Nuclear orientation of154EuFe,166TmGd and169YbFe and169YbGd

Nuclear orientation measurements at low temperatures have been carried out on radioactive isotopes of¹⁵⁴Eu and¹⁶⁹Yb in Fe and of¹⁶⁶Tm and¹⁶⁹Yb in Gd. Gamma-rays from¹⁶⁹YbGd, obtained by the decay of implanted and melted sample of¹⁶⁹IuGd, were observed to have smaller anisotropies of opposite signs compared with those from¹⁶⁹YbFe. This may indicate that for Yb polarized in Gd electric quadrupole interaction is comparable with magnetic dipole one. Mixing ratios of the transitions in¹⁵⁴Gd,¹⁶⁶Er and¹⁶⁹Tm have been made for two levels in¹⁶⁶Er. Our analysis showed that the 3^– to 3⁺ beta decay to the 1128 keV level in¹⁵⁴Gd has 65(20)% j=0.     

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.     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureT _f() is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f() is found to be weak, for (Y, Gd)Al₂ T _f() is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.     

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.     

The magnetic hyperfine field at Hg impurities in ferromagnetic Gd

The magnetic hyperfine field at dilute Hg impurities in Gd has been investigated by the conversion electron (e ^–)--time differential perturbed angular correlation (TDPAC) technique. The radioactivities^197m Hg and¹⁹⁹Tl were implanted into Gd foils by means of an isotope separator. TDPAC measurements were performed with the 165 keV-L-conversion electron—134 keV--cascade of¹⁹⁷Hg at different temperatures and with the 334 keV--158 keV-K-conversion electron cascade of¹⁹⁹Hg at 200 K.The regular site occupation probabilities were found to be 15(3)% for an annealed^197m HgGd sample and 29(5)% in unannealed¹⁹⁹TlGd samples.From the magnetic hyperfine interaction frequencies measured for the regular sites at 200 K the magnetic hyperfine fields |H _hf(¹⁹⁷HgGd; 200 K)|=256(13) kG and |H _hf(¹⁹⁹HgGd; 200 K)|=267(7) kG were deduced.On leave from the University of Lisboa, Portugal     

Magnetic properties of (La,Gd)Al2 alloys

Theac anddc susceptibilities of (La_1–x Gd _x )Al₂ alloys were measured for moderate Gd concentrations (0.01x0.08) between 0.05 and 4.2 K. Sharp spin glass like maxima in ^AC are observed at temperaturesT _f which scale asT _f /x 26 K for all samples. However, scaling of ^AC is not observed indicating that the RKKY interaction at short distances which becomes more important with increasing Gd concentration, is ferromagnetic in this system. The amplitudeV ₀ of the RKKY interaction is obtained from both the approach to the saturation magnetization and ^AC forT0, yielding a mean value ₀=(1.5±0.3) 10^–26 eV cm³ which is in agreement with that observed in other systems with Gd impurities. Furthermore, the inferred value of the exchange integral is in accord with other measurements (T _c depression, reverse resistance anomaly) on (La, Gd)Al₂. A superposeddc magnetic field rounds the maximum of ^AC and shifts it to lower temperatures. ^AC is completely flat below 4.2 K in a field which saturates the isothermal remanent magnetization, showing once again the correlation between the peak in ^AC and the onset of irreversible properties.This work is part of the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

The183Re ground state dipole moment and nuclear spin-lattice relaxation of Re in Fe

The hyperfine interaction of the system¹⁸³Re(70d)Fe has been investigated with the NMR/ON technique. With the hyperfine field valueB _hf(ReFe)=–76.0(1.5) T the ground state magnetic moment was determined as: (5/2⁺,¹⁸³Re)=+3.12(6) _N. The field dependent nuclear spin-lattice relaxation time has been measured. The result for the high-field relaxation rateR _exp=1.65(5)·10^–15 T ²s K^–1 is explained in terms of indirect spin-wave interaction.     

Temperature dependence of the electric field gradient of Hg in Bi

The temperature dependence of the nuclear quadrupole interaction of the¹⁸⁸Hg(12⁺) isomer in a Bi host has been measured by the TDPAD technique. Assuming that the T^3/2 law is valid for this system a fit at the high temperature region yields _Q(T=0)=132(6) MHz and B=7.2(6)×10^–5K^–3/2. The EFG of HgBi and the temperature dependence strength follow the general trend of other probes in Bi.Supported in part by the MINERVA Foundation, Munich, Germany and the Israeli Academy of Science, Jerusalem, Israel.Incumbent of the Arye Dissentshik Career Development Chair.Incumbent of the Ruth Epstein Recu Career Development Chair.     

