NMR study of the electric field gradient in the paramagnetic phase of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _Q = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _zz ): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.     

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.     

DPAC study of radiation damage in178W recoil implanted silicon

The annealing behaviour of radiation damage in¹⁷⁸W recoil implanted n-type Si is studied from 295 to 641 K by the differential perturbed angular correlation method (DPAC), using¹⁷⁸Hf as probe nuclei for the first time. Preliminary results suggest that oxygen-vacancy (O-V) pairs are observed, which give rise to a quadrupole interaction characterized by |V _zz|=5.41×10¹⁸ V/cm² (v _Q=2550 MHz). The probe nuclei also experience an electric field gradient (EFG) due to distant defects.     

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.     

Die Hyperfeinstruktur des angeregten 32P3/2-Zustandes von Natrium

The hyperfine structure of the 3²P_3/2-state in the Na(I)-spektrum was investigated by optical double resonance. Three zero fieldrf-transitions (ΔF=±1, Δm_F=0) were detected and thus the unambiguous interpretation of therf-spectrum was made possible. From an analysis of therf-spectrum one obtains the magnetic hyperfine structure splitting constant a=(18.5 _?0.2 ^+0.6 ) Mc/sec the electric quadrupole interaction constantb=(3.2±0.5) Mc/sec which yields an electric quadrupole moment Q(Na²³)=(0.138±0.025)·10²⁴cm².     

Electric field gradient at 12 N implanted into ZnO

The nuclear quadrupole interaction of short-lived β-emitter ¹²N(I?=?1, T _1/2?=?11 ms) implanted into a ZnO single crystal has been studied by means of the β-NMR method. We have observed a quadrupole splitting of ¹²N in ZnO at room temperature, from which the electric field gradient (EFG) q?=?+?(8.6 ±1.1) ×10¹⁹ V/m² was deduced assuming axially symmetric EFG with respect to the crystalline c axis. A first principle calculation does not reproduce the data under assumption of the lattice location of ¹²N at the oxygen substitutional site.     

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.     

NMR of Dy nuclei in ferromagnetic DyAl2 and Dy metal

The NMR spin echo of Dy nuclei in ferromagnetic DyAl₂ yields for the Dy¹⁶¹ hyperfine field constant, a=?845.2±0.3 MHz and quadrupolar splitting, 2P=420.0±0.3 MHz compared to a=?830.0±0.5 MHz and 2P= 387.8±0.5 MHz in ferromagnetic Dy metal. The different contributions to these parameters are discussed. A line, observed at 1273 MHz Dy metal, is attributed to Δm_I = 2 transition.     

Mößbauereffekt in Eisenstilben und FeCl2·H2O

Using Mößbauer effect measurements in the temperature range between 3 °K and 310 °K the magnetic fields at the nucleus in iron-stilbene, FeCl₂·H₂O and FeCl₃ are determined to beH _T=0=(250±10) kOe, (252±18) kOe and (468±10) kOe; a Néel-temperature ofT _N=(23±1) °K is measured for iron-stilbene. The electric quadrupole splittings atT=0 °K for iron-stilbene and FeCl₂ ·H ₂ O, ΔE=(+2.52±0.02) mm/sec and (+2.50±0.05) mm/sec, yield electric field gradients at the iron nucleus ofq _z=+9.7·10¹⁷ V/cm² and +9.6·10¹⁷ V/cm², whereq _z⊥H; Debyetemperatures of θ=162 °K and 188 °K are obtained. The energy of the excited 3d-electron levels in iron-stilbene is estimated to Δ₁=309 cm^?1 and Δ₂=618cm^?1 as deduced from the temperature dependence ofΔE. In contrast to the suggestion ofEuler andWillstaedt bivalence of the iron in ironstilbene is found; its composition is shown to be 4(FeCl₂ ·H ₂O)·stilbene.     

Time-dependent perturbed angular correlation study of181Ta in Mg

The quadrupole interaction of¹⁸¹Ta in Mg metal has been investigated at room temperature by time differential perturbed angular correlation method yieldingv _Q=127±7 MHz. The observed EFG is calculated to be 20.8×10¹⁶ V/cm².     

Bestimmung des elektrischen Kernquadrupolmomentes des ungeraden stabilen Strontium-87-Kerns

In order to determine the electric quadrupole moment of Sr⁸⁷ (I= 9/2) the hyperfine structure-splitting of the 5s5p ³ P ₁-state of the SrI-spectra was investigated by optical double resonance. By detection of high frequency transitions (ΔF=±1,Δm _F=0,±1) in an external magnetic fieldH ₀≈0 one obtains the hyperfine structure separations asv _F=11/2?F=9/2=1463·149 (6) Mc/sec andv _F=9/2?F=7/2=1130·264 (6) Mc/sec. From these frequencies one calculates the magnetic hyperfine structure-splitting constantA=?260·084 (2) Mc/sec and the electric quadrupole interaction constantB=?35·658 (6) Mc/sec. B leads to an electric quadrupole moment ofQ(Sr⁸⁷)=+0·36 (3)·10^?24 cm².     

