b, is slightly smaller than expected from the known systematics of deformation parameters in this mass region.     

NMR spin echoes and quadrupole effects in nickel single crystals

NMR spin echo measurements of naturally abundant⁶¹Ni have been performed with Néel-type nickel single crystals at 4.2 K. The NMR excitation condition could properly be chosen in order to get signals either from nuclei situated within magnetic domains or within domain walls. In both cases a quadrupole splitting of the NMR line could be observed. By applying an external magnetic field, the direction of the domain magnetization could be varied with respect to the fcc crystal lattice. From the variation of the quadrupole splitting with the crystallographic direction, the tensor of the electric field gradient (EFG) has been derived. The quadrupole splittings and the corresponding field gradients in the principal axis system of the EFG are: Δv_Q kHz, Δv_Q kHz, Δv_Q kHz, V_<111>=(6.6±0.5)·10¹⁸ V/m², V_<110>=4.6±0.5)·10¹⁸ V/m², V_<112>=(2.0±0.5)·10¹⁸ V/m².     

Intrinsic point defects in niobium: Trapping at100Pd and111In impurities observed by PAC

Radiation-induced lattice defects in high-purity niobium have been investigated in the temperature range of 30K to 540 K by means of - perturbed angular correlation (PAC) measurements using the radioactive probes¹⁰⁰Pd/¹⁰⁰Rh and¹¹¹In/¹¹¹Cd. Both probes were produced within the niobium samples by means of heavy-ion nuclear reactions. At the Pd impurities trapping of defects occurred during heavy-ion irradiation at about 30 K in two defined configurations: defect 1(Pd) withv _Q1=e ² qQ/h=42(±2) MHz, ₁=0 and defect 2 (Pd) withv _Q2=(±2) MHz, ₂=1. Two defects were observed at the In impurities in annealing stage III (around 250 K) after heavy-ion as well as electron irradiations: defect 1(In) withv _Q1=87(±1) MHz, ₁=0 and defect 2(In) withv _Q2=105(±2) MHz, ₂=0.65(±0.02). A third defect (defect 3(In):v _Q3=177(±2) MHz, ₃0.2) appeared above 260 K after heavy-ion irradiation only. The data are interpreted in terms of interstitial trapping at the Pd impurities and vacancy trapping at the In impurities. Information on the microscopic structure of defect 1(In) and 2(In) is obtained from a PAC-single-crystal experiment. For defect 1(In) axial 111-symmetry is found, which leads us to identify this defect with a monovacancy as nearest neighbor with respect to the In probe. Defect 2(In) is the trapped divacancy for which an orientation is found that is consistent with both vacancies being nearest neighbor to the probe but second nearest neighbors to each other.     

Low-temperature nuclear orientation of209,210At after recoil-implantation into Be single crystals

The recoil-implantation technique has for the first time been applied to prepare samples of dilute^209,210At in Be single crystals. The quadrupole interaction frequencies _Q=e²qQ/h of²⁰⁹AtBe and²¹⁰AtBe were determined by quadrupole-interaction nuclear orientation at temperatures down to 7 mK to be –208(23) MHz and –138(10) MHz, respectively. The ratio of quadrupole moments Q(²⁰⁹At)/Q(²¹⁰At) was determined to be 1.47(7).We would like to thank Prof. P. Kienle for his kind interest and support of this work and K. Assmus for the irradiations. This work was supported by the BMFT, Bonn, and the KFA, Karlsruhe.