Magnetic properties of thin epitaxial Gd films on Nb

We investigated the magnetic properties of Gd films by magnetization measurements with the transverse magnetooptic Kerr effect in the temperature range from 2 to 295 K and for magnetic field up to 0.3 T. The thicknessd _Gd of the Gd films was varied between 11 and 1000 Å. For thick films (d _Gd500 Å) the form of the hysteresis curves and especially the nonmonotonic temperature dependence of remanence and coercivity can be qualitatively reproduced with a simple model employing theT dependence of the easy-axis direction. TheT dependence of the saturation magnetizationM _s of the thick films (d _Gd>=150 Å) strongly resembles the behavior of bulk crystals, whereas for the thinnest filmsM _s approximately depends linearly onT up to the Curie temperatureT _C . The thickness dependence ofT _C follows approximately a power law with an exponent 1.6 close to the theoretical value.     

Investigation of the behaviour of the impurity atoms in Si by μ− SR-method

The dependence of the residual polarization of negative muons in p-type Si on temperature in the 4.2–270 K range has been investigated. Measurements were carried out in external magnetic field of 0.08 T transverse to the muon spin. The impurity concentration in the sample was 2 · 10¹³ cm^–3. Muon spin relaxation was observed at temperatures below 30 K. The relaxation rate atT=30 K is equal to 0.18±0.08s^–1. The relaxation rate grows with the decrease of temperature and at 4.2 K exceeds 30s^–1. The value of the residual polarization at zero timeP(t=0) is constant within the investigated temperature range.In the rangeT<30 K data on the relaxation rate are well described by the dependence =B·T^–q, whereq=2.75. Power dependence of may evidence the essential role of the phonon mechanism in the relaxation of the electron momentum of the acceptor center.The authors express their gratitude to V.B. Brudanin and I.A. Yutlandov for providing the sample, and to Yu.B. Gurov for advices.     

Electric field gradient at100Rh in HCP zirconium and gadolinium

The measurement of QI frequencies at¹⁰⁰Rh in Zr and Gd reveal that the electronic contribution to the EFG is quite large. The value of ¦K¦ is 8 for¹⁰⁰RhZr whereas it is 45 for¹⁰⁰RhGd. The temperature variation of¹⁰⁰RhZr neither follows T^1.5 nor has a linear T dependence.     

Gyromagnetic ratios of first 2+ states in126, 128, 130, 132Xe

Gyromagnetic ratios of first 2⁺ states in^{126, 128, 130, 132}Xe were determined by implantation perturbed angular correlations (IMPAC). The effective hyperfine magnetic field together with the transient magnetic field at xenon nuclei in iron was utilized to obtain the precession of the angular correlation. The precession due to the transient field was taken from systematics to be /g=–36±6 mrad. The effective hyperfine magnetic field was determined in an experiment on¹²⁶Xe in iron to be 900±200 kG. The results for theg-factors areg(128)=0.41±0.07,g(130)=0.38±0.07 andg(132)=0.37±0.05. Theg-factor of the 2⁺ state in¹²⁶Xe was determined in a separate experiment using a radioactive source of¹²⁶I to beg(126)=0.37±0.07, and was used as calibration for the IMPAC-data.     

Saturation of ionization edge absorption by donors in germanium

A study of saturation of the absorption and photoconductivity of Sb and P donors in Ge for radiation of 90 m wavelength, i.e., of energy very closely above their ionization edges is presented at T=9.3 K. Under these conditions negligible heating by the excess radiation energy is expected, which provides a convenient opportunity to study the kinetics of photoionization and recombination. From these measurements we have determined the donor capture cross section of electrons at 9.3 K to be _c=(1.2±0.7)×10^–12cm^–2, and the relaxation time from the 2s to the ground state as ₂₁=(5.8±1.0)×10^–10s. The saturation intensity of the absorption coefficient is around three orders of magnitude higher than the saturation intensity of the photoconductivity. We explain the nonlinear photoconductivity by the Debye-Conwell dependence of the mobility on the number of photoionized donors and compensating acceptors.     

Isomeric transitions between shape coexisting states in iodine isotopes withA=115, 119 and 121

Applying delayed- r.f. coincidences and generalized centroid-shift analysis, nanosecond isomers in some odd-A iodine isotopes are studied in-beam. The reactions¹⁰⁶Cd(¹²C,p2n) and^116,118Sn(⁶Li,3n) have been used. The following half-lives were determined:T _1/2(565 keV)=0.50±0.10 ns andT _1/2(837 keV) =0.90±0.20 ns in¹¹⁵I,T _1/2(687 keV)=5.50±0.50 ns andT _1/2(2418 keV)=4.0±0.50 ns in¹¹⁹I as well asT _1/2(812 keV)=0.35±0.20 ns andT _1/2(445 keV)<0.10 ns=">¹²¹I. A systematics of the absolute probabilities for the transitions deexciting theg _9/2 9/2⁺ and _11/2 11/2 ₁ ^– band heads in^115–125I is presented and discussed.Partly supported by the Bulgarian National Research Foundation under contracts PH14, and PH31 and Kernforschungszentrum Karlsruhe, Stabsabteilung Internationale Beziehungen     