The sign of the quadrupole interaction of the 828 keV state of115In in Zn

Sign and magnitude of the electric quadrupole interaction of the 828 keV state of¹¹⁵In in a Zn single crystal was determined by aβ-γ TDPAC measurement. The result:v _Q =e·'Q _I V _zz /h=?193(13)MH_z confirms the universal correlation between the electronic and ionic contributions to the total electric field gradient proposed by R.S. Raghavan et al.     

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.     

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.     

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.     

Mössbauer Characterization of Iron-Based Nanogranular Films

The electric field gradients (EFG) of the Hf₂Fe intermetallic compound were calculated using the full-potential linearized augmented plain-wave (FP-LAPW) method as embodied in the WIEN 97 code. The obtained values are compared with other ab-initio calculations and on a qualitative basis with the previously reported experimental data obtained from TDPAC. The calculated results, −23.1·10²¹ V/m² and 2.7·10²¹ V/m² for Hf 48f and Fe 32e position, respectively, are in excellent agreement with experimental data (23.4·10²¹ V/m² and 2.7·10²¹ V/m²), better than those reported in earlier calculations. The calculated EFG for Hf 16c position (4.2·10²¹ V/m²) is stronger than the experimental one (1.1·10²¹ V/m²).     

Interpretation of the 119Sn Mössbauer parameters

The variations of the ¹¹⁹Sn Mössbauer isomer shift δ are interpreted for tin compounds from a semi-empirical tight-binding calculation of the electronic density at the nucleus ρ(0). A molecular model is proposed in order to relate the variations of ρ(0) for the Sn(IV) chalcogenides to the changes in the Sn environment. The variations of the experimental values of the quadrupole splitting δ are linearly correlated to the values of the electric field gradients (EFG) calculated by the full-potential linearized-augmented-plane-wave (FP-LAPW) method. The value of the ¹¹⁹Sn nuclear quadrupole moment is found to be |Q| = 10.5 ± 0.2 fm². Finally, the relation between the EFG and the Sn environment is discussed for SnO.     

Die relativen Quadrupolmomente der ersten angeregten 2+-Zustände von Os188, Os190 und Os192

The angular distributions of the deexitationγ-rays following Coulomb-excitation of the first excited 2⁺-states in Os¹⁸⁸, Os¹⁹⁰ and Os¹⁹² were measured using a metallic Target of natural Osmium. The measured attenuation coefficients areG ₂(Os¹⁸⁸)=0.798±0.013,G ₂(Os¹⁹⁰)=0.917±0.030 andG ₂(Os¹⁹²)=0.940±0.030. As a general test the angular distribution of the 330 keV-γ-rays of Pt¹⁹⁴ was also measured. This distribution was found to be completely undisturbed. Assuming pure electric quadrupole interaction with the internal crystalline fields one obtains an interaction frequency ofΔv _Q =eQ V _zz /h=278±32 MHz for the 155 keV-state of Os¹⁸⁸. Because the electric field gradients acting on the decaying nucleus are the same for all isotopes, one can deduce the ratio of the quadrupole moments of the excited states. The result isQ(Os¹⁸⁸)∶Q(Os¹⁹⁰)=1.11 _?0.19 ^+0.28 andQ(Os¹⁹⁰)∶Q(Os¹⁹²)=1.03±0.30. The effects of the uncertainties in the effective field gradients and their possible asymmetries on the integral attenuation factors are discussed. For 1≧G ₂?0.75 these effects are found to be small.     

The electric field gradient at the nuclear site of177Lu in Zn and In

The electric quadrupole interaction frequencyν _Q =eQV _zz /h of¹⁷⁷Lu in single crystals of Zn and In has been measured by the method of low temperature nuclear orientation. The results are $$\begin{gathered} v_Q ({}^{177}Lu\underline {Zn} ) = - 180(5)MHz \hfill \\ v_Q ({}^{177}Lu\underline {In} ) = - 19(5)MHz. \hfill \\ \end{gathered} $$ With the known quadrupole moment of¹⁷⁷LuQ=3.39 (2) b we derive for the electric field gradientV _zz (Lu Zn)=?2.20 (5)×10¹⁷ V/cm² andV _zz (Lu In)=?0.23 (6)×10¹⁷ V/cm². The results are compared with magnetostriction measurements of silver single crystals doped with rare earth atoms.     

Second-order quadrupole splitting of the55Mn NMR signal in KMno4 powder

In a KMnO₄ powder sample the⁵⁵Mn NMR signal has been observed using a FT pulse spectrometer at magnetic fields between 0.6T and 2.1T. A second-order quadrupole splitting has been found. The quadrupole coupling constant 3e²qQ/(2I(2I?1)h) is (240±10) kHz, the asymmetry parameterη is less than 0.05.