Magnetic and electric hyperfine interactions in the system EuSm

Magnetic and electric hyperfine interactions in the system EuSm were investigated with the isomeric 11/2^– state in¹⁴⁵Eu by applying the time-differential perturbed angular distribution (TDPAD) method. The temperature dependence of paramagnetism was studied between 90 K and 1000 K by measuring the magnetic hyperfine interaction frequency _L=g_NN¹ (T)B_ext. The paramagnetic correction factor strictly follows the Curie-Weiss relation =1+C/(T-), withC=–50(2) K and =–29(5) K. This is compatible with a hyperfine field ofB _int(0)=–25(1) T, a valence of two for Eu in Sm, and antiferromagnetic order at low temperatures.The temperature dependence of the electric quandrupole coupling constant v_Q, investigated between 100 K and 400 K, can be reproduced by a linear temperature variation v_Q(T)=v_Q(0) (1-AT), with v_Q(0)=16.2(4) MHz andA=7.2(8)·10^–4 K^–1.The paramagnetic relaxation time _rel of the nuclear alignment is proportional to the temperature of the sample, with _rel T^–1=3.7(2) ·10^–9s K^–1.This leads to the Korringa relation _J T=const=5.1(5)·10^–11s K for the relaxation time of the 4f electronic spinJ. Assuming that the relaxation ofJ is mainly caused by exchange interaction between conduction electrons and localized 4f electrons at the Eu site, an exchange integral of |J _eff|=0.10(2) eV can be deduced.     

39K NMR study of the low temperature phase transitions in KLiSO4

The³⁹K NMR spectra, spin-spin (T ₂) and spin-lattice (T ₁) relaxation times of KLiSO₄ have been measured in the temperature range from 300 K to 90 K. The temperature dependence of the³⁹K (I=3/2) NMR spectra demonstrates the occurrence of a first order phase transition atT _c1=217 K which occurs without a change in the K⁺ site symmetry and another first order transition atT _c2=190 K which is connected with a lowering of the K⁺ site symmetry and the formation of three kinds of ferroelastic domains. From the angular dependence of the second order quadrupole shifts of the³⁹K NMR 1/2–1/2 transitions the electric-field gradient (EFG) tensors at the potassium sites were determined at 290 K, 204 K and 180 K. The symmetry of the ferroelastic phase is monoclinic and not orthorhombic as the K⁺ EFG tensors are tilted away from thec-axis belowT _c2. TheT ₁ data further show the freezing in of the slow reorientational motion 10^–8 s with decreasing temperature from 300 to 90 K.On leave from: J. Stefan Institute, E. Kardelj University of Ljubljana, Ljubljana, Yugoslavia     

Inclusive production of φ-mesons in neutron-proton interaction at 30–70 GeV/c

A signal of the-K⁺K^– decays has been observed in neutron-hydrogen interactions at energies of 30–70 GeV in an experiment using the BIS-2 spectrometer. The longitudinal momentum spectrum in the forward region,x _F<0·1 atp _T<1 GeV/c, can be well described by a power law dependence (1–x _F)^N withN=4·28±0·42. Thep _T ² -spectrum has been parametrized by the exp(–Bp _T ² ) law with the exponentB=(3·02±0·55) (GeV/c)^–2. The extrapolation of the measured cross section to the full kinematic region yields a value of (220±85) b in agreement with measurements from the proton beams.     

μ+ depolarization inAlGd alloys

The ⁺ depolarization rate in diluteAlGd alloys containing 50 and 450 atomic ppm Gd was measured in a transverse field of 80 Oe over the temperature range 6–300 K. For both alloys, A increased dramatically above 200 K, reaching values of 0.69 and 0.93 s^–1, respectively, near room temperature. The results are interpreted as providing evidence for a thermally-activated trapping mechanism.This work was supported by the U.S. Department of Energy.     

Nuclear spin lattice relaxation of191mIr in Fe

Thermal cycling of the lattice temperature was used to determine the nuclear spin lattice relaxation of^191m IrFe in polarizing fields of 0.05 to 1.3 T. At low temperatures, the relaxation time is not very much shorter than the lifetime of^191m Ir. In the first part of the paper, the master equation formalism is extended to include a finite lifetime. Our result for the reduced relaxation constant, ² C _K =(1.48±0.11)·10¹⁴ K s^–1 T^–2 (high field limit) is in serious disagreement with that of a spin echo measurement of¹⁹³IrFe, but fits much better into the general systematics. A comparison of relaxation rates for 3d-, 4d-, and 5d-impurities in Fe is given. As a by-product, a Kapitza conductivity constant ofl _K =1.5 mW cm^–2 K^–4±30% was found between Fe and dilute³He/⁴He.     

Unusual behaviour of conductivity and superconductivity in solid Sn−Xe mixtures

We have measured the normal statedc conductivity , the superconducting transition temperatureT _c and the critical superconducting currentI _c0 of frozen Sn–Xe mixtures in dependence of the Sn atomic fractionc. A new, high-T _c phase is observed forc between 0.58±0.02 and 0.74±0.02. This phase has aT _c-value of about 5.7 K, it shows a strong decrease in and inI _c0 and transforms into disordered Sn during annealing between 15 K and 50 K.It is a pleasure for us to dedicate this paper to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday.This work was supported by the Deutsche Forschungs-gemeinschaft